Drive Axle and Differential
The rear axle assembly transmits torque, supports vehicle weight, and allows wheel-speed difference in turns. Understand the differential: ring and pinion set final ratio, the differential case carries spider gears, and side gears drive the axle shafts. A conventional (open) differential sends torque to the wheel with less traction; a limited-slip or locking differential changes this behavior.
Key concepts: gear ratio and setup (pinion depth, backlash, bearing preload); the difference between ring-and-pinion whine (load-dependent, changes on coast vs drive) and bearing noise (constant or speed-related); axle seal and vent function; lubricant level and type; and wheel-end components (bearings, seals) on full-floating axles. Recognize that a bent housing or worn bearings can cause abnormal tire wear and vibration.
Traps: misdiagnosing differential whine as tire noise (tire noise usually changes with road surface, not throttle); ignoring a plugged vent that forces oil past seals; and assuming a locking differential is faulty when tire scrub in tight turns is normal. Also watch for confusing axle shaft/bearing noise with differential carrier noise.
Worked example: A bus howls on acceleration and quietens on coast. That load-reversal pattern points to ring-and-pinion wear or poor setup rather than a wheel bearing (which tends to be constant and speed-related). Verify lubricant level and condition, then check backlash and pattern before replacement.
Rule of thumb: noise that changes with throttle = gear set; noise that stays constant with speed = bearing or tire.