How a Car Works · Chapter 04
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 PROBLEM
Follow the two arcs through 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. The scrubbing eats tyres, upsets the steering and, on a slippery surface, can push the car straight on when you asked it to turn.
TAKEAWAYIn every corner the outer wheel travels further — a driven axle must let its two wheels disagree.
· THE TRICK
Inside the differential, the two wheel shafts face each other, each ending in a bevel gear. Between them, mounted on the ring gear the engine drives, sit small “spider” gears meshing with both. 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 — a mechanical see-saw, balancing speeds instead of weight.
READA racing transaxleGearbox and differential in one casing — inside the Cooper T51.TAKEAWAYThe spider gears keep the average of the two wheels equal to the input — always.
· THE CATCH
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.
READThe differential, in fullThe spider-gear trick and its fixes, with the two equations that govern it.TAKEAWAYAn open diff can only feed each wheel what the weakest can use — every fix patches this one honest weakness.
Limited-slip differentials, locking diffs and modern traction control — which simply brakes the spinning wheel — are all patches over the same weakness.
FOR THE SELF-CHECK