Re: pic: Why aren't omni wheels like this used?
 

Another primary reason these types of Omni Wheels will not see use in FRC is due to the roller thickness. Both the original and the "improved" wheels have parts of the rollers with near zero thickness. This comes with a number of problems including that the rubber rollers preferred for traction won't support a robot's weight on near zero thickness (bumpy ride) and as the wheels wear they will very quickly become non-round (very bumpy ride.)

Re: pic: Why aren't omni wheels like this used?
 

That's for sure! Neither of these designs could be expected to produce a perfectly smooth ride in real life, even after they were tweaked for durability and manufacturability. Even dualies can only be perfectly smooth if the wheel axle is held perfectly parallel to the ground (flat ground AND infinitely stiff robot frame), but with a good enough roller design, they can indeed usually be expected to produce a smoother ride than these two wheel designs would.

Engineering is all about trade-offs, however, and a design like these two would seem to strike a potentially useful balance between a traditional single-omni design and a dual-omni design: smoother than a single-omni, and lighter/more compact than a dual-omni. It still remains to be determined whether they could be produced at a low enough price point to compete with the current market leaders, of course!

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EDIT:
Now there's an odd idea. I was considering refining my design and printing it in pieces just to evaluate the concept, but then I remembered that many "impossible-to-make" designs are trivial when additive manufacturing is employed. What if a multi-nozzle 3D printer were employed: nylon for the structure, TPE for the rollers, and a soluble support material to maintain clearance where necessary?

Re: pic: Why aren't omni wheels like this used?
 

you mean like this one? :yikes:

Re: pic: Why aren't omni wheels like this used?
 

The white nylon rubs against the polycarbonte body of the wheel. Looking at the AM pdf it appears the roller contacts a flat face. I could be wrong on this, I'll leave it to someone with the time to open the cad to confirm.

-Adrian

Re: pic: Why aren't omni wheels like this used?
 

Polycarbonate. Hmm. Is there enough room between them to install a lower-friction (perhaps nylon or Teflon) shim?

Re: pic: Why aren't omni wheels like this used?
 

It would have to more like a film than a shim; the free axial motion of the roller is barely sensible. If you were willing to trim down (and presumably polish) the roller you could put a thin shim in place.

Re: pic: Why aren't omni wheels like this used?
 

I couldn't find any data for polycarb-on-nylon friction. I suspect it's not pretty.

Re: pic: Why aren't omni wheels like this used?
 

I concur. However, in many cases where omnis are used, the goal is either to allow a long wheelbase robot to turn (tank with 2 or 4 omnis), or to allow fine strafing adjustments so that a robot can line up on a game piece or a target after forward motion has completed or nearly completed (H/slide drive). In both of these cases, "not pretty" is still a whole lot better than "solid wheels".

I can certainly see roller friction while applying driving torque being a real problem with kiwi (three omni wheels in a triangle) or Killough (four wheels in a diamond configuration) drives. I almost suggested kiwi (implicitly using duraomnis due to our game's KoP) to one of our intramural design teams today; I'm glad I held back.

Re: pic: Why aren't omni wheels like this used?
 

Nationwide Plastics has polycarb-polycarb at 0.52 (unspecified, but I'd guess static) and Engineering Toolbox has nylon-nylon at 0.15-0.25 (static). Polycarb-nylon might be somewhere in between. Not too bad, really.

Re: pic: Why aren't omni wheels like this used?
 

The concept of these wheels are quite good as they provide close to a full concentric circumference, however they are complex to manufacture. A greater choice of dual and triple row omni-wheels are more readily available such as those by Vex, Andymark and Rotacaster Wheel which can provide better traction and better or equivalent ride quality.

Re: pic: Why aren't omni wheels like this used?
 

These are both fantastic omni ideas. I personally think any omni other than the standard "strait sized" omni would be a huge improvement. We are using the practice bot's drive train from last year, and replaced traction with omni wheels. Only downside, the bot can't be driven in the huge back lot outside.