Misalignment and vehicle control

Any serious impact on any part of the all-welded

body of the mono constructed vehicle will cause

distortion throughout the structure, which will not

necessarily be adjacent to the damaged area. Since

the front and rear suspension units are connected,

either directly or through the medium of a front or

front and rear subframe, distortion of the body

shell means a disturbance of the relationship of the

running wheels one with another. When this occurs

the result will be crabwise running in which the

car seems to travel in a sideways manner, commonly

called dogtracking. It is often necessary for

the driver of such a vehicle to struggle constantly

to maintain a straight course, and this becomes

greater as the speed increases.

Misalignment of car underbody can also cause

tyre wear and may affect the brakes and steering

control, which will not be cured however much

checking and adjustment is carrried out. These faults

can render a car outside the uniform standards

required by law in respect of the fitness of the

car for use on the road. A car with a damaged or

weakened underbody can be a menace to safety on

the road. Aside from the effects it has on steering

control and tyre wear, it can also place undue stress

on other mechanical parts of the vehicle. The

alignment of the engine with the clutch and

transmission may be affected. This could cause a

manual transmission to jump out of gear and might

result in permanent clutch failure. Any radical

change in the angle of transmission to the rear axle

may cause excessive wear in the universal joints,

noise in the rear axle or axle failure. Moreover,

whenever the underbody has been damaged by

collision there is a possibility of a broken hydraulic

brake line which could lead to complete brake

failure. Electrical connections also may become

broken. There are a number of modern cars where

the rear track varies substantially from the front

to the back and there are several which have up to

25 mm variation in track; these points are taken

into consideration when checking for distortion.

Checking a mono constructed body

Shell for underframe alignment

In order to make an alignment check of a mono constructed

underbody, the first step is to assess the

extent of the damage, the probable amount of force

involved which caused it, and the direction of that

force. This information will act as a guide by indicating

where to look for possible distortion. With this

knowledge a visual check should be made of doors,

bonnets, boot lids, roof and centre pillar positions for

any distortion which may be visible. Inconsistent

gaps around these panels show that the panel assembly

has been moved during the collision.

The underbody should be checked next. This can

only be done by jacking the car up and inserting

safety stands, or by using lifting equipment in the

form of a hoist which will allow a more thorough

examination of the underside of the vehicle. The subframes

and cross members can now be examined for

kinks or buckles with the aid of a portable light, but

where sighting is difficult or impossible the members

are examined by touch. This is not conclusive evidence

of distortion, but will generally be sufficient to

convince the repairer whether a more precise check

should be made by using the dropline method, the

gunsight gauge method, or a jig alignment system.

Major accident damage 439

Drop-line method of checking

Underbody alignment

For many years in the body repair trade the drop-line

method was the basic means of checking the alignment

of both composite and mono constructed vehicles.

When a car has been damaged the relationship

of the vital points, such as rear suspension spring

hangers, front hub centres, and front suspension

centres, must be checked on a flat floor surface with

the vehicle jacked clear of the floor, all four road

wheels removed, and safety axle stands in position.

With a plumb bob and length of cord, plot on

to the workshop floor a series of points common

to both right- and left-hand underbody members. At

least eight such points should be selected, four on

each side, which could include front and rear spring

shackle bolts, bumper bar bracket bolts, or any factory

formed holes, rivets, bolts, or intersection of

crossmembers common to both side members. On a

composite designed body the body bolts provide

ideal locating points. Variation in wheelbase can be

checked by plotting the centres of each of the four

wheels. The plumb-bob line should be held against

the centre of each point selected and the position

where it strikes the floor should be marked with a

pencil cross. The pencil is more easily seen if the

floor is previously chalked in the approximate area

where the plumb-bob will fall.

After all the points have been plotted and the

wheels replaced, the car can then be rolled away,

leaving the chalked pattern on the floor. These points

are joined diagonally by using a chalked length of

cord which is held tight and then flicked to the floor,

leaving a chalked straight line between each pair of

points. These lines will indicate if any variation has

occurred in the diagonal measurements. The next

stage is to establish a chalked centre line through the

diagonals. This is done by bisecting the lines joining

the front and rear pairs of points and using the

chalked cord to mark the centre line. If the underbody

is in perfect alignment, this centre line should

then pass where each diagonal intersects the centre

line. When the centre line does not pass within 3 mm

to these intersecting points, measurements must be

taken to establish which member is distorted. A further

check can be made by joining the sets of points

transversely. These transverse lines will be at right

angles to the centre line if the underbody is in correct

alignment (Figure 14.58).

Figure 14.58Drop-line check on front and rear

subframes of a Mini (Rover Group Ltd )

A–A 660.40 mm Front subframe front mounting

26.0 in set screws

B–B 412.75 mm Front subframe rear mounting

16.25 in set screws

C–C 1282.70 mm Rear subframe front mounting

50.50 in set screws

D–D 977.90 mm Rear subframe rear mounting

38.50 in block set screws

All dimensions taken at centre line of set screw or set

screw hole.

Owing to the inaccuracy of this system, the

increase in the complexity of design structure of

vehicles, and their critical suspension geometry, the

drop-line method has been superseded by the use

of alignment jigs, using either a bracket alignment

check system or a universal measuring system.

Gunsight gauge method of

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