ASE S5 Suspension & Steering (School Bus) cheat sheet

Recirculating-ball gear: steering shaft turns a worm gear that drives a ball nut; ball bearings circulate through return tubes, giving low friction and high torque multiplication.

Rack-and-pinion: pinion gear meshes with a toothed rack; one pinion rotation moves the rack a fixed linear distance, so steering ratio equals pinion rotations per rack travel.

Steering ratio = steering wheel degrees divided by road wheel degrees; a higher numeric ratio (e.g., 20:1) means less effort and slower response than a lower ratio (e.g., 14:1).

Inner tie rod end play is checked by rocking the road wheel while an assistant holds the steering wheel stationary; movement at the inner joint indicates wear, not normal gear lash.

Idler arm and pitman arm should be checked for vertical and horizontal play with the wheels straight ahead and the suspension loaded; any perceptible up-down idler movement exceeds typical spec.

Steering gear lash is adjusted only at the center (straight-ahead) position; adjusting off-center preloads the gear and causes binding at the ends of travel.

A steering wheel that is off-center after a toe adjustment indicates the toe was set without centering the wheel or the tie rods were adjusted unequally.

Wander and poor returnability with correct alignment commonly trace to excessive steering gear preload, binding ball joints, or low caster.

Collapsible steering column: an impact causes the intermediate shaft or column tube to collapse (mesh or slip design), absorbing energy rather than transmitting it to the driver.

Tilt/tilt-telescoping columns use a locking pawl engaging a toothed plate; a loose or popping column under load points to a worn pawl, spring, or shear pin.

Ignition lock cylinder release requires the key in the RUN or ACC position to retract the lock pin before the cylinder can be withdrawn.

The steering shaft universal joint (cardan joint) transmits torque through an angle; unequal joint angles on a two-joint shaft produce velocity fluctuation and require phased yokes.

Sector shaft and pitman arm are splined and indexed; the pitman arm must be installed with the steering gear at center and the marks aligned, or the steering wheel will be off-center.

Turn signal switch replacement on many columns requires removing the steering wheel and lock plate; the cancelling cam must engage the switch lever for self-cancel to work.

A horn that sounds when turning indicates a chafed horn wire in the column or a grounded slip ring, not a faulty horn relay.

Column-mounted key release and shift interlock systems must be checked for proper adjustment; a misadjusted interlock prevents key removal in gear or allows removal in gear.

Hydraulic power steering: engine-driven pump supplies flow and pressure; relief valve limits maximum pressure, flow control valve regulates flow to the gear.

Typical hydraulic system pressures run roughly 1000-1500 psi at full lock; the relief valve bypasses excess flow to prevent hose and seal damage.

Air in a hydraulic system causes whine, foam, and a stiff or jerky feel; bleed by cycling lock-to-lock with the engine off, then running, keeping the reservoir full.

A serpentine belt slipping under load causes assist loss at low speed or during parking maneuvers; check belt tensioner and belt condition before condemning the pump.

Electric power steering (EPS): a motor assists the column, pinion, or rack; the control module uses steering angle, torque sensor, and vehicle speed inputs to vary assist.

EPS torque sensors are typically non-serviceable; a steering angle sensor calibration is required after column or rack replacement to restore assist and stability control.

Fluid type matters: using the wrong power steering fluid can swell seals and change viscosity; always match the OEM specification (e.g., ATF vs. dedicated PSF).

A binding steering gear spool valve produces unequal assist left versus right; compare effort in both directions at the same engine speed to isolate the valve.

MacPherson strut combines the shock absorber and spring into one unit and serves as a structural locating member; the upper mount carries both spring and damping loads.

Short-long arm (SLA) suspension: upper and lower control arms of unequal length keep the tire contact patch more stable as the suspension travels.

Ball joint wear is checked by loading the joint (wheels on the ground or jack under the lower arm) and measuring axial and radial play against spec.

Coil spring free length and wire diameter determine rate; a collapsed or broken spring lowers ride height on one corner and changes camber.

Strut rod (tension/compression rod) controls fore-aft wheel location; worn bushings cause braking pull and clunking under acceleration and braking.

Sway bar (stabilizer bar) links opposite sides; a broken link or worn bushing causes body roll increase and a rattle over bumps, not a ride height change.

Bump stops limit suspension travel and protect the strut; a missing or deteriorated bump stop allows metal-to-metal contact and strut damage.

Camber on a strut suspension is often adjusted by an eccentric bolt at the strut-to-knuckle joint or by slotting the strut tower per OEM procedure.

Leaf spring: multiple leaves clamped together act as a beam; the main leaf locates the axle, and additional leaves increase load capacity and rate.

Solid axle with leaf springs uses shackles to allow length change as the spring flattens; a seized shackle restricts travel and can crack the spring eye.

Trailing arm and track bar rear suspensions: the track bar (panhard rod) controls lateral axle location; worn bushings cause rear steer and dog-tracking.

Rear coil spring suspensions use upper and lower seats; a misseated or broken spring changes ride height and can cause a lean that mimics a bent axle.

Rear air suspension (school bus) uses height control valves to maintain ride height; a leaking bag or valve causes one side to sag and the compressor to run continuously.

Rear shock absorbers control rebound and compression; a failed shock shows as cupping tire wear, excessive bounce after a bump, and fluid leakage.

U-bolts clamp the axle to the spring; they must be re-torqued to spec after spring or axle work, and uneven torque can shift axle alignment.

Rear wheel bearing play and axle end play are checked with the wheel off the ground; excessive play indicates bearing, seal, or axle wear and affects tire wear and stability.

Camber: inward or outward tilt of the wheel top from vertical; positive camber tilts the top outward, negative tilts it inward; excessive camber causes one-sided tire wear.

Caster: forward or rearward tilt of the steering axis; positive caster improves straight-line stability and returnability; too much can increase steering effort.

Toe: difference in distance between the fronts and rears of the tires; toe-in means fronts are closer, toe-out means they are farther apart; toe is the primary cause of feathered tire wear.

Thrust angle: the bisector of the rear total toe; if rear toe is unequal, the thrust angle is nonzero and the vehicle dog-tracks, requiring rear adjustment first.

Included angle = camber + SAI (steering axis inclination); comparing included angle side-to-side helps detect a bent spindle or knuckle.

Setback: one front wheel is rearward of the other, measured as the difference in caster or in wheelbase; excessive setback indicates frame or suspension damage.

Toe-out on turns (turning angle / Ackermann): the inner wheel turns sharper than the outer; incorrect turning angle indicates a bent steering arm or knuckle.

Alignment sequence: check tire pressure and ride height, then rear camber/toe (thrust angle), then front camber, caster, and toe last.

Tire size P235/75R15: 235 = section width in mm, 75 = aspect ratio (sidewall height as % of width), R = radial, 15 = rim diameter in inches.

Load index and speed rating are stamped on the sidewall; school bus tires must meet or exceed the OEM load index for the axle GAWR.

Tire placard (certification label) lists recommended cold inflation pressure; inflating to sidewall maximum is incorrect unless the placard specifies it.

Wheel lug nut torque must be applied in a star pattern and rechecked after the first 50-100 miles; loose or uneven torque can warp a hub or loosen a wheel.

Tapered roller wheel bearings require preload or end play per spec; over-tightening causes heat and premature failure, under-tightening causes play and wobble.

Tire wear patterns: center wear = overinflation; both edges = underinflation; one edge = camber; feathered = toe; cupping = shock or balance.

Out-of-round or unbalanced tires cause vibration at speed; balance weights correct imbalance, but a bent rim or separated belt requires replacement.

Dual wheel spacing: tires in a dual set must match in diameter within spec; mismatched diameters cause the larger tire to carry more load and overheat.

Air suspension leveling valve (height control valve) senses ride height via a linkage and adds or exhausts air to maintain the designed height under varying loads.

Air dryer removes moisture and oil from compressor output; a saturated dryer passes moisture to valves and bags, causing freeze-up and corrosion in cold weather.

Compressor governor controls when the compressor loads and unloads; a failed governor can cause continuous compressor run or no build-up.

Air bag (spring) inspection: check for cracks, chafing, and leaks with soapy water; a leaking bag causes slow sag and continuous compressor cycling.

Hydraulic suspension (e.g., Hydraulic Accumulator systems) uses pressurized nitrogen and hydraulic fluid; loss of nitrogen charge causes a hard ride and poor leveling.

Pressure protection valves isolate a failed circuit so the rest of the air system retains pressure; a tripped valve causes one corner to stay down.

Air line fittings and push-to-connect joints must be fully seated; a partially seated line leaks and can blow off under pressure, dropping the suspension.

Before working on an air suspension, discharge stored pressure per OEM procedure and support the frame; air bags can store energy and cause injury if released suddenly.