# Bumpy Ride with AM's 8" Mecanum wheels?

My team has assembled the 8" mecanum wheels from AM and we are getting a really bumpy ride…

the robot jumps all over the place and nearly shakes itself to pieces.
this happens both in slow speeds and in hugh speeds.

is this a common problem with the 8" wheels?
or maybe we did something wrong?

do the 6" mecanums from AM solve this problem?

thanks!
-Leav

Before you get too worried about it skittering around, try putting more weight on the wheels. I think you’ll also find that feedback-based control of wheel speed can smooth things out a lot.

I thought that the larger diameter wheels, that something has the smoother the ride should be.

Definitely add weight. A light mecanum (around 50 pounds) bounces a bit, but bring it up to 120 and it won’t bounce nearly as much.

Joe: from Royal Jesters, want to chime in on this discussion??

The problem is iinherent with a wheel of this design. The mathematical formula that the individual segments of the wheels, in effect, the optimal shape of the rollers is patented. The optimal shape is one that keeps a portion of the rollers in contact with the carpet at all times. Team 357 can give you the better low down on wheel/roller shape than I.

Thanks for the swift replies!

We added about 100 pounds of bricks to the robot, which helped, but did not alleviate the problem altogether.

i’ll try to get a video up so you can see for yourself how bouncy it is.

Rosiebotboss, do you think/know if the new 6" am which have a smoother shape solve this problem?

from my geometry class I seem to remember that the intersection between the relevant shapes in this design should be an ellipsoid but I’m probably mistaken because it doesn’t seem like someone could patent an ellipsoid

-Leav

go to www.team357.com and http://www.airtrax.com/ and look for the reference to their MIT/Inventeam project.

Airtrax is the “world leader in omni directional technology.” Check it out…their whell chair is prett neat and patented and going onto production (the last time I talked to the team leadership)

The 8" AM mecanum approximates the ideal roller shape with a few rubber cylinders; on the other hand, the 6" actually has a curved roller. Although I have no first-hand experience to back it up, I think the 6" will give you a smoother ride. I don’t think the difference is worth another \$300 though… just use locking nuts with the nylon inserts on your robot and tighten them and your electrical connections after every match and you should be fine.

There IS one gotcha…

The wheels are normally bumpy because the shift of ground contact from one roller to the next is not always smooth. As long as the rollers are at a 45 degree angle then everything should be good except…

There is only one wheel configuration that will work and give you all functions.
( I know they refer to them as “Right” and “Left” but that’s not necessarily the side they go on.)

looking down FROM on TOP…look at the top rollers and then look at the roller bolts. They should make an “X” pattern like this:

``````\   /

/   \
``````

If you don’t have this configuration, you can drive forward, backwards, left and right BUT you’ll have a terrible time trying to rotate and that will be really bouncy.

My two cents

Steve

you probably drove this around on tile surfaces. The carpet makes a HUGE difference! Get some carpet and the bricks and it should be fine. The wheels will bounce a bit naturally but should not be that bad. My team used 3 inch omni’s our 1st year and it bounced a bit, but was fine.

We’ve put together out 8" mecanum bot also, and it vibrates a lot too. We are considering putting electronics and such on a 3/8" sorbothane pads to absorb shock. Do you guys think this would help?

We used the 8" A.M. mecanums last year, it was definitely a bumpy ride even with a 110 lb. robot. This year we’re using the 6" A.M. mecanums and the ride quality is significantly better.

That said, we simply checked the tightness of the structural fasteners between every match last year and had no problems due to the vibration [not many problems at all - but that’s both luck, and a different thread topic]. Use loctite and nylock nuts where you can, and check the other bolts frequently and you should be able to cope just fine with the vibration.

Another thing we noticed was that after hard use, the 8" wheels wore-in some, i.e. the hard corners on the rollers wore off to a rounder profile. It isn’t perfect (still vibrates significantly) but they do wear-in after a while.

When designing any mechanum drive it is important to account for vibrations. Our team used the 6" wheels on an offseason robot and have to give the robot a suspension system for proper driving. Short of that there is really not too much that can be done other than tightening down your bolts and hoping for the best.