does a 39 tooth rear sprocket require a longer chain if i use the stock chain & front sprocket? i have a 38 on and it seems like i have more room for larger sprocket but people have said a 40 won't work with stock chain. i'm really not looking to change the front to a 13, because i added 21 inch tires and i'm looking to compensate for the height change without changing my final gear rating. another words, when i put the larger tires on i lost some accelleration and i'm looking to get back to the old (stock) gearing/acceleration. make sense?