How to lock a 3.25 omni

Hi, my team is using 3.25in omni wheels on our robot and we are wondering how we should lock them (how to stop the rollers from moving) because unlike a 4in omni, screws can not fit in the rollers


393 motor screws might fit. If not, you could try wrapping rubber bands around the wheel to stop the rollers.


I tried 393 screws and they didnt seem to work, maybe I have to force the screws in more because my omis are brand new?

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I’ve never tried to lock a 3.25" omni, so I don’t know if they fit; they may not. Wrapping a few rubber bands around it should work though.


1 word: superglue. If it doesn’t work, you aren’t trying hard enough.

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Looking at the design difference, it looks like there’s no true way to make it work. I think you’re stuck getting the 3.25" traction wheels, and you can likely shave the dimples off and add rubber bands around its circumference.


These wheels actually work pretty nice with 3.25" omnis as locking wheels. Can confirm

I don’t think you have to shave off the dimples because the form tiles deform to the shape of the wheel. If anything the dimples dig into the tiles more which could give more traction, I mean why else would they be there? I could be wrong though.

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just buy some 4-32 or m3 screws from home depot or smth and jam them in there

Last year our team used this type of tire, the performance is excellent. Optimal grip.20190718_192130


*W H E E Z E *

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There isn’t a big reason to lock the 3.25s. Most teams use the 393 motor screws which work but wear down the rollers pretty fast. The reason people lock 4inch omnis is because they are actually 4.125 whereas the 4 inch traction is 4 inches actually. This problem is only with 4 inch omnis since 3.25 ones are 3.25 as are the traction equivalent


in the vid you sent it looks like the bot was driving on wood. i’m pretty sure that wasn’t “optimal” grip even on the wood, but on foam you’re going to be taking even more losses than was apparent on the wood due to increased rolling resistance from the higher viscoelastic properties of the foam (formula for rolling resistance is Crr=sqrt(z/d) where Crr is coefficient of rolling resistance, z is sinkage depth, d is diameter of the wheel. after that it’s just Crr*N=F where N is the normal force and F is the rolling resistance. note that the formula i denoted for Crr has its limits and is defined by wikipedia as the “coefficient of rolling resistance for a slow rigid wheel on a perfectly elastic surface, not adjusted for velocity”).With that said, this formula should serve as more than accurate for purposes in VRC, since no one seems to care about rolling resistance anyways : (
anyways traction is a trade off with rolling resistance (which in turn reduces efficiency). more traction leads to a near proportional increase in rolling resistance (unless you’re using a very fun rubber compound that i’m more than happy to talk about if you’re interested) in vrc, so it’s very unlikely that your design would work “optimally” for both an incredibly rigid material like wood, and a very deformable (viscoelastic) material like foam. more testing wouldn’t hurt


In the video I shared, the surface is not wood, the surface is like rubber. In foamy tiles the grip is better due to the grooves in the wheels. It is true that in the video the surface does not offer an optimal grip as it happens with the foam floor. Now, why is grip better? Because the grooves get a little messy on the foam floor and they are 8 wheels (6 omni-directional and 2 track wheels).

i’m pretty certain that this surface doesn’t have even half the viscoelasticity as foam tiles (plus, even rubber isn’t as deformable as foam lmao. it’s usually 40A-80A on the shore hardness scale. foam like vex tile foam is typically much lower)

this is true to an extent, but it’s much more due to the deformable tiles than it is due to the grooves. research the relationship between deformability, rolling resistance, and traction to see what i mean.

y are grooves messy

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I know three ways of locking 3.25" omnis.

First is to screw tightly two 60T gears on the sides:


It’s easy to do, but takes a lot of space and could leave dents on the rubber rollers.

Second is to use small zip ties - takes some time to do and you need to be careful not to make it too tight.


Third method is to cut #32 rubber band and make a few figure 8 loops around the rollers like you see in the picture.

That work the best, but you need a lot of patience to tie all the rollers and check them regularly after that.

But why? Why do you need locked 3.25" omnis and 4.0" not gonna cut it?


Of course. From the beginning I meant that, another thing is that the behavior of the drive train on the foam floor is not shown in the video.

that’s my point; i don’t think a vid of it driving around on a 40-80A durometer floor really shows anything about its traction capabilities when we’re comparing it to bots on a 15-35A floor (not sure the actual hardness. my foam number is just an estimate since vex doesn’t list it, but most foams that are similar to the vex foams are around that area of hardness).

I never doubted it, his argument is correct.