[Dick Linn]

Instigator,

We're trying something similar, but only on one end, assuming that 2 wheel drive is good enough (time will tell). 4WD is heavy and 6 is even heavier. Plus 4WD can be less maneuverable if it is a simple scrub-steer 'bot. We're using the drill motors in low, so we aren't faced with the problem of getting a 40:1 reduction in speed at the end.

Try to make the wheels drive at the same ft/sec, so carefully measure the diameter ratios of the two types of wheels, under typical load conditions, and select the sprockets or gears accordingly. If you drive the smaller wheels off the axle shaft that the large wheel is mounted on, you can add a small sprocket there and a slightly larger one on the 10 inch wheel to get things evened up. That may reduce the number of large gears (read weight) that you need. If the wheels are exactly 12.5 and 10 inches, a 15-tooth on the 12.5 driving wheel axle connected to a 12-tooth on the 10-inch driven wheel axle should do it. A 12.0 inch tire would require either a 12 & 10 or 18 & 15 tooth sprockets. If the wheel is about 12 1/4, you are in between a 16:13 and a 17:14. These are assuming you're using commonly available sprockets for 3/8 (#35) chain.