07/09/2017 10:48am
Changing sprocket sizes or ‘gearing’ does not increase or decrease your bike's power. What it does do is change how the power is delivered. So, let's look at a 2005 GSXR750. The standard gearing is 17 Front and 43 Rear or 17/43.
So if you remove a tooth from the front to make it 16/43 what it will do is make the bike accelerate quicker but loose top speed slightly. The same effect happens if you increase the size of the rear.
If you want to do the opposite and make the bike more ‘cruisey’ or give it more top speed then you go bigger on the front or smaller on the rear or both.
Of course, you need to remember a few other things along the way. The bikes manufacturer will have already set the bike up to pretty much max out the top speed so just changing sprocket sizes alone is not going to make it much faster. If this is the case then you will need more power to pull the new gearing and for that, you will need more engine tuning. None the less the principles remain good.
The further you go, the more extreme things get. A good starting point for road and track day bikes is 1 down on the front and 1 up on the rear. There isn’t a race track, in Britain at least, where the straights are long enough to run standard sprocket sizes with a standard engine tune. Production racing back in the mid-nineties on a GSXR750 I was running 1 down on the front and 1, 2 or 3 up on the rear depending on the track. We do get customers particularly with 600’s asking for 2 down on the front and 4 up on the rear but actually, all they are doing is moving 2nd gear to where 1st used to be and then using 2nd to 6th instead of 1st to 5th! With a set up like this, the bike will wheelie almost on thought. Fine if that is what you are looking for but not necessarily the best set up.
With bigger 1000cc and 1200cc bikes, you want to be looking to use the torque as much as you can rather than gearing for revs. This decreases its tendency to high side and helps you drive it off the corners. It is not the least bit unusual for a Ducati twin around a tiny track like Brands Indy circuit to only use 1st to 4th! Gear it to use all 6 gears and you are constantly changing gears and it makes it almost unrideable. The bigger the engine the more you need to use the torque.
In the last few years racing a Ducati 1098 I was ‘over gearing’ it slightly. It wasn’t as revvy but made it much easier to ride and delivered better lap times.