Belt In Tube West Coast Drive Design Feadback

Nice looking design!

Some advice specific to belt in tube:

The biggest difficulty with belt in tube is assembly and maintenance. You will want access holes on the top of the tube above the pulleys. Even still, getting the shafts to line up with the pulleys and bearings requires some finesse. We are planning to move to Thunderhex which will hopefully help. You can check out the CAD for our 2019 drivetrain here. Our access holes have tabs that we bend up slightly to provide a smooth edge for the belt.

Once you have the belts inside the tubes, your entire team needs to be disciplined about keeping debris out. If metal shavings, rivet heads, or other things get in, your belt will get shredded. Make some polycarb covers for the access holes. We let our guard down this year, and a shaft collar ended up inside which shredded a belt. Fortunately this happened during practice at our shop.

I don’t have any experience with 3D-printed drive pulleys, so I can’t say how well they would hold up. The failure mode would likely be rounding of the hex bore. 30 tooth pulleys will be fine. We use 24 tooth, but have not tried it with anything larger that 6 inch wheels. We buy aluminum pulley stock from BB Manufacturing, which we machine and hex broach. You can find our pulley CAD here.

Good luck!

Is this an “issue”?

You could look at the load ratings for the belting and see if anything is under-rated. Heck you might be able to change the further belt to a smaller width to save space/weight.

If you are concerned about the fact that you’ve doubled your failure points (i.e. if you snap the rear belt, both front wheels are out of comission) you could run two lengths of belt- one from rear to center, one from rear to front.

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I am always super worried when teams cut that much out of their frame rails. That aluminum was there for a reason! But if you got away with it, maybe I’m being overly conservative.

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We only do it on one side, so it doesn’t really affect structural integrity much.

I feel like you’d be better served by putting the cutouts/covers on the underside. Servicing would require a little upskirt, but then the topside has full attachment potential. little things though.

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For a chain version I would want to use 25 or 25h chain. What recommendations are there for 25 vs 25h chain? If a chain did break how tough would it be to change with access holes at sprocket locations.

The 254 style double belt/chain run is what I was getting at. I’ve seen regulalrly loaded belts snap in drivetrains before without any other obvious cause for the damage so I think double loading the belt could certainly exacerbate stress on the belt to the point of failure within the timespan of an FRC season more regularly than single loading the belt would.

What you said about doubling points of failure is another issue that I forgot to include in my earlier post.


We ran #25 chain for the past 3 years without encountering any problems.

If you happen to snap a chain you will need to use the chain tool to repair but they will not fit through an access holes

I believe master links are also an option but I have 0 experience with it

IIRC when 971 ran belts they used large pulleys, wide belts, and center wheel gearboxes for precisely these reasons.

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You have to download and install it to a pc, and make some assumptions about torque, but Gates had their belt and pulley ratings in their Design IQ software:

IIRC, you need something like the kitbot size pulleys to be really within the Gates recommendations. (42 tooth, 15mm wide, 5mm pitch HTD belt). The only way you can get smaller is with 8mm pitch belt, which is harder to come by.

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I’m getting the feeling that a skid steer style drive that is not the KOP should just use chain.

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I am working on a chain version and will update the post when done. I am having a few issues with packaging of the last stage my problem is the vex 16t sprockets have a 1/4" thick protrusion on one side. There is a location where an 1/8 smaller would make it easier but I would rather not have to machine cots parts.

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What’s worth noting is that with a decent setup (which might just be a hex shaft with a few clever modifications), you could face off the extra space from those sprockets pretty quickly, using either a mill or a lathe. Since the face doesn’t need to be perfectly smooth or precise, you could probably manage a really quick turnaround on them and have a drivetrain’s worth done in a jiffy.

You could also try to use the AndyMark 17t dual sprockets, though I’m not sure how well that would package with the reduction in the tube.

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Not sure why you think that. As long as you can accurately machine the center to center distance or put a tensioner in, belts are a perfectly viable solution. Belts are lighter and more efficient than chain. Placing the belts in the tubes just requires more planning and understanding of the trade-offs. Not everyone will think the trade-offs are worth it, and that’s fine. We love our belt-in-tube drive train, and avoid using chain wherever possible.


For your packaging issues, maybe the WCP chain-in-tube sprockets might help:

If any has noticed the gearbox has one mounting screw located behind the 60t last stage spur gear. I could extend the gearbox side but maybe there is a better solution. This issue causes a very complicated assembly.

Also should I start a new thread for the chain version or add it to this thread?

Without looking at your gearbox in detail, could you make an access hole through the gear? I have done that in the past in a couple gearboxes that were in confined spaces.

I’m inclined to agree with this statement. They might work, but why risk it in the drive train.

30T Pulley stock here. Can’t recommend B&B enough.

We run 15mm belt in tube using 1x3" that is severely lightened; running a similar setup since 2017. They aren’t easy but we will never go back to chain…so heavy and ugly. :wink: