Problems with Vex Pro mecanum wheels

This year we chose to use the vex pro mecanum wheels (http://www.vexrobotics.com/vexpro/wheels-and-hubs/mecanum-wheels.html), but are disappointed with their performance. They travel forward backwards and rotate alright, but they strife VERY slowly. I understand moving sideways is slower, but this is moving maybe 40% of forwards speed. They are mounted properly (“X” on top) and are run by one CIM motor each, with a 9.87:1 gear ratio.

Does anyone know what might be the problem? On a side note, our practice robot has “homemade” mecanum wheels from a previous year, and it works well, so I assume the code is fine.

Are the rollers completely free to rotate, or are they a little “tight”? The roller friction is the big variable in mecanum wheel performance.

They are loose.

We used them this year and were very satisfied. Of course it will be slower sideways, but I don’t recall exactly how slow it is for us. (We also used 1 cim to each wheel, but I don’t remember the gear ratio)

The robot with our homemade wheels is also certainly noticeably slower sideways vs. forwards/backwards, but as slow as the vex wheels. I can’t remember how the andymark wheels performed exactly, but I do recall they were better than these vex wheels.

^This is the kind of speed I am hoping for…

While moving forward/back, your torque is the same as a normal wheel.

While strafing, your sideways torque is 1/SQRT(2) (~70%) of your normal torque since the wheels are driving at each other BEFORE the extra losses of friction in the rollers.

While on a diagonal, your acceleration is somewhere between 35-50% of your normal forwards acceleration since you’re only spinning half of the motors and also half of your your robot is (essentially) on caster wheels.

Translating torque into acceleration has a lot to do with roller friction and robot weight. Additionally, unless there are specific dead zones for sideways & forward, the code is robbing the robot of torque needed to strafe at reasonable speeds (since no joystick is ever perfectly sideways for long).

Our robot did not strafe as fast as the robot in that video, however wheel speeds will be different as well. We used the AndyMark wheels for a while before switching to the VexPro wheels. (Funny story, we changed them out Friday night at a competition, some of our other team members thought we were crazy… rightly so.) We like the VexPro wheels more than the AndyMark wheels because they seem to run smoother and are more solid. (Note: the AM wheels we have had already seen a fair amount of use, new wheels may act different.) As for speed I think the VexPro ones seemed a bit faster, but I’m only saying this from recollection, not fact.

What torque are you referring to in this context?

Torque available for acceleration.

*It’s the reaction force of the floor on the wheel which causes acceleration of the vehicle.

In the ideal case of no roller friction or axial free play, and a non-compliant floor, the acceleration force in the strafing direction is the same as the accelerating force in the forward direction.

The affordable mecanum wheels used for FRC do not have thrust ball bearings on the rollers. The rollers have axial free play. The rollers have spinning friction (especially when loaded). The carpet is compliant. That’s what makes the vehicle go slower in the strafing direction.

What do you mean by ‘compliant’?

Ok that makes sense. We did dedue it had something to do with the spinning wheels.

Any recommendations to make it better?

Definition 3.

Glue the rollers to their axles and forget about strafing.

Or this? http://www.chiefdelphi.com/media/photos/36998

Or this? http://www.chiefdelphi.com/media/photos/36998

LOL:D… Maybe we will try switching the wheels on Wednesday back to the Andymark one? We have to take the transmissions off to swap gears anyways.

I don’t think he’s joking around. I see no reason why a standard traction tank drive wouldn’t be more advantageous for you to use instead of mecanums.

Let the battle begin!

Make sure to check every roller, last year we had a strafing issue and it was because one roller was stuck. Our issue looked more like an intermittent speed issue is strafing.

I would also check if each wheel can rotate forward and backwards at full speed with the robot off the ground. Hopefully you have the encoders hooked up to each gear box to see the counts per second. At least this can sort out programing versus mechanical issues.

Just to be clear how loose our rollers are on our 2012 robot, the rollers will spin a couple time after flicking them.

That’s fine, but we could just put real traction wheels on if we are taking the transmissions of anyways.