pic: Off-Season Drive Project

This season taught me a lot about drivetrain design, so I tried to put what I learned into a base to build this off-season.

The 6 cims are geared 1:6 (inspiration from 1986) for a speed of about 14fps. There is no front plate on the gearboxes as there is only one reduction.

There’s a secondary frame that will hold the bumpers and create an octagonal frame perimeter (inspiration from 971) to help with friction pins.

The wheels are 4" Colsons with WCP hubs for their balance of traction and durability.

Total weight for everything seen here is 42lbs.

It seems like with the octagonal frame, you have room for 2" wide - if not wider - wheels. With the 4" Colsons coming in a 2" wide variety, is there any reason you opted to choose the 1.5" wide wheels?

Good catch. I had originally designed the drivetrain without the octagonal frame up top. Now that I’ve added it, I have room for the 2" wide wheels. The only qualm I have is that I won’t be able to use an e-clip to hold the wheels on the shaft because the WCP hubs are 1.5" thick. I suppose a screw, a washer, and some loc-tite would suffice. Thoughts?

We have used bolts and washers to hold wheels on the past few years with cantilevered axles, it works great and eliminates any ‘slop’ in the assembly.

It will be a bit more tedious than normal, but using a 1/2" snap ring would definitely be possible since you can fit a snap ring tool inside the small amount of space the Colson bore opens up. Alternatively, as you mentioned, a tapped hole with a screw, washer, and loctite will also be an effective method.

Another question - is your bellypan just not lightened yet, or are you using a solid piece of aluminum? If you don’t have the resources to waterjet a diamond-bellypan, I highly suggest using a solid piece of 6mm or 1/4" thick plywood as a bellypan. It’s much lighter than a solid aluminum plate, and will serve well as an electronics pan as well as providing the torsional stiffness the diagonals that a diamond bellypan provide.

This works well. I’d use a hex cap screw (which you can tighten with a wrench, not a screw driver/hex key) so you can get some extra torque on it.

EDIT: I was unaware you were referring to a socket wrench, not a regular wrench

My original post

In this particular scenario where there is a small, confined space for a tool to fit into, I’m not sure if you will be able to position a wrench in a way that allows you to get a significant torque advantage. A standard socket head should be sufficient to get the required torque, and the loctite will help in keeping things in.

MORT ran 2" wide colsons with this retaining method this year. We had issues on our practice bot with socket head screws and loctite not being enough to retain the wheel. YMMV, but I would recommend a hex cap screw.

No longer relevant to topic


Maybe I’m misunderstanding you, but I’m not understanding how you could fit a wrench inside the 1 3/16" bore of the wheel and hold it perpendicularly to the axis of rotation so that you can get the mechanical advantage to gain the extra torque.[/spoiler]

Socket…not adjustable…


This is our drivetrain with incorrect screws retaining the wheels.

We used large washers for cosmetic reasons, we did some machining on the face of the wheel and wanted to keep it covered.

Edit: Or as Akash pointed out, socket wrenches work in the set up you have CADed.

That makes more sense than a regular wrench - my apologies.

I revise my previous statement to say that, thanks to Eric’s example, a hex cap screw works equally as well any method of retaining.

The plan is to use either .0625" polycarbonate or garolite to allow us to leave it solid, maintain the weight advantage, and to easily mount and insulate electronics.


We’re partnering with another team to help with the waterjetting.

I’d seriously question that decision… Polycarbonate will be very floppy.

That is a thing of beauty. I can’t believe I haven’t seen it before. What’s the part number on those wheels, and do you have a link to any more pictures?

I don’t think there are build season photos on the team website yet, but you can find some random pictures of the drive on the team facebook page.

Here’s one, and there should be more if you keep clicking next Redirecting...

If there’s anything in particular you want to see, just ask!

Using a bolt to retain the wheels and take up all slop is a bad idea as it will preload the bearings. It’s also more likely to loosen the fastener.

Compared to two snap rings, etc. it’s definitely sub-optimal, but with regular maintenance (check the bolts before a match) and some Loctite, it’s a decent enough solution that will retain wheels and account for manufacturing tolerances. Just don’t over-tighten the bolts as the tighter they are, the more preloaded your bearings are, and hex bearings seem to be particularly temperamental in FRC.

By the way, the wheels in 11’s photo are 2" wide black Colsons.

I’ll third the notion of not using thin polycarbonate as a structural belly pan. Polycarbonate is great for some applications but in this one the thickness you need for polycarbonate to be sufficiently rigid makes it a poor choice compared to thinner sections of other materials, such as garolite. We still use an un-lightened 1/16th aluminium belly pan though this is suboptimal.

Kind of reminds me of an old 179 drive.

We have used a bolt to retain wheels for three years and have never had a single problem. We do not take up slop with them, we always leave a little slop just like you would with a retaining clip.