pic: Custom 3 CIM 2 Speed Gearbox

Here’s a gearbox I designed using vex/wcp parts. Including a 22-32 chain reduction and 4 inch wheels, it is geared for approximately 17 ft/s theoretically, in aim of around 15-16 ft/s. Low gear is as low as possible given the high gear ratio. with motors and all hardware, it only weighs 11 pounds, almost 2 pounds lighter than a WCP 3 CIM DS and only 2 pounds heavier than an AM 2 CIM Super Shifter. The motors are also all located at the furthest possible lowest point, the best one in terms of CG. The final design point is it’s height, it is only 4 5/8 inches tall allowing a relatively short sheet metal chassis to contain it.

This was my first custom gearbox design, so there might be issues with it, please don’t hesitate to point them out.

The CAD files, both Inventor and STEP are located at: https://drive.google.com/folderview?id=0B0soKk9hjXfLYXlQSU1KMXBKbkE&usp=drive_web

What spread are you using?

I take it this is designed to mount into a non WCD style drive? I bet you could probably flip the motors around and hang this over your wheels if you use a 24:60. Also I think your weight calculations are off. To be almost 2 lbs lighter than the DS Standard 3 CIM you’d have to weigh: ~2.8 - 2 = .8 lbs


and your probably around the same weight due the fact the internals + motor weight is about the same. Weight of 3 CIM DS Standard (Non WCD)


EDIT: Are you comparing weight to the 3 CIM WCD with PTO?

The spread is 3.64. It is for a sheet metal, non WCD chassis. I considered putting the motors over the wheels, but I decided that the CG benefits of lowering 17 pounds around 4 inches would be more beneficial. My weight estimate only says 10.999 lbs, but I was comparing it to the 12.94 value from http://www.wcproducts.net/wcp-shifter-3cim/ (Standard, non-WCD, with 3 14t motor pinions, motors and 1 pneumatic cylinder). I think mine has one or two fewer gears and is thinner, but .2 lbs can probably account for that. I wonder how they got 12.94 lbs?

Looks cool! I like the layout.

But then he’d loose the reduction with the chains.

I’m not sure that it will be a problem, but the pinion gear on the middle CIM is going to have twice the load the others would have. I know that some teams with a similar layout have the idler gear between a CIM and the next stage of reduction, as opposed to between two CIMs.

Makes sense. I normally value space savings vs CG. I’ve been a bit envious of 125’s setup this past year.

The “builder” on the site doesn’t work so hot, its mainly an approximation for shipping. We’ll be fixing that value in the coming months with a site update.

For this setup it wouldn’t really matter as you can just get a larger reduction in the chain setup. It would be a much larger deal if the OP decided to direct drive a wheel.

My FEA gave the middle pinion a safety factor between 3 and 5, but it will be something that we inspect regularly. We’re designing a drivetrain that should generally avoid pushing matches, so I’m not too worried about it, and if it does break, we’ll have weeks to fix it unlike in the build season. I think 254 may have done something similar last year (2013) in their gearbox with a dual loaded pinion, but I’m not certain if they did or if it worked well.

If I remember correctly, 254’s 2014 gearbox had a similar layout with the motor pinions.

Here is an image of 254’s [unpopulated] gearbox.