2x1x1/16 Belt-In-Tube

During the off season after having seen so many different chain-in-tube drivetrains, I decided to create my own iteration. All of the components used are Vex-Pro. I initially started with 25 chain, but I didn’t like the sound the chain made when it slapped the inside of the tube. I moved to 9mm belts for the weight savings, ease of ability to assemble, minimal efficiency gains, and the smooth and quiet operation it provides. At the moment the design is untested, so I am not sure how well the 9mm belts will hold up on a drivetrain in competition. The current iteration uses the plastic clamping bearing blocks, 30" of 2x1x1/16 aluminum tube, 24t 9mm belts, with a spacing of 12.4" C-C between axles. The weight with 25 chain was 2.768 lb, and the current weight with the 9mm belts is 2.562 lb. The only thing it needs is a set of the WCP cams to keep the belts taught.

Let me know what you think or how it might be improved upon.

Looks nice!

Last year we tried a very long (30 ish inches C-C) 9 mm drive belt and direct driven rear 6" HiGrip wheels supported by plastic bearing blocks. The front omni wheel was mounted in VersaBlocks with a WCP cam for tightening. Not sure if it was the length of the belt or that it was only 9mm wide, but the belt skipped a lot and make some awful noises, even with a lot of tension. Shorter belts may work better, but you way want to use 15mm belts. We ditched drivetrain belts for a while.

Our plastic bearing blocks cracked a lot with a full weight robot and minimally cantilevered wheels (wheel right at edge of bearing block with 1/16 acetal spacer). And this was a year without contact. I would test the plastic bearing blocks with weight on them. Maybe we were unlucky.

Other than the slapping of the chain, how was chain in a tube? What sprockets did you use? 18T? 16T?


The combination of 9mm wide belts and 24T pulleys, especially with 6" wheels, has been tried in FRC before by several teams with problems. Ratcheting and tensile belt failure are among the issues teams ran into.

My old team has done belt in tube for several years, eventually settling on 24T pulleys and 15mm wide belts, inside 3x1.5 tube. The 3x1.5 is used to leave clearance for rivets above and below the chassis, so if you attach your chassis differently you could use a smaller tube.

Using an adjustable tension system rather than exact centers presents some challenge as well - you need to get the tension exactly right. It’s very possible to over-tension the belts, or tension one side more than another, leading to issues. So be sure you do this right. That said, once you get the tension perfect, it’s going to stay that way if you did it right.

Very cool COTS solution!

I’d like to echo this. I’ve been on teams that ran belt for years with exactly zero failures until we moved to 9mm. I can’t imagine doing 9mm again with the 24T pulley.

15mm is the way to go.

It seems a lot of teams have had this problem, and some have not. The move to 4-cim drives to replace 6-cim might help mitigate it a bit, but I guess time will tell (or good testing).

Broaching pulleys from SDP-SI or using larger pulleys from Vex and turning down the flanges would allow you to use a slightly larger pulley and reduce the risk of belt breakage.

The chain-in-tube used 4x 16t #25 hubbed sprockets. This took much more finesse to assemble due to there being many acetal spacers between the sprockets. With the pulleys, I can just drop the assembly into the tube and add the 1/16" spacers to take up the tubing wall thickness when I install the plastic bearing blocks. If I use the 221 Robotics 17t double #25 sprockets it would help some, but the belts would still be much easier to assemble.

A little tip for using spacers with sprockets in a tube if you encounter this in the future would be to use a few small drops of super glue to connect the spacers to the sprockets. Why struggle with a bunch of small pieces when you can connect them together and have one assembly to drop down into the tube? :slight_smile: The bond is strong enough to get it assembled and if you want to take it apart later they aren’t hard to break apart.

Using this trick made assembling chain in tube so much easier last year.

Just came to a realization here - do the top & bottom of the side frame rails for a belt-in-tube design become (effectively) unusable? Putting even a rivet through the wall would interfere with the belt or chain, correct? How do teams account for this?

Teams either use bigger tubes for more clearance (with belt in tube designs - chain-in-tube takes advantage of the close fit), or they put two hole patterns down the length of the tube and just rivet to the opposite side of the chain.

What Chris said.


In 2014 my team was using a 9mm belt setup on the outside of the our aluminum tubing. That year we managed to snap almost a dozen belts. I couldn’t imagine having to re run the belts through the tuning each time they snapped.

Additionally the flying toasters (3641) did a similar design to what you explained, they were only able to compete because they had extra modular wheel assemblies. I’d recommend 15mm belts for anything related to your drive train.