Understanding Gearing

Indeed it isn't, but it sure as hell can baffle the hell out of us. Here is all you ever needed to know about the mathematical world of gearing

Posted: 22 November 2010
by Mark Graham

Like a great many things on your motorcycle (unless you have built your own gearbox entirely from scratch), the overall gearing will be factory-calculated. This will usually offer the best compromise between easy getaways in the lower ratios, optimum acceleration through the 'box and as high a top speed as possible, given the major limiting factor of the engine's torque.

But if you're after changing the riding characteristics of a bike, changing the gearing is a (reasonably) straightforward place to start.

Cast your mind back to pushbikes and Derailleur gears: if you were privileged enough to have two cogs at the front, five at the back and a chain linking any combination of those sprockets together - you had gear ratios.

If you went big cog at the front and small at the back it was a bitch to make any headway from standstill, but rockin' quick when you got going, yeah? Small cog front, big cog back was good for wheelies, and all the stuff in between was for finding a compromise between how hard/fast you wanted to pedal according to the speed you wanted to make.

And all of that was in turn dependent on how much torque (turning force) your spindly little legs could make. So far so simple.

Let's explain a few things...

GEAR RATIO is the ratio between the number of turns the gearbox sprocket makes to each single turn of the rear wheel and is calculated by: driven gear divided by the drive gear. Example: 40 tooth rear sprocket divided by 16-tooth gearbox sprocket = 2.5. So 2.5:1 is the ratio of turns the gearbox sprocket has to make to turn the rear wheel one revolution.

SHORT gearing, or SHORTENING the gearing  (sometimes called LOWERING the gearing) - fitting a smaller gearbox sprocket, or a bigger rear sprocket to drop the final drive ratio. Example: 40 tooth rear sprocket divided by 15-tooth gearbox sprocket = 2.66. So ratio is 2.66:1. Lowering the gearing means increasing the ratio. This lowers the speed the bike will achieve in each gear.

TALL gearing, or RAISING the gearing (sometimes called LENGTHENING the gearing) - fitting a bigger gearbox sprocket, or a smaller rear sprocket to raise the final drive ratio. Example: 40-tooth rear sprocket divided by 17-tooth gearbox sprocket = 2.35. So the ratio is 2.35:1. Raising the gearing means lowering the ratio. This raises the speed the bike will achieve in each gear.

SPROCKET CARRIER - the part the rear sprocket bolts onto, that slots into the rear wheel axle assembly.

CUSH DRIVE - the rubber inserts in the rear wheel/or sprocket carrier that limit transmission shock at take-off and at every gearchange. NB: some supermotard machines converted form motocrossers do not have a cush drive (motocross dirt acts as a cushion) and can therefore be very hard on transmissions, especially if ridden by monkeys.

PRIMARY DRIVE - the pair of gears (chain in ye olden days) that transfer torque from crankshaft to gearbox input shaft.

CLOSE-RATIO GEARBOX - a gear cluster that has ratios very closely stacked together to provide better acceleration through the gears and minimise loss of engine revs between gearchanges.

SOFT LINK - the link that joins two ends of a chain together - so-called because the pin ends can be peened over using a special tool.

Continue the lesson on gearing

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