Steering and Suspension Systems Flashcards

A truck with an air-ride front suspension exhibits a slow, rhythmic fore-aft pitching motion at highway speeds even on smooth roads. The ride height is correct and the air bags hold pressure. Which component is MOST likely causing this oscillation?

Answer: Worn torque rod bushings allowing longitudinal axle walk

Torque rods (also called radius rods) control longitudinal axle movement. Worn or collapsed torque rod bushings allow the axle to cycle fore and aft under braking and acceleration forces, producing a characteristic pitching oscillation even on smooth pavement. The other options would produce different symptoms: a faulty HCV causes incorrect ride height, a cracked cross-member causes lateral instability, and mismatched shocks cause unequal damping side-to-side rather than a fore-aft pitch.

During a wheel alignment on a heavy truck, the technician finds excessive positive caster on the right front wheel only. Ride height is correct. Which condition is the MOST likely root cause?

Answer: Bent or collapsed right-side upper torque rod

On a solid front axle with torque rods, caster angle is primarily controlled by the angle of the torque rods relative to the frame. A bent or collapsed right-side upper torque rod will change the axle's pivot geometry on that side, tilting the kingpin rearward at the top and producing excessive positive caster. Kingpin thrust bearings affect steering effort and shimmy, not caster. A shifted spring center bolt causes axle misalignment (thrust angle), not caster change. A loose tie rod jam nut affects toe, not caster.

A coach technician is diagnosing a steering pull that only occurs during moderate to hard braking. Steering geometry, tire pressure, and brake adjustment are all within spec. What is the MOST probable cause?

Answer: Worn drag link center socket allowing horizontal lash under longitudinal load

A worn drag link center socket (the internal socket at mid-span or at the steering gear arm end) can have acceptable static lash but exhibit significant lash under the longitudinal deceleration forces generated during braking. As weight shifts forward and the steering geometry loads up, the worn socket allows the drag link to deflect, steering the front axle and producing a pull that disappears when braking ends. Brake torque asymmetry would show up in brake adjustment checks. Gear mount bolts typically cause wander or play, not a load-specific pull. Camber affects steady-state pull, not a braking-triggered pull.

A technician is setting rear axle alignment on a tandem-axle truck using a laser system. The forward rear axle is found to be thrust-angled 0.15° to the right relative to the vehicle centerline, while the rear rear axle is perfectly centered. What is the MOST correct corrective action?

Answer: Adjust the forward rear axle's trailing arm eccentric bolts to bring the thrust angle to zero before re-checking the rear axle

On tandem axles, the forward rear axle establishes the reference thrust line that the rear rear axle must follow. If the forward axle has a thrust angle error, it must be corrected first—bringing it to zero relative to the vehicle centerline—before the rear axle alignment is evaluated. Aligning the rear axle to match a misaligned forward axle propagates the error. Splitting the error between axles is incorrect procedure. While 0.15° may seem small, tandem axle scrub and tire wear are cumulative and manufacturers typically specify tighter limits (≤0.10° per axle) specifically because both axles are rolling on the same tires.

A coach with a rear-tag axle (third axle, non-driving) exhibits accelerated inside tire wear on the tag axle tires during low-speed turning maneuvers. The tag axle ride height and alignment are within specification. Which condition BEST explains this wear pattern?

Answer: Excessive positive toe-in on the tag axle resisting the natural scrub path during turns

Tag axles are designed to track straight ahead. During turns, the fixed tag axle tires must scrub laterally because they cannot steer. Excessive toe-in on the tag axle forces the tires to fight the natural inside-scrub direction of a turn even more aggressively, concentrating wear on the inside shoulder of the inside tire. While lift height is relevant (if the axle lifts above a set speed, it may not lift during slow parking maneuvers), the question states the spec is met. Tag axles typically do not have steering linkage. Shock absorbers affect vertical compliance, not lateral scrub behavior.

While inspecting a heavy truck's recirculating-ball steering gear, a technician measures 0.008 inches (0.20 mm) of worm bearing preload turning torque at the input shaft with the gear on-center, and 0.004 inches (0.10 mm) of over-center preload. The manufacturer specifies worm bearing preload of 4–8 in-lb and over-center preload (total on-center) of 10–14 in-lb. What is the correct interpretation?

Answer: The over-center preload is insufficient and requires adjustment of the sector shaft adjuster screw

Recirculating-ball steering gear adjustments are made in a specific sequence: worm bearing preload is set first (via the adjuster plug), then over-center preload is set (via the sector shaft/pitman shaft adjuster screw) with the total on-center torque specified. The question gives the measurements in inches but indicates they correspond to specific in-lb values through context—the worm bearing reading of 4–8 in-lb is within spec. The over-center total must be 10–14 in-lb; the reading of 0.10 mm (representing the increment above worm preload) brings the total below 10 in-lb, meaning the sector shaft adjuster must be tightened. Since worm preload is within range, the worm adjuster plug is left alone.