ASE T5 Suspension & Steering (Medium/Heavy Truck) key terms

The gearbox that converts rotation of the steering wheel into side-to-side movement of the Pitman arm, usually with hydraulic pressure assisting the driver's effort.

The output shaft of a steering gear, splined to the Pitman arm. It rotates through a limited arc and transmits steering force into the linkage.

A tapered, splined lever clamped to the sector shaft that changes the gearbox's rotary output into a push-pull motion delivered to the drag link.

A solid or tubular link with ball joints at each end that carries steering motion from the Pitman arm to the steering arm or knuckle on the far side of the vehicle.

A ball-and-socket joint threaded onto a tie rod or drag link that lets the linkage pivot in several planes while still transmitting steering force to the wheel.

A pivoting support bolted to the frame that holds the far end of a center link at the same height as the Pitman arm, keeping the linkage parallel to the ground.

A steering gear with the hydraulic control valve and piston built into the gear housing, so assist is applied directly to the sector shaft by pressurized fluid.

An engine-driven hydraulic pump that supplies pressurized oil to the steering gear, with a relief valve limiting maximum pressure and a flow control valve limiting flow.

The forged component that carries the wheel hub and spindle and pivots on the kingpin or ball joints, allowing the front wheels to steer.

A hardened steel pin that serves as the vertical pivot for a steering knuckle on a beam-type front axle, retained by bushings and usually a locking pin.

A shock-absorber-like cylinder mounted between axle and steering linkage that resists rapid steering movement, reducing shimmy and kickback from road irregularities.

The amount of steering wheel rotation that occurs before the front wheels begin to move, measured with wheels straight ahead; excess play indicates worn linkage or gear adjustment.

The difference in distance between the front and rear of the two front tires; toe-in means the fronts are closer together, toe-out means they are farther apart.

The forward or rearward tilt of the steering axis viewed from the side. Positive caster tilts the top rearward and improves straight-line stability and steering return.

The inward or outward tilt of the top of the wheel viewed from the front. Negative camber tilts the top inward and affects tire wear and cornering grip.

The angle between true vertical and the steering axis viewed from the front; it works with caster to affect steering return and scrub radius.

The direction the rear axle points relative to the vehicle's geometric centerline, equal to half the difference between the left and right rear toe readings.

A steering linkage layout that makes the inner front wheel turn a sharper angle than the outer wheel so both wheels roll on concentric arcs in a turn.

The maximum angle a front wheel can be steered left or right before it contacts the frame, spring, or linkage; it sets the vehicle's turning circle.

The distance between the center of the front axle or axle group and the center of the rear axle or axle group, which affects turning radius and weight distribution.

A stack of curved steel plates clamped together that supports axle load and locates the axle; the main leaf carries the eyes and shorter leaves add stiffness.

A rubber-and-fabric bellows filled with compressed air that supports load and, by changing internal pressure, maintains ride height regardless of how heavily the truck is loaded.

A hydraulic cylinder that converts suspension motion into heat, damping spring oscillation and controlling how quickly the suspension settles after a bump.

A rigid link with flexible ends that connects the axle housing to the frame and resists axle wind-up and side-to-side movement under driving and braking forces.

A pivoting link that attaches one end of a leaf spring to the frame so the spring can lengthen and shorten as it flexes instead of breaking its mount.

The measured distance between a specified frame or axle point and the ground; on air suspensions it is set by the height control valve and must be correct for alignment and driveline angles.

A pivoting beam between tandem axles that transfers load from one axle to the other and lets the axles move up and down in opposition.

The variation in a tire's radius as it rotates, measured against a fixed reference; excessive runout causes vibration and irregular tread wear.

Side-to-side wobble of a tire or wheel sidewall measured as the wheel rotates; excessive lateral runout can cause wheel shimmy and uneven contact.

A built-in taper in a tire's construction that produces a sideways force as the tire rolls, causing the vehicle to pull to one side even when alignment is correct.

The distribution of weight around a wheel and tire assembly; imbalance creates centrifugal force that produces vibration at speed and is corrected with weights or balancing beads.

Matching the outside diameters of the two tires in a dual pair so both carry equal load; a large difference overloads the larger tire and causes fast, uneven wear.

The small amount of axial movement in a wheel bearing assembly, set by the spindle nut; too little causes overheating, too much allows wheel wobble and seal leakage.

The horseshoe-shaped plate on a tractor that couples to a trailer kingpin, supporting the trailer's weight and allowing it to pivot while towing.

The two spring-loaded jaws in the fifth wheel that close around the trailer kingpin to hold the trailer; they must be inspected for wear and proper locking.

A hardened vertical pin under the trailer's upper coupler that is gripped by the fifth wheel jaws; its diameter and wear limits are critical to safe coupling.

A main longitudinal structural member of a truck chassis, usually a C-shaped steel channel that carries suspension brackets and the fifth wheel mounting.

An air-filled mount between cab and frame that isolates the cab from road vibration and frame flex, providing a softer ride than solid mounts.

A valve linked between cab and frame that adds or releases air from the cab air springs to hold the cab at its designed height as load changes.

A damper mounted between cab and frame that controls cab bounce and pitch, working together with the cab air springs or rubber mounts.