ASE S5 Suspension & Steering (School Bus) key terms

The forward or rearward tilt of the steering axis viewed from the side of the vehicle, measured in degrees. Positive caster tilts the top of the axis rearward and helps the wheels return to straight ahead.

The inward or outward tilt of the top of a wheel from vertical, viewed from the front. Negative camber tilts the top inward; positive camber tilts it outward. It affects tire wear and cornering grip.

The difference in distance between the front and rear of the two tires on the same axle. Toe-in means the fronts are closer together; toe-out means they are farther apart. It controls tire scrub and steering feel.

The direction the rear axle points relative to the vehicle centerline, expressed in degrees. It is the average of the rear individual toe angles and determines whether the vehicle tracks straight down the road.

The tilt of the steering axis from vertical as seen from the front of the vehicle, formed by a line through the upper and lower ball joints. It affects steering effort and returnability.

The distance on the road surface between the point where the steering axis meets the ground and the center of the tire contact patch. It influences steering feel and stability during braking.

The lateral distance at the ground between the steering axis centerline and the tire contact patch center. Often used interchangeably with scrub radius on independent suspensions without a true kingpin.

The size of the smallest circle a vehicle can trace with its outer front wheel at full steering lock. It depends on wheelbase, track width, and how far the steering linkage allows the wheels to turn.

A linkage layout that makes the inner front wheel turn sharper than the outer wheel during a corner, so both wheels roll without scrubbing. It is built into the angle of the steering arms.

The horizontal distance between the center of the front axle and the center of the rear axle. It affects turning radius, weight distribution, and alignment reference measurements.

The distance between the centerlines of the two tires on the same axle. It affects vehicle stability, tire clearance, and suspension geometry calculations.

An alignment procedure that measures and adjusts angles on all four wheels, including thrust angle and rear toe, so the vehicle tracks straight and the steering wheel is centered.

A steering gearbox that uses ball bearings circulating in a nut and worm shaft to convert steering wheel rotation into pitman arm movement. Common on heavier trucks and buses for its durability and low friction.

A steering system where a pinion gear on the steering shaft turns a toothed rack, moving the tie rods directly. It is compact and gives precise feel, common on lighter vehicles.

The output shaft of a recirculating ball steering gear that carries the pitman arm. It rotates as the ball nut moves, transferring steering input to the linkage.

The lever attached to the sector shaft of a steering gear that converts rotary motion into the side-to-side motion of the steering linkage. It is splined and must be indexed correctly on installation.

The link that connects the pitman arm to the steering knuckle or center link on a conventional steering linkage. It transfers steering motion across the vehicle.

An adjustable link that connects the steering linkage to the steering knuckle arm. Turning its sleeve changes toe setting by lengthening or shortening the effective link.

The threaded clamp that joins the inner and outer tie rod ends and locks the adjustment. Loosening its clamps lets a technician set toe, then it must be retightened to hold the setting.

The joint at the inboard end of a tie rod that attaches to the rack or center link. It usually uses a ball-and-socket design and is a common source of steering play.

The joint at the outboard end of a tie rod that connects to the steering knuckle arm. It is threaded for toe adjustment and has a tapered stud that seats in the knuckle.

A bar that ties the left and right steering linkage together on a conventional parallelogram system. It carries idler arm and pitman arm motion to both tie rods.

A pivoting support on the passenger side of a conventional steering linkage that holds the center link and allows it to move side to side. Worn bushings cause steering wander and play.

The forged assembly that carries the wheel hub and spindle and pivots on the ball joints or kingpin to steer the wheel. It also mounts the steering arm and brake components.

A pivoting and rotating joint with a ball stud in a socket that connects the control arm to the steering knuckle. It allows suspension travel while the wheel steers.

A vertical pin on which a steering knuckle pivots on a solid axle. Bushings or bearings in the knuckle ride on the pin, and wear shows up as play or shimmy.

The flexible sleeve that isolates a control arm from the frame or axle and allows pivoting. Worn or cracked bushings cause clunks, poor alignment retention, and vague steering.

A stack of curved steel leaves that supports axle load and locates the axle on many rear suspensions. It provides both spring rate and fore-aft axle control.

The sleeve or bushing at the eye of a leaf spring that allows the spring to flex and the shackle to pivot. Worn bushings cause axle misalignment and handling complaints.

The pivoting link that attaches the rear eye of a leaf spring to the frame. It allows the spring to change length as it flexes and controls ride height.

The clamp that holds a leaf spring pack to the axle housing. Proper torque is critical because loose U-bolts allow axle movement and can cause spring or housing damage.

A rubber-and-fabric bellows that supports vehicle load using compressed air instead of steel springs. Changing internal air pressure changes ride height and spring rate.

A valve linked to the axle that adds or exhausts air from the air springs to maintain a set ride height as load changes. A misadjusted or leaking valve causes lean or ride height faults.

The distance from the ground or a specified frame point to the axle or suspension reference point. It must be set correctly before alignment and affects air spring pressure and handling.

A device in the air system that removes moisture and contaminants before air reaches the suspension and brake components. Moisture carryover can freeze valves and damage air springs.

The engine-driven hydraulic pump that pressurizes power steering fluid. Low pressure or flow causes hard steering, and a failing pump can whine or produce contaminated fluid.

The valve inside the steering gear that directs pressurized fluid to one side of the piston in response to steering wheel input. It provides assist proportional to steering effort.

The hydraulic oil that transmits pressure and lubricates the pump, gear, and lines. The correct specification matters because wrong fluid can damage seals and change steering feel.

A brake assist system that uses power steering hydraulic pressure instead of engine vacuum to boost braking. A steering pump or fluid fault can therefore cause a hard brake pedal.

A condition where the vehicle drifts side to side and needs constant correction. Common causes include excessive caster error, loose linkage, worn idler arm bushings, or tire problems.

The clearance between the gears inside a steering gear. Too much lash shows up as free play at the steering wheel; too little causes binding and poor returnability.

The small axial movement allowed in a wheel bearing assembly. Correct adjustment prevents overheating and wobble; too much play causes vibration and uneven tire wear.

The amount of side-to-side or up-and-down wobble in a rotating part such as a wheel, hub, or brake drum. Excessive runout causes vibration and uneven tire wear.

Side-to-side wobble measured parallel to the axle when a wheel or hub is rotated. It is checked with a dial indicator and must stay within the manufacturer's limit.

The raised center section of a hub that centers the wheel by matching its center bore. Proper pilot fit carries load and prevents wheel vibration and stud stress.

The specified tightening force applied to wheel fasteners, usually in a star pattern with a calibrated torque wrench. Incorrect torque can loosen wheels or distort brake drums and rotors.

The lip seal that keeps lubricant in and contaminants out of a wheel bearing and hub assembly. A leaking seal lets grease escape and allows water and dirt to damage bearings.