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Suspension and Steering, Page 3 of 4

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Steering geometry

Front wheel alignment (also known as front-end geometry) is the position of the front wheels relative to each other and to the vehicle. Correct alignment must be maintained to provide safe, accurate steering, vehicle stability and minimum tire wear. The factors that determine wheel alignment are interdependent. Therefore, when one of the factors is adjusted, the others must be adjusted to compensate.

Front-end alignment is best checked with sophisticated equipment, such as an alignment rack.

Caster angle
See Figure 16

Caster angle is the number of degrees that a line, drawn through the center of the upper and lower ball joints (or strut and lower ball joint) and viewed from the side, can be tilted forward or backward. Positive caster means that the top of the upper ball joint (or strut) is tilted toward the rear of the vehicle, and negative caster means that it is tilted toward the front. A vehicle with a slightly positive caster setting will have its lower ball joint pivot slightly ahead of the tire's center. This will assist the directional stability of the vehicle by causing a drag at the bottom center of the wheel when it turns, thereby resisting the turn and tending to hold the wheel steady in whatever direction the vehicle is pointed. A vehicle with too much (positive) caster will be hard to steer and shimmy at low speeds. A vehicle with insufficient (negative) caster may tend to be unstable at high speeds and may respond erratically when the brakes are applied.

Figure 16 A positive caster angle will have the lower ball joint pivot slightly ahead of the center of the tire and the strut or upper ball joint tilted toward the rear of the vehicle.
A positive caster angle will have the lower ball joint pivot slightly ahead of the center of the tire and the strut or upper ball joint tilted toward the rear of the vehicle.

Camber angle
See Figure 17

Camber angle is the number of degrees that the wheel itself is tilted from a vertical line, when viewed from the front. Positive camber means that the top of the wheel is slanted away from the vehicle, while negative camber means that it is tilted toward the vehicle. Ordinarily, a vehicle will have a slight positive camber when unloaded. Then, when the vehicle is loaded and rolling down the road, the wheels will just about be vertical. If you started with no camber at all, then loading the vehicle would produce a negative camber. Excessive camber (either positive or negative) will produce rapid tire wear, since one side of the tire will be more heavily loaded than the other side.

Figure 17 A positive caster angle means that the top of the wheel is slanted slightly away from the vehicle so that when loaded, the wheels will be approximately vertical, producing even tire wear.

A positive caster angle means that the top of the wheel is slanted slightly away from the vehicle so that when loaded, the wheels will be approximately vertical, producing even tire wear.

Steering axis inclination
See Figure 18

Steering axis inclination is the number of degrees that a line drawn through the upper and lower ball joints (or strut and lower ball joint) and viewed from the front is tilted to the left or the right. This, in combination with caster, is responsible for the directional stability and self-centering of the steering. As the steering knuckle swings from lock to lock, the spindle generates an arc, causing the vehicle to be raised when it is turned from the straight-ahead position. The reason the body of the vehicle must rise is straightforward: since the wheel is in contact with the ground, it cannot move down. However, when it is swung away from the straight-ahead position, it must move either up or down (due to the arc generated by the steering knuckle). Not being able to move down, it must move up. Then, the weight of the vehicle acts against this lift, and attempts to return the spindle to the straight-ahead position when the steering wheel is released.

Figure 18 Steering axis inclination, in combination with caster, is responsible for the directional stability and self-centering of the steering.
Steering axis inclination, in combination with caster, is responsible for the directional stability and self-centering of the steering.

Toe-in
See Figure 19

Toe-in is the difference (in inches) between the front and the rear of the front tires. On a vehicle with toe-in, the distance between the front wheels is less at the front than at the rear. Toe-in is normally only a few fractions of an inch, and is necessary to ensure parallel rolling of the front wheels and to prevent excessive tire wear. As the vehicle is driven at increasingly faster speeds, the steering linkage has a tendency to expand slightly, thereby allowing the front wheels to turn out and away from each other. Therefore, initially setting the front wheels so that they are pointing slightly inward (toe-in) allows them to turn straight ahead when the vehicle is underway.

Figure 19 Toe-in.
Toe-in

Rear suspensions
See Figures 23, 24, 25, 26 and 27

There are three basic types of rear suspension: independent, semi-independent and live axle. Each of these suspension systems has their own distinctive variations, but the general principles and component types are relatively similar to that of front suspension systems described earlier in this chapter.

Independent rear suspension systems may be found on both rear, front, and 4-wheel drive vehicles. They utilize control arms which allow one wheel to move separately from the other wheel.

Semi-independent rear suspension systems are often found on front wheel drive vehicles. These systems utilize a cross member, which connects to two trailing arms. Despite the fact that there is a solid connection with the cross member and the trailing arms, the cross member will twist with each up and down movement of the wheels. This twisting action provides not only semi-independent movement, but also a stabilizer effect.

Live axle rear suspension systems are usually found on rear and four wheel drive vehicles. These systems consist of leaf or coil springs utilized in conjunction with the live axle, which is the differential axle, wheel bearings, and brakes operating as a unit.

Rear suspensions, in general, can be much simpler than front suspensions since all they have to do is support the rear of the vehicle and provide some sort of suspension control. However, some rear suspensions, especially those found on sports cars, are quite complex.

Figure 23 The semi-independent axle used on many of today's front-wheel-drive vehicles.
The semi-independent axle used on many of today's front wheel drive vehicles.

Figure 24 This is a strut suspension with coil spring, shock absorber and strut combined in one assembly. This assembly attaches to the body and wheel spindle. In this type of suspension the lower control arm, strut and rear axle usually mount on some sort of sub-frame, which is attached to the body of the vehicle.
This is a strut suspension with coil spring, shock absorber and strut combined in one assembly. This assembly attaches to the body and wheel spindle. In this type of suspension the lower control arm, strut and rear axle usually mount on some sort of sub-frame, which is attached to the body of the vehicle.

Figure 25 This is an independent rear suspension used on many sportier vehicles. Coil springs are used between the control arm and the vehicle body, and the control arms pivot on a cross-member and are attached at the other end to a spindle. A shock absorber attached to the spindle or control arm absorbs vibrations.
This is an independent rear suspension used on many sportier vehicles. Coil springs are used between the control arm and the vehicle body, and the control arms pivot on a cross-member and are attached at the other end to a spindle. A shock absorber attached to the spindle or control arm absorbs vibrations.

Figure 26 This is a non-independent rear suspension. It differs from other similar designs in that coil springs replace leaf springs, and strut rods and control arms serve to position the rear axle.
This is a non-independent rear suspension. It differs from other similar designs in that coil springs replace leaf springs, and strut rods and control arms serve to position the rear axle.

Figure 27 This is a basic leaf spring rear suspension with shock absorbers to control vibration as well as up and down axle movement.
Click on picture to enlarge view

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1998 W. G. Nichols - Chilton's Easy Car Care