Know This – Motorcycle Gearing

…In theory

What is it that drives your bike forwards? Horsepower? Torque? Nope, what shunts your bike up the road is a thing called ‘thrust’ – the force of the back tyre acting on the road – and thrust is a product of available torque and the gear ratio through which it is being applied.

A typical bike’s first gear ratio will be around 3:1, so in first the output shaft will be rotating at a third crank speed (rpm) and force is multiplied by three. A sixth gear ratio is typically around 1:1, so there is a third the thrust available in top and this is why your bike accelerates harder the lower the gear you are in.

All the gearbox ratios are further increased a given amount by the ‘final drive ratio’ – the ratio between front and rear sprockets.. This ratio re-multiplies the tractive force to the wheel. So, the bigger the difference between the smaller front sprocket and the larger rear, the more force and acceleration you have in each gear. By simply changing sprocket sizes we can alter our gear ratios and increase acceleration, but, as ever, there is a trade off.

Lowering the gearing (increasing the ratio) and multiplying thrust also results in the back wheel rotating more slowly in relation to engine speed. And, of course, wheel speed equates to road speed. So, if you were to lower the gearing too far, your bike would obviously suffer in terms of speed.

To make sure gear ratio lingo is as complicated as possible, the term ‘lowering the gearing’ means increasing the ratio – and vice versa. This is because the ratio in question is a ‘reduction ratio’. What you are actually lowering is the speed the bike will do in each gear.

In practice

As with many aspects of set-up a road bike’s gearing is a compromise, often as much a product of politics as performance. A manufacturer may choose a final-drive ratio to pass drive-by noise tests, offer generous mpg figures, keep the top speed comfortably short of the redline for reliability purposes, or create mind-blowing top speed headlines where a nice tail wind is available.

On road bikes, compromise can be a good thing and it might just be that your bike’s gearing is already spot-on for you and your riding style, but in many cases the gearing is a shade too high. After all, how often do you use top gear right up to the redline? Often a bike won’t even pull to the redline in top – a perfect excuse to tinker with your sprockets.

A tuned bike, making many more horsepower than standard, may need the final-drive reduction ratio lowering if a blistering top speed is your quarry and it might help keep the front wheel on the deck in the lower gears. But for each extra mph you want your gearing to pull, an incrementally larger rise in horsepower is needed. When the available thrust at peak power can push the bike no faster, then that’s it and raising the gearing will have negative effect.

Generally tinkerers want to drop the gearing to give stronger acceleration in a speed range the bike is most often used. Where standard gearing means your bike is well shy of the redline, full throttle in top, dropping the ratio a tad will actually make the bike faster.

Making changes

A smaller front sprocket, with fewer teeth, will raise the reduction ratio and lower the gearing and top speed. A larger rear will do likewise. Bigger/smaller will obviously have the opposite effect.

If you do want to change your bike’s gearing, it’s worth knowing what the result will be before you go ahead and this isn’t difficult. If, for instance, your top speed at the redline is 150mph and you reduce the size of your front sprocket from 16 to 15 teeth, then top speed will fall in direct proportion. Dividing 15 by 16 we get 0.9375 and if we multiply the top speed (150) by this we get your new top whack of 140.6mph – much slower, but you’ll get there quicker.

Making changes at the back will result in smaller increments of change. Take a 45-tooth rear, add a gnasher, and you’ve got 45 ¸ 46 = 0.98. Multiply 150 by this figure and the loss of speed is just 3mph. A tooth change at the front results in around three times the change as one at the back.

There are physical limits to how big and small you can go with front sprockets. Below 15 teeth, a front sprocket will tax the chain, causing excessive wear, and could mean the chain has trouble clearing the swing-arm. Often the ideal ratio for a bike will be the result of changing both sprocket sizes, combining the above calculations to find the exact change you want.

Most bikes these days electronically calculate speed by gear position and engine revs or a gearbox speed sensor. Thus, if you change the gearing, the speedo will be out. The answer is to calculate the error and re-mark the speedometer. If your speedo’s digital you’ll need a pretty gymnastic mind

On the road

Lower gearing will give a more ‘lively’ ride. You’ll need less revs to get off the line and first gear will be over more quickly. The front wheel will be more likely to leave the ground too as the extra thrust accelerates you faster.

Higher gearing will give a more relaxed ride, with cruising speed at correspondingly lower revs. More clutch slipping will be needed from a standstill and first gear will feel ‘longer’. Acceleration in top gear may feel rather sluggish.