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[Bennett548]

You mention reducing the diameter of the outer wheels, have you considered increasing the diameter of the middle wheel?

I'm thinking that versawheels with blue nitrile might have a large enough OD to give a decent amount of rock with 4" Colson wheels on the outside.

Alternatively, you could use 4" wheels on the back and middle and a 3.5" wheel on the front. That gives you the same amount of rock as a 1/8" center drop.

Edit: You can also use a vex 16 tooth sprocket on the 3.5" wheel down from the 17 tooth sprocket on the 4" wheels to try to get the surface speeds of the wheels more closely matched. This should reduce wheel wear.