"Modern Automotive Technology," Vocabulary from Section 11

Although there are design variations, all tires have the same basic parts: beads, body plies, tread, sidewall, belts, and liner. Asymmetric tires have a different tread pattern on the inner edge than the outer edge: the outer tread pattern is designed to improve cornering on dry surfaces, while the inner tread pattern improves traction on wet surfaces.

Many tires have a maximum recommended inflation pressure of 32 psi (220 kPa). Some tires are self-sealing (for small punctures) and some have an extremely stiff sidewall so that they can run flat into a repair shop. During a flat (slow leak reduces inflation pressure) or a blowout (instant rupture and air loss), a safety rim with small ridges ensures that the tire beads remain on the wheel.

Tire rotation involves changing the location of the tires on the vehicle to ensure maximum tire life. Normally, the front or rear tires wear differently. Rotation helps even out tire wear and prevents premature failure of any one tire.

In an independent suspension system, each wheel is attached to its own suspension unit, so the movement of one wheel has minimal effect on the other wheels. In a nonindependent suspension system, the right and left wheels are attached to the same solid axle, so when one wheel hits a bump in the road, its upward movement causes a slight upward tilt of the other wheel (a semi-independent suspension minimizes this effect with a flexible axle).

A suspension leveling system maintains the ride height and vehicle attitude (height ratio between front and rear of body) with changes in the amount or distribution of weight in the car. An automatic system uses height sensors to turn on a compressor to either add or bleed air out of the shocks or springs to counteract the load change.

Sticktion is a term used to describe the initial resistance to movement between rubber seals or bushings and metal parts. Sticktion can afflict parts of the suspension system and affect ride smoothness.

The steering system allows the driver to safely maneuver the vehicle. Early steering systems were simple mechanisms consisting of a series of shafts, rods, and gears that transferred steering wheel motion to the wheels and tires. Modern steering systems include more sophisticated hydraulics and electronics to improve precision.

The steering column normally bolts to the underside of the dash; it sticks through the firewall and fastens to the steering gear assembly. Most vehicles use a collapsible steering column to help prevent driver chest and face injury during a collision. These columns can also tilt so that the steering wheel can be positioned at different angles and heights.

The steering linkage is a series of arms, rods, and ball sockets that connect the steering gearbox to the knuckles. The linkage used with a worm-type gearbox is called a parallelogram steering linkage; it typically includes a pitman arm, center link, idler arm, ball sockets, and two tie-rod assemblies (and sometimes, a steering damper).

In addition to the steering wheel, column, and shaft, a rack-and-pinion steering system includes pinion gear (rotated by the steering wheel and shaft), rack (long steel bar with teeth that meshes with pinion gear), gear housing, and tie-rods (to connect the rack with the steering knuckles).

When the steering wheel is turned, the weight of the vehicle causes the front tires to resist turning. This twists a torsion bar (rotary valve mechanism) or thrusts the pinion shaft (spool valve mechanism), which moves the control valve to align oil passages. Pressure then acts on the power piston, which pushes the rack and turns the front wheels.

There are two common types of brake assemblies used on modern vehicles: disc brakes (often used on the front wheels) and drum brakes (often used on the rear wheels). A disc brake assembly includes a caliper, caliper cylinder, brake pads, and rotor; a drum brake assembly includes a wheel cylinder assembly, brake shoes, and brake drum.

Several principles apply to the operation of a hydraulic system:
--it can be used to increase or decrease force or motion.
--liquids in a confined area will not compress, while air will.
--when pressure is applied to a closed system, it is exerted equally in all directions.

Brake fluid is one of the most important components for a functioning brake system. It must have the following characteristics: correct viscosity (free flowing at all temperatures), high boiling point, low freezing point, and water tolerance; its noncorrosive quality should lubricate to reduce wear on metal and rubber parts.

An anti-lock brake system (ABS) uses wheel speed sensors, a computer, and a modulator unit to prevent skidding during hard braking. To prevent a collision, a driver might press down hard on the brake pedal; without ABS, too much hydraulic pressure will cause the wheels to lock and the tires to skid. When tires skid, they decrease their friction with the road surface and reduce vehicle control. For maximum stopping power, the tires should almost, but not quite, skid.

Caster is the forward or rearward tilt of the steering knuckle (spindle support) when viewed from the side of the vehicle. It is a directional control angle that offsets road crown pull (steering wheel pull caused by hump in center of road) and returns the wheels to the straight-ahead position.

Just like caster, camber is measured in degrees starting at the plumb line. If the wheel is aligned with the plumb line, camber is zero. Most manufacturers recommend a slight positive camber setting so that the tops of the wheels tilt outward when viewed from the front of the vehicle. This prevents tire wear, loads the larger inner wheel bearing, and aids steering.