How does a car gearbox work?

The principle

A manual car gearbox. An engine spins fast, but on its own it doesn’t push very hard: a typical small car engine makes about 200 Nm of turning force. To pull away from a standstill or climb a hill you need more force and less speed. The gearbox does that swap, just like the gears on a bike.

Step 1: The clutch lets go. Between the engine and the gearbox sits the clutch: two discs pressed hard together. When you push the clutch pedal, the discs move apart, so the engine keeps turning but nothing pushes the gears any more. Now the gears can be changed without a fight, a bit like easing off the pedals for a moment while you shift on a bike.

Step 2: All the gears always turn. Let the clutch back in and the engine turns the input shaft. Its gear turns the layshaft, a second shaft with one gear for every gear of the car. Each of those is always meshed with a partner gear on the output shaft, the one that leads to the wheels. The trick: those partner gears sit on bearings and spin loose around the output shaft. In neutral they all turn, but the output shaft stays still and the wheels get 0 Nm.

Step 3: A collar locks one gear. Between the loose gears sit sliding collars that turn with the output shaft. Move the gear lever and a collar slides sideways until its little teeth (dog teeth) grab one gear. Now that gear, and only that one, drives the shaft. Before the teeth meet, small cone-shaped rings rub together to match the speeds, so the teeth slide in instead of grinding. In 1st, a small layshaft gear drives a big output gear: the engine turns about 3.5 times for one turn out, so about 200 Nm becomes about 700 Nm.

Step 4: Higher gears go faster. Each higher gear uses a pair where the output gear is smaller. 2nd is about 2:1 (about 400 Nm), 3rd about 1.4:1 (about 280 Nm). Top gear, often 5th, is about 0.8:1: the output turns faster than the engine and gives only about 160 Nm, which is plenty for cruising and keeps the engine calm and quiet. Real numbers vary from car to car, and a final gear in the axle multiplies them again.

Step 5: Reverse adds one gear. For reverse, a small extra gear (the idler) sits between the layshaft and the reverse gear. Every gear turns its neighbour the opposite way, so one extra gear flips the direction and the car rolls backwards. It is about 3.3:1, so reverse is slow and strong.

Manual or automatic? Other gearboxes do the same job in different ways.

• Manual (this one): simple, light, cheap to fix and fun to drive. Downside: you work the clutch yourself, which is tiring in stop-and-go traffic.
• Automatic (planetary gears): shifts by itself and is smooth and relaxed. Downside: heavier, more complex and pricier to repair.
• Dual-clutch (DCT): two clutches with the next gear already waiting, so shifts are lightning fast. Downside: can be jerky at walking pace and costly to fix.
• CVT: belts on cones that change size, so there are no steps at all and the engine stays at its best speed. Downside: the “rubber band” feel, and it can’t take as much force.