How a Car Works · Go deeper5 min read

The racing transaxle: gearbox and diff in one casing

Put the engine behind the driver and the gearbox no longer needs a long shaft to reach the wheels. In 1959 Cooper bolted gearbox, final drive and differential into one casing right between the rear tyres — and drew the layout every Formula 1 car, every supercar and most racing cars still use.

The half-metre drivetrain

A front-engine car strings its drivetrain along the whole chassis: engine at the nose, then a gearbox, then a propshaft running the length of the car under the driver, and finally a differential between the rear wheels. The Cooper T51 deleted the middle of that sentence. With the engine already sitting over the rear axle, its gearbox and differential could live together in a single aluminium casing — the transaxle— hung directly off the back of the engine. Engine, clutch, gearbox, differential and driveshafts all within about half a metre. Cooper's first units were reworked Citroën road-car internals, one more thing rivals laughed at right up until the championship table came out.

One casing, three jobs: the gear pairs choose a ratio, the final drive multiplies it, and the differential splits the result between the rear wheels — no propshaft anywhere.

Three jobs, one casing

Inside the casing, power makes three quick transformations. First the gear pairs pick a ratio — low gears trade speed for torque on corner exits, high gears trade it back on the straights. Then the final drive multiplies everything once more, by a fixed ratio. Finally the differentialsplits the result between the two rear tyres while letting them turn at different speeds through corners. In a front-engine car those three jobs are separated by metres of spinning steel; in a transaxle the power crosses the whole chain in centimetres. No propshaft mass, no shaft inertia to spin up, and almost no driveline losses — a few percent more of the engine's power simply arrives at the road.

Ratios you could tune to the track

The layout had a practical gift: with the gear cluster hanging off the very back of the car, mechanics could open the casing and swap complete ratio sets between practice sessions. Gearing is a pure trade — shorter ratios multiply torque for corner exits, longer ones stretch top speed — and the right answer changes with every circuit. Monaco wants punch out of hairpins; Monza wants a long, tall top gear. Teams travelled with boxes of gears and made that call at every race.

Why everyone copied it

Within two years of the Cooper's championship, every front-engine Grand Prix car was obsolete — and the transaxle went with the engine. It is on every Formula 1 car since, on essentially every mid-engine supercar, and even on some front-engine road cars that mount a transaxle at the rear purely for its weight distribution. When one layout wins the argument this completely, it stops being a design choice and becomes simply how the machine is built.

Go deeper: the gearing trade in one linefor engineers

Torque at the wheels is the engine's torque multiplied by both ratios in the casing:

T_{wheels} = T_{engine} \times r_{gear} \times r_{final} \times \eta

with efficiency $\eta$ around 0.95 or better for a transaxle's short path. Road speed moves the opposite way — divide engine speed by the same two ratios. A shorter final drive multiplies every gear's punch and trims every gear's top speed at once, which is why it was the single most-swapped part in the box.