Is there any obvious disadvantages to small (like 2") wheels for a drivetrain. Our thinking is: Smaller wheels= less gearing=Lighter=More room for other stuff. Obviously they will wear quicker, and not be able to navigate non flat floors, but are there any other disadvantages?

Remember that the standard bearings used for most FRC drive axles has a 1.125" OD.

For the most part no. Wear and discontinous/uneven floors are two of the main issues. Also, you have to consider how you’re going to deliver power to the gears, since any mating gears/sprockets/pulleys will have to be less that 2" in diameter (preferably a good bit less so that when the wheels compress a little but, there is still some space between any snags on the field (like tape that is starting to peel up) and your power-transmitting component.

Another consideration is friction. I won’t do the exact equations because they are nuts, but basically it boils down to the contact patch. According to the Hertz Solution for surface contact, the width of the contact patch is a function of the ration of the two mating radii. Meaning, the larger the radius of the wheel, the closer its radius is to the radius of the ground (technically infinite, but the relation still holds). So, the contact patch (area interfacing with the ground) will be larger if your wheel has a larger diameter. While high-school physics (and most introductory undergrad classes) would have you believe that F = u*N, where u is the coefficient of friction between two materials, this is a bit simplified. Really, the coefficient of friction relies on a number of things, but the area of the contact patch is one of them. Then, holding the patch fixed and the materials constant, friction varies ~linearly with the pressure (normal force of contact over area of contact), also called the stress, applied.

This isn’t perfect either though, because as the normal stress increases, both surfaces, in our case the carpet and the wheel tread, will deform more, increasing the are of the contact patch, and changing the coefficient of friction. So, friction is a very complex problem, even though we can often get away with simplifying it to understand the concept. Changing your wheel diameter doesn’t just affect clearances, it also affects the way the wheel interacts with the floor, and the friction provided. For FIRST applications, and in general, we can only describe so much by theory, so the best way to test if the wheel diameter changes friction on your specific design is probably to go out and test it. Hope this helps!

Also check the max load that the wheels can handle. If you’re using small wheels designed for intakes, they may not be able to handle the weight of the robot plus all of the dynamic loading associated with a drivetrain.

Generally speaking larger wheels will create more friction than smaller wheels, all else being equal. More generally: the lower the contact pressure the higher the coefficient of friction when working with polymers.

I both like it and I don’t.

Less gearing and less weight is great, but I think there is a minimum size wheel that will work for an FRC robot running on carpet.

I could conceive of a robot that actually runs on 2" or even 1" dia wheels, but I think carpet and foreign objects would give it much more problems than a 4" wheels.

Also, wheel speed gets a little crazy with small wheels. 1" diam wheels, circumference 3.1", to go 14 ft/sec (168in/3.14in) = 53.5 rps which is 3210 rpm. That’s really humming for a wheel. A car going 100 mph only sees wheel rpm around 1500.

But the best advantage of smaller wheels is that you get to move the contact patches closer to the corners of the robot and gain stability. Large wheels bring the contact patches closer to the CG and hence have less stability in stopping and starting.

The main disadvantages are ground clearance and difficulty to power. The outputs on most gearboxes require more ground clearance than 2in wheels allow for direct drive, meaning you’re basically going to be using some elaborate chain or belt run using very small sprockets to power them. It also means your chassis ground clearance will likely be <.75" unless you’re using some exotic hub and bearing configuration instead of the standard 1.125" bearings that most teams use (smaller bearings would also likely be weaker and more prone to breaking).
I would also suspect the total surface area in contact with the ground would be quite a bit less than if you used a larger wheel (unless you use a lot of them, but that would defeat the “weight savings” you might gain).

At that point it almost becomes easier to skip the gearboxes completely and just do all your reduction with belts directly from the motor output shaft to the wheels.

You also bring your CG down as the belly of your robot gets closer to the floor.

Right. I think that’s what I was trying to say, just not in the most direct way.

As noted by several, getting the torque to smaller wheels, and keeping them on an irregular surface are the biggest concern. Another big one is that small wheels of the same material wear faster than larger wheels.

I agree with most other posts in this thread. I think you should add higher rolling resistance to the list of disadvantages.

Just like most things in design, and FRC design in particular, choosing drive wheels is an optimization problem. This means that the best solution will very from team to team, and will definitely depend on what you want out of your drivetrain.

If you are having trouble seeing the disadvantages of small wheels (or almost any design option for that matter), I recommend imagining the far extreme case.

So for the case of drive wheel size, lets imagine that a team builds a robot that runs on 0.25" diameter wheels. What struggles did they face to build this drive? What struggles will they face keeping it running? What will its performance be like?

Hopefully from evaluating this idea, you can clearly see many of the design and performance hindrances that others have mentioned in this thread, and maybe you can backtrack from there to find a point at which these disadvantages are outweighed by the advantages for your team. I think my team has found a good balance this year using 3.5" OD by 1.25" wide colsons.

Every time a wheel turns, it moves the circumference of the circle, so the larger your wheel, generally the faster you will go if you applied the same hearing to the smaller one. There is a sweet spot for wheel size, so I would generally stick above 4" and go for either the 6" or 8" for a quicker robot. I would definitely not build a chassis with 2" wheels for FRC… It probably wouldn’t turn out well, you will most likeley need multiple axles of 2" wheels, which actually would increase weight.

Going from a 4" to an 8" wheel and doubling the gear reduction within your gearbox results in the exact same linear top speed for the robot. The 8" wheel is just heavier in terms of the wheel itself and needing more reduction to get down to a usable speed.

Rather than discussing the impracticability of 2" wheels, I’m interested in the potential advantages and disadvantages of 4" wheels as opposed to 6" and 8".

I currently favor 4" sized wheels (particularly colsons) over larger variants primarily due to the lower CoG, weight, and gearing they offer. This is assuming a flat field, of course. If there were some kind of barrier or irregular surface like 2012 or 2016, it would be a different story.

What I mean is having the same gear reduction for the 4" and 8" wheels. The 8" will make the robot travel more distance in less time if they are geared the same. This can be an advantage for teams who want to bump robot speeds to 15fps +

There are some wild, unstated, assumptions here. You also seem to ignore the option of changing gearing in the gearbox…

Why do you want to make gearing changes with wheels and not gears/sprockets/pulleys?

It only took swapping 4 gears to change our drive speed (as opposed to swapping 6x wheels, rebuilding our entire chassis to fit them, etc). Cheaper, faster, and easier than changing wheels!

Smaller wheel diameters allow for a longer wheel base, more/better packaging options, and result in lower rotating mass (resulting in faster drivetrain response, but I don’t know if one would notice it).

I really liked 4in wheels for our robot this year, it let us stash air tanks and the battery in our side-pods, saving lots of interior space and improving serviceability.

http://i.imgur.com/8MlpxLZ.jpg

The CG argument is a little silly… your motors/bellypan/etc can be located low in the robot regardless of what size wheels you have unless one is holding themselves to an arbitrary design practice. Every year I rotate our transmissions as far as possible to get the motors nice and low in the chassis.

You can’t say that this is true universally. Gearing too high will reduce acceleration and driving times might actually increase.

What you’re saying is true, but doesn’t really apply here. Our gearboxes take fast-spinning motors and slow them down to usable wheel speeds. The larger your wheel, the more you need to slow it down (i.e. higher reduction) to get to the same speed. Therefore if a 10:1 ratio with an 8" wheel gives you the speed you want, a 5:1 ratio (which is smaller and lighter) will give you the exact same speed with a 4" wheel.

IF you’re describing a steady state with no acceleration, negligible air resistance and similar inefficiencies, OK.
But as soon as you start describing a dynamic (e.g. from a stop, move 20 feet and stop) system, no. Not at all. Doubling the wheel size does not automagically double the torque from the drive motors.