Suspension and Axle Assemblies
This domain tests suspension types, axle geometry, air-suspension controls, and the alignment checks that explain irregular tire wear and poor tracking. Expect questions on U-bolt service, ride height, slider pins, and thrust angle.
Leaf spring systems
A leaf spring assembly includes the main leaf, leaves, center bolt, U-bolts, hangers, shackles or slippers, equalizers, bushings, and pins. Loose U-bolts let the axle shift, which breaks center bolts and changes alignment; re-torque is required after the initial service interval the OEM specifies, and many OEMs call for new U-bolts rather than reuse.
Air suspension
Air bags carry load, a height control valve with linkage to the axle maintains ride height, and a dump valve releases air for coupling or unloading. Ride height is measured between the axle and frame at the specified points. Incorrect ride height changes driveline angles, load distribution, and suspension travel. Leaks are found with soap solution at bags, fittings, and valves; shocks control rebound but do not carry load.
Axle geometry and alignment
Camber and toe are built into the axle; thrust angle describes whether the axle is square to the frame centerline. Misalignment comes from bent beams or spindles, worn bushings, loose U-bolts, shifted sliders, or frame damage. Symptoms include dog tracking, scrub, and feathered or diagonal tire wear.
Slider tandems
Locking pins engage holes in the frame rails; air release retracts them. Inspect pins for wear, holes for elongation, and the air circuit for leaks. A partially engaged pin set can shift under load.
Worked example
Both tires on one axle show feathered tread. Reasoning: feathering points to a toe or thrust condition rather than inflation. Check ride height first, then bushings and U-bolt torque, then measure axle parallelism and thrust angle. Repair the cause - a bent axle or worn bushing - rather than adjusting the symptom.
Traps that cost marks
Adjusting the height valve linkage to hide a sagging bag, reusing U-bolts, torquing U-bolts with an impact only, and blaming inflation for wear caused by thrust angle.