what size sprocket to run with 18" tires ona 250R

[mmartin]

I'm currently in the middle of my build on my baby. An 86 ATC 250R. I am changing the rear tires and rims from the stock 20" tires to 18x10" tires. The sprockets that I currently have on the trike is 13 tooth and 39 tooth. Does anyone know if that is the stock size sprockets? And also does anyone know how to figure the ratio so that I can maintain my gearing to the ground with the new tires?? Or what size sprockets would be optimum for the 18" tires ?? Any help would be greatly appreciated.

[Red Rider]

Yes, 13/39 is the stock gearing.
To figure out your ratio of gearing-to-tire size, I've always used the following: Rear tire size/(Rear sprocket teeth/Counter-shaft sprocket teeth).

So in your case: 20/(39/13) = 20/3 = 6.66
With a 14 tooth counter-shaft sprocket & 18" tires, you'd have: 18/(39/14) = 18/2.79 = 6.45
Or a 36 tooth rear sprocket & 18" tires would yield: 18/(36/13) = 18/2.77 = 6.50

A counter-shaft sprocket will be cheaper than a rear sprocket, so a 14 tooth counter-shaft sprocket will get you back pretty close to stock gearing.

[mmartin]

Thanks for the help