Tiny wheel

[asid61]

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.

[JohnFogarty]

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.

[DjParaNoize-]

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.

[Oblarg]

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.

[orangemoore]

Originally Posted by JohnFogarty

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

Do you mean 3 inches?

[EricH]

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.

[cadandcookies]

Originally Posted by JohnFogarty

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.

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.

[asid61]

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.

[Mike Marandola]

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

[R.C.]

Originally Posted by Mike Marandola

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

3.25" VersaWheel Dt's

[BJC]

Originally Posted by JohnFogarty

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.

Originally Posted by DjParaNoize-

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.

Originally Posted by Oblarg

Generally, from what I've seen, you essentially make them out of solid delrin and they're plenty strong.

Originally Posted by EricH

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

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

[Mike Marandola]

Originally Posted by R.C.

3.25" VersaWheel Dt's

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

[Joey Milia]

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.

[Maxwell777]

Originally Posted by JohnFogarty

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.

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.

[smistthegreat]

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