Mismatched wheels

[Jared Russell]

This only works as long as the difference in diameters of the driven wheels is very small. I've seen plenty of cases where a team achieved a 1/8"-3/16" center drop by having all axles in line and using a slightly larger custom center wheel(s) than on the corners.

A 1" difference in diameter is much larger than I would recommend for this purpose.

[Kevin Sevcik]

You'd kill your battery pretty quickly. Driving straight forward, the motors would have to provide enough torque to overcome the static or kinetic friction of the wheels you're slipping. Even without much weight on them, the outer wheels will take a decent amount of torque to slip like that.

I think the reason it works in the case of slight differences in diameter is because there's enough give in the carpet-tread system, that the tread and carpet just elastically deform to accommodate the differential as they roll from starting contact to full contact to releasing contact. The incoming tread and carpet is always undeformed, so it's not like there's any build up of differential. Elastically deforming the tread and carpet for that little bit still requires some force, but it's almost certainly less force than sliding friction.

[Ken Streeter]

Originally Posted by Jared Russell

A 1" difference in diameter is much larger than I would recommend for this purpose.

Agreed.

However, we've done this before, with a not-too-difficult proviso, that can make it work perfectly:

The trick is to change the belting / chain / gears (whatever it is that connects your center axle to the outer axles) so that the linear speed of the wheels is the same. This is a matter of adjusting the pulley / sprocket / gear sizes for the different-sized wheels. Essentially, you'll have a different reduction ratio to the 9" wheels than you would to the 8" wheels. In theory this is easy.

In reality, it's a little trickier (but not too much), in that you need to source (or make) pulley / sprocket / gear sizes that give the desired ratios. However, that's not an unsolvable problem.