62 Section 1 Introduction to Collision Repair
Copyright Goodheart-Willcox Co., Inc.
■ Double crown—Curved in two directions, from top
to bottom and also from side to side. Double-crown
panels will not flex.
■ Reverse crown—Curved in rather than out, forming
a concave surface. Reverse-crown panels will not
flex.
See Figure 4-7 for examples of each of these
types of crowns.
The degree of elongation and compression of
the grains in a crown determines the strength of the
crown. Higher crown (greater curvature) means greater
strength. Combination crown means that a panel has
at least two types of crown (low and high). Because
most panels have combination crown, it is useful to
think of crown in terms of areas rather than panels.
In this way, you can identify one section of a panel as
a high-crown area and another section of the panel
as a low-crown area. Different types of crown require
different repair methods.
In addition to bending, some body panels are
formed by stretching. In a stretched area, the grains
are elongated. The metal in a stretch becomes slightly
longer and thinner. Figure 4-8 shows the grain in a
stretch. Stretching will increase the surface area of
the stretched metal. Stretches are formed adjacent
to bends when making complex shapes like double
crowns.
Collision Factors
In addition to the types of metals used in vehicle
construction and the way these materials are shaped,
there are several collision factors that determine the
amount of damage that occurs when a vehicle is
involved in a collision. These factors include force,
inertia, target, impact angle, surface area of contact,
and vehicle height. Each of these factors will be
discussed in the following sections.
Force
Force can be defined as any action between two
objects that changes their relative condition as to
motion or rest. In collision repair, force can be thought
of as the energy exchanged when two objects collide.
There are a number of ways collisions can occur. A
parked vehicle can be hit by another vehicle, a vehicle
can run into an unmovable object, or two vehicles
moving in opposite directions can collide. In all these
examples, the force or energy from one object impacts
on the other object.
The force of a moving object is a product of the
weight and speed of the object. If a golf ball were held
one foot above a car hood and then dropped, the hood
would not be damaged. However, if a baseball were
held one foot above the hood and dropped, the hood
would be damaged. Finally, if a bowling ball were held
one foot above the hood and dropped, the hood would
Reverse crown
High crown
Double crown
Low crown
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Figure 4-7. Low crown is nearly flat, high crown has
noticeable curvature, double crown is curved in two
directions, and reverse crown has a concave curve.
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Figure 4-8. Stretching increases the surface area of a
panel. Note the elongation of the grains in the stretch.