pic: Belt Drive Chassis

We are trying to develop a new system to replace the #35 chain that we have used in the past. Typically we would run two short runs of chain each start from our drive wheel in the middle. I am trying to design a system that uses a timing belt and one long run. The drive wheel is still in the center. Currently I am designing the system with 3/8 .2 pitch timing belt, with 28 tooth pulleys. The center wheel is dropped 1/8". I have two idler gears to adjust the tension and to draw the belt into the drive gear. I am wondering if I should step up to a larger belt, also do you think there is enough contact between the belt and drive pulley?


I obviously have no more experience with timing belt than you do, but I figured if you haven’t seen this whitepaper by 234, you may find it useful.

234 and 612 both said in that thread they successfully used 15mm belts without problems, with 5mm pitch. .2 pitch is about the same so that makes sense.

I really like the sliding tensioners that also help the belt engage the sprocket. If it works as planned it should save a lot of weight over chain that way.

I’m sure you know but Gates has in the past given teams offers for free drive belts in the KoP. I think they allow teams to get some specific kinds, so it would make sense to model around those particular sizes to save money.

One thing to consider regarding use of a single belt per side…if you do have a very rare belt failure your bot will basically be dead on the field while use of two belts per side still allows decent mobility should one belt fail on a given side. Good for you to consider using a belt though…987 switched 3 years ago to belts and loves them.

Definitely worth considering, though it’s also worth noting that a direct driven center wheel can alleviate this problem somewhat.

Glad to hear some other teams have been using belts very well!

My honest advice is to stick with chains, but that is another story…

As to your question about enough belt wrap on the center pulley, I would have to say no, not enough. Remember you are trying to transmit all the power to both end axles from this wrap. With less than 90 degrees of wrap, you are asking for slippage. The high tension needed to keep this from slipping will add drag from the idlers, and the low number of engaged teeth will be hard on the belt. It is always difficult to try to drive from the center of a belt or chain path. You might go with just one idler, and bring it around to create much more wrap on the center pulley. Is there any reason you do not want to use to use two belts? This only involves one additional pulley, eliminates the idlers, and as already mentioned it prevents a fully dead side.

Another option would be to use a double sided belt, and take the belt through an “S” shape in the center with two driven pulleys, no idlers. This would require an output shaft arrangment with two driven shafts turning in opposite directions, which would be easy to do with a gear pair.

I would go to two belts, it makes belt wrap easier, and if your outer wheels slide in and out, you don’t need any idlers or pensioners touching the belt.

Just curious, Have you considered going to 25 chain for weight reduction?

As you guys have some experience with these belt drives…

How does the belt drive handle high torque situations, such as a pushing contest vs. another robot? Can it handle a high-torque transmission setup?

Also, assuming the belt is inside rectangular tubing, how easy/difficult would it be to change a belt if you do have that rare belt failure? I can see pulling out the axles of the end wheels to wrap a belt around the pulley relatively easy, but the axle of your center wheel is also the output shaft of your transmission–seems very time consuming to change if you’re in a playoff situation.

I think I too would go with 2 belts on each side–you would guarantee a solid 180-degree wrap on each pulley, plus the redundancy of multiple belts.

Good point on the drive axle.

A timing belt can handle any high torque job if it is sized properly. They power Harley motorcycles for Pete’s sake. Just a matter of looking up the load rating of the belt, and estimating the load in your system. But these ratings only apply with sufficient wrap on the pulleys, which is also part of the belt specs. Keep in mind that your motor/transmission can only exert a certain amount of torque before it stalls, and the the wheels will only see a certain amount of torque before they slip on carpet. If your belt can handle these torques, its good.

Another example of the use of a timing belt is in many cars. I have had to replace the timing belt in my Volkswagen GTI before, but that was entirely due to fatigue as opposed to straight force.


There are some “minimum wrap” recommendations on drive belts (and chain) that you should look up. I believe they are listed on the Gates web site.

As others have suggested, two belts per side with a 180 degree wrap gives excellent coverage, but it does require extra width in the design.