65 Chapter 4 Fundamentals of Collision Damage
Copyright Goodheart-Willcox Co., Inc.
All collision damage found in a vehicle is caused
by force from the side (lateral) or force from one end
(longitudinal). If you understand these two basic ways
that force can cause plastic deformation in a panel, you
will have a much easier time examining and repairing
damage. In summary, when a force acts on a panel it
will first move the panel. If no movement is possible,
the force will create buckles in the panel. Longitudinal
force causes folds and buckling, lateral force causes
stretching in low-crown panels and collapse and arrow-
heads in high-crown panels.
Inertia
If an object is not moving, energy, or force, is
required to make it move. If an object is moving, force
is required to make it stop. Inertia is the tendency of
an object to remain at rest or to remain in motion. This
means that a moving object will stop or a stationary
object will move only if sufficient force acts on it. Let’s
look at a stationary object first. Consider a large rock
sitting near a road. The inertia of an object is deter-
mined by the weight of the object and the strength of
any attachments holding the object in place. The inertia
of the rock is simply a function of its weight—there are
no attachments holding it in place. If a subcompact
car moving at 10 miles per hour (16 km/h)—lightweight
vehicle, slow speed, and therefore, a small force—hits
the rock, the rock will not move. The force of the small
car is not great enough to overcome the inertia of the
rock. However, the rock would move if it were hit by
a large sport-utility vehicle moving at 50 miles per
hour (80 km/h). Why? The sport-utility vehicle—heavy
vehicle, high speed, and therefore, a large force—has
enough force to overcome the inertia of the rock. To
move an object, a force greater than the inertia of the
object must be applied.
As mentioned previously, inertia also influences
objects that are already in motion. Unless sufficient
force acts on it, a moving object will stay in motion.
Some people launch boats this way. The boat is placed
in the bed of a pickup truck with the tailgate down.
The truck is backed toward the water at a high rate of
speed. Near the water’s edge, the truck’s driver slams
on the brakes. The truck stops at the water’s edge, but
the boat slides out of the bed and lands in the water.
That is an example of inertia. Both the truck and the
boat were moving at the same speed. The force of the
brakes stopped the truck. However, the boat was not
attached to the truck, so the braking force did not act
on it. The boat stayed in motion until it hit the water.
For a collision example, think of a vehicle as
consisting of three sections: front section, center
section and rear section. On a unibody vehicle, the
front section includes all parts ahead of and under the
hood, the center section is everything that is between
the rocker panels, and the rear section is all parts
behind and under the deck lid. On a pickup truck, the
three sections would be the engine compartment, the
cab, and the bed.
Figure 4-14 shows a vehicle colliding with an
unmovable wall. The impact event begins and ends
in a split second. At the moment of impact, the front
section collapses but the center and rear sections
are still in motion, due to inertia. When the center
section can no longer move forward, the forces at
work may cause the rear end of the vehicle to rise off
the ground.
This upward deflection of the rear section may
cause a buckle in the roof at a pivot point. Usually this
pivot point is where the center pillar meets the roof.
This damage to the roof is called inertia damage. The
center section may also deflect upward. The roof may
move forward, causing inertia damage at the wind-
shield pillar.
The amount of inertia a section has is determined
by the weight of the section. A heavy section has
more inertia than a light section and is more likely
to cause inertia damage. A long section, such as a
pickup truck bed, can act as a lever when it rises up
during a frontal impact. The upper front portion of a
pickup truck bed often hits the back of the cab during
a front-end collision.
Inertia is also at work in a side impact. In
Figure 4-15, a vehicle is sliding sideways on icy pave-
ment and hits a tree. The inertia of the tree is great
enough to resist the force of the vehicle—the tree does
not move. The center section of the vehicle stops, but
the front and rear sections continue moving for a split
second. This causes inertia damage to the front and
rear sections of the vehicle. Inertia damage is only
found when a vehicle is moving with at least moderate
speed and comes to an abrupt stop. A gradual stop will
not cause inertia damage.
Goodheart-Willcox Publisher
Figure 4-14. When the front section of the vehicle
has stopped moving, the rear section may rise off the
ground.