Explained
How a gearbox works.
Why cars need gears
An engine only produces useful power across a limited band of speeds, while a car's wheels need to turn anywhere from a crawl to motorway speed. A gearbox bridges that gap. By choosing which pair of gears carries the power, it changes how many times the wheels turn for each turn of the engine, and in doing so it changes the torque, or twisting force, delivered to the wheels.
The rule is simple. When a small gear drives a large one, the large gear turns more slowly but with more torque, in proportion to the number of teeth. In the gold pair in this model, a 12-tooth gear drives a 30-tooth gear, a ratio of 2.5 to 1. The output shaft turns 2.5 times slower than the input, and, ignoring friction losses, with 2.5 times the torque. That's what first gear is for: getting a heavy car moving from rest.
Always in mesh
It's tempting to imagine gears sliding in and out of contact when you change gear, and some very early gearboxes did work that way. Modern manual gearboxes are constant-mesh designs. Every forward gear pair is meshed all the time. The driven gears spin freely on the output shaft on bearings, so most of them are just idling round. To select a gear, the selector fork slides a collar along the shaft. Teeth on the collar, called dogs, engage with matching teeth on the side of the chosen gear and lock it to the shaft, so that pair now carries the power.
Before the dogs can engage, the gear and the shaft need to be turning at the same speed. That's the job of the synchroniser, a cone clutch built into the collar that rubs the two parts to the same speed in a fraction of a second. It's why a modern gearbox changes smoothly without the double-declutching drivers once needed.
Teeth, shafts and bearings
The teeth themselves are carefully shaped, usually with a curve called an involute, so that they roll against each other smoothly and the speed ratio stays constant through each tooth's contact. The shafts are held in rolling bearings in a rigid housing that keeps the gears exactly the right distance apart, and oil splashed around inside carries away heat and stops the teeth from wearing. Press play to watch the three pairs in this model turn at their three different ratios at once.
Parts glossary
- Gearbox housing
- A cast aluminium or iron case holds the shafts in exact alignment and keeps the gears bathed in oil. Here it is cut away so you can see inside.
- Input shaft
- The input shaft is turned by the engine through the clutch. The gears fixed to it drive their partners on the other shaft.
- Drive gears
- These gears are fixed to the input shaft. A small drive gear turning a large driven gear trades speed for torque, which is what you need for pulling away.
- Driven gears
- Each driven gear is always in mesh with its partner but spins freely on the output shaft until it is locked to it. The gear ratio is simply the ratio of the tooth counts. Fun fact: In most modern manual car gearboxes the forward gear pairs stay in mesh all the time; changing gear just locks a different gear to its shaft.
- Output shaft
- The output shaft sends the chosen speed and torque on towards the wheels. In this two-shaft layout it turns the opposite way to the input shaft.
- Bearings
- Ball or roller bearings let the shafts spin with very little friction while holding them precisely in place against the forces from the gear teeth.
- Selector fork and collar
- Moving the gear lever slides a collar along the shaft with a fork. Its dog teeth lock the chosen gear to the shaft, and a synchroniser matches their speeds first so the change is smooth.
