ES 250/2 gearbox sprocket

[liquidgold]

I will shortly require a new gearbox sprocket but seek advice as to the right one. Twenty five years ago I ran one of these bikes and always thought it was slightly over geared with the standard 21 tooth sprocket. I know there is a big gap between 3rd and 4th gear which will remain but is there any advantage in running a 20 tooth sprocket to bring the revs up a bit? Anyone with any experience of playing with sprockets? John

[Puffs]

If you replace a 21T front sprocket by a 20T one, the engine will make 5% more RPM at the same speed. (The 20T sprocket has to rotate 1 full turn (=20T) + 1T to cover the same distance as one rotation of the 21T sprocket, hence it has to rotate 1/20 = 5% more.)

On my ETZ251 I have recently replaced the 21T front sprocket for a 20T one. I did that to compensate for a 110/90-16" rear tire, which is quite a bit bigger than the standard 3.25"x16". That rear tire should bring the revs down by 5.4%, hence the 1T smaller front sprocket should take it back roughly to the standard gearing.

It is definitely an improvement, it was too long before. YMMV.