I would probably drop down 1-2 tooth on the bottom sprocket (gear). You could get the same effect going up one size on the upper sprocket, but your chain might not allow you to go any bigger. You could get a longer chain i guess, but that's a waste of money IMO, because you still have to buy a different size gear.
If you knew the diameter of both the 8 and 9 tooth drivers, as well as your gearing on your machine, i could figure out all the ratios for you and come up with the closest match.
umm. 8/9. track speed is 88.8% of what it was originally. Diameter doesn't matter. It's units. one goes 8 units/rev, another goes 9. Since the track has a set amount of drive lugs, you see that the ratio is just the ratio of the drive teeth.
I the upper has 20 (but could be 15) and the lower has 35. This is according to the OEM Breakdown picture.... It says on the upper sprocket: Sprocket, Spline (20/15 teeth) (13W). So I'm not sure which number refers to the number of teeth on the sprocket. (I'm trying to do this without getting my hands dirty)
Check out this site, you can plug in the numbers with the different drivers and come up with the same effective gear ratio.
You will need the center to center distance on your gears. If you know the pitch of the chain you can make sure that the gearing your going to change to will work with the existing chain or if you will need a new one.
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