The automotive chassis provides the strength necessary to support the vehicular components and the payload placed upon it. The suspension system contains the springs, the shock absorbers, and other components that allow the vehicle to pass over uneven terrain without an excessive amount of shock reaching the passengers or cargo. The steering mechanism is an integral portion of the chassis, as it provides the operator with a means of controlling the direction of travel. The tires grip the road surface to provide good traction that enables the vehicle to accelerate, brake, and make turns without skidding. Working in conjunction with the suspension, the tires absorb most of the shocks caused byroad irregularities. The body of the vehicle encloses the mechanical components and passenger compartment. It is made of relatively light sheet metal or composite plastics. The components which make up the chassis are held together in proper relation to each other by the frame.

FRAMES
Learning Objective: Describe the function, construction, and types of frames used on wheeled vehicles.

The separate frame and body type of vehicle construction (fig. 8-1) is the most common technique used when producing most full-sized and cargo vehicles. In this type of construction, the frame and the vehicle body are made separately, and each is a complete unit by itself. The frame is designed to support the weight of the body and absorb all of the loads imposed by the terrain, suspension system, engine, drive train, and steering system, and the body merely contains and, in some cases, protects the cargo. The body generally is bolted to the frame at a few points to allow for flexure of the frame and to distribute the loads to the intended load-carrying members. The components of this type of frame are as follows (fig. 8-2):

The SIDE MEMBERS or rails are the heaviest part of the frame. The side members are shaped to

Figure 8-1.- Separate frame and body.

Figure 8-2.- Components of a typical frame design.

accommodate the body and support the weight. They are narrow toward the front of the vehicle to permit a shorter turning radius for the wheels and then widen under the main part of the body where the body is secured to the frame. Trucks and trailers commonly have frames with straight side members to accommodate several designs of bodies and to give the vehicle added strength to withstand heavier loads.

The CROSS MEMBERS are fixed to the side members to prevent weaving and twisting of the frame. The number, size, and arrangement of the cross members depend on the type of vehicle for which the frame was designed. Usually, a front cross member supports the radiator and the front of the engine. The rear cross members furnish support for the fuel tanks and rear trunk on passenger cars and the tow bar connections for trucks. Additional cross members are added to the frame to support the rear of the engine or power train components.

The GUSSET PLATES are angular pieces of metal used for additional reinforcement on heavy-duty truck frames.

With this type of frame construction, the body structure only needs to be strong and rigid enough to contain the weight of the cargo and resist any dynamic loads associated with cargo handling and cargo movement during vehicle operation and to absorb shocks and vibrations transferred from the frame. In some cases. particularly under severe operating conditions, the body structure may be subjected to some torsional loads that are not absorbed completely by the frame. This basically applies to heavy truck and not passenger vehicles. In a typical passenger vehicle. the frame supplies approximately 37 percent of the torsional rigidity and approximately 34 percent of the bending rigidity; the balance is supplied by the body structure. The most important advantages of the separate body and frame construction are as follows:

Ease of mounting and dismounting the body structure.

Versatility; various body types can be adapted to a standard truck chassis.

Strong, rugged designs are achieved easily; however, vehicle weight is increased.

Isolation of noise generated by drive train components from the passenger compartment through the use of rubber mounts between the frame and the body.

Simplistic design that yields a relatively inexpensive and easy manufacturing process.

Frame members serve as supports to which springs, independent suspensions, radiators, or transmissions may be attached. Additional brackets, outriggers, and engine supports are added for the mounting of running boards, longitudinal springs, bumpers, engines, towing blocks, shock absorbers, gas tanks, and spare tires.