[littleraw]

my friend has a 20" bmx bike. He has a 40 tooth sproket and dosnt know what size cog he needed for dirtjumping. can someone tell me a good gear ratio?

 

[phlegm]

2.5-2.7:1

 

[littleraw]

whats that meen?

 

[Boxxer]

Divide 2.5 through 2.7 into 40 and that will give you rear cog size.

 

[Bicyclist]

For instance, a common drivetrain combo is 25-9. To get the ratio, you divide the bigger # (the chainring) by the smaller # (the cog).

So, 25-9 yields a ratio of 2.778, which is a good starting point (it's the standard ratio nowadays).

 

[ServeEm]

I run 30/11, nice and quick

 

[stevop007]

just stick with a 30 tooth up front and a 11 tooth in the rear