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Student Workbook

LV25 Drive Shafts (2)

kap all phase 2 & 3 6/11/03 11:36 am Page 19

Student Workbook for Technical Certificates in

Light Vehicle Maintenance and Repair

MODULE LV25 DRIVE SHAFTS (2)

Contents

Page Page

Advantages of traction control 15

Vibration 8

Drive Layouts (Nomenclature)

Advantages of four wheel drive 9

Disadvantages of four wheel drive 11

Advantages of rear wheel drive 12

Disadvantages of rear wheel drive 12

Advantages of front wheel drive 13

Disadvantages of front wheel drive 14

Introduction

Drive shafts and related systems are generally very reliable However, when faults do occur it is important to understand the effects that such faults will have on the vehicle and be able to recognise the symptoms

Drive Shaft Inspection and Test Methods

As with most diagnostic procedures, a visual inspection should be carried out first This would involve checking that the drive shaft is correctly located and all the components associated with it are in good order

The areas that should be checked are the driveshaft gaitors, the constant velocity joint, the tripod, the damper and the shaft itself When carrying out an inspection on the gaiters they need to be checked for splits, cracks and

correct location

There is little to check with the damper other than its position, to ensure that it

is not damaged and that it is securely fixed to the drive shaft

The shaft itself is simply a steel tube Visually all you can do is make sure it is correctly located and it is not damaged in any way Look for any signs to indicate if anything is rubbing against it such as an under body panel

Although components such as under body panels are just made of plastic, the process of components rubbing against a drive shaft for long periods of time will wear a groove in the drive shaft This groove will weaken the drive shaft and make the chance of a drive shaft failure occurring more likely

If further examination of the drive shaft is necessary the drive shaft will need

to be removed Once it has been removed several checks should be made One of the first checks would be to measure the drive shaft run out This should be carried out by placing the shaft on two V blocks and placing a dial gauge centrally on the shaft which is then rotated through 360º Any

fluctuation of the dial gauge needle indicates the drive shaft is bent

The tripod should now be inspected Before it is removed from the drive shaft its orientation should be marked to ensure it fits back onto the drive shaft in the same position that it was removed from This is necessary as incorrect reassembly can lead to the drive shaft being out of balance The balls on the end of the tripod should be inspected first, they should have a smooth surface and have no signs of pitting If there are any signs of pitting they need to be replaced If pitting is present the faces that the balls run against inside the tulip will also need to be checked

There are many needle rollers in between the balls and the tripod and it is possible for these to pit They should be inspected and replaced if there are any signs of pitting or overheating

The old grease should be thoroughly cleaned off to make sure there are no foreign particles present in the grease This is important, as any foreign

particles in the grease will lead to exaggerated wear and reduce the life of the components Upon reassembly refer to the manufacturers’ specifications to ensure the correct grease is used

The position of any matchmarks should be noted, ready for reassembly

At the other end of the drive shaft is the constant velocity joint This should be checked to ensure that there is no play present It should have a smooth action and no binding should occur

When the joint is stripped down all of the components should be cleaned and inspected Once the components are clean it will be relatively clear if there is any unusual wear, namely pitting, although heat damage can occur (blueing and general discolouration) The shaft should be a tight fit on the inner face and there should be no backlash present If any backlash is present the worn component(s) should be replaced

Upon reassembly the correct grease should be used It will normally be a grease with a high graphite content; always use the manufacturer’s

recommended grease as it has to cope with high temperatures and loads that

an inferior grease would be unable to If the grease degrades prematurely, component failure will not be far behind

When reassembling the components on the drive shaft all the spring clips and circlips should be replaced Reusing clips greatly increases the chance of a drive shaft failure and for the small amount they cost it is well worth replacing them

Common Faults

Common faults associated with drive shafts and related components are grease leaks, noisy operation, vibration, and loss of drive to the road wheels

Grease leaks

Grease leaks from drive shafts are relatively common and split drive shaft gaiters are usually the cause They are easily identified as there are normally signs of grease on the car body or suspension arms because it sprays out due

to centrifugal force and the gaiter will also be covered in grease This needs

to be rectified reasonably quickly as the grease not only lubricates the joint but

it also cools it If there is a substantial reduction in the available grease within the gaiter the lubrication affect will be reduced along with the cooling affect

Both of these problems will lead to premature drive shaft failure

Noisy operation

Noisy operation is a fault more commonly associated with front wheel drive vehicles The most common noise experienced is ‘clunking’, which occurs in phase with wheel speed and is most prominent when cornering If a clunking sound is present, the outboard constant velocity joint is most likely at fault and

it will need replacing It may be the case that the whole drive shaft will need replacing, as separate constant velocity joints aren’t always available

Replacement drive shafts are regularly purchased from outlets on a return basis, so the old drive shaft will need to be kept

Drive shafts and related components do wear out over time and periodically the drive shafts of any front wheel drive vehicle will need replacing However

it tends to be the more powerful front wheel drive vehicles that wear out drive shafts on a regular basis along with vehicles fitted with modified engines, as often the drive shafts are not upgraded to cope with the increase in torque Noise often precedes complete failure This tends to be when the vehicle has been driven with a split drive shaft gaiter If the gaiter is split, not only does the essential grease get displaced, but also dirt and grit can enter the area where the moving parts are

Dirt and grit cause wear to occur at an accelerated rate and so it is important

to make sure the drive shaft gaiters are kept in a good state of repair The clips holding the gaiters in position can occasionally break or become loose causing the gaiter to move on the shaft If this occurs, dirt and grit can access the moving parts which can lead to premature joint wear Make sure the clips are in position firmly and replace them if they are in a poor condition Ensure that good quality metal clips are fitted as it is common for plastic cable ties to

be used on replacement gaiters and they are unsuitable

Vibration

Drive shafts can become imbalanced and this is most likely caused through bending of the shaft Vibration will be felt through the steering, clutch pedal and through the car body The vibration will increase relative to road wheel speed and is usually cured by replacement of the drive shaft, although a slight imbalance can be cured by on vehicle wheel balancing

Loss of drive

Drive shafts do break and although is rare on standard road cars it is not uncommon on modified vehicles as they are often driven hard Revving the engine to 6000 rpm and sidestepping the clutch will take its toll!

If a drive shaft has broken, there is little that can be done except replace it Drive shafts can become dislocated This is normally caused by mechanical failure although altering the suspension set-up can cause the distance

between the hub and differential to increase during rebound to the point

where the drive shaft pops out If this happens at high vehicle speed the consequences can be serious as the drive shaft will ‘whip’ and destroy most things in its path

Drive Layouts (Nomenclature) Features and Benefits

Four-wheel drive vehicles have a number of advantages over two wheel drive vehicles and fall into two main types:

• full-time 4WD (permanently engaged)

• part time 4WD (driver selectable)

It should be noted that 4WD systems are fitted to cars of different designs for differing reasons A high performance saloon with 4WD uses the system to achieve better acceleration and handling, whereas 4WD fitted to an off-road vehicle is primarily aimed at improving the vehicles ability to cross very rough, undulating terrain A 4WD system fitted to a performance saloon is seldom suited to improving the vehicle’s ability in rough terrain and often damage can

be caused if the vehicle is used this way The following points highlight the advantages and disadvantages of 4WD systems in a general sense and the suitability for use should always be taken into consideration

Advantages of four wheel drive

As all four wheels are driven, the chances of wheel spin occurring are

considerably reduced This will lead to improved acceleration and good off-road capability In the case of off-off-road vehicles with differential locks, even if three wheels lose traction, providing one of the wheels has good grip the vehicle will continue to move

As all four wheels are driven the tyres tend to wear evenly On two wheel drive vehicles the driven axle always wears out first, providing the suspension geometry is correct on all four wheels

As engine braking can occur on all four wheels the vehicle will slow down faster as a result

Vehicles with permanent four-wheel drive systems often handle better on corners This is because all four wheels are driven and can “pull” the vehicle round the corner at higher speeds, as loss of traction is less likely to occur

As 100% of the vehicle’s weight is distributed over the driven wheels the system utilises the vehicle’s weight to increase the tractive effort that can be applied and as a result an improvement in handling and acceleration is

achieved

Exercise 1

Working from the figures supplied in the diagram below, complete the table for

a front wheel drive vehicle:

Disadvantages of four wheel drive

As there are more mechanical items that need to be fitted to four-wheel drive vehicles they are often more expensive

Servicing is also more expensive as there are more oils to change and more components to inspect and maintain

As four wheels are driven there are more frictional losses This leads to a reduction in power at the road wheels and an increase in fuel consumption The clutch often needs replacing at more frequent intervals due to the extra loads placed on the assembly

Vehicles with four-wheel drive have more complex drive trains and so there are more things to go wrong

Off-road vehicles often have a selection lever for the driver to choose whether the vehicle is driven in four-wheel drive or two-wheel drive mode Four-wheel drive should only be used off-road as drive train wind-up can occur on hard surfaces

Transmission wind-up is when the front wheels have revolved at a different rate to the rear wheels and the components connecting them together (the drive train) ‘twists’ or loads up This seldom happens off-road as the tyres can slip on the soft surfaces relieving any wind-up that may have occurred If four-wheel drive is used for any substantial distance on a hard surface, serious damage can occur to the drive train often leading to costly replacement of components Additionally it has been known for injuries to occur when drive train wind-up is present and the vehicle is jacked clear of the floor (wheel free) The wind-up will release with considerable force

Always ensure that four-wheel drive is disengaged (part time 4WD vehicle) before jacking up

Advantages of rear wheel drive

As rear wheel drive vehicles have less mechanical components when

compared to four-wheel drive vehicles they cost less to manufacture

(relatively)

It is easier to drift a rear wheel drive vehicle round corners and in certain driving conditions this is desirable, i.e track motor racing

Often a good weight distribution is achieved with rear wheel drive vehicles as the weight of the differential and drive shafts is at the rear, the engine at the front and the gearbox in the middle The handling qualities of a car that has

an even 50/50 weight distribution are often superior

Rear wheel drive vehicles never suffer from torque steer as all the torque is transmitted to the back wheels

Disadvantages of rear wheel drive

Sometimes rear wheel drive vehicles can be “tail happy” This is when the rear of the vehicle kicks out during heavy acceleration or cornering For the inexperienced driver this can lead to a spin occurring, which is obviously very dangerous

Rear wheel drive vehicles do not have as high a level of traction as four

wheels drive vehicles and so cannot accelerate as hard

The prop shaft and gearbox in the centre of the floor pan results in less

passenger compartment space Vehicles with front engine front wheel drive layouts avoid this problem

The engine tends to be mounted longitudinally and so often the bonnet has to

be longer or the space in the passenger compartment reduced With four wheel drive and front wheel drive it is possible to fit the engine transversally

The 1932 BUCCIALI TAV12 - one of the first front wheel drive road cars

Advantages of front wheel drive

More passenger cabin space due to the absence of any transmission tunnel

The engine can be mounted transversely which can lead to a reduced length

in bonnet or increased cabin space

Front wheel drive vehicles generally have good traction as the bulk of the vehicles weight is sitting directly over the driven wheels

Front wheel drive vehicles often have good steering stability as the vehicle follows the thrust angle that the steered and driven wheels generate

Disadvantages of front wheel drive

Poor weight distribution as the vast majority of the weight is over the front wheels only

Complicated suspension and drive shaft arrangement as the driven wheels are also the steered wheels

As the engine tends to be fitted transversely very large engines such as V12s tend not to be fitted to front wheel drive vehicles

Front wheel drive vehicles often suffer from torque steer This is a steering snatch that occurs during acceleration due to different length drive shafts Some front wheel drive vehicles have an idler shaft fitted in the middle of the vehicle to prevent this from becoming a problem, or sometimes the longer shaft (which is more susceptible to twist) has a larger diameter

Front wheel drive vehicles tend to wear out front tyres at an increased rate This is due largely to the fact that the front tyres have to cope with both

steering and driving/braking forces