How Automotive Suspension and Steering Works - Page 2

Spindle

Something has to tie the control arms, ball joints, springs and steering system together, as well as provide a mounting for the wheel assembly. This is referred to as the spindle. The spindle is typically a forged metal component that has provision for mounting the ball joints at top and bottom. In the middle the spindle is fitted with an axle hub, to which the wheel and its bearings are attached. The spindle also has a fitting to mount the steering components, stabilizer bar (if any) and anti-sway bar.

As the car moves down the road the wheel pushes the spindle up and down and the steering rotates it vertically right and left. This motion is transmitted through the ball joints and control arms, as well as the steering components and stabilizer assemblies.

Shock Absorbers, Anyone?

The above components are in place to allow the car to move safely and predictably down the road, but something is needed to keep the wheels from bouncing wildly when they hit bumps in the surface or dips in the roadway. If not checked, a wheel's tendency to bounce would allow the springs to expand and contract (rebound) over several oscillations, much like a Slinky.

The component that prevents this action is the shock absorber. When a shock is operating properly it controls the rate of spring rebound and compression, thus assisting in limiting body sway.

The purpose of a shock absorber is to keep the tire in contact with the pavement!

Shock absorbers were used early on and have gone through a number of designs over the decades. During the 20s and 30s a popular shock absorber was the "knee action" type, a complicated hydraulic device that had a tendency to leak and wear out. This was universally replaced by the telescopic shock absorber, or "double acting" shock absorber.

The telescopic shock consists of an outer casing, central piston rod, upper and lower fluid reservoir cylinder, piston, control valve, fluid fill and provision for mounting on the car. When the shock is extended the piston is pulled upward, forcing fluid through the control valve into the lower chamber. When compressed the fluid is forced through the piston valve into the upper chamber, and so on.

The passing of the fluid through the control valves slows down the movement of the piston, thus placing a damping action on the movement of the springs. Most shocks have valving that is calibrated for an average load condition and are not adjustable, but some aftermarket shocks have adjusting devices to control the amount of damping action.

Steering System - Manual

All manual steering systems contain the same basic components, although individual designs vary tremendously. The most popular systems are the recirculating ball worm and nut design and the rack and pinion. The basic components are as follows:

Steering Wheel

The steering wheel is the only visible component in everyday use. The wheel itself is attached, usually by means of a spline, to the steering shaft.

Steering Shaft

The steering shaft is a hardened steel rod that transmits rotation of the steering wheel to the rest of the system. Depending upon the car's size and engine configuration the shaft may be solid or articulated (utilizing U-joints or other coupling devices) to clear around engine bay components.

Steering Gear (recirculating ball and nut)

The purpose of the steering gear is to transform the rotation of the steering shaft into a means of moving the rest of the steering components in order to turn the wheels.

In recirculating ball worm and nut designs the components are mounted in a gear case that is normally attached to the car's frame. The components consist of the ball nut, worm gear, sector shaft and sector gear. The steering shaft is connected to the input shaft of the gear case, either directly or through the use of a universal joint.

As the steering shaft turns, the input shaft's worm gear moves the ball nut along the worm gear. The ball nut's teeth, intersecting with the sector gear's teeth, forces the sector gear to rotate the sector shaft. All components are set in bearings and lubricated either with grease or special oil. (Some manufacturers built steering gear cases with just a worm gear driving the sector. This configuration works well but wears quickly. To solve this problem, manufacturers turned to the ball nut, in which ball bearings follow the worm gear grooves, dramatically lowering wear rate.)

Pitman Arm

At the bottom of the sector shaft is mounted the pitman arm. This arm is splined to the sector shaft and rotates with it. The arm is in turn mounted to the steering linkage.

Linkage

Typical linkage consists of a rod that runs horizontally under the car, usually called a "drag" or "center" link. The pitman arm connects to one end and as the pitman arm moves the link follows from side to side. Attached to the link are the steering arms.

Steering Arms

Steering arms, commonly called "tie rods" are attached to the center link. These have ball joints at both ends so that any up and down movement of the wheels can be accommodated without binding. The tie rods are designed to be adjustable in length so that alignment adjustments can be made.

And Now, Rack and Pinion Steering

Rack and pinion steering systems differ from recirculating ball systems in that there is no gear case, pitman arm and center linkage. Instead, the steering shaft connects to the rack assembly, which in turn connects directly to tie rods out to the wheels.

Inside the rack assembly is a toothed rack that mates to a mating pinion gear that is driven by the steering shaft. As the pinion is turned the rack moves back and forth horizontally. The rack assembly is mounted to the car's subframes.

The chief advantages of rack and pinion systems are cost, compactness and less overall "slop" in the steering system due to fewer joints. Such systems give a more precise "road feel," a desirable element in modern vehicle design.

Power Steering

Power steering systems utilize essentially the same components, with the addition of an engine-driven hydraulic pump and associated equipment. In most systems the pump pressurizes hydraulic fluid (basically automatic transmission fluid) and pumps it to either the steering gear case or a power cylinder attached to the center linkage. Valves in the system direct the pressurized fluid in the proper direction when the steering wheel is rotated.

Power assist greatly reduces steering effort. Such systems were introduced in the 1940s and gradually took the place of manual systems during the 1960s. If the power steering pump fails for any reason, or if the engine stops running, the driver always has manual control of the steering, although effort may be considerable.

Alignment Terms

All front suspension components must be designed to keep the wheels in the proper orientation. Otherwise, tire wear would be unacceptable, steering effort could be difficult and the car might be unstable or difficult to control.

Engineers go to great lengths to design front suspension systems to provide ride and handling as well as tire life. The geometry of suspension/steering systems is classified by the following terms:

Caster

The spindle on which the wheel is mounted is not set vertically between the control arms. Doing so would cause an unstable rotation about that axis. Therefore, manufacturers specify an amount of slant off the vertical, measured in angular degrees. The slant can be either forward or rearward, expressed as "positive" or "negative" caster angle.

Camber

Camber is the amount of wheel tilt, at resting height, off the vertical. Positive camber refers to the top of the wheel tilting to the outside of the car, while negative camber refers to the top of the wheel tilting to the inside of the car. Most cars are aligned so that there is slight negative camber.

Toe-In, Toe-Out

Front wheels are generally not perfectly parallel to each other, due to the way the suspension's geometry changes during turning, forward movement and its adjustment to the curvature of the road surface (all roads have a "crown," or a convex shape to shed water to the sides). Engineers specify a degree of Toe-in - the wheels are slightly closer together at the front than at the rear - or Toe-out - the wheels are farther apart at the front than at the rear.