Sprocket change

[ccRider246]

I ready to change sprockets and chain and I have a general question. I currently have the stock sprockets for my 81 750F - 18/46. A friend has a new set of Sunstar 17/42 sprockets that can be had very reasonably. My question is, what does the 17/42 do for me with regard to gear ratios? I have a DID 50VA 108 link chain. So I would be dropping 1 tooth in front and 4 teeth in back. This should give me slightly lower rpm at 70 mph right?

[woodsrider250-2]

'in general' 1 in the front is like 3 in the back....So it's nearly a wash (1tooth change).  Proven by the math.  17/42=.40  18/46=.39 so you're looking at a .01 taller final drive.  I'd GUESS that the chain should be fine if you decide to switch.  Now 18/42 would net a .42 which would be a .03 difference.  I'm once again guessing (more involved math here involving tire dia/speed/etc), but each .01 probably nets 2-300 RPM change at 60mph.

[ccRider246]

Thanks, that confirms what I was thinking. I'm going to give it a shot. I don't really think I will notice much difference in performance but if I reduce the rpm at highway speeds by a few hundred that would be a nice benefit. I will change them in the morning and find out.

[ccRider246]

Just a follow up, I made the sprocket change from 18/46 to 17/42. I had to shorten the chain from a standard 108 to 107. At 75 mph I now run at 5500 rpm where I was a 6000 rpm with standard sprockets.

[woodsrider250-2]

Cool, so .01 could be roughly considered 500rpm at cruise for these bikes....Good rough figure to have in the brain DB.