pic: Inverted 3 CIM 2 speed gearbox with PTO

[traxxasracr1]

Originally Posted by Jared

I really like this design - having a quick swap gearbox is useful.

We did the same thing in 2014 with a 3/8" female hex made from a steel hex sleeve from SDP. There are two things to consider with this type of design: the gearbox flexing away from the frame and the alignment of the output shaft with the wheel shaft.

The fit of our female hex was sloppy, so we definitely needed two bearings on the gearbox shaft and two bearings on the wheel shaft. Getting the alignment of these four bearings is crucial for this design to work. The same applies for the mounting holes, but these could always be slotted if alignment is an issue. We had issues at first when we were using Vex hex shaft that was bent. As of 2015, the shafts are MUCH better, but there are still two very different versions of the hex shaft sold by Vex.


Your setup to hold CIMs will help decrease the moment on your center plate, but there will still be a moment on the 1/4-20 bolts and the wall of your tube. Be aware that different CIMs have different OD's. The wall of the frame tube is pretty thin, and I'd be worried about the holes being enlarged or the wall being bent, though I have heard of teams getting away with a setup like you have. We welded inserts into the frame rail that were connected to both walls of the frame tube, but this may have been overkill.

we haven't had any problems with alignment between the plates in the gearbox, but I do like the idea of slotting the holes, we will consider that if we have problems later on.