Custom Chassis Problem

Hello friends

We will make our custom chassis for the first time this year, but we have a problem.

belt or chain ? Which is more useful for us?

thanks for your answers

The main reasons for choosing either:

  • Belts are lighter and slightly more efficient/quieter. This means they can generally run at higher speeds.

  • Chains are more forgiving in sprocket spacing and easier to mount, as can be split. A better option if drilling the drivetrain holes without a mill.

Chains too can handle more load in general, but a correctly selected belt will do fine for drivetrains. Either can do the job, but make sure to keep spares of whatever you choose.


I just want to add that when using a belt you need to use a relatively large pulley(around 40 T IIRC) to make sure there are enough teeth engaged(because the load rating of an HTD5 belt tooth is smaller than a #25 chain). Thats the reason the KOP pulleys are as large as they are.
Wiyh a sprocket, we use a 16T #25 sprocket with no issuses so that is something to concider.
Also wheel size may factor in, and ground clearence for the reason I stated above.


@Jonathanshani makes an amazing point, getting enough wrap on your pulley is vital to avoid skipping. We originally used 18T Pulleys on our drivetrain this past season, and it went horribly, the second there was weight on our bot we couldn’t move because anything would cause our belt to skip. We switched to 25 chain and never looked back, they were easier to tension, without tensioners, so we loved them! I think we may try to pursue belts in the future, but intentionally adding a tensioner is necessary! It’s also important to think of where your failure point may be when assessing chains or belts. I’m definitely not an expert, but throughout our matches we would have to switch our gearbox multiple times during competition, we figured this was originally a manufacturing error and something had just gone wrong, but we also hypothesized this may be because when we stall our drivetrain(which we didn’t have current limits on), instead of skipping a belt, our failure point was our gears. After a couple of matches our gears would look more like metal circles which was a big hassle. Again, I don’t know if this is actually the cause, just a guess but a good thing to keep in mind!

The JVN calculator is your friend when it comes to calculating drive gear box ratios and motors and can be a great reference for preventing such disasters in gear boxes.

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Can I ask why the decision to switch to a custom chassis? I am not trying to discourage it, we use a custom chassis, I am just generally curious as to the reasoning your team has chosen.


Gear teeth shearing is the main reason we switched to all steel gears in drivetrain gearboxes. Aluminum is fine for low load applications, but I would personally never use aluminum gears in a drivetrain gearbox.

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Belt, belt or belt.

BTW if you don’t think you can do exact pully to pully spacing you shouldn’t attempt a custom chassis.


I’m starting to think that they’ve got steel pinions on aluminum gears or something (not a great idea). I can’t imagine aluminum gears failing that often.

We did have steel falcon pinions on 7075 aluminum gears

We generally always do belts. They just don’t need tensioning., and dealing with chain can sometimes be a real PITA. The downside is that you need to be really precise about where you put your bearing holes. We figured out the magic on this this year (use calipers, measure twice, use a non-spring-loaded punch, and then use your phone to zoom in on where the drill bit is actually centered.)

I have the same question. The same decision process that you used to select between multiple chassis options should be used to select between multiple transmission options.

I also encourage you to not make these binary decisions (Option A or Option B only). This is prone to leading to “Us and Them” thinking and defensive or emotional responses. Instead embrace Stephen Covey’s The 3rd Alternative philosophy.

When my teams are approaching a decision like this, I require a minimum of three options to select from and encourage five options. The evaluation of multiple options really helps the team discover what the underlying problem is that they are trying to solve.

DM me if you’re interested in a more structured Engineering Trade Study process that I share with my teams. It was the primary method for making these types of decisions when I worked on satellite and missile programs at Lockheed Martin, and it works very well at FRC scale.

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Belts are so much nicer to use once installed correctly. We did our first custom chassis this year and it turned out great, we won FiM state championship. With that being said Vex blocks can be your friend and help get the tension correct if your holes are slightly off. We used a mill for spacing but still needed to use them. The thing about Vex blocks is they have 1/16" offset to them. So if you use them on all 3 holes you’ll end up with a 1/8" of high center which is a little much in our opinion kop chassis is 1/8" so we went with a center centered middle hole and up with no vex block and let the blocks move the end wheels up 1/16". We went with 30t pulleys and had enough wrap not to slip even during hard defense. With that being said we did have to do lathe and mill work on the pulleys and a shaft to run 30t pulleys on the WCP flipped gearboxes, they’re designed to use chain. Hopefully this helps you out in your decision

We switched from belt to chain a few years ago. Primary we use chain for compactness. Generally speaking we have also found chains to be more reliable as well. We are currently running chain in tube which is the most compact option and not possible with belts. The chain in tube is small and self contained enough that we are able we are able to swap out an entire side of the drive train in 3-5min.

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How do you not tension your belts? Do you do the exact center to center distance that a calculator would say? Or do you generally undersize the dimension by some amount?

I meant “re-tension.” Once you have it, belts don’t stretch enough over the course of a season that you have to fiddle with it. (As long as nothing slips, which we’ve had happen – you actually need to attach bearing blocks to the tube, or put the bearing directly in the tube, instead of just relying on clamping pressure). Chain was a nuisance with that.

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Pro tip: use the andymark default drivetrain spacing and belts. It’s excellently sized and never had a belt issue, and I’d you do somehow strip belts, other teams are likely to have spares


If you want to go a bit more custom, you could also open the CAD provided and look at the tolerances AndyMark uses and then implement that for your own belt lengths and pulley sizes.

I typically don’t use any sort of tolerance. If the C-C distance is 5” for a belt, then I do exactly 5”. The CNC is accurate enough to cut that without issue. Never had a belt slip either

What size pulleys were you using, that could have potentially been our problem, we used 18 tooth pulleys :grimacing: