Checking underbody alignment
Another method of checking underbody alignment
for distortion is that of using gunsight frame gauges
(Figure 14.59). These gauges consist of two sliding
bars which can be adjusted by either pulling out
or pushing in to make them fit correctly across
a chassis frame or underbody. At the middle of
the gauge is a sighting pin which always remains
central irrespective of the distance to which the
gauge is extended, and which is used to establish the
datum line (the vehicle’s centre line). There are
adjustable hanging rods on both ends so that the
gauge may be raised or lowered. These hanging rods
are also used to attach the gauge to the underbody or
frame at symmetrical points on it, which may be
factory formed holes, mounting points, or any part
of the underframe which has identical positions on
both sides of the vehicle (Figure 14.59a). Never
440Repair of Vehicle Bodies
hang gauges on movable parts such as control arms,
springs, torsion bars, or any mechanical moving
parts, as false readings will be obtained. A minimum
of three gauges is essential, but the more gauges
used the easier it will be to determine exactly where
the distortion is located, as sighting along four or
five gauges is quicker than relocating three gauges
back and forth along the length of the vehicle.
When setting up the gauges to check underbody
alignment, the in-line condition will be when all
sighting pins are in line and all gauge bars are parallel.
If one gauge must be lowered to clear some
obstruction on the underbody, all other gauges must
be lowered a corresponding distance to maintain
a level sighting. The condition where the gauge bars
are not parallel (Figure 14.60a) indicates that the
underbody has moved up or down at some point
along its length. The condition where the sighting
pins are not in line, while the gauge bars are still
parallel (Figure 14.60b) indicates that the underbody
members have moved to one side only (sway).
A twisted underbody is recognized by the gauge
Figure 14.59(a) Gunsight gauge frame (b) diamond
detector gauge (Blackhawk Automotive Ltd )
Figure 14.60Frame gauges showing distortion
(Blackhawk Automotive Ltd )
Major accident damage 441
condition where sighting pins are out of line in addition
to the bars not being parallel (Figure 14.60c).
As well as these basic checks an additional
gauge is available, known as a diamond detector
gauge (Figure 14.59b), which is specially made to
fit on the top or bottom of the self-centring gauges.
This provides extra sighting pins to help to detect
sideways movement (sway) and diamond damage
(when the underbody has been forced into a
diamond shape), which may not be evident when
using self-centring gauges only. The only indication
of a diamond condition would be that the
two pins of the diamond detector gauge would be
out of line with all other self-centring pins. Having
determined the type of damage, the actual location
is found by visual inspection. The main advantages
gained by using this system of alignment check
are the speed at which it can be set up to diagnose
distortion, and the convenience it offers in making
regular checks during repair procedure.
Chief precision alignment gauge system
The principle of the self-centring siting pin is
used by Chief in their alignment gauge system.
These gauges are hung underneath the car from
specific data reference points, giving an instant
alignment check according to the siting on the
pins (see Figure 14.61 and Section 14.2.13).
Vehicle alignment checks using a
Digital measuring tool
This is a lightweight aluminium measuring tool
which can be used to check body alignment on
any part of a vehicle body. By extending the
tool, its measuring range covers from 2 mm up to
3683 mm. The measurements are displayed on an
LCD digital readout, in either metric (millimetres)
or imperial (feet and inches), and are accurate to
1 mm. It is supplied with pointers, cones and
magnetic bases so that accurate measurements can
be taken on various parts of the vehicle without
difficulty (Figure 14.62). This measuring tool is
used for comparative measurements when checking
a vehicle for damage and when estimating a
repair. While a vehicle is on a jig, the tool can be
used to double check measurements taken by the
jig’s own measuring system. It also can be used for
Figure 14.61Chief E-Z Liner II showing hanging gauges, self-centring (Chief Automotive Ltd )
442Repair of Vehicle Bodies
Underbody alignment using
Jig fixtures
Using an alignment and repair system is the only
method of guaranteeing a comprehensive and accurate
check on the alignment of the underbody of
a vehicle. The monocoque body is produced on a
production line using a series of quality controlled
jigs and fixtures for holding all panels in the correct
position for spot welding. To the production engineer
this is the only way of consistently reproducing
a basic shell fitted with all its mechanical parts in
the correct places. When it comes to repairing a
body which has been damaged or distorted in an
accident it is only logical that accurate alignment
jigs should be used to ensure a safe, roadworthy
repair and one which has the appearance of a new
vehicle. The introduction of systems of body jigs
was a major step forward in body repair techniques.
These systems were designed and developed to cope
with the ever-increasing call on the accident repair
trade. Every vehicle presented a different problem
but, equally, every body shell had certain points
which always could be relied upon to be accurately
controlled during factory production (Figures 14.65
and 14.66), and it was around these points that all
manufacturers of alignment equipment designed
their products. This approach and the enthusiastic
cooperation of the vehicle manufacturers at all
stages ensured that when a car was mounted on the
equipment, the damage of distortion showed up in
an unmistakable fashion. The products now available
to the body repairer include the following:
1 Alignment systems that incorporate bracket
mounting (see Figure 14.67)
2 Alignment systems that incorporate either a
mechanical (see Figure 14.68) or a laser light
measuring system (see Figure 14.69)
3 Dual equipment, which incorporates both measurement
and bracket systems
4 Cassette-type multifunctional alignment and
repair systems (see Figure 14.70)
5 Alignment systems that incorporate either a
mechanical or an electronic measuring system
(Figure 14.71).
The manufacturers are continuing their research to
develop appropriate body repair equipment as car
design and methods of assembly are influenced by
technological progress.
Figure 14.62Digital measuring tool with
accessories (Stanners Ltd )
Figure 14.63Checking a vehicle’s wheel base with
a digital measuring tool (Stanners Ltd )
Figure 14.64Digital measuring tool being used during
repair to check vehicle suspension points (Stanners Ltd )
measuring a vehicle’s wheelbase and comparing it
with manufacturer’s data (Figure 14.63). In addition
it can be used to check the critical suspension points
before, during and after repair (Figure 14.64).
Major accident damage 443
Figure 14.65Manufacturer’s alignment points