116 Body and chassis alignment checks

Body and chassis alignment checks will be necessary if the vehicle has been involved in a major

Table 1.1 Summary of function and application of soundproofing materials

Function

Acoustic materials

Application

Insulation

Loaded PVC, bitumen, with or without foam or fibres base, mineral wool

Floor, bulkhead dash panel

Damping

Bitumen or mineral wool

Doors, side panels, underside of roof

Absorption

Polyurethane foam, mineral wool, or bonded fibres

Side panels, underside of roof, engine compartment, bonnet

collision, but overall alignment may also be necessary if the vehicle's steering and ride characteristics do not respond to the expected standard of a similar vehicle when being driven.

Structural misalignment may be caused by all sorts of reasons, for example, if the vehicle has been continuously driven over rough ground at high speed, hitting an obstacle in the road, mounting steep pavements or kerbs, sliding off the road into a ditch or receiving a glancing blow from some other vehicle or obstacle etc. Suspicion that something is wrong with the body or chassis alignment is focused if there is excessively uneven or high tyre wear, the vehicle tends to wander or pull over to one side and yet the track and suspension geometry appears to be correct.

Alignment checks should be made on a level, clear floor with the vehicle's tyres correctly inflated to normal pressure. A plumb bob is required in the form of a stubby cylindrical bar conical shaped at one end, the other end being attached to a length of thin cord. Datum reference points are chosen such as the centre of a spring eye on the chassis mounting point, transverse wishbone and trailing arm pivot centres, which are attachment points to the underframe or chassis, and body cross-member to side-member attachment centres and subframe bolt-on points (Fig. 1.9).

Initially the cord with the plumb bob hanging from its end is lowered from the centre of each reference point to the floor and the plumb bob contact point with the ground is marked with a chalked cross. Transverse and diagonal lines between reference points can be made by chalking the full length of a piece of cord, holding it taut between reference centres on the floor and getting somebody to pluck the centre of the line so that it rebounds and leaves a chalked line on the floor.

A reference longitudinal centre line may be made with a strip of wood baton of length just greater than the width between adjacent reference marks on the floor. A nail is punched through one end and this is placed over one of the reference marks. A piece of chalk is then held at the tip of the free end and the whole wood strip is rotated about the nailed end. The chalk will then scribe an arc between adjacent reference points. This is repeated from the other side. At the points where these two arcs intersect a straight line is made with a plucked, chalked cord running down the middle of the vehicle. This procedure should be followed at each end of the vehicle as shown in Fig. 1.9.

Once all the reference points and transverse and diagonal joining lines have been drawn on the

Fig. 1.9 Body underframe alignment checks

floor, a rule or tape is used to measure the distances between centres both transversely and diagonally. These values are then chalked along their respective lines. Misalignment or error is observed when a pair of transverse or diagonal dimensions differ and further investigation will thus be necessary.

Note that transverse and longitudinal dimensions are normally available from the manufacturer's manual and differences between paired diagonals indicates lozenging of the framework due to some form of abnormal impact which has previously occurred.

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