Changing the front and rear sprockets of a motorcycle has the affect of changing the gear ratio. That means you will either gain more power/torque but lose top end speed or you will lose top end speed but gain more power/torque. The gear ratio is computed by dividing the number of teeth on the rear sprocket by the number of teeth on the front sprocket. This means that for example a motorcycle with a 51 tooth rear sprocket and a 15 tooth front sprocket has a gear ratio of 51/15 = 3.4. You can affect this ratio by changing the number of teeth on either the front sprocket or the rear sprocket. Adding more teeth to the rear sprocket is the same affect as lowering the number of teeth on the front sprocket. The opposite is also true in that lowering the number of teeth on the rear sprocket is the same affect as adding more teeth to the front sprocket.
A 12 tooth front sprocket and a 48 tooth rear sprocket is the same as having a 60 tooth rear sprocket and a 15 tooth front sprocket. Notice the example math below (gear ratios are written as 1:ratio):
- 48/12 = 1:4.0
- 60/15 = 1:4.0
The gear ratio is identical so which one does it make sense to increase or decrease? This depends on your target gear ratio. First, it is better price wise to buy another front sprocket rather than a rear sprocket however changing one tooth on the front sprocket is equivlent to changing more than one tooth on the rear sprocket. The effects are increased when changing the front verse rear. The other issue with the front sprocket is that it can only be changed within a small range of teeth, you can't buy a 50 tooth front sprocket for example so in some cases you may want to change both or only the rear sprocket.
- 51/15=1:3.4
- 51/14=1:3.6
- 54/15=1:3.6
The above shows you how to achieve a 3.6 ratio when changing either the front or the rear sprocket. Notice that you must add 3 teeth to mimic the effect of removing one tooth from the front sprocket. This is where you have a more fine grain ability to tune your gear ratio when changing the rear sprocket as opposed to the front sprocket.
So what is the gear ratio? First, the higher the Gear Ratio the more power/torque you will have and the lower the gear ratio the more speed/top end that you will have. The "1:ratio" means that for every 1 revolution of the front sprocket the rear sprocket will turn 1/ratio. So a 1:4.0 ratio means that for every 1 revolution that the front sprocket rotates the rear sprocket or tire basically will turn 1/4.0 = 0.25 turns. You know that the speed at which the front sprocket can rotate only changes between high and low RPM and which gear the transmission is in.
That means that for any 4th gear without modifying the transmission the front gear can only turn at a certain interval at its highest RPM range. This means that changing the gearing then gives you either more speed or more power. The tire can rotate more closer to the number of revolutions as the gear box turns giving you more speed or you can get more power because the leverage the motor has when every revolution only turns the rear sprocket a small amount for each of its rotations. However with more speed you have less power because the motor loses this leverage and with more power you lose speed because the shaft only rotates to a fixed amount and will be turning the rear sprocket less at the highest RPM.