07/09/2017 10:43am
Changing sprocket sizes can result in needing a longer or shorter chain but there are sometimes ways in which you can avoid having to change the chain and there is a quick way to work it out.
Let's look at the 2005 GSXR750 which is geared 17/43. Add the number of teeth on the front and rear sprocket together and in this case, you get 60. You can usually get away with a difference of 2 points in either direction and still use the same chain. In this case that would be 58 – 62. So any combination of sprocket sizes that adds up to between 58 and 62 should work for this bike. So if you wanted 16/45 it adds up to 61 and it will work.
There is no perfect rule here because there are so many different types of bikes with different chain lengths and different adjusters but this rule works fine in 98% of cases. Ducati’s have very short chain lengths and a totally different type of adjustment but it should still be good to about 1.75 instead of 2.
Once you know this you can learn to work within the parameters. So say you had the same bike and needed to gear it 16/47 this gives a final drive ratio on 2.938 but adds up to 63 so may not fit. But if you geared it 15/44 instead it has a ratio of 2.933, almost exactly the same gearing but it adds up to 59 so it will fit!
Armed with this information, a gearing chart and a large box of sprockets you can normally get it to work without having to change the chain. If you don’t have a gearing chart then just divide the number of teeth on the rear by the number on the front to get the final drive ratio. 15/44 is 44 divided by 15 or 2.933. 16/47 gives you 2.938 and so on.