4-cim vs. 6-mini-cim drivetrains

inb4 someone brings up 6-8x 775pros.

But hey, who likes saving weight anyways.

And if/when they burn out, that’s even more weight saved! Gotta consider the mass of the magic smoke :wink:

What about the mass gain from lifting a heavier robot?


I must have slipped a digit or something when I did the calculation a few years ago; I got something about a tenth of that. Yes, that’s significant.

It would be interesting to test the CIM with the max power test but with the target speed 4664 rpm, where the power output curve crosses 215W on the fast side.

Dang it Paul. I thought we were done innovating in the drivetrain gearbox. It’s pretty hard for us to refute the logic of a 6 Mini CIM drivetrain, especially since we see so much heat fade. Sounds like a great off-off-season project to get the students ready for the season.


I made a calculator error in my calculations below. Ari Meles-Braverman pointed it out to me in a PM. The pressure for the PV calculation is actually 222 psi, not 55.56 psi. see the bolded items in my quote below for the corrections.

For us viewers at home, what would the math be if the CIM uses a bearing instead?

Paul can correct me if I’m mistaken somewhere, but the equivalent CoF of normal ball bearings* is usually somewhere between 0.001 and 0.0015 (compared to 0.1 for the bushing). So the bearing friction force would be between 0.05% and 0.075% of the CIM torque at max power, compared to 5% for the bushing.

Those numbers increase when you take into account the fact that the bearings are actually being used on a MiniCIM, which has a lower torque at max power. Multiplying by the ratio of torques of the two motors, it should work out to about somewhere between 0.086% and 0.129% of the MiniCIM’s torque at max power.

*If the MiniCIM uses roller bearings, they usually have a equivalent CoF of about 0.002, so they would have a friction force of 0.174% of the MiniCIM’s torque at max power. Even then, that’s still much less than the CIM with bushings.

Just to understand where you’re coming from in the bulk of these posts… Are you comparing 1 CIM to 1 miniCIM, or 1 CIM to 1.5 minCIMs.

Are you factoring in some assumed heading as well? Shifting down the peak power over time curve.

If this becomes a priority, and you don’t have a well-worn CIM handy for a dyno test, I’m sure I could convince my team to swap 2 off of our 2017 drive train.

Does anyone have spec data on CIM or miniCIM rotor inertia?

I ran a quick calc using the AndyMark 2013 FRC drivetrain kit and the rotor inertia spec for the 2018 FRC BLDC motor. I assumed 6 motors, a 150 lb robot, and ignored the moment of inertia of the wheels and gears (just wanted to compare rotor inertia to robot mass inertia).

  M = mass of robot in kg

  R = radius of wheel in meters

  G = motor speed divided by wheel speed

  J = rotor inertia Nm/sec2

  N = number of motors

  Jm = inertia of robot mass reflected back to rotor
M: 150 * 0.4535924$
R: 6/2 * 0.0254$
G: 11.53 $ "AndyMark 2013 drivetrain"$
J: 0.0013 * 0.00706155$ "2018 FRC BLDC rotor"$
N: 6$
Jm: (M*R^2)/G^2;

254 has found that swapping in new CIMs for old ones after a couple of events (or a few dozen hours of practice driving) leads to noticeable improvements in acceleration. Can’t say how much of that is due to the bushings vs. the brushes or other factors, but I’d suspect it’s mostly the former.

This is really interesting. Out of curiosity, have you quantified it in any way? E.g. time-to-distance before and after swapping motors? I realize the actual effect will manifest differently with different robots/drivetrain configurations, but I’m wondering if this is something that more teams should be considering/chasing down or if this is more like a few percentage points of efficiency that is only going to be apparent once many other drivetrain factors are optimized.

We haven’t quantified it, but we’ve noticed the same trend.

Older CIMs also start to definitely smell like burning (very scientific).

Ever taken apart a CIM from a well used drivetrain? The brushes and commutator get pretty filthy with carbon buildup. I don’t know how much it affects performance, but apparently RC hobbyists clean their motors regularly to keep them in peak condition. I think for most of our applications, that performance hit is pretty negligible. Optimizing other parts of the robot is usually a higher priority.

If VEX still has their motor testing setup available, it would be interesting to compare a fresh motor to itself after a lot of use. Maybe a CIM degrades faster than a miniCIM due to its bushing, or maybe they degrade at the same rate?

Out of curiosity, how does inertia “propagate” through gear ratios? You define G but it seems to go unused?

Thank you for pointing that out. I will correct it.

My educated guess is that most of the CIM degradation is from the large amount of heat cycles they go through, and the insulation breaking down.
A CIM is only class B winding insulation (130deg C) vs a Mini-CIM with class H (180deg C).
We (971) started doing a basic check of motor resistance to gauge the health of a CIM, I’ll have to ping someone to get those numbers.

Entertainingly I still have access to a Dyno at work that can handle a CIM, but no one is paying me this time :stuck_out_tongue:
(I’ll do it when I get bored, which probably won’t happen for awhile).


Usually when such insulation breaks down, it typically leads to catastrophic failure modes rather than small but observable decreases in performance.

It’s not uncommon to see it break down in the tightly wound corners w/ slightly higher resistance (and the bending potentially weakens the enamel some). This can cause a single winding to short out, just decreasing the amount of copper in play.