Rubber bands on the regular wheel - middle ?

We are using 3 wheels on each side. 2 Omni wheels on far ends and regular 4” wheel in the middles. Planning shaft encoders on the far end wheel. We have seen teams use rubber bands and zip ties in the middle wheel.

  1. Does it need these extra rubber bands and zip ties
  2. Is the diameter different between 4” Omni and regular ?
  3. if we use rubber bands and zip ties - is there possibility that the shaft encoder gives wrong values- as there is a chance the far wheel does not touch ground when the middle wheel’s zip tie raised it a little bit ?

Thanks.

  1. Yes, or it won’t touch the ground
  2. Yes, omnis are a little bigger
  3. Yes, there is a chance to that

I would, however, not do that. You can instead use something called a “locked omni-wheel”. You can make one by adding screws in between the rollers and wheel hubs. I will attach an image on how to do that. They will have the same diameter as regular omnis, and they have WAY more traction than rubber bands or traction wheels. The screws stop them from moving side-to-side. Use locked omnis.
5c140dd550378_IMG_2576.JPG

There is a difference between the diameter of 4” traction wheels and 4” omnis. This is why you see the term “locked omnis” a lot. They replace this middle wheel so that there is no need for using rubber bands. In my opinion, this is much easier than using rubber bands.

Because the diameters are different, the rubber bands are needed to equalize the wheel contact.

As long a the middle wheel doesn’t slip and has pressure to turn, the shaft encoders will be accurate. You may always have issues with wheel slip, and increasing their diameter should only help this, as long as they are making constant contact.

@TaranMayer That’s my picture! I used locked omnis on my 4 wheel drive because they get more traction than 4” traction due to several factors, which I won’t really go into unless it’s requested.

Thanks !
@TaranMayer , @Got a Screw Loose - so you just screw it into those spaces ? Wouldn’t this actually increase the diameter of the Omni - slightly ?

Thanks.

Slightly? Sure. It compresses pretty easily though, and as long as you do it to both of the wheels, the diameter will stay constant, and that’s what’s important. The rollers will push out a little bit, but not enough to cause any issues. And if you have to have one wheel larger on a side, the middle is the best choice.

In short, yes, it will change the diameter, but not enough to make a real noticeable difference. You’ll be safe.

Thanks … looks like locked omnis are closer to regular Omni’s size than the other traction or regular wheel. We will try it out.

The other method used since time immemorial for 6-wheel bots is to mount the center wheels sliiiiightly lower than the outer four, by as little as 1/4".

But that would create a center tipping point and mess up shot consistency if you have an even gravity distribution, right? How do those teams make up for that? Just put a bunch of weight on one side?

Usually robots wind up fairly consistently heavier on one end than the other. If not, you can always shift the battery placement a little to guarantee it.

You can also remove the tire from the wheel, place a few rubber bands around the wheel one layer thick, and replace the tire. That made up the difference nicely and had no chance of the rubber bands falling off the robot some of my students did it with.

Good idea, but it sounds like a pain to execute. Is it as bad as I think it is?

Somewhat, yes, unfortunately. Use the wider rubber bands if you try. The narrower ones will tend to roll as you slip the tire back on, and that will make the tire uneven.