Question on Gearing ratio

[Mike R.]

I have an 01 cr-125 and stock gearing is 13/51=3.98 but, I have thinking about going to 12/49 =4.08 this will help bottom end which i like the idea of. My question is I am no chain guru so will I need a longer chain or shorter? What length will I need going to 12/49?
Is there any problems going to a 1 tooth smaller front as like premature slider wear or rubbing on the swingarm?
Thanks

 

[Jaybird]

Without going into the specific physics involved, the largest countershaft sprocket you can use to obtain a given ratio, the better.
If you are looking for a 4:1 ratio (nice choice for a 125) go for the 13/52 instead of the 12/49.

If you have a good deal of chain adjuster left with your current ratio, you may be able to mount the 52 with the same legth chain. But always order your chain longer and use a breaker to make sure.