Legality of Replacing Motor Cartridge with Rubber Bands

I’ve been noticing in some robot reveals that they’re not actually using gears to spin their flywheels. Instead, they replace the cartridge with a spacer containing rubber bands which grips the inside gear. For a better explanation, watch this video. Do you think that using the legal vex parts makes this legal, or is it tampering with the motor?

Please review this thread: Legality of having a motor like this

or tl;dr: Follow up to motor cartridge q&a : Robot Events


Mom I’m famous! ⠀ ⠀ ⠀

This is completely legal because it adheres to the q&a requirements, which state that the motor modification must be made with vex legal parts (which these are).


by the way ben bless your heart we were struggling to figure it out and your explanation video was very :ok_hand:


No problem! That’s why we make them.

But beware, the GDC will not consider this a “normal motor use” !

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This has been the central issue to the “direct drive cartridge (DDC)” issue, the subtext has to do with VEX Robotics awesome return / replace policy. VEX stands behind their products because they know their engineering is sound and they go to great pains in the manufacturing process. So if it breaks they replace it. (Which is why they are also a fan favorite of the First FRC world)

The chances of breaking a motor with a DDC is pretty decent. And VEX doesn’t want to be on the hook to replace the motors (I’ll guess that the cost is for 2 motors and the labor / tracking / shipping puts it in the $200 range for each motor) for a large number of teams.

So that prompts the “normal motor use” disclaimer in the rules.


Vex stands behind their products because they don’t wanna get sued. Despite their “great pains” they didn’t think to add a diode to prevent ESD.

There is literally no evidence that the chances of breaking a motor with a 3600 RPM adapter is any higher than the chances of breaking a motor with a cartridge.

The cost for 2 motors + shipping is about $115. You can tell because it costs about $115 to order 2 motors.


My team lost a motor as they attempted various configurations for a “DDC”. I suspect small bits of their friction materials got into areas of the motor where they weren’t expected.



I’m not going to dive into the ESD diode engineering again. “Gonna get sued?”

As far as the RMA cost is you are missing is the rest of the RMA process. They need to log the RMA in, handle it in receiving, ship out another motor and incur the shipping for that (you paid for the first and they pay for the second). I’m sure there are also internal processes to figure out why this part failed, what batch it was from to see if there is a pattern of defects from a batch that they need to worry about. The whole Quality Assurance cycle stuff that you’ll learn about.

I’ll bet you a fried pie that the original engineering was based on having a cartridge to support the motor gear (that tiny white gear deep inside the motor). We all know that that the cartridge supports that gear pretty well ( sun gears and all that tight tolerance engineering that is in those cartridges)

We also know from decades of building robots and mechanical things that cantilevered loads off of a motor is a bad thing. We also know that a tiny amount of offset can create wobble and at high speeds (say 3600 RPM) the wobble can create friction and heat.

I’ve seen the current direct drive ideas and I’ll guess they are not that straight, so there will be wobble, wobble that is currently not in the VEX designed and built cartridges. All of that is introducing a new failure point opportunity.

Remember, the only time wobble is fun is from the 1970s advertising slogan said, “Weebles wobble, but they don’t fall down.” (Many mentors are now laughing and most of the roboteers are going “What is a Weeble”)