The Motor Vehicle 2010 Part 14 pot

If the final drive ratio is 4 : 1, then the braking torque exerted on each road wheel is twice the brakingtorque exerted on the brake drum by the brake, that is, the total brakingtorque

936 The Motor Vehicle

road This not only increases the speed of onset of aquaplaning, but alsohelps to maintain a high coefficient of friction between tyre and road.Incidentally, a useful rule of thumb for estimating aquaplaning speeds fortyres without treads is:

Aquaplaning speed = 9 × √tyre pressure

It is based on experimental data and the fact that recommended tyre pressuresare a function of, among other things, the load on the tyre and the area of itscontact patch with the road

Aquaplaning occurs when the film of water on the road is driven by theforward rolling motion of the wheels into the wedge-shaped-gap between thetyre and the leading edge of its contact patch with the road At the criticalaquaplaning speed, the pressure in this wedge of water has risen to the point

at which it is high enough to support the vehicle Therefore, the tyres thenride up on to the film of water, which, of course, has a coefficient of frictioneven lower than that of ice, so the car is floating and will respond to neithersteering nor braking forces

An important aspect of design for active safety is the minimisation ofdriver stress and fatigue Another is provision for warning the driver ofdanger as early as possible before the situation becomes critical To this end,good all round visibility and efficient lighting at night are, for instance, two

of the measures that can be taken Others include the installation of devicessuch as electronic detection systems for warning the driver that he is becomingdrowsy: some of these depend on the monitoring of eyelid movements andothers of pulse and steering wheel movements Thirdly, the design should besuch that, should the car become involved in an accident, its occupants will

be, so far as practicable, protected from injury due to collapse of the structure.Fig 36.10 The HITS (Head Impact Test System) rig used by MIRA for assessing the occupant-friendliness of interior components and trim

937 Vehicle safety

36.6 Structural safety and air bags

Since it is neither practicable nor desirable to build vehicles as strong astanks, their basic structures must be designed to collapse in a controlledmanner in an accident A prime consideration is to prevent the steering wheelfrom being thrust back and crushing or penetrating the driver’s chest or neck

or, perhaps, even breaking his jaw Among the measures originally adoptedwere the inclusion of telescopic or concertina type collapsible elements in

the steering column In some early instances, the lower end of the steering

column tube was coarsely perforated, so that it would collapse when

subjected to heavy axial loading

Another of these measures was the incorporation of two universal joints,one at the lower end of a shortened steering column shaft and the other onthe steering box, the section between them being set at an angle relative tothe axis of the steering column In the event of a front end impact, the sectionbetween the two universal joints would displace laterally instead of pushingthe upper part of the column back towards the driver

Subsequently, two further changes were made One was to increase thearea of the hub of the wheel, to reduce the intensity of loading locally on thechest The other was to reduce the stiffness of the rim of the wheel, so that,

if the driver was thrown forward on to it, it yielded rather than severelydamaging his rib cage

Later, gas-inflated bags were installed in the steering wheel hub, Fig.36.11 These are supplementary safety devices, as they are effective only inconjunction with correctly adjusted seat belts They can be inflated by airbut, to obtain rapid deployment, inflation using chemicals producing nitrogen

or other gases are more commonly used Correctly tensioning the belt isimportant, otherwise it will fail to guide the driver in a manner such that hisface comes down on to the air bag instead of slithering over it and strikinghard objects beyond In the USA, failure of drivers to fasten seat belts hasbeen the cause of serious injuries, which has led, unjustifiably, to doubtsbeing expressed regarding the effectiveness of air bags

For the protection of front seat passengers, air bags are installed behind apanel in the dash fascia, and side air bags may be embodied in the seatsquabs An advantage of the latter site is that it moves with the seat when itsposition is adjusted, so the bag can be smaller than if it were stowed, forexample, in the door Moreover, in the door, it could be more vulnerable to

impact damage Mercedes has developed what they term window bags, 2 m

long, for the protection of the heads of all the passengers, which otherwisecould be injured either by hitting the side window or by intrusion These arestowed in the sides of the roof, and deploy in 25 ms Bags suspended fromthe cant rail and extending the full length for protecting the passengers in

both the front to the rear seats are sometimes called curtain bags.

In general, because the occupants’ heads start further away from the bagsthan do their shoulders, side bags at or near shoulder height, for protectionagainst side impacts, should open earlier than those for either window bags

or those for frontal impacts To meet this requirement, Toyota have developed

a system in which pellets of a chemical that generates mostly argon gas areused for inflation The sensors are mounted low in the centre pillars and theair bags are stowed in the front seat squabs

938 The Motor Vehicle

Padded lid Bag

Casing

Inflator

Dash

Steering wheel pad

Enhancer Squib

Bag

Gas generant Screen Inflator

Fig 36.11 Two Toyota gas bag installations: left, in the dash for the front seat passenger and, right, in the steering wheel hub, for the driver In both instances, an electrically

fired squib generates the heat to fire the pellets which generate the gas As the bags inflate, they push away the padded trim panels beneath which they are housed

Since the primary impact may be over within 10 ms, all the bags have todeploy within 20–30 ms To obtain rapid deployment, most manufacturersemploy pellets of sodium azide which, when heated, produce large quantities

of nitrogen to inflate the bags Sodium azide is a salt of hydroazic acid(N3H3) Initially, air bag deployment was mostly triggered by decelerationforce acting on some very simple form of mechanism, such as a ball in atube, mounted adjacent to, or within, the steering wheel hub Subsequently,electrically fired gas generators have been triggered by computers in response

to its receipt of appropriate deceleration signals The deceleration sensorsare usually mounted on a front transverse member of the vehicle structure

An advantage of this system is that the whole sub-assembly, including thegas generator, can be housed compactly within the steering wheel hub assembly,and the deceleration sensor can be placed in any position where it will bemost effective, Figs 36.11 and 36.12

Perforations in all bags allow the gas to leak out at a rate that increaseswith internal pressure, thus modifying their spring rates so that the occupants’heads do not rebound violently This at least reduces, and hopefully evencompletely obviates, the possibility of spinal whiplash damage Moreover,the deflation and collapse of the bag, within a few ms after inflation, leavesthe steering wheel relatively clear of obstruction so that the driver will have

a better chance of regaining control after the impact In the event of a multiplecollision, the air bags are, of course, effective in only the first impact

Steering wheel

939 Vehicle safety

36.7 Passenger compartment integrity

The compartment that houses the driver and passengers should remain intactafter an accident Four measures are necessary: one is to incorporate crushzones at the each end of the car; the second is to stiffen the door and itsimmediate surroundings so that, in the event of a side impact, it will not bepenetrated or deflected violently inwards and strike the occupants; third, thedoor trim must be soft or side air bags must be installed so that, if theoccupants are flung against it by the lateral acceleration, they will not beseriously injured; and fourth, the door frame and not only its joins but alsothose between the pillars and cant rail must be strong and stiff enough toreact elastically to absorb the shock loading

Basically, the occupants must be housed in what amounts to a strong cage,which will protect them also if the car rolls over This generally entails theuse of substantial fillets, and perhaps the fitting of reinforcement plates, atthe joints between the pillars and the cant rails and sills With the currentneed to reduce overall weight, the use of thin gauge high strength ductilesteel, instead of the traditional thicker gauge high ductility material for structuralmembers and some body panels can help to improve both crushability andintegrity of structures

It is important to design so that the loads due to an impact (whether front,rear or side) are, so far as practicable, spread uniformly throughout thewhole structure and that the proportions of all the principal members of thecage containing the occupants are adequate to react those loads elastically.Diagonal and transverse members may have to be incorporated under the

of door pillar) Weight

Front passenger’s air bag

Fig 36.12 Top, mechanically actuated bag firing mechanism: bottom, electrically

actuated alternative The latter has the advantages of greater compactness of the parts that may have to be accommodated in the steering wheel hub and the sensors and electronic control unit can be sited in the most appropriate positions

940 The Motor Vehicle

floor and, possibly, in the roof to transfer some of the loads from one side tothe other especially, although not solely, for catering for side or offset frontalimpacts

If the shock to the occupants is to be reduced significantly, a considerableproportion of the total kinetic energy of the moving vehicle must be absorbed

by the crush zone as it collapses At the front, the space between the grilleand engine is inadequate for absorbing that energy, except in very minorcollisions Consequently, in the more severe accidents the engine will bepushed back, and it is important to prevent it from thrusting the dash and toeboard back until they strike the occupants and possibly trap them in theirseats Consequently, the engine is generally mounted in a manner such that

it will be deflected downwards and slide under the toe board In particular,

if the engine is on a sub-frame, the attachment of the longitudinal members

of that frame to the toe board and front floor can be designed to shear, toenable the whole installation to slide back under the floor Even so, the dashand toe board structure must still be stiff enough to prevent significant engineintrusion into the saloon At the rear, there is more space for a crush zone, butthe fuel tank must not be ruptured, which is the reason for the modern trendtowards installing fuel tanks much further forward than hitherto

Ideally, the structure should collapse progressively at a constant rate, as if

it were a sprung buffer, Fig 36.13 One design method that has been successful

is to bow the longitudinal members so that they either spread outwards orcollapse progressively inwards when heavily loaded in compression Another

is to incorporate vertical swaged grooves in the side walls of straight members

so that they collapse in a controlled fashion Ideally, the swages would bedistributed alternately, along each side, over the length of the longitudinalmembers of the frame or sub-frame However, the zig-zag, or concertinatype of collapse thus aimed at is extremely difficult to achieve in practice.Once the first kink has formed, usually at the foremost swage, the member

is already bowed and therefore is more likely to continue to do so than toconcertina One manufacturer has notched the corners of the rectangularsection longitudinal members to initiate progressive collapse Each notch

Swages

Fig 36.13 Diagrammatic representation of front longitudinal frame member carrying the suspension and engine The lengths of the swages, in each set of four (in the top, bottom and two sides of the frame), become progressively smaller, from the foremost

to the rearmost, so that the frame will offer progressively increasing resistance to collapse in a frontal impact The lower diagram shows it only partially collapsed

941 Vehicle safety

extends from the corner only a very short distance down one face and a longdistance across the other face However, one should be wary of introducingnotches in such structural members subject to fatigue loading, since cracksare liable to be generated by and spread from the stress concentrations thusinduced

It is preferable to encourage simple bowing by siting all the swages alongeither the outer or the inner face rather than the top and bottom of each member,

to cause both to bow respectively either inwards or outwards If both bow outwards, the restriction imposed by the body panelling attached to them will help considerably in providing a progressive reaction to the crushing force, If they bow inwards, they are similarly restricted, but perhaps by the presence of the engine between them Inward bowing, however, tends to absorb more energy per unit of length of collapse This might or might not be what

is desired, hence crash testing is essential for proving designs

An aspect that should not be overlooked is that swaging the sides of thelongitudinal members will reduce their stiffness for reacting to side loads.This need not be serious if the ends of the vertical swages terminate short ofthe junction with the top and bottom plates, each of which will then become,

in effect, a separate U-section member The ends of the arms of each Uterminate where the swages begin, Fig 36.14 Incidentally, box sectionlongitudinal members can be welded fabrications Alternatively, they could

be square section tubes, the swages being produced by hydroforming, usinginternal hydraulic pressure to expand the tube into a mould

36.8 The problem of the small car

In an impact with a large car, a small car is inevitably at a disadvantagebecause the inertia of the former is greater than that of the small car Moreover,the provision of a crush zone of adequate length at both the front and back

of the small vehicle is, of course, much more difficult For this reason, theprinciple of designing for the engine so that, when thrust backwards, it slides

A useful rule of thumb is that a length equal to 16 multiplied by the thickness of the metal represents the maximum length that is stable on each side of each angle under compression, the measurement being taken from the inner face in each corner or, for the fabricated section, the centres of the bends

942 The Motor Vehicle

down beneath the toe board and floor is the only practicable course.Furthermore, maximum use should be made of transverse members to distributethe loads appropriately between all the longitudinal members, including thebody panelling, in a manner such that they are all equally stressed, as in Fig.36.15

An interesting feature in this illustration is the pair of gusset struts, oneeach side, between the front transverse member and each longitudinal sidemember If an impact occurs as indicated by either of the two thick arrows,the corner affected by the impact will be pushed back The gusset strut willstabilise the front end of the side member so that, assuming it is designed tocollapse concertina fashion, it will not bow Moreover, the transverse memberwill tend to pivot about the opposite corner, which will be stiffened by thegusset strut It therefore will offer more resistance to the pivoting movement,and therefore a larger share of the impact loading will be transferred to thatside than if there were no gusset member there At the rear, the design is suchthat the spare wheel will help to take some of the loading from a rear endimpact and transfer it to the main structure

At the rear, the main requirement again is to utilise transverse members tothe best advantage Also important is a robust C-pillar and a good supportingstructure for the rear axle Double skinning the rear quarter panels canenormously strengthen that part of the structure, although this does raiseproblems as regards repair to minor damage In general, the overall strengthand integrity of the occupant cage may need to be higher than that of a carwith long crush zones front and rear

Fig 36.15 Below: plan view of a Toyota frame designed to spread the loads imposed

by front and rear end impacts uniformly throughout the structure The combination of the front transverse member and the diagonal members, A and B, one on each side, triangulate the front end of the frame to constrain it to collapse concertina fashion, as

shown in Fig 36.13 Scrap view above: elevation of a different frame, showing how

the loads are distributed as viewed in a vertical plane The triangulation struts shown

in this example are fitted in the door frames

943 Vehicle safety

36.9 Side impacts

As regards side impacts, there is not enough space within doors to serve as

a crush zone, so the emphasis is on the use of transverse members betweenthe sills and cant rails to share the loading between the structural elements onboth sides of the vehicle Within the doors themselves, horizontal beams theends of which are securely fixed to the front and rear frame members of eachdoor are widely used However, it is difficult to make them stiff enough tohelp much unless the frame and especially its waist and bottom rails are verystiff, so that vertical or diagonal beams can be fixed to them to support thecentre of the horizontal ones The longer the door, the more intractable is theproblem Of particular importance is that the B-pillar be strong enough toprevent it and, with it, both doors from being pushed inwards in a sideimpact situation

In general, if the central portion of the outer panel of the door is thrustinwards, it will tend to pull not only its front and rear edges, but also thewaist and bottom rails towards each other Consequently, all these membersmust be adequately stiff Another measure that has been adopted, for example

by Volvo, is to fill the space between the outer and inner panels of the doorwith a plastics honeycomb If the hexagonal elements of the honeycomb arefairly thick, the filling as a whole will offer significant resistance to penetration.Moreover, it also transfers some of the loading radially outwards to the doorframe members and thus further reduces the tendency towards penetration ofthe door It would appear, however, that structural stiffening alone will not besufficient to satisfy future legislation, so the installation of side air bags tosupplement the door stiffening measures will probably be inescapable Armrests which could be forced against the vulnerable areas of the lower ribs ofthe occupants, should not be installed

36.10 Smart air bags

Some early work with air bags revealed shortcomings, but these have beenovercome First, the occupants of the car must be accommodated in fullysupportive seats, with their seat belts fastened Second, the bags in front ofthe driver and passengers must not deploy in any situation other than aserious frontal impact Third, because the impact in a crash is usually over inabout a tenth of a second, the deployment of the air bags must be accuratelytimed Deployed too soon, they might strike the occupants’ faces and causethe driver to lose control earlier than he might otherwise have done and, iftoo late, they may be ineffective

Research to overcome these problems has demonstrated that first the preciseshape of the impact acceleration pulse must be determined This is a function

of the crush characteristics of the front end of the car Then the characteristic

of the performance characteristics of the bags is ascertained, so that thedeployment and collapse can be synchronised with that of the pulse Gas-inflated bags deploy in about 20–30 ms, but they have perforations in them

so that they subsequently deflate to enable the driver to maintain controlafter the impact In any case, if they did not deflate, the heads of the occupantsmight bounce back from them, possibly causing neck injury

An outcome of this research is the development of computerised controlsfor regulating not only the deployment, but also the tensioning of the seat

944 The Motor Vehicle

belts These are the smart air bags referred to in Table 36.1 Signals transmitted

to the computer include seat belt tension, rapidity of brake application, andthe deceleration detected by a sensor mounted on a front transverse member

of the structure of the vehicle

If the belts are too loose, the occupants are accelerated forwards beforebeing suddenly restrained by them, which can cause injury Incidentally, theacceleration sensor for side air bag control is generally mounted at the base

of the door pillar Testing is now carried out initially using computer programs,which are followed by full-scale crash tests both to prove the validity of thecomputer modelling and to enable any fine tuning necessary to be done.Smart air bags are still under development, so further sophistication can

be expected A recent advance has been the provision of sensors and acontrol system that will inhibit deployment of bags in front of empty seats.This will reduce costs for the owner, since only those for the occupied seatswill need to be reinstated A further refinement that has been proposed isautomatic assessment of the size and weight of each occupant and his or herbelt restraint status, and setting the deployment characteristics accordingly.Yet another factor that can be brought into the equation is the direction andseverity of the crash

Compartmented air bags have been produced that could be selectivelyinflated, according to the severity and direction of the impact, and perhapsthe weight of the occupant of the seat, or whether a child seat has been fitted.Following instantaneous assessment of the weight of the occupant relative tovehicle speed or the severity of the impact and seat belt status, such a systemmight be able to inhibit deployment if it is unnecessary

TRW Automotive has developed what they call a heated gas inflator (HGI),

in the form of a vessel containing a weak mixture of hydrogen and air at apressure of 175 to 310 bar, as a substitute for explosive pellet type inflators Thisdevice would be difficult to accommodate in a steering wheel hub, but itwould be suitable for passenger and side air bags Two or more such devicesmight be used for multiple rates of deployment or for compartmented air bags.Further in the future, we might see radar-based systems for gauging theclosing speed of the car with the vehicle ahead, or any other object withwhich the car might be approaching, and setting the air bag control systemappropriately This could entail also the fitting of an acceleration sensor inthe crush zone

Current provisions for adjustment of the driver’s seat and steering wheelcould, if he had short legs and a long body, place him too close to thesteering wheel for safety in the event of air bag deployment It thereforecould become desirable to mount the pedals on a base plate that could bemoved horizontally to adjust its position relative to the seat This, togetherwith the usual seat adjustment facility, would give the driver the means ofpositioning himself optimally in the horizontal sense relative to his steeringwheel and other controls Vertical adjustment of either the steering wheel orseat might also be desirable, however

36.11 Seat belts

Ideally everyone would have a safety harness of the two shoulder strap (fourpoint) type, worn by aircraft pilots and rally drivers However, this is commonlyregarded as too restrictive to be acceptable by the motoring public It is also

945 Vehicle safety

more costly than the adjustable combined lap and single shoulder strap (threepoint) type harness The latter type is satisfactory if the lap belt fits snuglyround the pelvis, the upper anchorage for the shoulder strap is low enough toprevent strangulation or damage to the neck of the person it is supposed toprotect, the whole harness is a reasonably close fit around the body, and theoccupant can instantly release himself from it in the event of, for example,

The shape of the seat bucket must be such as to prevent him from sliding

down through the lap belt, sometimes termed submarining, and the combined

effects of both the lap and shoulder straps should restrain him from beingshot either forwards or upwards out of the seat Another requirement is thatwhen the belt is retracted, the buckles must be in a position such that, whenthe occupant is seated, he can easily reach them The upper buckle is usuallymounted on the B-pillar, and has to be pulled down and snapped into thesocket beside the seat pan Attachment to the B-pillar may present a problem,especially to cater for the seat’s being slid forward for the benefit of driverwith short legs To overcome this problem, the belt is in some instancescarried on the otherwise free end of a short arm pivoted to the B-pillar.Alternatively, either a manually or electrically actuated adjustment devicemay be installed

As previously indicated, if belts are not fitted snugly, violent accelerationforces can propel the occupant forward at high speed until the slack in thebelt is suddenly taken up and he strikes it with such force as to injure him

To avoid this situation without having to rely on the occupant’s making theappropriate adjustment, Toyota offer a system incorporating an automaticdevice for increasing the pre-tension in an emergency, Fig 36.16 When anelectronic sensor detects an acceleration rapid enough to throw the occupantsviolently forward out of their seats, the electronic control causes gas at highpressure to be released into a cylinder which is part of the seat belt tensioningmechanism A piston in this cylinder pulls a cable wound round the beltpulley, which it rotates to pull the harness tight Subsequently, the gas escapesthrough the clearances around the piston and cable rapidly enough to releasethe tension so that the occupants can, for example in the event of a fire,immediately unlatch their harnesses and escape Without seat belts, or even ifthey are inadequately tensioned, the occupants can be thrown violently invirtually any direction

Small children are best belted into rearward-facing safety seats of appropriatesizes Such seats can be anchored either to the adult seats or to the dash orbacks of the front seats This implies ensuring, at the design stage, thatsuitable anchorages are provided and that seat backs are strong enough totake the weights of both a front seat occupant and a child plus its safety seat.Where air bags are installed, care must be taken to ensure that they will notstrike the children if they deploy

946 The Motor Vehicle

36.12 Improvement of active safety

Active safety embraces the ergonomic design of the vehicle for ease ofcontrol by the driver without his becoming fatigued, as well as the moreobvious features such as harmonisation of the steering, braking, tyres suspensionand handling characteristics, to reduce the likelihood of his losing control.There are five main requirements:

1 That while the motorist is at the wheel, he can readily verify thatdriving conditions are safe

2 In every situation, all control responses should be proportional to thedriver’s input

3 All responses of the vehicle must be instant as well as accuratelyreflect the input

4 The vehicle must be dynamically stable

5 Drivers must be able to recognise when limits of stability are beingapproached

Belt web

Belt reel

Gas at high pressure

Plunger

Pretensioner cable

Fig 36.16 Toyota automatic seat belt tensioner In the event of an impact, gas is discharged from the horizontal cylinder on the right, into the chamber above the plunger, which it forces downwards Dragging the pre-tensioner cable with it, the plunger thus pre-tensions the seat belt for the duration of the impact, after which the tension is progressively released as the gas escapes through the clearances round the cable and plunger

947 Vehicle safety

Modern measures for improving dynamic stability include anti-lock brakesystems (ABS), traction control systems (TRC), and vehicle stability control(VSC), sometimes called vehicle dynamics control (VDC) None of these,however, extends the critical limit at which the tyres lose their grip on theroad Under normal conditions, all three systems are dormant, automaticallycoming into operation only in emergency situations, when they are neededfor the avoidance of an accident Provided the vehicle accelerates, brakesand turns consistently with the driver’s input, he should be able to avoidaccidents in all normal driving conditions

Different drivers behave in different ways in an emergency Some willslam on the brakes, others will steer out of trouble, some will do both, whileothers will, if appropriate, accelerate to avoid the problem These variationsshould be taken into consideration by the designer but, of course, it is impossible

to cater for drivers who freeze and do nothing: only passive safety can helpthese

36.13 Tyres, suspension and steering

The grip of the tyres on the road determines how rapidly the car will accelerate,where it will go and at what point it will stop Very rough roads may causethe tyres to bounce clear of the surface and therefore to lose their grip.Smooth roads when wet will tend to have a low coefficient of friction, so thecar may slide on a corner or the effectiveness of the brakes may be significantlyreduced Water on roads of any sort will lead to aquaplaning at some criticalspeed The more efficient the tread in squeezing water from the contact patchbetween the tyre and the road, the higher will be the critical speed as regardsaquaplaning This and other aspects of tyre design are covered in Section41.12

Suspension design is dealt with in detail in Chapters 42 and 43, so we arenow concerned only with the safety aspects As regards safety, the function

of the suspension system is to keep all four tyres on the road, to maintain aflat and stable ride, to keep the attitude of the wheels relative to the road inthe optimum position under all dynamic conditions, and to limit vehicleposture changes when cornering, braking and accelerating In other words, itmust reduce to a minimum changes in the position of the centre of gravity ofthe vehicle due to pitching and rolling

Steering performance is affected by suspension layout, tyre characteristicsand the centre of gravity of the vehicle, all of which affect the inherenttendency to over- or understeer These aspects are covered fully in Chapter

41 and the two just mentioned A summary of geometrical characteristics thatinfluence steering is illustrated in Fig 36.17 The steering should be firmand stable in the straight ahead condition, and the feel, or feel-back, of thesteering control is an important requirement as regards safety

During departures from this central position, with both manual and powerassisted systems, the feel-back should increase linearly, to give the driver apositive indication of the angle through which he has turned the wheel Somedesigners favour an increase in the rate over the last degree or so to full lock,

so that the driver has a positive indication that he is approaching the limit ofwheel movement Consistency of feel-back improves with increases in thestiffnesses of the links and mountings of steering mechanisms

948 The Motor Vehicle

36.14 Electronic control systems in general

Electronic control can be exercised either by a central electronic unit (ECU),

or individual electronic control units can be incorporated, as sub-systems toeach of the controls, such as steering, brakes, etc., where they can be used totransmit information to, and receive it from, the other sub-systems The lattergenerally offers the advantages of compactness and because, provided thecentral computer can be eliminated, the wiring harness can be simpler andinstallation easier

Because electric motors are amenable to electronic control, they are now

being considered for use as actuators However, it might be more practicable

to substitute electric motor drivenfor engine-driven hydraulic pumps, the former being potentially both lighter and more compact

36.15 Electric power assisted steering

Electronically controlled electrohydraulic power assisted steering systems have been

developed by, for example, AB Automotive Electronics, Delphi, EchlinAutomotive Systems and TRW Lucas Steering Systems In general, the directinput signals are vehicle speed and the torque applied by the driver to thesteering wheel Power assistance is provided by a 12 V permanent magnetbrushless electric motor which, in turn, drives a hydraulic pump to actuatethe assistance mechanism This type of motor is relatively quiet, powerfuland compact Moreover, by virtue of its low inertia, its responsiveness tochanges in demand is good

On the basis of signals indicating the temperature of the motor and currentflowing through it, the ECU regulates the speed of the hydraulic pump tothat appropriate for exercising control safely and efficiently As the steeringwheel is rotated further from the straight ahead position, the ECU applies aprogressively increasing current, and thus correspondingly increases thehydraulic assistance

A closed centre hydraulic control valve and engine driven pump take aconstant supply of energy at a fixed level from the engine, so the advantage

Kingpin

inclination

Vertical line through centroid

of wheel bearing assembly Direction

of roll

Kingpin offset

Kingpin trail

Caster trail

949 Vehicle safety

of the electronically controlled pump is that the power demanded is no morethan is required to cater for the instantaneous operating conditions At idle,for instance, the current can be as little as 0.5 A and, in most operatingconditions, it will be around 1–2 A Only under extreme conditions will thedemand become higher Consequently, energy requirement for power assistance

is reduced to a minimum

The implication, of course, is that, with the electronically controlled pump,the fuel consumption will be lower For the manufacture one advantage isthat a single power assistance sub-assembly can be common to the wholerange of vehicles manufactured, from sub-compact to minivans Another isthat, for development work on the test track, the unit can be tuned with a lap-top computer to try out various steering characteristics

Perhaps the most significant benefit that can be obtained is that, by virtue

of electronic control, it becomes possible to do things that would be impossiblewith mechanical or hydraulic control For instance, the compromises inherent

in conventional mechanical steering geometry can be obviated, because theelectronic system could control each wheel independently

Even the mechanical linkage between the steering wheel and gear could

be eliminated and replaced by a drive-by-wire system Aircraft are operated

in this way, so worries about failure would appear to be unfounded Variousways of getting around this problem, such as dual or triple control circuits,are available With such systems, the computer monitors all the circuits and,

if one is found to be malfunctioning, the computer switches it off and relies

on those that are functioning normally At the same time, a warning signalindicates to the driver that his steering system urgently needs attention

36.16 Brakes

Except to cater for their deterioration in service, there is no point in installingbrakes the torque capacity of which significantly exceeds the maximumadhesion limit of the tyres For safety, the vehicle should slow and stop in amanner consistent with the input by the driver to the pedal In emergencybraking, the attitude of the vehicle should, as previously indicated, remain sofar as practicable constant In all circumstances while the vehicle is slowing

or stopping, control should be easily maintained, and the performance of thebrakes should not vary with the length of time they are applied This isespecially important in emergency situations and during descents of long,steep inclines because, under these conditions, the friction elements tend tobecome very hot and brake fade could therefore occur

The attainment of all these aims is greatly facilitated if the braking effort

is divided between the front and rear wheels in proportion to both the front–rear weight distribution and the limiting adhesion of the tyres, which mayvary with their vertical and lateral deflections There should be no lag betweenpedal and brake application, and the feel-back from the pedal should accuratelyreflect the degree of braking applied at the road surface Finally, the performance

of the brake linings, or pads, should be consistent

36.17 Automatic braking and traction control

Automatic brake systems (ABS) have been covered in detail in Chapter 39and traction control systems (TRC) and limited slip differentials (LSD) in

950 The Motor Vehicle

Chapters 31 and 39 If the wheels lock, the coefficient of friction between thetyres and the road becomes lower than when they are rolling, and the vehicle

is liable to become unstable and skid To prevent the wheels from lockingwhen the brakes are applied, the sensor in a simple ABS system signals to acomputer the speed of rotation of the wheels In the more primitive systems,

as soon as the speed of any wheel is reduced to the point at which it is about

to lock, the computer signals the brake control to reduce the hydraulic pressure

to all four brakes In modern advanced systems, however, the pressure isreduced for only the brake of the wheel that is about to lock With either form

of ABS, therefore, brake control in an emergency is greatly simplified: allthe driver has to do is to push as hard as he can on his brake pedal

Limited slip differentials, Sections 31.3 to 31.7, help to prevent the totalloss of traction that occurs with simple differential gears when a drivenwheel on one side spins freely, for example on ice or in very soft ground Italso ensures that some torque is delivered to the inner wheel of a vehicle that

is cornering tightly, and thus it increases the overall tractive potential.Traction control systems prevent the wheels from spinning if the torquetransmitted to any wheel rises above that which can be transmitted by thetyre If one or more of the wheels spin, the consequent loss of coefficient offriction between their tyres and the road tends to cause the vehicle to becomeunstable and go out of control The sensor for detecting the onset of wheelspin is usually common to both the ABS and TCS systems but, of course, forthe latter function, it sends a signal of impending wheel spin, instead ofwheel lock, to the electronic control On receipt of such a signal, the computerorders application of the relevant brake until the tendency to spin is nullified,and thus maintains the vehicle in a stable condition

With four-wheel drive (4WD), the torque output from the engine isdistributed to four instead of two wheels, so the tractive force deliveredthrough each of the tyres is halved Consequently, the tendency to wheel spin

is correspondingly reduced To prevent torque wind-up to the drive-line,most modern 4WD systems have a differential or limited slip device betweenthe gearbox output and the drive shafts to the front and rear wheels

36.18 Recently introduced advanced systems

Four-wheel steering, although costly, has some advantages as regards stabilityand can increase ease of parking With the application of computers andelectronics to vehicle control systems, it is now possible to have active four-wheel steering In other words, the four-wheel steer system can be made toadjust the angle of the rear wheels to compensate for any force input fromone side, such as a sudden gust of wind or some other tendency to causeover- or understeer It also helps the driver to keep closely to his intendedcourse

36.19 Suspension control

Suspension performance can be improved too by an advanced safety measure.This is an electronic control system by means of which the characteristics ofthe dampers are adjusted automatically in relation to the speed of the vehicleand roughness of the road One such system is the Toyota electronicallymodulated suspension (TEMS), which also includes a two-way switch on

951 Vehicle safety

the dash for enabling the driver to select damping for either normal or sportingoperation For very many years manually adjustable dampers have, of course,been available, but very few drivers have the skill needed to make the appropriateadjustments manually

For the future, a further development could be active suspension, in whichelectronically controlled hydraulic jacks keep the body at all times at aconstant height and attitude relative to the road Some of these systemsdispense with suspension springs However, it seems more likely that thoseused in conjunction with them will ultimately preferred since, if the staticweight of the car is supported by springs, malfunction of the hydraulic jacksand their control system would not be so catastrophic In general, activesuspension has the advantage of utilising to the full the tyre performancepotential However, it is costly, it consumes energy and its durability andreliability remain open to question Consequently, for general application, itsfuture would appear to be in doubt

36.20 Ergonomic considerations and safety

Since ergonomic measures are taken before the accident, we shall categorisethem as active The driver’s seating position is of prime importance, in that

he has to use his eyes to obtain at least 90% of the information he needs fordriving safely As regards the position of the seat itself, this must be such thathis view of the scene outside the car is obstructed as little as possible bycomponents such as rear view mirrors or windscreen pillars; similarly hisview of the instruments, switches and other controls must be clear; at thesame time, he must be able to reach all his controls easily, with a minimum

of effort, and without being distracted from what is happening on the roadahead Visibility of indicator and warning lamps under brilliant sunlight is aconsideration sometimes overlooked All controls should be easy to operate,and instrument and other indicators easy to read In other words, the aim is

at enabling the driver to remain relaxed and comfortable throughout hisjourney, and thus minimising fatigue

Also important is his view of the four corners of his car Radar devices toindicate the proximity of obstructions have been suggested However, even

if they could be offered at acceptable costs, their effectiveness in trafficmoving at even relatively modest speeds would be open to question Drivers’rear view mirrors in many instances fail to cover a range of vision wideenough to include cars overtaking from all possible angles On the nearside,the view through the mirror should include the nearside wheel, for easeparking, as well as the road behind In general, mirrors should not be so widethat they are in danger of being struck by the mirrors of passing cars or otheritems such as gate posts Although some of the points raised here are relativelyunlikely themselves to cause accidents involving death or injury, they arerelevant as regards driver fatigue, which can have serious consequences.Driver fatigue is affected by, among other things, the climate: in coldcountries, an electric seat warming system may be desirable, but when it isvery hot, ventilation of the seat cushion and squab, as well as the saloon,may be more appropriate Air conditioning is even better, and can remove theneed for seat conditioning in any climate In hot countries, air conditioning

is generally regarded as essential, as also, of course, is interior heating andventilation in cold conditions

952 The Motor Vehicle

With VSC Without VSC

Fig 36.18 Left, diagram showing the path typical of an oversteering car driven beyond the limit of adhesion of the wheels on the road, compared with, right, one

driven in the same manner but equipped with vehicle stability control

953 Vehicle safety

As regards the squab, the most important requirement for comfort issupport for the lumbar region of the occupant regardless of his or her sizeand, preferably, this support should be adjustable Appropriate selection ofthe shape and position of the lumbar support can also reduce some of thepressure on the cushion Provision for adjustment of the angle of the squab

is, of course, also highly desirable Another requirement, and one that is notwidely appreciated, is cushioned, yet firm, support for the lower end of thespine, just above the coccyx It is in this region that spinal damage is mostlikely to be sustained in the long term as a result of wear and tear Because

of variations in the sizes and physical proportions of drivers, biaxial adjustment(vertical and longitudinal) of the positions of both the seat and steeringwheel can contribute significantly to comfort

For the prevention of whiplash injury of the spine in a rear end impact, theconventional headrest is not fully effective According to Volvo, the spinemay be affected throughout its length, so the shape and restraint offered bythe whole of the seat back is relevant To this end Volvo have been concentratingon:

1 Controlled resilience of the squab cushion and installation of a newrecliner system to reduce the severity of a rear end impact pulse on thespine

2 Reducing to a minimum movement of all parts of the spine relative toeach other, by providing good support from the base right up the spine

to the head, so that, throughout the impact, the curvature of the spinechanges as little as possible

3 Reducing to a minimum the forward rebound of the occupant from theseat into the seat belt

The outcome of design on these three principles is what Volvo calls theWHIPS seat In a rear end impact with a conventional seat, the occupant isfirst accelerated forward This causes him to sink into the resilient trim onthe squab and then rebound forward against the seat belt In the meantime,his head is supported by the head rest With Volvo’s new system, the forcesbetween the body of the occupant and the seat squab activate the WHIPSsystem The new recliner allows the squab to move backwards, and thereforereduces the forces between it and the body without increasing the distancebetween the head and its restraint Additionally, the seat squab bends backwards

to reduce further the g-force on the body The overall result is a reduction

also in the energy available to induce rebound

A safety seat introduced by Saab has what they term the Pro-tech aligning head restraint Their aim is at providing uniform support throughoutthe whole length of the spine In a rear end impact, this seat back absorbsenergy by allowing the lower part of the spine and back to sink into it in acontrolled manner, an action likened by Saab to the catching of a ball in apadded, gloved hand The rearward movement of the occupant’s body isreacted by a pressure plate in the squab This plate is connected to the lowerend of a vertical pivoted lever, the upper end of which pushes the head restforward until the moments about the pivot balance, to counteract the tendency

self-to whiplash, Fig 32.20 Saab claim that, with this system, the head rest can

be set in a lower position than would otherwise be safe

For the rear seats of some of their models, Saab provide vertically adjustable

954 The Motor Vehicle

head rests which can be lowered by the driver when it is not carrying passengers

in them In this way he is assured of the best possible range of rearwardvision If left in the lowered position, however, they are very uncomfortable

so, as soon as passengers enter the seats, they are obliged to elevate them tothe position in which they will provide adequate protection from spinalwhiplash

36.22 The pedal controls

Inappropriate pedal arrangement can have a significant effect on driver fatigue

as well as directly cause accidents To enable the driver to differentiate easilybetween the brake, clutch and accelerator controls, the pad on the acceleratorpedal should be positioned directly beneath the ball of the driver’s right foot,when resting in its natural position It should be slightly convex so that thedriver can easily pivot his foot about his heel to depress the pedal, with justenough friction between the sole of his foot and the pedal to enable him tomaintain a steady throttle opening when needed The range of angularmovement must be large enough to avoid jerky operation of the throttle and,over virtually all its range, the feel-back should be linearly progressive andproportional to throttle opening In some instances, when the throttle is justcracking open, the movement of the pedal relative to that of the throttle may

be increased to avoid jerky take-off from rest

For rapidity and ease use in an emergency, the brake pedal must be directlybeneath the driver’s left foot It should be significantly larger than the throttlepedal, so that the two are readily distinguishable from one another, and in

Axis of pivot

Fig 36.20 The Saab seat designed to prevent spinal damage due to whiplash At the lower end of the pivoted lever is a pad against which, during a rear end impact, the shoulders push to swing the head rest at the upper end of the lever forwards about the pivot to support the head Stops, not shown here, limit the motion of the lever

955 Vehicle safety

order that the pressure per unit area of the sole needed for application of thebrakes is not too high Obviously, since the loads applied to the brake pedalare much greater than to the throttle control, it must be both stronger andstiffer Brake pedal travel should be proportional to the degree of brakingapplied, and the length of travel must be large enough to provide the necessaryfeel-back, but without calling for excessive movement of the upper leg.The clutch and brake controls should be well separated so that the drivercan neither confuse one for the other nor accidentally push both downsimultaneously However, for comfort while cruising, there should be spaceadjacent to the break pedal for the driver to rest his foot clear of the pedals.Within reasonable bounds, the feel-back can be such as to give the driver apositive signal that the clutch is fully disengaged For example, with amechanically actuated clutch, a toggle linkage may be appropriate so that theresistance to pedal depression suddenly almost disappears when the clutch isfully disengaged

is converted into heat Again, just as when driving the car the torque of theengine produces a tractive effort at the peripheries of the driving wheels, so,when the brakes are applied the braking torque introduced at the brakedrums produces a negative tractive effort or retarding effort at the peripheries

of the braking wheels As the acceleration possible is limited by the adhesionavailable between the driving wheels and the ground, so the decelerationpossible is also limited Even so, when braking from high speed to a halt, therate of retardation is considerably greater than that of full-throttle acceleration.Consequently, the power dissipated by the brakes, and therefore the heatgenerated, is correspondingly large

When a brake is applied to a wheel or a car, a force is immediatelyintroduced between the wheel and the road, tending to make the wheel keep

on turning In Fig 37.1 this is indicated as the force F; this is the force which

opposes the motion of the car and thereby slows it down The deceleration is

proportional to the force F, the limiting value of which sepends on the normal

force between the wheel and the road, and on the coefficient of friction, or

of adhesion, as it is called Since the force F does not act along a line of

action passing through the centre of gravity of the car, there is a tendency forthe car to turn so that its back wheels rise into the air The inertia of the car

introduces an internal force F1 acting at the centre of gravity in the opposite

direction to the force F The magnitude of the inertia force F1 is equal to that

of the force F The two forces F and F1 constitute a couple tending to makethe back wheels rise as stated Since actually the back wheels remain on the

W S

W1 + Q W1 + W2 = W W2 – Q

S F F

Fig 37.1 G

O

957 Brakes

ground, an equal and opposite couple must act on the car somewhere so as

to balance the overturning couple FF1

This righting couple is automatically introduced by the perpendicular

force W1 between the front wheels and the ground increasing by a small

amount Q while the force W2 between the back wheels and the ground

decreases by an equal amount Q The forces +Q and –Q constitute a couple which balances the overturning couple FF1 The magnitude of the latter is F

× OG, so that other things being equal the smaller the height OG the less the

overturning couple The magnitude of the righting couple QQ is Q × SS, so

that the greater the wheelbase SS the less the force Q, that is, the less the

alteration in the perpendicular forces between the wheels and the ground.When going down a hill the conditions are changed From Fig 37.2 it will

be seen that the vertical force W, the weight of the car, can be resolved into two components H1 and K The component K is the only part of the weight

of the car that produces any perpendicular force between the wheels and theground, and is, therefore, the only part of the weight giving any adhesion.Thus on a hill, the adhesion available is necessarily less than on the level

The component H1, however, tends to make the car run down the hill, and if

the car is merely to be kept stationary, a force H equal and opposite to H1must be introduced by applying the brakes The forces H and H1 constitute

an overturning couple, which is balanced by an increase L in the perpendicular

force between the front wheels and the ground, and an equal decrease in therear

If, instead of being merely held stationary, the car has to be slowed down,

then an additional force F must be introduced between the wheels and the ground by applying the brakes harder An equal inertia force F1 is thenintroduced by the deceleration of the car This inertia force acts at the centre

of gravity of the car, and together with the force F constitutes an additional

overturning couple, which is balanced between the wheels and the ground.The perpendicular force between the front wheels and the ground is thus

increased by an amount L + Q, and that between the rear wheels and the

ground is decreased by the same amount Thus, on a hill, the decelerationpossible is less than on the level for two reasons First, the maximum

perpendicular force between the wheels and the road is reduced from W to K, and secondly, part of the adhesion is neutralised by the component H1 and isnot available for deceleration

If the rear wheels only are braked, the conditions are still worse, because

the force producing adhesion is still further reduced by the amount L + Q.

A little consideration will show that the opposite action occurs when the

958 The Motor Vehicle

car is being driven forward The perpendicular force between the front wheelsand the ground is then decreased, and that between the rear wheels and theground is increased, so that from the point of view of adhesion the rearwheels are a better driving point than the front wheels This is particularly sowhen accelerating up a hill

The extent of this alteration in the weight distribution depends directlyupon the magnitude of the deceleration, which, in turn, assuming the brakesare applied until the wheels are about to skid, depends upon the coefficient

of adhesion between the wheels and the road When that coefficient is lowthe maximum deceleration is low also, and the weight distribution is alteredonly slightly Under these conditions the relative effectiveness of the frontand rear wheels is in the ratio (approximately) of the weights carried bythese wheels, and if the weight carried by the front wheels is only a smallpart of the total weight little will be gained by braking them

The decelerations possible with modern braking systems are, however,high enough to make the braking of all the road wheels desirable and this is

a legal requirement in most countries

37.1 Two functions of brakes

Two distinct demands are made upon the brakes of motor vehicles First, inemergencies they must bring the vehicle to rest in the shortest possibledistance, and secondly, they must enable control of the vehicle to be retainedwhen descending long hills The first demand calls for brakes which canapply large braking torques to the brake drums, while the second calls forbrakes that can dissipate large quantities of heat without large temperaturerises It may be pointed out that the same amount of energy has to be dissipated

as heat when a car descends only 400 yards of a 1 : 30 incline, as when thesame car is brought to rest from a speed of 35 mph Thus heat dissipationhardly enters into the braking question when emergency stops are considered,but when descending long hills the problem is almost entirely one of heatdissipation

37.2 Braking systems

A driving wheel can be braked in two ways: directly, by means of brakesacting on a drum attached to it: or indirectly, through the transmission by abrake acting on a drum on the mainshaft of the gearbox, or on the bevelpinion, or worm, shaft of the final drive A brake in either of the latterpositions, being geared down to the road wheels, can exert a larger brakingtorque on them than if it acted directly on them If the final drive ratio is

4 : 1, then the braking torque exerted on each road wheel is twice the brakingtorque exerted on the brake drum by the brake, that is, the total brakingtorque is four times the torque on the brake drum Thus, brakes acting on theengine side of the final drive are much more powerful than those acting onthe wheels directly A transmission brake, however, gives only a single drum

to dissipate the heat generated, whereas when acting directly on the roadwheels there are two or more drums Also in many vehicles a transmissionbrake would be badly placed as regards heat dissipation, but in commercialvehicles it can sometimes be better in this respect than wheel brakes sincethe latter are generally situated inside the wheels and away from any flow of

959 Brakes

air The transmission brake has the advantage that the braking is dividedequally between the road wheels by the differential but the torques have to

be transmitted through the universal joints and teeth of the final drive andthese parts may have to be increased in size if they are not to be overloaded.The transmission brake at the back of the gearbox is fixed relatively to theframe so that its actuation is not affected by movements of the axle due touneven road surfaces or to changes in the load carried by the vehicle Invehicles using the de Dion drive or an equivalent, the brakes are sometimesplaced at the inner ends of the drive shafts and here again the torques have

to be transmitted through universal joints and also through sliding splineswhich may cause trouble

In present-day vehicles the wheel brakes are usually operated by a footpedal and are the ones used on most occasions; they are sometimes referred

to as the service brakes The brakes on the rear wheels can generally be

operated also by a hand lever and are used chiefly for holding the vehicle

when it is parked and are consequently called parking brakes but as they can,

of course, be used in emergencies they are sometimes called emergency brakes.

37.3 Methods of actuating the brakes

Considering manually-operated brakes, the brake pedal or lever may beconnected to the actual brake either mechanically, by means of rods or wires,

or hydraulically, by means of a fluid in a pipe Before considering theseconnections, however, we must deal with the brakes themselves

The construction is somewhat similar to that of a fluid flywheel and theunit is generally placed between the gearbox and the front end of the propellershaft but it can be incorporated with the gearbox The chief drawbacks ofthis type are that it is difficult to control the braking effort precisely and thatwhile it can provide large braking efforts at high vehicle speeds it can supplyvery little at low speeds and none at all when the road wheels are not rotating.Thus is can be used only to supplement a friction brake and so such devices

are often called retarders rather than brakes.

The electric brake is, in effect, an electric generator which, being driven

by the road wheels, converts kinetic energy into an electric current andthence, by passing the current through a resistance, into heat

The ‘eddy current’ brake employs the same principle as the eddy current

960 The Motor Vehicle

clutch described in Section 24.21 The rotor is coupled to the road wheels,being often mounted on a shaft that is interposed between the gearbox andthe propeller shaft, and the stator is mounted on the frame of the vehicle Theheat generated is dissipated chiefly by convection but this may be augmented

by some kind of fan which may be incorporated into the rotor

This type of brake suffers from the same drawback as the first type offluid brake, namely, that it cannot provide any effort at zero speed and can

be used only to supplement a friction brake A fairly large number of suchbrakes are in use at the present time, as retarders, and have been quitesuccessful

The vast majority of brakes are friction brakes and these may be subdividedinto: (1) drum brakes and (2) disc brakes, according to whether the brakedmember is a drum or a disc Drum brakes are still widely used and areinvariably expanding brakes in which the brake shoes are brought into contactwith the inside of the brake drum by means of an expanding mechanism.External contracting brakes are now used only in epicyclic gearboxes.The principle of the internal expanding rigid-shoe brake is shown in Fig.37.3 The brake drum A is fixed to the hub of the road wheel (shown in chaindotted lines) by bolts which pass through its flange The inner side of thedrum is open, and a pin B projects into it This pin is carried in an arm Cwhich is either integral with, or secured to, the axle casing, a rear wheelbrake being shown The brake shoes D and E are free to pivot on the pin B.They are roughly semicircular, and in between their lower ends is a cam M.The latter is integral with, or is fixed to, a spindle N free to turn in the arm

Q of the axle casing A lever P is fixed to the end of the cam spindle, andwhen this lever is pulled upon by a rod which is coupled to its end, the camspindle and cam are turned round slightly, thus moving the ends of the brakeshoes apart The shoes are thus pressed against the inside of the brake drum,and frictional forces act between them, tending to prevent any relative motion.This frictional force thus tends to slow down the drum, but it also tends tomake the shoes revolve with the drum The latter action is prevented by the

pin B and the cam M The pin B is therefore called the anchorage pin The

magnitude of the frictional force, multiplied by the radius of the drum, givesthe torque tending to stop the drum, that is, the braking torque

The reaction of this braking torque is the tendency for the shoes to rotate

A B C

D

P E

A B

C D

Q P

N M

R

M

Fig 37.3 Internal expanding rigid-shoe brake

961 Brakes

with the drum, so that this reaction is taken by the pin B and cam M, andultimately by the axle casing and the members which prevent the axle casingfrom revolving, that is, the torque-reaction system Most modern car brakes

do not have actual pins for the shoe anchorages but, instead, have simpleabutments against which the rounded ends of the webs of the shoes bear andare kept in contact by springs, but in lorries a separate anchorage pin is oftenprovided for each shoe, as indicated in Fig 37.4 which shows a design of theKirkstall Forge Engineering Company The anchorage pins are seen at A and

B and are carried in the projecting arm C of the brake anchorage bracket Thelatter is a force fit on the end G of the axle case, and a key is provided toprevent any rotation The actuating cam D is now of S shape, which provides

a greater amount of expansion of the shoes and a more constant leveragethan is provided by the simple cam shown in Fig 37.3 The cam D is integralwith its shaft and is supported in needle roller bearings, one of which is seen

at E The pull-off springs H are now single-leaf springs, which are easier toremove and replace than coil springs The seats F, to which the road springsare bolted, are formed integral with the brackets C

The cam expanding mechanism described above is simple in constructionand fairly satisfactory in action but there are others and two are shown inFigs 37.5 and 37.6 The first, Fig 37.5, is used in heavy lorries and is avariation of the S-cam described above, it is actually a double crank andconnecting rod mechanism, it provides a greater movement with less friction

R

W T

Fig 37.5 Brake shoe expanding Fig 37.6 Wedge-type actuator

mechanism

Fig 37.4

S D

H

C

A B

962 The Motor Vehicle

than the ordinary cam; when the S-cam is used friction is often reduced byemploying rollers at the shoe ends against which the cam surfaces bear Inthe second example, Fig 37.6, a wedge T is used and is pulled inwardstowards the centre of the vehicle by the rod R in order to apply the brake Thewedge operates through rollers that reduce friction and forces the plungers

or tappets U and V apart The body W that houses the tappets may be fixed

to the backplate of the brake assembly in which case the forces applied bythe wedge to the shoes may be unequal or it may be free to slide and thenforces will be equalised Equalisation of the shoe forces can be obtainedalthough the housing is fixed by leaving the wedge free to rock or slidesideways and an example of the latter is shown in Fig 37.31

37.5 Elementary theory of the shoe brake

Consider the simple shoe shown in Fig 37.7 An actuating force W will give rise to a normal force P between the shoe and the drum (this force is shown

as it acts on the shoe) and this normal force will produce a frictional force µP

if the drum is rotating as shown Now the shoe is in equilibrium under theaction of the forces shown, together with the forces acting at the pivot, butthe latter have no moment about the pivot and consequently the clockwise

moments due to the forces P and µP must be balanced by the anti-clockwise moment due to W Hence we get the relation—

M µQ

963 Brakes

so that the expression for the braking torque is—

conventional brake the leading shoe will become the trailing one if the direction

of rotation of the brake drum is reversed, and vice versa.

An actual brake shoe acts in a similar manner to the simple one consideredabove, the only difference being that the frictional force µP will act at a

larger radius than the radius of the brake drum and this will accentuate thedifference between the torques developed by the shoes

In a brake of the type shown in Fig 37.3, however, the expanding cam

will not apply equal forces to the shoes but will apply a greater force to the

trailing shoe Taking the data assumed above and supposing that a totalactuating force of 1000 N is available then the cam would apply a force of

767 N to the trailing shoe and only 233 N to the leading shoe The totalbraking torque would then be 8000 Nm If, however, the whole 1000 Navailable for actuation had been applied to the leading shoe alone then thebrake torque would have been 17 144 Nm, that is, more than twice as greatand this result can be obtained by making both shoes leading shoes andapplying 500 N to each

If the actuating mechanism were of the type that applies equal forces tothe shoes then each actuating force would be 500 N and the total braketorque developed by the shoes would be 8571 + 2608 = 11 179 Nm Thus afloating or equalising actuating mechanism gives an increase in brake torquefor a given actuating force but it has the disadvantage that the wear of theleading shoe (assuming the shoes to have linings of the same material) would

be 3.29 times that of the trailing shoe The brake with two leading shoeswould not suffer from this drawback and, as has been seen, gives an evengreater brake torque Such brakes are, therefore, widely used, particularlyfor front wheels When hydraulic actuation is used it is a simple matter tomake both shoes leading ones for the forward direction of rotation; the brake

is arranged as shown in Fig 37.9, two actuating cylinders, connected by apipe, being used instead of one cylinder For the reverse direction of rotationboth shoes would be trailing shoes and the brake would be rather weak Forthis reason it is usual to employ the two-leading-shoe brake in the front wheelsonly, the rear brakes being the conventional leading and trailing shoe type

964 The Motor Vehicle

When the brake actuation is mechanical it is not so simple to make bothshoes leading ones but a relatively simple mechanism has been developed byGirling and the principle of this is shown in Fig 37.10 The expandingmechanism does not act directly on the shoe but on one arm of a bell-crankwhich is freely pivoted on a pin carried by the shoe The other arm of thisbell-crank bears against a fixed anchorage (shown cross-hatched) and theshoe itself can bear on this anchorage and on another one at the top, asshown It should be clear that, supposing the arms of the bell-cranks to be all

equal in length, the force in the strut will be equal to the actuating force W

and this force will act on the bell-cranks as shown; also that the bell-crank

at the bottom will press on the anchorage with a force W and the anchorage

will press back equally on the bell-crank as shown The resultant force on

each bell-crank will thus be a force R as shown and these forces will press

the shoe into contact with the drum If the drum is turning clockwise the shoewill now move round clockwise very slightly until it bears on the anchorage

at the top and is thus a leading shoe while if the drum is turning clockwise the shoe will turn anti-clockwise and will bear on the anchorage

anti-at the bottom, being once again a leading shoe Thus, by employing twoshoes, each with the bell-crank and strut mechanism, a brake which is a two-leading shoe brake for either direction of rotation and which requires onlyone actuating mechanism is obtained

37.6 Brake shoe adjustments

In order to take up the wear of the brake linings and to enable the clearancebetween the shoes and the drum to be adjusted the anchorages on which theshoes pivot are frequently made adjustable so that the shoes can be movedoutwards In the example shown in Fig 37.11, the brake shoes bear on theends of tappets carried in a housing that is fixed to the backplate of the brakeassembly These tappets can be forced outwards by screwing in the adjustingwedge, thereby reducing the clearance between the brake shoes and drum.The adjusting wedge is roughly conical but the cone is actually a series offlats; this enables the pull-off springs to lock the adjustment positively

A design by Lockheed suitable for heavy duty brakes is shown in Fig.37.12 The housing G is bolted to the backplate H and the tappets A and Bhave screwed inner members D and E that are prevented from rotating bytheir engagement with the ends F of the brake shoe webs The outer members

R

W

W

W W

R

965 Brakes

of the tappets have teeth K formed on them so that they can be rotated fromthe outside by means of a simple crown wheel J and thus move the innerportions of the tappets outwards Other designs of adjusting mechanisms can

be seen in Figs 37.14 and 37.15

An alternative point at which an adjustment can be made is between theactuating mechanism and the ends of the brake shoes

37.7 A modern rear-wheel brake

The brake shown in Fig 37.13 is suitable for the rear wheels of a car because

it incorporates a handbrake actuation It is a Girling design The shoes bearagainst the flat faces of an abutment carried by the backplate at the bottomand against the ends of the plungers of the hydraulic actuation cylinder at thetop They are held in place by two springs, only the bottom one being shown.Flat strip springs S at the middle of the shoes bear against the webs and holdthe shoes against projections (two for each shoe) formed on the backplate.The handbrake actuation is by means of a bell-crank lever B and a connectinglink or strut, made in two parts E and F The bell-crank lever is pivoted on theupper end of a pillar C and when its long arm is pulled inwards as indicated

by the arrow the short arm D applies a force to the right-hand shoe and thereaction of this force moves the upper end of the pillar C to the left, thismotion being possible because the pillar is carried at its lower end on aflexible rubber moulding The motion of the pillar is transferred through thestrut EF to the left-hand shoe and so the shoes are applied with equal forces.The ends of both the parts E and F of the connecting link are flat but themiddle parts are cylindrical, E being hollow and F having a screw threadformed on it on which there is a nut G, rotation of which will alter theeffective length of the strut The flat end of the member E pivots on the pillar

C and a ratchet lever (shown in dotted line in the upper view and seen belowthe strut in the main view) is also pivoted there A slot in a short arm of theratchet lever is engaged by the end J of the spring that surrounds the pillar sothat the spring tends to rotate the ratchet lever in an anti-clockwise direction,this motion is limited by a small shoulder formed on the bell-crank The longarm of the ratchet lever can engage teeth formed on the outside of the nut G.When the bell-crank is rotated to apply the brake the spring causes theratchet lever to follow up the motion and if, due to wear of the linings, this

Fig 37.11 Wedge-shaped

adjustment device

K

A F

966 The Motor Vehicle

is greater than normal the lever will rotate the nut G and take up the wear.When the brake is released, the shoulder on the bell-crank will move theratchet lever back to its original position The actuating force applied to thelever B and the force in the strut both tend to pull the pillar up against thebackplate and so a roller K is provided to reduce friction when the pillarmoves during the brake actuation

An automatic adjusting mechanism made by the Lockheed Company isshown in Fig 37.14 The actuating cylinder is single-acting, i.e it is closed

at one end and its piston has an arm A secured to it This arm carries a pin

B that engages a bell-crank lever C which is free to turn on a fixed pin D Theend E of the bell-crank forms a pawl that engages the teeth F of an adjustersleeve that is free to turn inside the piston but must move axially with it Thetappet G is screwed into the adjuster sleeve but is prevented from rotating bythe brake shoe that it engages When the piston is moved outwards (to theleft in the diagram) the pin B rotates the bell-crank and if the movement isgreater than normal the pawl E will ride over one or more teeth of theadjuster sleeve F so that when the brake is released and the piston movesback the pin B will rotate the bell-crank and turn the adjuster sleeve so as totake up the wear

Some designers prefer to use adjusting mechanisms that operate onlywhen the brake is applied and the car is moving backwards and an example

of this is shown in Fig 37.15 The brake is a duo-servo type in which oneshoe is used to apply the other through the adjusting turnbuckle whether the

967 Brakes

E

D C

B A

F

brake drum is rotating clockwise or anti-clockwise The shoes are preventedfrom rotating very far by a fixed abutment C, against which one of the shoeswill abut when the brake is applied An expanding mechanism operated bythe handbrake linkage may also be incorporated between the upper ends ofthe shoes For forward motion of the drum, which is assumed to be anti-clockwise, the shoe A is the servo shoe and it will be brought into action bythe left-hand piston of the actuating cylinder The frictional force on the shoewill cause it to rotate slightly with the drum and so acting through theadjusting turnbuckle it will apply the shoe B, causing it to bear against theabutment C at the top The force Q applied to this shoe will be considerablygreater than the force P applied by the piston to the upper end of the servoshoe For backwards motion the shoes will change their functions and theshoe B will become the servo shoe The slight rotation that it then gets willcause the lever D to rotate about its pivot E on the shoe because the upperend of the lever is constrained by the wire D which is anchored at its top end

If undue wear has occurred the rotation of the lever D will be sufficient tocause the pawl at its end to rotate the nut of the turnbuckle and so take up thewear

Another example of a duo-servo brake is shown in Fig 37.16 (whichillustrates the principle), and in Fig 37.17 which shows the complete brakeand all its components The brake is a Girling design and suitable for heavylorries In the diagram, the left-hand side shows the service actuation and theright-hand side the parking actuation but the brake is actually symmetrical.Double-acting hydraulic cylinders at top and bottom provide the serviceactuation and bring the shoes into contact with the drum when, assumingclockwise rotation, the left-hand shoe will move round until it comes up

C

B F

D

E Q A

C

Fig 37.15 Girling duo-servo brake

with auto adjuster

Fig 37.16 Girling two-leading-shoe brake

968 The Motor Vehicle

excluder Dirt

Grub screw Tappet

Crown wheel

Spring plate

Circlip Adjuster

pinion

CirclipSeal

Carrier strut

Screw adjuster

Shoe carrier

Dirt shield

Rubber grommet

Steady post

Bridge pipe

Heat shield

Ball

Bleed screw

Cylinder body

against the abutment at A, and the right-hand shoe will come up against theadjuster at B Both are fixed points, A being part of the backplate while thebody of the adjuster which carries the tappets B is fixed to the backplate Foranti-clockwise rotation the abutments A and B will be interchanged Theservo action is thus effective for both directions of rotation

The parking brake actuation is provided by a wedge-type expander whichmoves the tappets D outwards and through levers C applies forces to thecentre of the shoes The lever C fulcrums on a flat formed on the tappet Band lies in a plane that is slightly offset from the central plane of the shoes

It will be seen from Fig 37.17 that the wedge of the expander is free to slide

in a slot formed in the plunger of the drawlink and so the forces that areapplied to the levers C will be equalised The adjusting mechanism for thetappets B is duplicated and operated by small bevel gears on the same principle

as that shown in Fig 37.12

Fig 37.17 Girling heavy-duty drum brake

969 Brakes

37.8 Disc brakes

Brakes using flat discs as the friction surfaces have been used in the past butuntil the last three decades have not been so successful They are now almostthe commonest type for the front wheels of cars and are often used for therear wheels and on some light vans The earliest disc brakes wear made onthe same lines as a multiple-plate clutch but most present-day designs use asingle disc and almost always have sector-shaped friction pads of relativelysmall area The great advantages are, first, that despite the small area of thepads compared with the area of lining of a drum brake occupying approximatelythe same amount of space the rises of temperature are smaller and consequentlythe linings are less subject to fade and their life is comparable with that ofdrum brakes Secondly, the action of the disc brake is unaffected by theoccurrence of wear or by expansion due to rises of temperature, both ofwhich are sources of trouble in drum brakes

Perhaps the simplest construction is the fixed calliper double piston typeshown in the diagram Fig 37.18 The disc A is secured to the wheel hubwhich rotates on bearings on the axle casing or stub axle depending onwhether a rear or a front brake is concerned A calliper member C is bolted

to the member B, which will be referred to as the mounting, and two pistons

D and E are carried in cylinders formed in the calliper These bear on padsconsisting of a metal plate to which friction material facings are bonded Themetal backplates fit in recesses in the calliper so that they are prevented fromrotating with the disc The callipers of such brakes usually have to be made

in two parts to enable the cylinders to be machined and also must haveopenings through which the friction pads can be removed for replacement.Their actual form is therefore more complex than as shown in the diagram

The brake torque provided by a disc brake of this type is given by T =

2µpaR where µ is the coefficient of friction, p is the fluid pressure, a is the area of one cylinder and R is the distance from the point at which the frictional

force acts to the wheel axis, this may be taken to be distance from the wheelaxis to the centre of area of the pad Calculation will show that to obtain thesame torque as from a drum brake occupying the same volume the forcesapplied to the pads of the disc brake must be much higher than those applied

to the shoes of the drum brake This is for two reasons, firstly the radius R

is necessarily less than the radius of an equivalent drum brake and secondlythere is no servo action in the disc brake because the frictional forces do not

A

B R

E D

Fig 37.18

970 The Motor Vehicle

help in the application of the brakes as they can do in drum brakes It isconsequently imperative that the axial forces on the disc shall be balancedand this is clearly so in the arrangement shown; it can however be obtained

in other ways and these are considered later

Although the axial forces are balanced there is an unbalanced tangentialfrictional force on the disc and this will bave to be supported by the wheelbearings By using two callipers placed diametrically opposite each other thebrake torque can be doubled and the tangential forces be balanced although,

of course, the volume of fluid that has been displaced to apply the brakeswill be doubled This arrangement is seldom used It is also imperative toprovide automatic adjustment for wear in disc brakes and this is usually donevery simply A rubber ring placed near the pad end of the actuating cylinder

is carried so that when the piston moves outwards the ring distorts enough toallow the normal clearance to be taken up without any slip occurring betweenthe ring and the piston but if the movement is more than this, slip occurs andwhen the fluid pressure is released the piston retracts only by the amount ofthe distortion and thus the normal clearance is restored

Some methods of balancing the axial forces without using two cylinders

are shown in Fig 37.19 In (a) the calliper is carried on two links G pivoted

at one end to the calliper and at the other end to the mounting B; it cantherefore float sideways so as to equalise the axial forces The design shown

in (b) uses a single pivot but has the disadvantage that if the friction facings

are initially of uniform thickness they will wear to a wedge shape and therewill be a waste of friction material but this can be avoided by making the

facings wedge shaped initially In (c), the calliper is allowed to slide along pins fixed in the mounting and in (d) the calliper is again carried on a single

pivot but this is now placed approximately tangential to the periphery of thedisc and in its central plane If the axis of the pivot is offset from the centralplane, then the tangential forces on the pads will have a moment about theaxis and the balance of the axial forces will be upset but the magnitude of theeffect is not likely to be great The pads will also wear to a wedge shapewhen the pivot is offset but again this can be allowed for by making themwedge shaped initially In all these examples only one actuating cylinder isrequired and this is one of their chief advantages as it avoids having to putone cylinder inside the road wheel where it cannot always be adequatelycooled

The disc itself can be a simple flat one but this can sometimes lead to

A

X X

O B

B

A B

Fig 37.19 Floating calliper disc brake arrangements