Hi Vex Forum,
My team is designing a mecanum drive for the first time this year, and I was wondering if we could have some tips on how to design it. I know that we should be very careful with our geometry, and that it is a bit of a programming challenge, but other than that, not much.
On another note, coming from FRC, I’m excited to be back in VRC and are ready for this challenge!
Thanks!
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- Don’t use 2" mecanum wheels, they have less than 0.1" of clearance between the plastic hub and the rollers. Use the 4" ones.
- VEX V5 mecanum wheels are pretty inefficient; expect there to be issues the first time you use them.
- There are two variations of the mecanum wheel: left and right. However, the left mecanum wheels do inherently not go on the left and the right mecanum wheels do not inherently go on the right. Instead, you place two LEFT mecanum wheels on the front left and back right, and two RIGHT mecanum wheels on the front right and back left.
- Mecanum bases and x-drives have interchangeable code, so if you ever decide to switch you can reuse your old code.
- Mecanum bases strafe slower than they go forward (somewhere around 80% of the forward speed when strafing)
- When putting mecanum wheels on your base, you want your entire base to be around 32 holes wide or greater depending how big the game elements are each year. I personally utilize a 35 hole wide base when using mecanum wheels. The space in between each drive channel (the two c-channels that the wheel axles travel through) is 5 or 6 holes if I remember correctly. If you want to gear the wheels you will have to add another hole width-wise to what you already have.
- From what I’ve tested, 4" mecanum wheels struggle to strafe when they are at speeds at or above 280 RPM or if the center of gravity is not approximately centered, although I’m not entirely sure if the speed fact is completely true. It could have just been that I built the fast mecanum base really poorly.
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And to add on to that,
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when going diagonal, I would suggest locking/ making the wheels move at like 1% because otherwise the robot will go more forward than diagonal
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for the best results, make sure the base is as square as possible
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Yes the speed factor is true. Especially with mechanum wheels and four motors, there is a delicate balance between torque and acceleration. If you go above 280 rpm on a four motor drive, it will have (in theory) a higher top speed, but the drive will not have enough torque to accelerate the robot quickly, especially with mechanum wheels and strafing with them. That is why a 267 rpm mechanum drive would be an ideal balance between torque and acceleration. Now if you want to go faster than this, you could do 300-350 rpm with six motors, but you couldn’t do mechanum drive. Also you definitely want to have a very central center of mass and center of gravity, otherwise your strafing could pulse and be very inconsistent.
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An easy way to visualize the correct placement is having the rollers all pointing to the center of the robot
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This is when looking at the robot from the TOP.
Another way of looking at it is will the robot spin in place without the motors turning? If so, the wheels need to be swapped right vs left.
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