3.25 Omni and Traction on same robot, + Gearing question - Spin Up

Here’s what I’m thinking -

Use 2 3.25 omnis in the front of my robot, and 2 3.25 omnis in the back
And have 2 3.25 Traction wheels in the center

Will this work with the different types of wheels and sizing and everything?

Would this be advantageous in Spin Up? Reducing horizantal sliding from pushes, but retaining the clean turns with the front and back wheels being omni. I need an honest opinion because it’s something i”m considering doing.

Could someone pose the disavatages to this? Any teams doing the same thing?

My other question is about the gearing on the chassis. Are all the wheels on one side supposed to be connected (in terms of their gearing) or should they be separate? I dont know how to better phrase this, but I can try again. Let me know if you dont understand what I’m asking

a general rule is that all your wheels need to have the same tangential velocity, so that they all work together in unison and don’t end up fighting each other.

tangential velocity of a wheel is rpm*circumference, which basically means that if all your wheels are the same size and moving at the same speed, you’re good to go. You could theoretically mix and match different wheel sizes if you move them at different speeds to achieve the same tangential velocity, but this is virtually never going to be worth the effort and I do not recommend it.

As far as your specific plan, a 6 wheel drive with traction center wheels is a very common and very good drive layout. I will caution against using 3.25" traction wheels though, in my experience the shape of the tire meant that the wheel was only making good contact with the ground half of the time, and the traction on them is not very good. Last season I had a lot of success using a custom 3.25" traction wheel:

but if you don’t want to spend the time and effort to make these, the 3.25" traction wheels do technically work, I just didn’t like them when I tried them.

The main benefit of a 6 wheel traction middle setup is as you suspect, prevents unwanted drifting, protects against sideways pushing, and has very clean and tight turning. The disadvantage is that you can no longer do controlled drifting while driving, which a lot of people enjoy doing, though I was never bothered by that myself.

good question, all the wheels on one side should definitely be linked together, ideally with gears. The reason being that if for whatever reason one of the wheels should be lifted above the tiles or lose traction, the power from the motors will be directed towards the other wheels still in contact. If the wheel being lifted had it’s own dedicated, unlinked motor, that motor would not be doing any work to drive the robot. It also just ensures all the wheels are moving perfectly in sync together.


You are amazing. I know this is unnecessary, but thank you my homie. @Xenon27

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What size flex wheel should I use?


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And what kind of glue is suggested? You have to stick the flex wheel on right?

I don’t understand how you wrapped the flew wheel around the traction wheel. How did you get two layers?

glue is not allowed in the construction of vrc robots.

what I did was carve out the inside of a 3" flex wheel, leaving only the outside ring. Then I stretched it over the grinded down 3.25" wheel hub, and used polycarbonate discs to prevent it from slipping off of either side. The tension of the rubber being stretched and the retaining discs are more than enough to keep the tire on.

I’m not sure what you mean by two layers, there’s just a single flex wheel rim used.


sorry are these not two separate layers? Can you explain what they are?

I also dont see the polycarb discs, could you point that out for me?

ah I see, the polycarb is pretty clear so it’s hard to see, but that seam that you see as being between layers is actually just the edge of the polycarbonate disc. Everything inside that is polycarb, held on by the screws.


OHHH, I see it now, thank you so much. It took looking at it from another perspective but I see it now lol.

Were you able to get the modified wheel to be accurate on how round it was? or was it a little lopsided?

Here is a pic of mine if this helps. There is no polycarb plate on the wheel in this picture.


it was round enough, there is a little unevenness from my poor job of cleaning up the inside of the tire, but I also made the wheel slightly larger than 3.25" so that it really dug into the tiles, and I had no issues with the shape of it. To make the hub I took the normal 3.25" traction wheels and just ground the tire around halfway down on a belt sander.

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Do you recommend having the center wheel slightly larger? Wouldn’t that make it difficult for the robot to move since the other 4 outer wheels are smaller? Especially when the robot doesn’t have much weight on it and you are coding things like odom or PID with just the chassis @Xenon27

yes, but only very very slightly. Just to ensure it digs into the tiles properly.
and for a light bot, don’t actively try to make it bigger, just be sure not to make it smaller than the omnis or it won’t have as good traction as it could. I think my wheels were around 3.35" in diameter if I’m remembering right, and my bot was pretty heavy so it pressed them into the ground much more firmly than a lighter bot would.


It also looks like the harvard-westlake reveal uses a slightly different form of flex wheel traction wheel, that consists of some sprockets inside the hollowed out flex wheel instead of the 3.25 traction thing Xenon27 did. Might be a little simpler, you may want to ask on that thread.


would 3.5” for the modified center wheels be fine with 3.25” for the others? ALso, will I have any issues when the robot is just a chassis?

Try It And See GIF - Try It And See - Discover & Share GIFs
But generally it would not work. All of the wheels should be approximately the same size.


yeah that will be too large, for now you should just try and make approximately the same diameter. Slightly slightly bigger is fine, but don’t go too much bigger and definitely don’t go smaller.

@Benn OR @Xenon27 Could one of you explain the approximate sizes of your modified wheel? I’m currently working on mine, but I don’t know how to make it approximately 3.3" - 3.35". What was the diameter of your traction wheel after shaving it down? What was the thickness of your shelled flexwheel?