How a Car Differential Lets Wheels Turn at Different Speeds
In a corner, the outer wheel travels farther than the inner one. A differential splits engine power between them. See open, limited-slip and locking types.
In this article
A differential is a set of gears that lets the two driven wheels on one axle spin at different speeds while both still receive power from the engine. It is needed because, when a car turns, the outer wheel travels farther than the inner wheel in the same time. Without a differential, one wheel would have to skid or hop through every bend.
The problem it solves
Wheels that are not driven can turn freely and each find their own speed. Driven wheels are different, because both are connected to the engine through the same shaft. Wikipedia's article on the differential puts it simply: the outer wheels must travel farther than the inner wheels in a turn, so the drive wheels need a way to turn at different rates.
The mechanical idea is a gear train with three shafts, where the speed of one shaft is the average of the other two. In a car, the shaft from the engine feeds two output shafts, one for each wheel.
Inside an open differential
The common layout uses a ring-and-pinion pair. The pinion, a small gear on the end of the driveshaft, meshes with a large ring gear. This turns the power through 90 degrees, from the lengthwise driveshaft to the sideways axles, and since the pinion has fewer teeth than the ring gear, it also provides a gear reduction.
The ring gear carries a small set of gears inside a case. When the car goes straight, they do not turn relative to each other and both axles move at the same speed. In a corner, the outer wheel's side gear turns faster than the inner one, and the small gears rotate to let that happen while still passing on torque. Gear Patrol, a motoring site, describes the effect as the faster outside wheel's gear spinning around the slower inside gear while the car turns.
The weakness of the open design
An open differential tends to send power to whichever wheel has less grip. Wikipedia says its drawback is that the wheel with less traction can spin. Motor1 gives the same explanation: when one wheel loses grip, torque flows to it and it spins freely. On a wet or icy surface, or when one wheel is lifted, the wheel with grip may get little drive.
Limited-slip differentials
A limited-slip differential (LSD) restricts the difference in power sent to each wheel. Motor1 describes three common types:
- Clutch-based: clutch packs sit behind each side gear. A spring keeps them lightly loaded in normal cornering. When one wheel starts to slip under power, the packs clamp and more torque goes to the wheel with grip.
- Torsen: uses worm gears to direct torque to the wheel with more grip, without clutches. Wikipedia says it was patented by Vernon Gleasman in 1958.
- Viscous: links the two outputs through a chamber of thick silicone fluid, which keeps the wheels at similar speeds.
Motor1 says Porsche first commissioned limited-slip differentials in the 1930s for its race cars. An LSD still lets the wheels turn at different speeds in a corner. It only limits how far apart they can go.
Locking differentials
A locking differential forces both wheels on an axle to turn together regardless of grip. According to Wikipedia, it can usually be unlocked for normal driving and is mostly found on off-road vehicles. Locked wheels cannot follow different paths in a bend on a hard surface, so the unlocked mode is used for everyday driving.
Brakes can imitate a limited-slip effect
Motor1 adds that many modern cars with open differentials use the brakes on a slipping wheel to copy what an LSD does. Braking the spinning wheel makes it harder to turn, so more torque can pass to the wheel that is gripping.
Not only for the rear axle
Cars with all-wheel drive often have a differential between the front and rear axles as well, since those axles also travel different distances in a turn. Gear Patrol notes that a locking centre differential forces the front and rear axles to turn at the same speed, while the individual wheels can still turn at different speeds if they have open or limited-slip differentials.
A long history
Gear trains of this kind are old. Wikipedia says the earliest verified use of a differential is in Joseph Williamson's clock of 1720, and notes that a differential may have been used in the ancient Greek Antikythera mechanism, with the dates given as about 100 BC or about 80 BC. For vehicles, it lists Onésiphore Pecqueur, who included a differential on the driven rear axle in a steam wagon patent of 1827, and James Starley, who patented one for a quadricycle in 1876. Carl Benz built one into his Patent-Motorwagen in 1885.
Summary
- Cornering makes the outer wheel travel farther than the inner one.
- A differential lets the two driven wheels turn at different speeds.
- An open differential is simple but sends torque to the wheel with less grip.
- Limited-slip designs reduce that problem.
- Locking differentials force both wheels together and are mostly for off-road use.
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