Tank Drive Inconsistency

We’re having a strange issue with our prototype tank drive where the right side motor set goes slower than the left side but only when going forward. Any ideas why?

Friction? A way to investigate is to interchange left and right wiring.

If the problem is unchanged, it means … ?
If the problem moves to the other side, it means …?

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Which gearboxes and motors are you using? Do you have a picture of the drive?

Make sure each side of wheels turn manually by hand in a similar way when the robot is unpowered. Otherwise it probably is a issue assembling gearboxes or otherwise mechanical.

I’ve seen gearboxes that were assembled with the bosses backwards, and also ones where the press-on rings were somehow too pressed on causing issues before, for example.

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To add to the list:

Are both motors new? Motors will perform slightly differently based on usage and direction of rotation.

If you pot the robot up on blocks, put a mark on the left and right wheels and video it spinning in place for 15seconds (to calculate rpm) is there an appreciable difference between them?

Lastly, is all the visible chain/belt tension the same between the two sides?

Do you believe this is a programming or mechanical issue?

My team had a similar problem last year that we found to be due to a loose wire. Not sure if that is your problem but if you can’t find a solution it might be something to check.

It would definitely be helpful if you would tell us what hardware and software you’re using.

In any case, the first thing to check is hardware. Put the robot on blocks, power off, and turn the wheels by hand. Is one side resisting more than the other? If so, track that down. There are many possible problems, but the #1 cause in my experience is a misassembled gearbox. Many FRC gearboxes feature gears with bosses, which is to say a standoff built into the gear. These bosses must face the bearings. More generally, look at whether anything is flat against a bearing somewhere.

If hardware seems OK, check out your controls. Unplug all but one of your motors, and test to see what happens. Cycle through each motor.

If you’ve done all that and not found the problem, post as many details as you can, and preferably some video of what’s going on.

This solution is a little bit out there, but it happened to us …

On one side of our drive train, we had installed 13T pinions onto our CIMs. On the other side were 14T pinions. We had no idea at the time, they looked pretty much identical, we only discovered the mistake after a full (virtual) season of frustration, when we disassembled to prep for 2022.


Thanks to everyone here! We managed to solve the issue by replacing both CIM’s on that side. Really thanks!

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This reminds of when we took apart our gearbox after the season and found that one of the two motors didn’t have a key in the shaft. :frowning: The drivers told us something wasn’t right, but we just blamed the software.

Follow up on this! Don’t just put those motors back in the parts box - track down which one is the problem and resolve it, or take both of them permanently out of service (that is, chuck 'em). You certainly don’t want to swap motors during an event and find out you installed what should have been a known lemon.

We found out one motor with bad brushes and marked it as so


Did you check the pinion gears? If they do not match the pinion gears on the other side, there may have been nothing wrong with the motors. Have you tested the motors to see if one or both are bad? Any bad motors should go in the trash immediately to prevent tripping up someone later.

Wouldn’t a different-sized pinion have cause equal (or at least similar) issues both in forward and reverse?

One would think so, but that’s not what we found. One direction was much more pronounced than the other (I don’t recall which).

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