Advice: how a motorcycle gearbox works

There are dogs in your motorcycle engine's gearbox – and they bite. They’re called dog teeth and they’re one component of the rotating, interlocking and sliding mechanism that is a constant mesh sequential gearbox.

Most riders will never want to see inside a motorcycle gearbox. If they are looking at the innards of a manual transmission, it's generally while their mechanic is explaining just how bad it can be to make a gear change from sixth to second and expect engine braking to do the rest. The sight of a macerated gear set will haunt you for a lifetime, especially after you see the repair bill.

So for most of us the gearbox is a clutch and a lever on the left side of the bike. The clutch disengages the engine from the transmission, we click up or down to select the next gear, then release the clutch for everything to reconnect.

That works for me but some want to actually understand what's happening inside the motorcycle gearbox.

The basics are fairly straightforward: the clutch connects the engine to the gearbox and the output shaft – the one with the little sprocket at the end you put the chain over – connects the gearbox to the rear wheel.

As you pull in the clutch lever, a hydraulic or (on older bikes) cable-operated action separates the sets of clutch plates so the engine and gearbox are disconnected. As you shift the gear lever, it mechanically moves a drum in the gearbox that in turn engages one of three (in a six-speed box) “selector forks”.

These selector forks are mounted on two shafts, along with the cogs or gears. Some gears are fixed to those shafts, others can be slid sideways by the forks to engage – using those “dog teeth” on the side of the cogs – with the fixed gear.

Simple, innit? Just don’t open the gearbox to take a look at all those bits and pieces. Like Pandora’s box, you may not get everything back in…