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This pages attempts to explain commonly used suspension terms.
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Shock Absorbers
- In the past, a wide variety of direct and indirect shock absorbing devices were used to control spring action of passenger cars. Today, direct, double-acting hydraulic shock absorbers and shock absorber struts have almost universal application.
- The operating principle of direct-acting hydraulic shock absorbers is in forcing fluid through restricting openings in the valves. This restricted flow serves to slow down and control rapid movement in the car springs as they react to road irregularities. Usually, fluid flow through the pistons is controlled by spring-loaded valves. Hydraulic shock absorber automatically adapt to the severity of the shock. If the axle moves slowly, resistance to the flow of fluid will be light. If the axle movement is rapid or violent, the resistance is stronger, since more time is required to force fluid through the openings. By these actions and reactions, the shock absorbers permit a soft ride over small bumps and provide firm control over spring action for cushioning large bumps. The double-acting units must be effective in both directions because spring rebound can be almost as violent as the original action that compressed the shock absorber.
Coil Springs
- Compression type coil springs may be mounted between the lower control arm and spring housing or seat in the frame. Other front suspension systems have the coil springs mounted above the upper control arms, compressed between a pivoting spring seat bolted to the control arm and a spring tower formed in the front end sheet metal. When coil springs are used in both front and rear suspension, three or four control arms are placed between the rear axle housing and the frame to carry driving and brake torque. The lower control arms pivot in the frame members and sometimes support the rear coil springs to provide for up-and-down movement of the axle and wheel assembly.
Leaf Springs
- Front leaf springs are used with solid axle beams in most truck applications. Corvettes use single-leaf, filament-wound, glass/epoxy front and rear springs mounted transversely; i.e., they are crosswise to the vehicle's centerline. Rear leaf springs are used on trucks and some passenger cars. Single leaf or multi-leaf springs are usually mounted longitudinally over the front axle beam or under the rear axle housing. The spring center bolt fastens the leaves together, and its head locates the spring in the front axle beam or saddle on the rear axle housing. U-bolts clamp the spring firmly in place and keep it from shifting. Eyebolts, brackets, and shackles attach it to the frame at each end. Leaf springs also serve as control arms, locating the rear end in position and transferring force to the chassis.
Torsion Bars
- Torsion bar suspension uses the flexibility of a steel bar or tube, twisting lengthwise to provide spring action. Instead of the flexing action of a leaf spring, or the compressing-and-extending action of a coil spring, the torsion bar twists to exert resistance against up-and-down movement. Two rods of spring steel are used in this type of suspension. One end of the bar is fixed solidly to a part of the frame behind the wheel; the other is attached to the lower control arm. As the arm rises and falls with wheel movement, the bar twists and absorbs more of the road shocks before they can reach the body of the car. The bar untwists when the pressure is released, just like a spring rebounding after being compressed.
- Adjusting the torsion bars controls the height of the front end of the vehicle. The adjusting bolts are located at the torsion bar anchors in the front crossmember. The inner ends of the lower control arms are bolted to the crossmember and pivot through a bushing.
Shock Absorber Struts
- A strut is a structural piece designed to resist pressure in the direction of its length. On typical "MacPherson Strut" use, the shock absorber is built into the strut. Most shock absorber struts are hydraulic units. Some MacPherson systems used on Ford vehicles are equipped with low-pressure, gas-filled shock struts. They are nonadjustable and nonrefillable. Like the hydraulic shock struts, faulty units must be replaced as an assembly. Another similar front suspension system is called the "hydraulic shock strut." This strut serves as a shock absorber and replaces the upper control arm. The coil spring, however, is located between the lower control arm and the body structure instead of being mounted directly on the strut.
Sprung and Unsprung Weight
- "Sprung" weight is a term used to describe the parts of an automobile that are supported by the front and rear springs. They suspend the vehicle's frame, body, engine, and the power train above the wheels. These are quite heavy assemblies.
- The "unsprung" weight includes wheels and tires, brake assemblies, the rear axle assembly, and other structural members not supported by the springs.
Sway Bar
- Some cars require stabilizers to steady the chassis against front end roll and sway on turns. Stabilizers are designed to control this centrifugal tendency that forces a rising action on the side toward the inside of the turn. When the car turns and begins to lean over, the sway bar uses the upward force on the outer wheel to lift on the inner wheel, thus keeping the car more level.
Control Arms
- A control arm is a bar with a pivot at each end, used to attach suspension members to the chassis.
- When coil springs are used in both front and rear suspension, three or four control arms are placed between the rear axle housing and the frame to carry driving and brake torque. The lower control arms pivot on the frame members and sometimes support the rear coil springs to provide for up-and-down movement of the axle and wheel assembly.
- A-arms are control arms with two inboard pivots, giving strength. Some front end designs use control arms instead of A-arms, usually to save weight and add adjustability.
Camber Angle
- Camber angle is the angle made by the wheel of your Scion vehicle. More specifically, it is the angle of verticle tilt of the wheels and tires when viewed from the front or rear of the vehicle. Camber must be taken into consideration in the design of steering and suspension components. If the top of the wheel is further out than the bottom (that is, tilted inwards), it is called positive (+) camber; if the bottom of the wheel is further out than the top, it is called negative (-) camber.
- Camber angle alters the handling qualities of the suspension. Negative (-) camber improves grip during cornering maneuvers. The negative angle provides the outer tire which is taking the greatest proportion of the cornering loads, at a better angle to the road, increasing the contact patch of the tire and transmitting the forces perpendicular to the tire, rather than through a shear force across it. On the other hand, for maximum straight-line acceleration, obviously the greatest traction will be attained when the camber angle is zero (neutral) and the tread is flat on the road. Proper management of camber angle is a major factor in suspension design, and must incorporate not only great geometric models, but also real-life behavior analysis of the components such as flex, distortion, elasticity, etc. What was once an art has now become much more scientific with the use of computers, which can juggle all the variables mathematically instead of relying on the designer's intuitive feel and experience, and as a result the handling of even low-priced automobiles has improved significantly.
- In older cars with double wishbone suspensions, camber angle was usually adjustable, but in newer models with McPherson strut suspensions it is normally fixed. While this may reduce maintenance requirements, if the car is lowered by use of shortened springs, this changes the caster angle (as described in McPherson strut) and can lead to increased tire wear and even impaired handling. For this reason, individuals who are serious about modifying their car for better handling will not only lower the body, but also modify the mounting point of the top of the struts to the body to allow some fore/aft movement for caster adjustment. Aftermarket plates with slots for strut mounts instead of just holes are available for most of the commonly modified models of cars.
- Another reason for negative camber is that a rubber tire tends to roll on itself while cornering. If the tire had zero camber, the inside edge of the contact patch would begin to lift off of the ground, thereby reducing the contact patch. By applying negative camber, this effect is reduced, thereby maximizing the contact patch.
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