A differential splits the engine's drive between the two wheels of an axle, yet lets them turn at different speeds. Use the buttons to look inside it and follow the torque from the driveshaft, through the gears, out to both wheels.
A car differential is the set of gears in the middle of an axle that sends the engine's drive out to both wheels. Its trick is that it does not force the two wheels to turn at the same speed: it lets them differ, so the wheels can follow the different paths they take whenever the car goes round a corner.
The drive arrives on a driveshaft and turns a small pinion, which meshes with a large ring gear bolted to the differential carrier. Inside that carrier are the spider gears and two side gears, one splined to each half shaft. As the carrier spins, the side gears carry the drive out to the wheels.
From the outside a differential is just a bulge in the middle of the axle. The pinion, the ring gear, the carrier and the little gear set hidden inside it are all out of sight, so cutting it open shows how one input can drive two outputs that are free to turn at different speeds.
One idea holds it together: the drive is shared out through gears that are allowed to rotate relative to each other. Going straight they lock up and both wheels turn together; in a turn they spin, quietly absorbing the difference so neither wheel has to skid.
The chain starts at the driveshaft, which turns the small drive pinion. Because the pinion is much smaller than the gear it drives, it has to turn many times for each single turn of the output — which is how the axle trades speed for the turning force the wheels need.
The pinion meshes with the large ring gear and drives it round. Besides gearing the drive down, the way the two gears meet bends the path of the torque through ninety degrees, so power arriving along the car turns to run out sideways to the wheels.
The ring gear is bolted firmly to the differential carrier, so the whole carrier turns with it. The carrier is the case that holds the small internal gear set, and from here the drive is about to be split between the two half shafts.
Inside the carrier, two spider gears sit on a cross pin and are free to spin about it. As the carrier turns they are carried around like a small planet system, and in normal driving they hardly rotate on their own, keeping the two sides matched.
Each spider gear meshes with a side gear, and each side gear is splined onto one half shaft so the two must turn together. The drive therefore passes from the carrier, through the spider gears and side gears, out along both half shafts to the wheels.
When the car goes straight, both wheels cover the same ground, so both half shafts need the same speed. The spider gears then stay almost still, the whole gear set turns as one locked unit, and the differential behaves as if the axle were one solid shaft.
Turning forces the two wheels to travel different distances in the same time. Because the half shafts are now free to differ, the spider gears spin on the cross pin, letting one side gear speed up while the other slows down. The outer wheel runs faster, the inner slower, and both keep driving without skidding.
It lets the two wheels on the same axle turn at different speeds while both keep being driven. That matters every time the car turns, because the outer wheel travels a longer path than the inner one.
In a turn the outer wheel follows a bigger circle than the inner wheel, so it has to cover more ground in the same time. If both wheels were locked to one speed, one of them would have to skid.
The pinion is the small gear on the driveshaft and the ring gear is the large gear it meshes with. Together they gear the drive down and turn it ninety degrees so the torque reaching the axles is at the right speed.
They link the two side gears and let them differ. Going straight they barely rotate, but in a turn they spin on the cross pin, which is what allows one side gear to speed up while the other slows down.
The whole gear set turns as one unit. The carrier, ring gear, spider gears and side gears all rotate together, and both half shafts turn at the same speed.
It is a differential with a clutch or a set of gears added so the two wheels cannot differ too much. If one wheel loses grip, the limited-slip unit still sends torque to the wheel with traction.