Chapter 3 of 8
The engine, chapter 2 showed, only works in a narrow band of speeds — and it cannot even idle without turning. The wheels start from zero. Between those two stubborn facts sits the transmission: a friction disc that lets two spinning things meet gently, and a box of gear pairs that trades speed for force like a set of levers. This chapter is the surface tour; the demo underneath lets you drive the numbers yourself.
Put the engine’s usable speeds next to the wheels’ and the problem is obvious: they barely overlap. An engine below about 1,000 rpm stalls; a wheel at 1,000 rpm is doing roughly 140 km/h. Connect them rigidly, one-to-one, and the car could never stand still with the engine running, never pull away, and never use the engine’s strong band at more than one road speed. Everything in this chapter exists to translate between those two bars.
Pulling away is the impossible case: the engine must spin, the car must not — and then, smoothly, both must agree. The clutch solves it with two plates pressed together by springs. Pedal down, they separate and the engine spins free. Ease the pedal up and the plates kiss and slip, dragging the car into motion while friction burns off the speed difference as heat. A launch lasts a second or two of deliberate slipping; a gear change, a fraction of that. It is the one part of the drivetrain designed to wear out instead of the parts around it.
The full story: friction, slip and the five-lever box — with the maths
Mesh a small gear with a big one and you have a crowbar that never stops prying: the big gear turns slower, but pushes harder, and the trade is exact. Chain two such pairs — a 3.6:1 first gear into a 3.9:1 final drive — and the engine’s twist arrives at the wheels multiplied fourteen times. That, not raw power, is why a modest hatchback can spin its tyres from rest. Fifth gear makes the opposite bargain: less twist, more road speed per rev, a quiet motorway cruise.
Plot the force each gear can put on the road against road speed and a geared car draws a falling sawtooth: first gear starts ferocious and runs out of revs almost immediately, each higher gear catches the baton lower down and carries it further. The car accelerates on whatever force is left above the drag curve — and where the last tooth finally sinks into the drag, that is top speed, full throttle, zero gain. Every gear change you have ever felt is a hop from one tooth of that saw to the next.
Drive the sawtooth: one engine, five gears, live tractive force