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Gearing on a Thunderace
Original article submitted by Yeasshhh

There is lots of talk about gearing but most people still don't know what going up or down a tooth actually does.

Now I know that you know that in standard gearing, the final drive ratio is 17 / 46 - that's 17 teeth on the front sprocket and 46 on the rear. Going up 2 teeth on the back seems to be the standard procedure but what does it actually do?

A few figures...

Our bench mark ratio is 2.706. This means that the front sprocket has to revolve 2.706 times to make the rear wheel revolve once. The higher this ratio is, the faster the acceleration and the lower the top speed. The lower this figure is makes the reverse true.

Front/Rear SprocketFinal Drive Ratio
Front/Rear SprocketFinal Drive Ratio



So we've got a lot of figures but it's not relevant. What we need to show is the gain or loss that you get if and when you change the gearing.

If we add another column onto this table, we get this:

Front/Rear SprocketFinal Drive RatioGain/loss over standard
17/452.6472.6470003 % loss
17/472.7682.2912 % gain
17/482.8244.36067 % gain


Front/Rear SprocketFinal Drive RatioGain/loss over standard
16/452.8133.95417 % gain
16/462.8756.24538 % gain
16/472.9378.53658 % gain
16/483.00010.86474 % gain

So, what does this mean? Starting at standard gearing, if you go up 2 teeth on the back then your acceleration should be 4.36067% faster, technically speaking... So, if your 0-60 is 3.5 seconds then this change in gearing should take it to 3.3471655 seconds. Or thereabouts.

Or, if your top end is 170 mph in standard gearing and you go up 1 tooth at the rear then it will drop to 166 mph. Go up 2 teeth, then it will drop to 162 mph.

Or, if you go to 16/48, your top speed will drop to 151mph, acceleration will be 10.86474 % greater and the side effects will be enormous! Acceleration will be 10% faster!

Here's the working.

2.706 - 2.647 = 0.059

0.059/ 2.706 x 100 = percentage gain/loss.

Here's the reality.

In racing circles, these ratios are really important. The mechanic (and usually the rider) wants the rider to be topping out at the fastest part of the track at full throttle, at the top of the power curve, in top gear. So you change the size of the sprockets accordingly. It's faster than stripping gearboxes.

On a practical note, it is always better to increase the size of the rear sprocket as the circumference of the sprocket increases - your chain will last longer. If you go smaller on the front, the circumference decreases and this will wear out your chain.

As for changing chains and sprockets together, well it's a bit of an old wives' tale. The story goes that they wear into each other and if you change a sprocket then it will wear out your chain. Not strictly true - I'm not going to do the maths for you but would you like to know how often the same tooth goes into the same link of the chain? About once every couple of hundred revolutions. This means that every link gets to fit every tooth and every tooth gets shaped to fit every link so changing a sprocket is hardly going to bother it. We're not talking blue printing here are we so it's much more fun to buy a new rear sprocket with an extra tooth and stick it on. What's it going to cost you and how much fun are you going to get back?

How much further can it go? Will your bike pull power wheelies if you go up two teeth? Don't know if you can calculate this as we are all fat, pie eating gits and it depends on so many factors but you could put the bike in a more powerful part of the rev range that allows you to ride easier. F'rinstance, if you are always on the motorway in the mornings and you want to get past traffic and 80 mph equates to 6000 revs, then going up two teeth will mean that you are sat at 6262 revs. If this puts you closer to the faster part of the engine and then you are going to accelerate 4.36067 % faster anyway then it could be useful.

Then again, you are on an 'Ace and I don't even drop a cog to overtake. I just use the twisty thing on the right and it fucks off anyway!

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