Drive Motor RPM cartridges discussion

A discussions about motor cartridges that was started in this thread: Personal Vex Team Quotes - #2199 by 1422_WI

Would it be better to gear up 200rpm or gear down 600 rpm?

Advantages and Disadvantages ?

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  • 600 RPM Direct
  • 200 RPM Direct
  • 100 RPM Direct
  • 600 RPM Geared Down
  • 200 RPM Geared Down
  • 100 RPM Geared Down
  • 600 RPM Geared Up
  • 200 RPM Geared Up
  • 100 RPM Geared Up

0 voters

so having a higher output (600 rpm) would be more precise and would be better for autonomous in theory, but seeing as how it comes out of exactly the same motor device with the same amount of electricity input, wouldn’t that just over complicate the system and maybe add unnecessary friction?

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I am a bit confused by this notion. I understand that it has something to do with slop but don’t really understand.

As for autonomous you can simply use tracking wheels in order to get accurate position tracking.

Using tracking wheels does not change how accurate small movements of the motors are, it just accounts for slip in the wheels or if the wheels arent contacting the ground for an extended period of time.

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The 600 rpm cartridge has one planetary stage, that has a 1:6 ratio (3600/6=600) while the 200 rpm cart has two stages, 1:6 and 1:3 (3600/6 /3=200). Thus it’s more direct to gear down from 600 rather than gear up from 200


3.25" wheels have a circumference of 10.21". For each 1 degree of rotation (without external gearing), you get 0.028 inches of linear movement. For 4" wheels, each 1 degree of rotation will get you 0.035 inches of linear movement. For reference, the legal thickness of polycarbonate for Vex is 0.0625 inches. I’m not sure that any autons require that level of precision for straight-line driving.

Turning may be more sensitive to small changes (e.g. if you over- or under-turn and then drive a long distance you’ll experience greater end-position variance) would be better. However, there are better ways to solve this particular problem than motor cartridge selection.


To try to explain slop:
Slop is when a motor turns 5 degrees, but 4 of the 5 degrees are used to tension a chain or move an axle in a lock bearing to a point where it has to turn the wheel/sprocket. The slop is wasted motion. If a chain and sprocket system has 30 chain links too much, the motor must move those 30 chain links before the receiving sprocket is given motion. If the motor moved a distance of 35 links, the receiving sprocket moved a distance of 5 links.
Output distance = Input distance - slop
5 = 35-30
Realistically, you won’t have more than one link of slop in your chain, but the axle bearings have a little bit of motion freedom from not having locked contact. Using gears eliminates chain slop, and screwing your gears to your wheels eliminates axle slop.

A 12t sprocket being fed 600 rpm would have less slop than a 36t sprocket being fed 200 rpm because the 12t would lose a tooth distance of about 1/3, whereas a 36t might lose a tooth distance of 1.

Hope that helps.


Jess is replying, we’ll see if she has any wisdom on this

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I don’t think cartridge matters too much, just make sure the speed is something you can control. There is plenty of things you can do to mitigate slop outside of cartridge that will affect slop more (like not using chain, bolting gears to wheels, threading axels lock gears to shafts, square inserts in your motors, etc).

For deciding ratios, I’ve been using @Chris_3553B gear ratio sheet.



once I built a 25x25 bot with 600rpm 4m direct omni drive
that was fun to send down the halls

… you’re kidding, right?

Generally, it is a good idea to stick with green motors. Unless you need to do a quick task like making a ring intake then use the blue motors. While speed is something you want, too much of it is bad for drivetrains as it makes your bot weaker compared to other bots so you can get pushed around. Also having too much speed can cause certain parts to come loose and make the bot harder to control. So my take on this discussion is that you want to throttle down on motors to say 400 500 rpm in between that range unless it’s absolutely dire. You generally stick with green motors and it is best to throttle them up but the blue ones are so fast that throttling them down is the best choice.

I learned all this when I built a Kiwi drive with 600 rpm motors and drove it through the halls of my school needless to say it was fun but in the end things were falling apart.

The blue ones also have less torque i think so a heavy load will put a lot of stress on the motors
I always use green direct or green geared up because there’s more torque for things like carrying mogos up a platform

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imo blue > green since theres less slop and slightly lighter. Using blue cartridges generally means you’ll have to gear up, which is also good because slop is reduced. Gearing down 200 for extra speed also means increased slop, which can reduce autonomous precision.


I thought gearing up made it go faster and have more slop

It depends, people use the terms interchangeably.

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motor cartridge doesn’t really matter, what matters is the ratio and wheel size.

200 rpm cartridges on a 7:5 ratio on 4" wheels has a very similar amount of speed and torque as 600 rpm cartridges on a 3:5 ratio, using 3.25" wheels.

The differences in cartridges are very minimal, and likely won’t effect your overall performance in a noticeable way. Of course, you need your cartridge to work for your particular gear ratio, using direct drive 200 rpm might work but direct drive 600 rpm is not going to be particularly usable.

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*for most teams and for most games, and definitely not on anything other than 2.75" wheels.