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