I’ve been playing around with the idea of a 3 CIM, 2-speed gearbox that is the height of the CIM for the past couple weeks and this is what I came up with.

Gearing:

Low gear: 8.23:1, 8 ft/s real-life speed with a 3.5” diameter wheel.

High gear: 3.43:1, 19.23 ft/s real-life speed with a 3.5” diameter wheel.

Shifter Spread: 2.40

I am well aware that 6 CIM drives are falling out style and that mini CIMs are all the rage these days. Therefore, this gearbox was designed so that replacing the CIMs with mini CIMs and swapping from 14 to 13 tooth pinions will result in nearly identical speeds.The CAD shows the CIM configuration because the packaging for the shifter cylinders looks better next to the longer CIM motors.

Overall I’m very happy with how the flat CIM gearbox’s packaging and ratios turned out. My main gripe with this gearbox is some of the cost inefficiencies, like having to use a VEX shifter shaft with the AndyMark dog gear.

The model can be downloaded here. Any feedback would be greatly appreciated and I’m more than happy to answer any questions anyone has.