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Chapter 3 Vehicle Construction
and traps the test dummies feet, or if the hood moves
back into the passenger compartment.
There are two tests used to determine the side impact
rating. In one test, the vehicle is stationary and a 3000-lb
sled moving at 38 mph hits the driver’s door at an angle
of approximately 60°. The movement of the test dummies
is measured. In the other side impact test, the vehicle is
stationary and a 3300-lb movable barrier moving at 31 mph
slams perpendicularly into the driver’s door. The barrier has
the shape of the front bumper, fenders, grille and hood of
an SUV or pickup truck. The barrier is not rigid; it deforms
on impact. As with the frontal offset test, the damage to the
vehicle and the forces on the test dummies are evaluated
to arrive at a good, acceptable, marginal, or poor rating.
During the roof strength rating test, the vehicle is
parked under a large metal plate. The underside of the
vehicle is supported so the suspension does not compress.
Hydraulic force pushes the metal plate on the side of the
roof over the driver’s head. The roof is compressed 5".
During the compression the windshield will crack and the
side windows will blow out. The amount of force required
to cause 5" of crush is measured and compared to the
vehicle weight. A force-to-weight ratio is then calculated.
If a force equal to or greater than 4 times the vehicle
weight is required to cause the compression, a ratio of
4:1 or more, the vehicle is given a good rating. If the
force-to-weight ratio ranges from 3.25:1 to 3.9:1, the
vehicle is given an acceptable rating. Vehicles with
ratios of 2:5:1 to 3.24:1 are rated as marginal. If the ratio
is below 2.5:1, the vehicle is rated as poor.
The rear impact rating is not determined by vehicle
deformation; instead it is based on the potential for head,
neck, and upper body injury to vehicle occupants. There
are two testing methods used to determine the rear impact
rating. The first testing method, called the geometry test,
looks at where the headrests would contact the back of
Figure 3-24. These assembly line employees are
positioning the front windshield on a newly assembled
vehicle. (Ford)
an average person’s head. A contact location above the
center of the head is good, at the center of the head is
acceptable, and below the center of the head is either
marginal or poor. The second testing method used to
determine the rear impact rating involves performing a
crash test. Crash dummies are belted into seats mounted
on an impact sled. The sled is then accelerated to
simulate a rear impact in which a stopped vehicle (0 mph)
is hit in the rear, causing it to accelerate to 10 mph (a 10
mph change in velocity). An impact of this type will cause
the occupants’ heads to snap back. Injury to the crash
dummies is measured to calculate the rear impact rating.
Summary
There are several ways to classify vehicles. In
addition to year, make, and model, vehicles are
often classified by construction type, body type,
and drive train configuration.
A full-frame vehicle has a body and a separate frame.
A unibody vehicle is constructed of sheet metal
panels that are spot welded together to form the
main body structure.
The vehicle’s drive train produces power and
conveys this power to the drive wheels.
A vehicle’s body style is determined by the
configuration of its body panels.
Vehicle designers constantly strive for ways to reduce
vehicle weight without compromising occupant safety.
There are three basic types of steel used in
automobile construction: mild steel, high-strength
steel, and advanced high-strength steel.
Laminated steel is made of two layers of steel
bonded together with a solid layer of adhesive.
Aluminum is only about one third the weight of
steel, but it can be just as strong.
Magnesium is lighter than aluminum, but it is just
as strong.
Plastic can be broken down into three general
categories—flexible, semi-rigid, and rigid.
Laminated glass is made of two layers of glass
with a layer of plastic in between them. Tempered
glass is a single layer of glass and is used on all
windows except the windshield.
A structural part increases the strength of the
vehicle. A non-structural part does not make a
vehicle any stronger.
The outer body assemblies are readily visible when
looking at the vehicle. These assemblies may or may
not be structural parts, depending on vehicle design.
The inner body assemblies are not easily visible
from outside the vehicle. Most are structural parts.
There are four basic types of safety ratings: frontal
impact, side impact, roof strength, and rear impact.