Having trouble understanding gear ratio problem

Last year our team used an 8.45:1 gear ratio with the intention of being fast, and we were having troubles. During competition either one side or both sides of our drive train would stop moving and then come back online in about 15-20 seconds. Working with the technicians at competition, we came to the conclusion that the gear ratio we used was too low for our robot and it was overloading the fuse, causing it to reset. We would like to still be able to move quickly, but not cause any complications on the robot.

One solution we thought of was trying to use a WCP dual speed gearbox. What would be the best dual speed gearbox to use? Also, what spread would you recommend?

If not the dual speed gearbox, are there any other gearboxes/ratios you would recommend? All feedback is appreciated.

The problem is incomplete…

What motors (number and type) and wheels (size and type) did you have in the drivetrain? It could be as simple as adding another motor, or changing your wheel size.

For example, if you were running 4" wheels, you could bump up to 6" or 8" wheels and adjust your gearing down. If you’re running 2 motors per side, and your gearbox allows it, you could add a motor.

We used tank drive with three 6 inch blue nitrile plaction wheels with a 2 CIM motor gearbox on each side.

In that case, I’d look into the math on using 8" wheels, and what kind of ratio you’d end up with for the speed you want.

You’re familiar with at least some of the various calculators for that sort of thing that are around, yes?

We tried using the JVN calculator, but it suggests even smaller ratios than the one used last year. To our knowledge as of now after working with technicians at competition, the small gear ratio was the issue.

Please confirm that you built a drop-center chassis.
That way your three wheels are all in the same plane!

Did you implement a current limit?
If so, what value did you choose?

Do you know what your robot (playing) weight was at competition?

I’d recommend using the JVN calculator spreadsheet to just play with your options a little bit. With your current described set-up you’re geared for about 16 ft/s free speed - a little on the fast side, but others have run this in the past too without trouble.

If you dropped to 4" wheels that could be the easiest fix and you will still be moving at a decent speed, maybe a little slower than you’d like though -

Some other questions -

  • How heavy is your robot with bumpers and battery?

  • When you turn your drive wheels by hand are they hard to turn, or is one side harder to turn than the other?

  • Which gearboxes are you using to drive the robot? Kit of parts toughboxes or something else?

  • When was the last time you checked your batteries? Do you have a Battery Beak available to make sure your batteries aren’t the issue? Old batteries can mask themselves like a lot of other mechanical / electrical problems. If you can, take a look at your log files from a match where you had issues and post one of those here as well so we can help see a bigger picture.

  1. We did build a drop center chassis
  2. I don’t believe we had a current limit
  3. We were around 150 at playing weight
  1. Around 150
  2. They are equally relatively easy to turn by hand
  3. KOP tough boxes
  4. The batteries aren’t the issue, we do have a beak

You probably want one, or a voltage ramp, or both. You’re trying to pull 100A through a 40A breaker when you start up/push.

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Next thing I’d do is replace those snap action breakers in your power distribution panel for the drive side that’s failing. Put brand new ones in just to rule those out. I’ve seen ones that progressively fail easier every time and just wear out.

A screen capture of a log file of one of your matches where this happened would tell us a lot too, whenever you can get to your driver station laptop.

And yes on the programming side some kind of voltage ramp should help. You can manually simulate this by easing up / down on the joystick instead of slamming from 0 to 100. Drive around for 3-4 minutes and then repeat with aggressive driving and see if the problem pops up.


Thanks for the feedback!

This is definitely the likely answer, but it helps to work backward from the symptom to check all possible root causes. As an example:

Key Symptom: individual circuit breakers tripping.

  • Potential Causes:
    • Faulty (internally broken) circuit breakers
    • Too much current draw
      • Potential Causes:
        • Motors are faulty (internally broken)
        • Motors have too much load
          • Potential Causes:
            • Excessive friction in gearing mechanisms
            • Gear ratio from motor to ground too small
              • Potential Causes:
                • Gearbox chosen ratio too small
                • Gearbox-to-wheels sprocket ratio too small
                • Wheel diameter too big
            • Excessive friction against ground
              • Potential Causes:
                • Center drop not “dropped” enough

Note that the current limit solution addresses the symptom partway through the chain, but not at the root cause. Sometimes it make sense to solve a problem this way, sometimes it does not. The advice I generally give is to solve the problem as “deep” as possible - address as close to root cause as you can. Only put a “bandaid” further up the chain when it’s not practical to fix root cause.

For clarity: In this context, “practical” to me means “can you easily swap out the gear ratio?”. If you were at competition, with a sharp deadline, and with no spare parts available, the answer might be “no”. Therefore, go address the symptom with a software-defined current limit. However, if you’re at the shop and can wait for parts shipment, address it with a change in gear ratio.


@Jame Your driver station logs should pretty easily help to diagnose this issue as well. Can you pull the driver logs and/or post a screenshot of them? We’re looking for the PDP slots to be turned on and the image to be ‘match length’ of one of the matches you saw the issue.


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