Differential Gear

How a differential works

When a car turns, the outer wheel rolls a longer arc than the inner wheel. If both wheels were rigidly locked to the same shaft, one would have to slip. The differential solves this by routing the driveshaft's torque through a clever arrangement of bevel gears that allows the two output shafts to spin at different speeds while still receiving torque.

The classic open differential consists of:

• A ring gear driven by the driveshaft's pinion (the source of torque).
• A carrier bolted to the ring gear that holds the spider gears.
• Two or four spider gears (small bevel pinions) free to rotate on a cross-pin in the carrier.
• Two side gears, one per axle shaft, that mesh with the spider gears.

When the car drives straight, both side gears rotate at the same speed and the spider gears don't spin on their pin — the whole assembly turns as a block. When the car turns, the spider gears walk around the side gears, letting the outer side gear lead the inner one.

             Ring gear (driveshaft input)
                     |
             ┌───────┴───────┐
             │   Carrier     │
             │               │
   Side ─────┤   ●──Spider──●  ├──── Side
   gear L    │      (pin)      │    gear R
   (left     │               │     (right
    axle)    └───────────────┘      axle)
                     |
             Both axles share torque
             but can spin at different speeds

Why an open diff strands you

The mechanical constraint of an open differential is that the torque on both output shafts is always equal. The reaction torque on a spider gear must balance, so whatever torque pushes the left side gear must equal what pushes the right. If one wheel sits on ice and reacts only 5 N·m of friction, the wheel on dry pavement also receives only 5 N·m — and the icy wheel free-spins at twice the average shaft speed.

This is the canonical "one-wheel-peel" failure. It's why a 4WD truck with two open differentials can be defeated by a single diagonally-opposite pair of slipping wheels: each axle has only the gripping wheel doing work, but if even one of the two grippers loses traction, that axle goes to zero.

Differential variants

	Open	Clutch LSD	Torsen (helical)	Torque vectoring	Locking
Speed differentiation	Free	Limited by clutch preload	Bias when load applied	Active control	None when locked
Torque split (slipping)	50/50 ≤ low side	~70/30 to 80/20	~80/20 (3:1 bias)	0/100 to 100/0	0/100 possible
Sensing	None	Speed-difference	Torque-difference	Yaw, throttle, steering	Driver or speed
Wear	Low	Clutch packs wear	Helical gears, durable	Solenoid clutches	Mechanical, brutal
Cornering on dry road	Smooth	Slight stiction	Smooth	Tunable	Binds and skips
Cost	$	$$	$$$	$$$$	$$ (manual) – $$$$ (electric)
Use case	Commuter cars	Sport sedans, light trucks	AWD performance, Audi quattro	BMW M, Acura SH-AWD	Off-road, locked rear

Torsen ("torque-sensing") differentials use helical worm-style gears whose meshing geometry creates internal friction proportional to torque load. Under straight-line load, they bias torque to whichever wheel has more grip; under coast or zero-load conditions, they behave like an open diff. The Audi quattro and several Toyota off-roaders use Torsen center diffs precisely because they need no electronic actuation.

Planetary differentials in trucks

Class-8 trucks and tractors often use a planetary final drive at each wheel hub after the bevel-gear differential. A typical arrangement: the driveshaft enters the differential at maybe 1500 RPM and 2000 N·m, the differential exits to half-shafts at 1500 RPM and 1000 N·m each, then a planetary set inside each wheel hub reduces 4:1 to 375 RPM and 4000 N·m at the wheel.

This split keeps the differential and half-shafts compact (low torque, high speed) while the planetary reduction multiplies torque close to where it's used. A direct-drive single-stage at the differential would require massive ring and side gears — heavy, expensive, and hard to package.

Worked example: cornering speeds

A car with a 1.5 m track width takes a 25 m radius corner at 50 km/h. The outer wheel traces radius 25.75 m, the inner traces 24.25 m. The differential must allow:

• Outer wheel: v = 50 km/h × (25.75 / 25.00) ≈ 51.5 km/h
• Inner wheel: v = 50 km/h × (24.25 / 25.00) ≈ 48.5 km/h
• Speed difference: 3 km/h, or about 6% of average
• For a 0.65 m tire radius: outer = 350 RPM, inner = 330 RPM, Δ = 20 RPM

That 20 RPM difference is exactly what the spider gears absorb by walking around the side gears. Without a differential, both wheels would be forced to 340 RPM and one would skid through the corner — the same scrubbing you feel when a locked rear axle drives on dry pavement.

Common failure modes

• Spider gear pin walk. Heavy off-road use can hammer the cross-pin loose; symptoms include clunks on torque reversal and metal in the gear oil.
• Side gear thrust washer wear. The bronze washers behind the side gears wear thin under sustained one-wheel-spin, letting the side gear walk axially and chip.
• Ring-and-pinion mesh wear. Incorrect preload or backlash from a sloppy rebuild causes whining (wrong backlash) or howling (wrong preload).
• LSD clutch pack glazing. Conventional ATF in a clutch-type LSD makes the plates chatter; the correct GL-5 with friction modifier eliminates chatter but the clutches still wear out around 100,000 km.
• Locker engagement under power. Engaging an air-locker while a wheel is already spinning can shear the dog-clutch teeth — always engage at low speed before commitment.
• Heat from sustained slip. Towing a stuck trailer with one wheel spinning can cook the gear oil past 200 °C, breaking down the EP additives and leading to rapid wear.

Picking a differential

• Daily commuter. Open is cheapest, smoothest, lightest. Modern stability-control systems brake a slipping wheel to mimic an LSD electronically.
• Sporty AWD. Torsen or torque-vectoring up front, mechanical LSD or e-LSD at the rear.
• Off-road truck. Selectable lockers front and rear; open or limited-slip center diff for highway smoothness.
• Drift / circuit cars. Mechanical clutch-pack LSD with adjustable preload — predictable, tunable, replaceable.
• EVs. Many EVs replace the rear differential with two independent motors, eliminating the bevel diff entirely while gaining true torque vectoring at zero added weight.