Chapter 4 of 8
One driveshaft arrives from the gearbox; two wheels leave. That sounds like plumbing, not engineering — until the car turns. In every corner the outer wheel travels further than the inner one, so a driven axle must push both wheels and let them turn at different speeds at the same time. The differential is the 19th-century gear puzzle that does it, and it hides in every car you have ever ridden in.
Follow the two front-to-back arcs in a corner: the outer wheel’s path is simply longer. If both driven wheels were bolted to one solid shaft, one of them would have to scrub — dragged sideways across the tarmac, chirping and hopping, fighting the car’s own engine. Karts and some farm machinery really do live with this. Cars cannot: the scrubbing eats tyres, upsets the steering and, on a slippery surface, can push the car straight on when you asked it to turn.
Inside the differential, the two wheel shafts face each other, each ending in a bevel gear. Between them, mounted on the ring gear that the engine drives, sit small “spider” gears meshing with both. Driving straight, the spiders do not spin at all — they just carry both wheels around together. In a corner they rotate gently in place, feeding exactly as much extra speed to the outer wheel as they subtract from the inner one. The average of the two wheels always equals the input: a mechanical see-saw, balancing speeds instead of weight.
The same see-saw has a famous failure mode. An open differential splits twist equally — so neither wheel can ever receive more force than the weaker one can use. Put one driven wheel on ice and it spins uselessly while its twin, on good tarmac, receives the same near-zero effort: the car goes nowhere in a haze of wheelspin. Limited-slip differentials, locking diffs and modern traction control (which simply brakes the spinning wheel) are all patches over this one honest weakness.
The spider-gear trick and its fixes, with the two equations that govern it
See a racing transaxle — gearbox and differential in one casing