Differential

The differential is the gear assembly within the driven axle (or transaxle) that distributes engine torque to both driven wheels while allowing them to rotate at different speeds during cornering. This speed differentiation is geometrically necessary — during a turn, the outer wheel must travel a longer arc than the inner wheel, requiring it to rotate faster. Without a differential, both driven wheels would be forced to rotate at the same speed, causing one wheel to skid during cornering, producing strong understeer, tyre scrub, and transmission stress. The differential uses a set of small bevel gears (spider gears) that rotate on a cross-pin mounted in the differential carrier; the carrier rotates with the ring gear driven by the propeller shaft or pinion. The spider gears mesh with side gears splined to each axle shaft. When both wheels encounter equal resistance, the spider gears do not rotate on the cross-pin and both axle shafts turn at the same speed. When wheel speeds differ — during a turn or when one wheel loses traction — the spider gears rotate on the pin, allowing speed differentiation between the two sides. Open differentials (the most common type) send torque to the path of least resistance, which in a traction-loss situation means sending all torque to the spinning wheel and none to the wheel with grip — the fundamental limitation that limited-slip differentials address.

The ring and pinion gear set in the differential also changes the rotational axis of the drive by 90 degrees on rear-wheel-drive vehicles — the propeller shaft enters longitudinally but the axle shafts exit laterally — while providing a final gear reduction ratio that multiplies torque (typically 3:1 to 4:5:1 on passenger vehicles). This final drive ratio is the last multiplication stage in the drivetrain before the tyre contact patch, and it determines the vehicle's acceleration-to-top-speed balance. The hypoid geometry of the ring and pinion — where the pinion centreline is offset below the ring gear centreline — allows the propeller shaft to be lowered for better packaging and crash performance but requires the extreme-pressure gear oil (GL-5 specification) that standard gear teeth do not demand. Limited-slip differentials add a clutch pack or viscous coupling that partially locks the two axle shafts together when a speed differential develops, sending torque to the wheel with grip rather than the wheel that is spinning — essential for performance driving and useful for low-traction conditions. Electronically controlled differentials on modern performance vehicles can vary locking bias in real time based on sensor inputs.

Whining or howling noise from the rear axle: A tonal noise that varies with road speed rather than engine speed indicates ring and pinion gear wear or incorrect gear mesh (backlash) from worn carrier bearings. A whine that changes character between acceleration and overrun indicates gear tooth contact pattern problems. Differential noise is progressive — address it early before gear tooth damage becomes severe enough to require complete axle rebuild.

LSD chatter on tight turns: As described in the Differential Fluid entry — incorrect or degraded fluid in limited-slip differentials causes clutch pack chatter. Fluid change with the correct LSD specification typically resolves this before clutch pack damage occurs.

Clunking on acceleration or deceleration: A single clunk when pulling away from a standstill, or a double clunk when transitioning between acceleration and engine braking, indicates worn differential carrier bearings, excessive ring and pinion backlash, or worn spider gear thrust washers. Clunking that worsens on corners indicates worn wheel bearings rather than differential internals.

Level 1 — Noise Monitoring and Leak Inspection: Listen for any tonal whining from the rear axle area that varies with road speed. Inspect the differential cover gasket and axle shaft seal areas for gear oil staining at every underbody inspection. Differential gear oil has a strong sulphurous smell — any similar odour from underneath the vehicle suggests a leak. Check gear oil level at the filler plug if leakage is suspected.

Level 2 — Fluid Replacement at Specified Intervals: Replace differential fluid at the manufacturer's interval — typically 50,000–100,000 km, more frequently for towing. Always use the correct specification — GL-5 for open differentials, GL-5 plus LSD friction modifier (or integrated LSD-rated fluid) for limited-slip units. Check the axle breather vent to ensure it is not blocked — a blocked vent causes pressure buildup and forces gear oil past axle seals.

Level 3 — Professional Service for Internal Repairs: Ring and pinion replacement requires specialist knowledge of gear mesh adjustment — setting correct backlash and tooth contact pattern requires precision measuring tools and experience. Incorrect mesh accelerates gear wear immediately. Internal differential work is a specialist repair best performed by a transmission or axle specialist rather than a general workshop.