The Gearbox on your Motorcycle

It’s easy to take your gearbox for granted through thousands of changes. Simply flick the lever with your foot to select two different, rapidly spinning gear ratios and the motorcycle leaps forward as its road speed is matched to the engine’s sweet spot in its rev range.

But every slick gearchange relies on a whole sequence of events from precisely engineered components. As you more the lever you turn a shifting arm or spindle that is attached to a selector drum. This has grooves or tracks machined into it to guide the selector forks, which then more the toughened steel gear ratios into a different pairing.

Those ratios have matching ‘dogs’ machined into them that look like a ring of 4 to 5mm pegs.

As the drum rotates, its grooves or tracks pull one fork out then another in to shuffle the ratio, while a detent spring gets the gears to mesh cleanly by stopping the drum over or under-rotating. A return spring brings the lever back to its start position for the next change.

To get a good solid shift the dogs may be machined with an ‘undercut’ or 3 to 5 degree slope on their reverse side that pulls the two ratios together.

Honda’s MotoGP bikes have a seamless gearbox designed, rumored to cost over 13 million Thai Baht (or around US$ 390,000) which uses complex electronics to eliminate the pitching back and forth which reduces grip. But we think the next sportsbike development is dog ring designs, where the spinning ratios are hollowed out and therefore lighter and easier to shift, saving a tenth of a second each time, with the dog ring within the ratio itself.

Problems start when poorly executed changes mean the dogs and forks begin to wear, or the various springs start to lose their tension. The gearbox feels slacker as you have to keep pressure on the lever to get the drum to move, or wait momentarily for the worn dogs to mesh fully each time. With 40 to 50 gearchanges per lap at a typical race circuit, lost time soon racks up.