Traction Limited Drivetrain

I was wondering how teams have built traction limited drivetrains, especially with 775s. Do most teams just use wheels with less friction that slip on the carpet? If so, what wheels? Or can you use flat belts that slip on the pulleys?


Traction-limiting is generally accomplished by using a lower gear ratio so that the robot is slower, but the output torque is higher. I would not recommend lower-traction wheels or slipping belts, as they would cause your robot to be more prone to be pushed around on the playing field. If you’re looking into 775Pro drives, there’s a good chance that you’re going to end up wanting a power-limited “speed” gearing and a traction-limited “pushing” gearing.

In general, assuming standard FIRST-ish wheel COF (i.e. ~1), it is not possible for a 150lb robot to be simultaneously traction-limited and geared for a high top speed.

The transition point from traction-limited to motor-limited, in my experience, generally occurs somewhere between ~10-14 FPS (the actual value will vary depending on the specifics of your drive). It’s worth noting that this transition is not a “smooth” one - because of the difference between static and dynamic wheel COF, there’s quite a huge difference (in terms of brownout/circuit breaker issues) between having not quite enough torque to slip the wheels, and having just enough to do so.

It’s probably a bad idea to switch to low-COF wheels for the express purpose of becoming traction limited, as this, well, compromises your robot’s traction. It is never a good idea to intentionally cause your drive belts to slip. As Whippet mentioned, the general solution is to either gear lower, or to use a two-speed gearbox. There are also software solutions, such as current limiting, that can help protect against the same problems.

I don’t mean to be discouraging, but if you’re asking questions like this, I think I can safely say that you are probably not ready to experiment with a 775Pro drive, and should stick to CIMs. Keep your approach simple until you’ve mastered the fundamentals - once you know the ins and outs of an ordinary CIM-driven tank drive, then you can make an informed decision about whether to experiment with a more ambitious (and much more complicated/potentially failure-prone) setup.

I’d rather spin my wheels than pop my breaker and if you don’t want to commit to the weight / complexity of shifting this isn’t a bad solution. Going with smooth wheels like colsons instead of wheels with more a tread that interacts with the carpet can help to adjust where your traction limit is. 33 ran slicker (and fewer) wheels so they could have a traction limited 15 fps drive this year.

With 775s it’s more complicated than just being traction limited though. You can search the plethora of threads on it - short answer is that I don’t recommend it.

I wouldn’t really call Colsons “low-COF wheels.” They’re not super-high-traction in the same way W-tread or Plaction wheels are, perhaps, but they’re plenty grip-y.

Well, I wouldn’t call them low-COF either, but when it comes to typical COTs options they’re on the lower end of the spectrum - especially compared to Ws and plactions. Not sure what you’d consider low-COF (mecanum or omni maybe?), but I wouldn’t really consider going with those just to get them to slip more easily.

The point was you could gear slightly higher if you run a less grippy wheel - not much higher, but it could be the difference between being traction limited at 13 or 14 fps which can make a difference in gearing choice. The better solution is to current limit in code to not pop your breaker.

Our team is a veteran team and have been quite successful with our 2 speed, 4 CIM drivetrain in the past couple of years. Reading many chief threads on 775 drivetrains and seeing that having 6 775s would have a smaller form factor and higher output than 4 CIMs, some members of my team have been considering working on a transmission over the summer.

This year, like many other teams such as 254, used Talon SRXs to current limit our floor gear pick up mechanism. I understand that this is a potential method to avoid burning the 775s; however, I am afraid this would be “unnatural” on the driver for the power to suddenly be cutoff. I am trying to understand hardware solutions for avoiding browning out and burning the 775s.

If you’re on a veteran team that’s had success with a 2-speed drivetrain*, chances are you probably have a mentor/older student who can help you with things like wheel/reduction selection and traction limiting. As great as CD is, it’s always betterto have things explained in person.

As far as 775pro drive trains, there have been a number of threads in the topic since the season ended. Use the search feature or Google to read through those, then if you have more questions you can come back here to ask them. Beat of luck!

*This is one reason why it’s helpful to us for you to put your team number in your profile. It helps the community know whether we should be guiding rookies to build within their means or pushing veteran teams to expand their horizons.

This is 100% the best way to avoid brownout/breaker issues, and is easy to do with Talons (I don’t know why anyone would use any other controller for their drive motors at this point, tbh).

If your description of your team is accurate, then there are almost certainly people on your team with more knowledge on the subject who would could explain to you the fundamentals of traction-limiting (and why being traction-limited alone probably isn’t great insurance for keeping 775pros healthy on a drive), and who would also be better-posed to ask for/receive/process meaningful advice on how to implement a 775pro drive.

Again, I don’t mean to be discouraging or insulting - but your original post doesn’t come across as one written by someone who’s familiar enough with FRC drives to be seriously considering designing a 775pro drive.

We ran 8, 2.5" Colsons powered by 4 RS775s. Geared for 15 fps, parked against a wall, it more or less couldn’t spin the tires (no surprise here). The one interesting thing that if one of the wheels got ever so slightly light (robot tips up a bit) then we could get that wheel to spin, more like a slip.

Another interesting tidbit. Under max acceleration, the front wheels draw less current than the back wheels. Also higher encoder counts.

Has anyone tried siping a Colson?

Makes perfect sense, considering the center of mass of most robots is almost certainly higher than the center of the wheels.

You’re correct. It was kinda cool to record it. We weren’t expecting it since we were doing an experiment on how much current limit effected maneuverability. Gave us a new appreciation for said phenomena.

We found implementing current-limiting on the drivetrain through the TalonSRX didn’t negatively impact driver feel this year.

We were also only clipping from ~60A stall down to 40A or 45A, with ~20fps 4-cim drive on a 70lb robot. YMMV with a full weight robot. YMMV if you try to engage in pushing anything around.

Actually, the CoM/CoG only has to be higher than the contact patches of the wheels to experience an increase in contact force on the wheels you’re accelerating away from.

Also, OP, note that traction limiting a 775pro drive train is not enough to keep from smoking the motors. If you run it at peak power (around 40-50A for most of the period), it will die well before the match is over, and long before it’s going to trip a 40A breaker. See the Vex test data.

You sure about this? I’m imagining the free-body diagram and given that the force is transmitted to the body of the robot through the wheel axles, I would expect the direction of the resulting torque to depend on which side of that plane the COM is on.

Perhaps I should get a pencil and paper.

Edit: Indeed, this is true - the action of the wheels on the robot can’t be thought of purely as a force acting on the wheel axles in the forward direction, they exert a torque as well.

We’ve done that. It was easier than we expected. Helps to make a jig, to keep the sipe spacing consistent.

I recall seeing siped Colsons on a 25 robot several years ago. They used different siping patterns to adjust the CoF so that it is different in the tangential (push) and axial (scrub) directions, and different for center vs. corner wheels in a 6WD.

Did you feel it was worth while? We like using the Colsons.

696 siped their Colsons this year using their 4-axis mill. They came out looking very nice and (according to them) they had an almost 40% increase in traction. Discussion here.

1923 also used siped Colsons on their comp bot this year that they cut on a bandsaw. They said they observed a noticeable (~15%) improvement in traction over their non-siped practice bot. Discussion here.