EXPERT’S GUIDE TO... GEARBOXES
Dogs, cogs, forks and gears – just what is going on within the depths of your bike’s gearbox?
Your gearbox is one of the forgotten heroes of the engine. A modern bike has to be able to work seamlessly over a huge range of speeds, from climbing a hill two-up, to cruising at modern motorway speeds, or lapping a racetrack. Changing the gearing of the engine is the only way to give this flexibility.
Inside a motorcycle gearbox are two shafts; an input shaft driven by the engine’s clutch and an output shaft that is connected to the front sprocket or shaft drive. These shafts carry gears of different sizes, just like a bicycle. Alternative gears are freely rotating on the shaft, they get drive when they need it from a neighbouring gear that is fixed to the shaft, but can slide into contact with the freely rotating gear, via teeth on the side called dogs. Only one input and one output gear are meshed at any one time, giving drive to the rear wheel.
When you press or lift the gear lever this rotates the grooved selector drum. The grooves move selector forks that sit in between the gears, disengaging the current gears and pushing two more to engage, which feeds in the next gear.