Tiny wheel
 

So I've seen teams use 4" wheels usually, and in some cases 3.25" wheels, but what stops teams from using wheels as small as 2" or 2.5"? Just out of curiosity.

I would think tread wear and not being able to sit closer to the ground due to 2x1 geometry would be the main reasons. However, I feel like you could reduce gearbox weight a little bit by using smaller reduction ratios.

Re: Tiny wheel
 

Not being an expert in physics here, but if you go any smaller in wheel size I'm betting the wheels wouldn't support the weight of the robot.

I've never even seen FTC teams use smaller than 4" wheels really.

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I believe that having 2" wheels would not be able to hold the weight of a 100~120lbs. But I haven't tested it so, I don't know really. Interesting though.

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2'' or so wheels aren't at all unheard of in FRC - you see them in some octanum module designs. Generally, from what I've seen, you essentially make them out of solid delrin and they're plenty strong.

Re: Tiny wheel
 

Do you mean 3 inches?

Re: Tiny wheel
 

You could use 2" wheels, and have them support the robot. You'd just need more of 'em, most likely.

There are other reasons, though. Like finding a sprocket that small--and adjusting the gearing to compensate for torque/speed. Or even finding a wheel that small.

But how about this one: ground clearance.

Let's assume for a moment that you run 2" wheels on a chain/sprocket or belt/pulley system. You use 1"x1" square tubing as your frame. What's your ground clearance to the bottom of the tubing? How about to the sprocket or pulley? Bonus question: What does FIRST have a nasty habit of putting under the field carpet? (Answers: Assuming the axle is in the center of the tubing vertically, about 0.5". Pulley or sprocket, probably less as it'll be close to the size of the wheel. And the bonus answer: supports for field elements, with some form of ramp/other protective thingy next to it, also under the carpet, often about 0.5" thick counting carpet.) And for an extra bonus, there is a penalty for damaging the field, and there's a higher-than-usual chance that you do that when you're that low.

Not to mention any obstacles to go over.

Re: Tiny wheel
 

I think that if you ran a solid aluminum cylinder you could probably support a robot. Heck, I'm willing to bet that it is physically possible to make a robot with 2" wheels. I believe 118 even used <2" omni wheels this year for their actuated system.

I think there are a couple of factors at play. One is definitely packaging-- standard FRC bolt patterns and interfaces don't necessarily fit on a 2" wheel. Then there's making sure you have enough ground clearance (most teams seem to have a clearance >1", more with games that have actual field obstacles), which means you are probably going to have to do a custom frame (good luck using the AM14U for this, or "normal" 1x2 tube). Ultimately there are a lot of interesting design challenges that might go into it (I might designing something with <2" wheels this summer to get a feel for this, actually).

The other is a lack of COTS parts-- the majority of teams don't spend the time or resources to machine their own frames, much less their own wheels. For them, there's little reason to move away from cheap and commonly available kit frame and wheel options.

There's also frankly not been a game that demanded "microwheels," or even really encouraged going outside of COTS components for wheels in recent years.

There's also a tradeoff for effort to design a custom solution versus the (relatively small) amount of weight saved.

Re: Tiny wheel
 

Okay, I can see load being a problem. Colsons from Robot Shop, 2" diameter are rated at 75lbs. Could be okay depending on robot weight and how many spares you have. 2.5" wheels are rated for 140lbs, which is perfect.
Ground clearance this year was almost nonexistent, but yeah, in past years there's usually some kind of obstacle.

Re: Tiny wheel
 

Kika Mana uses pretty small wheels for their swerve. I'm not sure exactly how small though.

Re: Tiny wheel
 

3.25" VersaWheel Dt's

Re: Tiny wheel
 

I find it very interesting that so many people associate smaller with weaker. This is definitely not a hard and fast rule. Nick has provided a number of good reasons why teams do not use smaller wheels. In general, an excellent reason to go with smaller wheels is to eliminate stages of gearing in the geartrain making the robot lighter, simpler, and more efficient. 4" wheels are actually at the point where only one reduction is needed to get to a non-stupidly fast drive speed. From this point of view 2" wheels do not allow us to remove any further stages and come with their own set of challenges; some of which have already been mentioned.

Cheers, Bryan

Re: Tiny wheel
 

Ah, the tread pattern was worn off. They aren't too small then.

Re: Tiny wheel
 

The smallest wheels I've seen used were used on 973's swerve drive in 2012.

192 ran wheels just under 3.5in this year and will most likely be going with 3in for the offseason and next year. 3in is probably as small as we'd go for use on a WCD, might go smaller with a swerve drive though. Small wheels make a huge difference when trying to make the drive gearboxes small and light.

Re: Tiny wheel
 

Why wouldn't it? Arent the rollers on Mechanum and Omni wheels about 1"? And doesn't the robot usually only sit on four of the rollers at once?

The only thing (I think) that would hold you back from going to 2" or lower is finding a way to power and support the thing.

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I'm pretty sure 525 and 1551 have used some of the ridiculously small Colsons on octacanum pods in the past.

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696's WCD in 2013 had 3" Colsons originally. After we couldn't get over the carpet bump,we went to 3.4" custom wheels.

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Were designing a swerve with 2-3 inch wheels right now. It so much easier to get the gearing right. The drive Base is a bit larger, almost on par with west coast. You can definitely support a robot with a aluminum wheel that small. Obstacles could be a problem.

Re: Tiny wheel
 

Just be sure that you bring spares if you are running wheels that small.

All your tread wear will be focussed to a smaller surface area of tread so a 2" wheel's tread will burn off twice as fast as a 4" wheel's and three times faster than a 6" wheel. After seeing photos of some post-competition racing slicks I would shy away from anything smaller than 4" and even then I would be worried.

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You could of course mitigate the wear by going wider but then you're increasing size again.

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Agreed. Rapid tread wear is the main drawback with small wheels. We burned through three sets of 4" wheels last year, and I got tired of changing them (not trivial in a dead axle design).

Re: Tiny wheel
 

They told me that they grind the tread off, purposefully.

Re: Tiny wheel
 

Yeah, I heard that too.
Tread wear. With WCD it shouldn't be as bad to swap out wheels.

I for one would love to see the work of art that a 192 swerve would be.

Re: Tiny wheel
 

as Brian said earlier I have no idea where the small wheel weakness thing is coming from???

In fact I would argue in many cases they are stronger, as side-load has less of the lever effect on the axle interface. In the world of unicycles (spoked wheels) small wheels are often substantially stronger.

When it comes to tread wear I think people need to adapt more NASCAR philosophy. Run the compound that works well for you change it often, if you're going six cim a lot of smart people (610) will argue that traction limited drive is actually the way to go.

Re: Tiny wheel
 

We ran 6 3" colson wheels. After 2 regionals there is no visible loss in diameter or pushing power.

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2.5" on or 2012 swerve.

Smaller wheels don't usually happen because of ground clearance.

Re: Tiny wheel
 

Only thing that hasn't been mentioned thus far is tread wear. Smaller wheels turn more revolutions per distance traveled, thus the tread will wear down faster. Probably not an issue with Colsons.

There may also be something to be said about traction when dealing with non-smoothed treads (i.e. Blue Nitrile or Orange Roughtop). Larger wheels have more contact patch with the carpet fibers vs tiny wheels, thus there are more fibers to push against, meaning increased traction in the forward direction. There is a tradeoff point of where this benefit is removed on very large wheels (> 4"-ish?) However, this is only conjecture based upon findings that 4"x1.5" wide roughtop gets more traction than 4"x1" wide roughtop on FRC carpet (I swear there used to be a whitepaper somewhere...).

Re: Tiny wheel
 

Actually fox46 and Rob both brought that up earlier on this very page. Accelerated tread wear is definitely another drawback to smaller wheels.

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Wow - good memory! Yes, we used 2" Colsons in 2012.

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Larger wheels/more contact patch supplies more grip due to the "cleating" effect where the "pinion" of the wheel has physical exertion on the "rack" that is the carpet. Like gears, there is more than static friction meshing the treaded wheel to the fiberous carpet.

If you were only just calculating straight static friction, wheel diameter and even wheel number has zero effect, because while you are increasing contact surface area, you decrease weight per unit surface area in a 1:1 fashion. All that matters in that calculation is coef of friction and weight of robot (f=mu*N)

Re: Tiny wheel
 

This is incorrect in FRC (and the real world). As has been stated on CD multiple times in the past, contact patch has a large effect on apparent CoF.

Re: Tiny wheel
 

Smaller wheels, around 2" diameter, are a gamble. As stated before, they are a bit sketchy with robot weight. But, I have tried to start a swerve design with 2-2.5" wheels. Using smaller wheels on swerve can expand and limit your choices on how you can effectively use weight and room on your robot. I can't really think of how you can effectively, or efficiently, use <2.75" on an extrusion frame drive base.