Drivetrain Speeds

I’d be curious to hear the results of other team’s experimentation with NEOs. We swapped from 4 big CIMs to 6 NEOs on our 2019 robot for IRI and were popping wheelies like nobody’s business. We ran 8" pneumatic wheels this year which probably didn’t help the situation and went to a 12.75:1 ratio on our drive train gearbox. We tried to program our way around the issue with some success. It really came down to our driver having to be more careful about throttling up and down, especially when playing defense so we didn’t end up on top of other robots.

Upon inspection this sheet looks a lot more in depth than what we used last year but overall founded on the research we did that led us down this path to begin with.

Thank you both for the number crunching and time it took for this. @AriMB @JesseK

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The truly optimal driving speed for most teams is the drivetrain that the team can put together the speediest.

For lots of teams, that is the kitbot in default configuration or buying a COTS gearbox.

The time some teams spend debating gear ratio optimization could go into drive practice and that drive practice helps teams go a lot faster on the field (pieces scored per match, thus scoring more points).

In short, more drive practice will go a lot further to improve your on-field performance than gear ratio optimization for certain sprint distances.

Use the kitbot


And remember that there are multiple gear ratios for the toughbox that allow you to use different sizes of wheels and not be super slow or too fast. It’s a fantastic gearbox, but I feel like sometimes teams don’t realize they have options to buy different ratios for it.


There is a lot of hype on this thread about brushless motors. Don’t get too caught up in it. They offer a few more percent efficiency, and lighter weight, but they aren’t going to miraculously change your drivetrain performance*

Instead, choosing your drivetrain speed is always a compromise between top speed and low-speed torque. It may seem counter-intuitive, but higher gear ratios (lower “free speed”) will actually give you faster sprints over short distances because you get more torque at low speed. The problem with a high-ratio gearbox is that it runs down the side of the motor torque curve more quickly, so you run out of torque at high speed, which ultimately is why it has a lower “free speed”.

To put the concept into actual numbers we have to start talking about voltage drop, current limiting, mass of the robot, traction limiting and efficiency (friction) of the drive train. Luckily there are several simulators (not JVN calc) on CD that can take this all into account for you. Below is a plot that we made with our own simulator while designing the 2018 robot. You can see that:

  • the Andymark Kitbot with 6" wheels keeps up with every other option for the first ~2s (0.5m), but then starts to slow down.
  • the EVO Slim single speeds, which are geared a little faster, start out slower than the kitbot, but eventually overtake it after ~2m
  • the 2017 geartrain (which was a 2 speed ballshifter is the first to reach all distances, but only if we assume it shifts perfectly at 0.25s

Shifters help avoid the compromise. We ran shifters this year because we wanted to be traction limited at the low end (to push against defense), and also as fast as possible in longer sprints. But shifters have their own issues. They need a reliable pneumatic system, and the shift time itself can impose an acceleration and controllability penalty (our robot would sometimes kick to the side as one side shifted a fraction of a second before the other). Lots of top-level teams are choosing single speed drive shafts these days for those reasons.

*an exception is brake mode. Brushless motors have way brakier brake mode.

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Did you run a NEO drive this year?

The hype isn’t about a generic brushless motor versus an equivalent brushed motor - it’s about the specific experiences FRC teams are having with these motors compared to CIMs.

+1^. Brendan is correct on the generalities, and Chris on the specifics. NEOs are better than CIMs for efficiency, size, and weight because they are brushless. They also perform better than CIMs in FRC drive trains because they offer more low-end torque at about the same free speed.

Look here.

And here.

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Yep, we switched mid build season from 3 mini-cims to 2 neos, primarily for weight reasons. And yes, the Neos offer a bit more peak power (maybe 10%, per @Richard_Wallace’s links). But on the whole, 10% isn’t going to win you more matches is all I’m saying.

(but we’ll still keep using them because, hey, 10%).

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A few % at top speed and at stall. More like 20% at 1000 RPM. That can make a big difference in quickness. Ask my team’s driver, or your team’s driver, or any NEO team’s driver. They can feel it.

You can SEE it. If you are used to the way a 3 minicim drive train accelerates, and you watch a neo drivetrain geared the same accelerate, the quickness difference is substantial. I got to see this on several practice versus comp bots and it was startling.

Tom and Richard are talking about what I was alluding to. It’s hard to deny what’s happening in front of you, even if it’s not immediately apparent from a quick glance at the stall / free speed numbers.

We also came back after matches with luke-warm motors after playing defense all match. Before our CIMs would be hot. On top of all the other things we saw - robot turned better, accelerated quicker and could push anyone. I don’t see how we’ll ever go back.

Not having used NEO’s this past year but we are beginning to explore them.
I am wondering why people went from 6 miniCIM to 4 NEO - vs 6 NEOs?

is it thought that more than 4 gives no more advantage? (Since you were already using 6 pdp ports)

Apparently 2468 (in another post) felt 6 were great - Would using 6 give you the low end power to eliminate need for 2 speed? is the choice between 6 pdp ports or adding a 2 speed transmission?

A MiniCIM is ~230W, a NEO is ~400W, so 4 NEO is more power than 6 MiniCIM.

We did some testing and found that the difference in acceleration and time to distance between 4 and 6 NEO is negligible for a full weight robot, since you’re either traction limited or voltage limited (as you draw more current, the voltage sags more, so you can’t get much more power out of the battery).

How about low end pushing power? and with a ~16 fps free speed?
Really appreciate the quick and databased response! As we have run swerve for most years an answer that says 4 is enough is convenient - yes I know we could (and you did) run 6 drive on swerve but that is a lot of pdp ports.

I also appreciate theat swerve is more about manueverabilty and agility than pushing - but exploring.

Did you gear faster when you went from 4 to 6 NEO? I haven’t tried it, but I think the best ratio for minimizing time to ~15 ft. would be about 30% less for 6 NEO than for 4. That is, if your robot is traction limited with 4 NEO at a certain ratio, it will still be traction limited at 30% smaller ratio with 6 NEO, and it will be quicker.

I think the same can be said of the difference between 6 MiniCIM and 4 NEO.

If we get a flat field in 2020, I expect to see some 6 NEO drivetrains with 3.5" wheels, and some 8 NEO drivetrains (many of them swerve) with 3" wheels. All single reduction with reasonably small ratios, somewhere between 3.5 and 4.

We are moving into an era of zippy FRC drivetrains. Humans, keep off the field!

When you say 8 neo drivetrains - I assume you are thinking differential swerve!

Also interested in the current implications of motor counts - 6 vs 4 Neos still may help in brownout and or thermal management of NEOs?

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There’s going to be some help, sure, but it’s a matter of diminishing returns. The Spark Max/NEO is already current limited to about 100A peak, which means that brownouts with four NEOs is less likely than with four CIM or mini-CIMs. As for thermal management, NEOs are so much more efficient than CIMs or miniCIMs, and (when mounted against an aluminum motor plate) so much better at dissipating that heat, that thermal overload is also less of an issue to begin with, so there’s less to be gained.

Unless the general caliber of drivers increases or a game favors short heavy robots or [added in response to @GraveFurball’s comment] a team spends time in shoving matches, I expect that the great majority of teams will likely find that four NEOs provide all the drivetrain power they can handle.

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We had a 4 neo drive