74 Section 1 Introduction to Collision Repair
Copyright Goodheart-Willcox Co., Inc.
Look inside the paper box. One side of the box is
collapsed and the other side is stretched. This type of
damage can also be found if lateral force is applied to a
unirail. A side impact will bend a unirail at a pivot point.
This type of damage is obvious, but unibodies can also
have subtle damage. If a component has moved during
the collision, either from collision force or inertia, there
will be clues. Some of these clues include misaligned
panels, broken spot welds, cracked paint, split sealer,
open seams, buckles, and bulges.
W W W W W W O O O O O O R R R R R R K K K K K K I I I I I I N N N N N N G G G G G G S S S S S S M M M M M M A A A A A A R R R R R R T T T T T T
Y Yo Yo Yo u u u hould sh sh sh ou ou ld ld ld al a a lw lw lw ays ay ay s s consider co co ns ns id id id er er th t t t he he he bend b b b en en d- d- d
versus ve rs us ki k -k in in k k k rul ru le le wh w he he n n assessi as se ss in in g g
unibody damage. If a bent unirail is
s p s tra a ig g hte e ned ed p rope ope rly, y, the e repa epa ired ed rail a
will be as strong as an undamaged rail.
However, if the impact force is great enough to cause a
kink, the damaged rail is not repairable, even if it is not
high-strength steel.
Front Impact
Figure 4-34 shows how longitudinal force from a
front impact moves through a unibody. The unibody
is designed to absorb collision forces. The unirails
are made with convolutions. Some impact force is
absorbed at each convolution. In this way, a unibody
gradually absorbs collision force. Figure 4-35 shows a
collapsed lower front unirail.
When examining a unibody vehicle for front impact
damage, start at the rear and look for panel misalign-
ment. If collision force reached the passenger compart-
ment, look for wrinkles in the floor. To find the wrinkles,
either remove the carpet from inside the vehicle or
raise the vehicle on a lift and look at the floor from the
outside. If the cowl has been moved back, the front
door will not fit properly. The door may drop when it is
opened, or the gaps around the door may be tapered
when the door is shut. Figure 4-36 shows a unibody
vehicle that has been hit on the bumper. Both unirails
were affected. The unirails have moved to the side,
causing sidesway at a pivot point. The unirails are
designed to move in this way from a front hit.
A high frontal impact is shown in Figure 4-37. The
lower unirail is undamaged, but the radiator support
is buckled. The damage does not extend to the cowl.
Figures 4-38 show a unibody with extensive impact
damage that extends to the cowl
Goodheart-Willcox Publisher
Figure 4-35. Impact force on the front bumper caused
this lower front unirail to collapse.
Goodheart-Willcox Publisher
Figure 4-33. Fold the paper as shown to represent
a unirail. Note that the first fold should be in the “up”
position for this model to work properly.
Mercedes-Benz
Figure 4-34. The unibody is designed to absorb
impact force by collapsing.
Fold