Understanding the Automotive Chassis System
The term "chassis" is used to designate the complete car minus the body. The chassis therefore consists of the engine, power-transmission system, and suspension system all suitably attached to, or suspended from, a structurally independent frame. Although this construction is widely used, an almost equal number of automobile makers employ a design in which the frame and body are welded together to form an integral unit.
Usually of all-welded steel construction, the frame may consist of either (1) box-girder side rails with reinforced center X; (2) full-length box-girder side rails with box-girder cross members (ladder type); or (3) center X construction with no side rails, braced front and rear with box-girder cross members.
The chief design requirements of the automobile frame, whether it be structurally independent or an integral part of the body, are that it provide great strength with minimum weight. It must be rigid enough tc absorb the road impacts and shocks transmitted by wheels and axles, and it must be able to withstand the torsional stresses encountered under operating conditions.
To save weight, side members are made deepest at the location of greatest bending moment, tapering off as the bending moment decreases. The frame is made narrower at the front to allow the front wheels to turn when steering; it also features a "kickup" at the rear to lower the center of gravity of the car and still allow sufficient room for effective rear-spring action.
Photo by CZmarlin
The front wheels of most passenger cars are independently suspended from the frame. Independent suspension reduces the front-end vibration associated with the rigid front axle that formerly was used, and it also improves vehicle riding and handling qualities. The movement of each front wheel is, within the limitations discussed below, completely unaffected by the movements of the other.
The most common independent suspension system mounts a steering-knuckle-and-wheel-spindle assembly between upper and lower pairs of nearly parallel control arms. The inner ends of the control arms pivot in rubber-mounted steel bushings secured to the frame; the outer ends terminate in ball joints that support the steering knuckle and wheel spindle. Because the lower arms are longer than the upper, the relation of their up-and-down movements is such that, in turning maneuvers, the outside and more heavily loaded wheel remains more nearly vertical with respect to the road surface.
Front suspensions may incorporate either torsion bars or coil springs. Torsion bars, one on each side, run parallel to the front-to-back center line of the vehicle. A torsion bar is a steel member, usually cylindrical, that absorbs front-wheel deflections by twisting about its own horizontal axis. One end of the torsion bar is fastened rigidly to the frame at some point toward the rear of the car; the other end is linked to the suspension system so that the shaft alternately twists and untwists in response to the vertical movements of the front wheel.
When coil springs are used, they are mounted under compression between the frame and the upper or lower control arms. In addition, a stabilizer bar often is linked to the lower control arms to balance tire loading and to prevent excessive sway when the car is cornering. Whenever one spring deflects more than the other, the stabilizer equalizes the deflection by transferring part of the load to the other tire.
Although a few American cars feature independent, or swing-axle, rear-wheel suspension, the majority use a fixed rear axle suspended from either laminated (layered) leaf springs or a coil-springs-trailing control-arm arrangement. Whichever suspension system is used, it must be designed not only to absorb road shocks but also to provide a means for absorbing the torque reactions resulting from driving and braking.
When laminated leaf springs are used, one end of each spring is fastened to the frame of the car by a pivot joint. The other end is connected to the frame by a shackle, or swinging joint, that compensates for the changes in over-all length that occur when the spring flexes. Connection bushings are steel sleeves mounted in oil-resistant rubber. Leaf springs usually are clamped to the rear-axle housing with U-bolts at a point approximately midway between the ends of the spring.
In a coil-spring rear-suspension system the springs are mounted under compression between the frame and the axle housing. Because of the nature of coil springs, transverse (crosswise) radius rods are used to restrict sidewise movement of the axle housing relative to the frame. To absorb torque reactions, special torque bars are installed between the axle housing and some reinforced point on the frame just ahead of the axle housing.
The shock absorber is a hydraulic damping device that controls the oscillations of the springs and prevents their being excessively compressed or expanded. Most commonly used is the direct-acting type, involving a double-acting piston-and-cylinder arrangement. Rear shock absorbers are installed between the axle housing and the frame; front shock absorbers usually are mounted inside the coil springs between the lower control arm and the frame.
Optional-equipment rear shock absorbers are available that provide adjustable load-carrying capacity, an especially useful feature for station-wagon owners. In one design the upper portion of a hydraulic shock absorber is surrounded by a metal-encased rubber boot that can be inflated with air from a connection inside the vehicle. By varying the air pressure within the boot from approximately 30 to 90 pounds per square inch, the driver can have a soft, comfortable ride when the vehicle is empty or a ride that is firm and controlled when the vehicle is heavily loaded.
All American automobiles are equipped with two independent brake systems: 4-wheel hydraulic service brakes operated by a pedal and mechanical parking brakes usually operated by a lever. In some older cars the parking brake consists of a brake-drum-and-band arrangement mounted on the propeller shaft directly behind the transmission. More commonly, however, the parking-brake system consists of steel cables and linkage that mechanically actuate only the rear-wheel service-brake shoes. By incorporating a positive mechanical lock, most automatic transmissions provide what is, in effect, another parking brake. Its engagement is controlled manually through the transmission-selector lever.
When the service-brake pedal is depressed, the master cylinder transmits equal pressure through high-strength tubing to hydraulic cylinders at all four wheels. Each wheel cylinder then forces a pair of brake shoes outward against the revolving drum with sufficient force to slow or stop the car. Maximum braking is attained when the pressure of the shoes against the drums is such that the wheels do not quite lock. Brake shoes are faced with a friction material that is either bonded or riveted to the shoes.
Depending on many variables—including the heating properties of lining and drum materials, car speed, deceleration rate, and (least significant) ambient temperature—the surface temperature at the drum while braking may exceed 1000° F (538° C). The forward motion of the car causes a flow of cooling air to sweep over the drum, effectively removing most of this heat.
Brake-drum material is usually cast iron; however, some manufacturers are taking advantage of aluminum's high heat conductivity and are using ribbed aluminum drums lined with cast iron at the wearing surfaces. Brake-drum ribbing has two functions: (1) it improves the radiation of heat away from the brakes by increasing the surface area of the drum, and (2) it provides an additional structural stiffness that enables the drum better to resist high-temperature distortion and accompanying brake fade.
Braking system improvements include the following: the provision of an extra margin of safety through the use of independent hydraulic systems for front and rear brakes; the introduction of self-adjusting brakes as standard equipment on a great many cars; the adoption of disk brakes long popular on European cars; and the use of linings of sintered (specially heat-treated) metal for heavy-duty braking service. Increasingly, American cars are being equipped with power brakes to reduce the pedal effort.
Wheels and Tires
The automobile wheel has progressed from the original wooden-rim-and-wire-spokes affair borrowed from the bicycle to the present all-steel safety wheel designed specifically for automotive applications. Made of two steel stampings welded together, the modern wheel combines great strength with relatively light weight. Most compact cars are equipped with 13-inch-diameter wheels (33 cm); all other American cars use either 14- or 15-inch wheels (36 or 38 cm).
Tires also have undergone some radical changes over the years. For example, pressure has dropped from the 65 pounds per square inch used in early tires to the 24 pounds per square inch now commonly recommended for low-profile tires. The tubeless tire, introduced in 1955 and now standard equipment, has added considerably to automotive safety.
Increasingly, automobile manufacturers are regarding the tire as an integral part of the car and not just as an accessory that is fitted arbitrarily into an already completed design concept. More attention is being paid to the skidding characteristics of tires, and the scientific designing of tire treads and casings is contributing importantly to both automobile handling and passenger safety. Tires claimed to be virtually punctureproof are available to the American motorist. Some feature a separate inner chamber that assumes the load if the regular tire casing loses its air; others utilize a gummy substance stored inside the tire to seal off leaks.
In tire construction, the term "ply" refers to one of the layers of rubber-impregnated cords that provide a tire casing with stiffness and strength. Thus a 4-ply tire has four layers of cord beneath its rubber tread face. By increasing the diameter of the tire cords, engineers have been able to design a 2-ply tire (introduced in 1961) with the performance and safety characteristics of a conventional 4-ply tire. In addition, the 2-ply tires provide a softer, more comfortable ride.
Once popular as a tire cord material, cotton has now been largely replaced by synthetic fabrics, such as nylon and rayon. Cotton deteriorates when moisture leaks through small cracks in the sidewall rubber and is therefore less desirable than the synthetic materials.
Through a suitable mechanism and connecting linkage, the steering gear permits the driver to control the vehicle's direction of travel by simultaneously changing the angle of both front wheels. Rotary motion applied to the steering wheel rim is transferred via a steel shaft to a lubricant-filled steering mechanism bolted to the frame of the vehicle. Here it is translated into the lateral movement required to position the steering-wheel linkage.
The translating mechanism may be one of several types: cam and lever, worm and sector, worm and roller, or recirculating ball nut. Most American cars now employ the recirculating ball nut. In this design, recirculating steel balls cause a grooved nut to move along the steering-shaft worm gear when the steering wheel is turned. The nut, in constant mesh with a toothed sector, imparts a rotational movement to the output shaft of the steering-gear mechanism.
A short steering arm (called the Pitman arm) is splined to the other end of the output shaft. It links the translational mechanism to the steering linkage via a ball-joint connection.
Steering tie rods and drag links, of varying lengths of steel rod or thick-walled steel tubing, transmit Pitman-arm movements to a steering knuckle at each road wheel. Ball joints throughout keep friction low and ensure freedom of linkage movement in any direction. Both the steering mechanism and the geometry of the steering linkage offer a mechanical advantage that makes steering comparatively effortless for the driver. In American cars, the steering ratio—which is the ratio of the gear reduction provided by the steering-gear translational mechanism—ranges from 15.7:1 for power-assisted steering to 24.1:1 for manual steering.