right im using a 1988 zx10 engine which runs an 18inch rear wheel with a 160/60 tyre with a 47 tooth rear sprocket as standard on the bike, but i will be using 13inch rear wheels with proberly 195/55 or 60 profile. What size sprocket would be best to fit on my chain diff. Is there any formular to work out the ratio's? I cant go for a 47 tooth as it leaves very little space to tighten the chain up.

Bear in mind you can get smaller front sprocket sizes which make things much easier as you can use a smaller rear sprocket = saved weight & more room for everything around it. I run 14:40 with twin zx10's & 600kg, but I'm running 18" rims with 285 45's. with less weight but single engine I would try around 3:1 or just below with the 13" rims hence 14:42 or thereabouts.

That all sounds about right.

On my grasser I'm using 16:38, but that gives 95mph in 2nd gear.

The engine will produce around 140bhp when complete if not a little more. Top speed is not all that important to me as long as it will hit about 120, im more concerned about acceleration.

42 tooth is what i had in mind for space so i will proberly go for that. I'll just have to cut a few different sizes out and just go for trail and error.

Cheers for the advice guys.

If you use 195/55 13 wheels, the zx10 motor which has the same gearbox internal ratios and primary drive as the ZZR1100 according to my haynes manual. 14 tooth front sprocket and 42 tooth rear sprocket then the theoretical top speed would be 145mph, assuming a 11750 redline same as the zzr1100.
14/45 would give you 135mph and 14/47 would be 130mph, I think the 14 tooth front is about the smallest you want to go really, also they are readily available, anything smaller would need to be a custom jobbie and probably not recomended either

Cheers
David

[Edited on 11/5/07 by Minicooper]