Smaller Diameter Wheels - What's the advantage?

[Jamie Kalb]

Originally Posted by JamesCH95

The advantage of larger wheels is a larger contact patch, which generally means more grip. Remember that real coefficients of friction are generally inversely related to contact pressure.

Do you really get a larger contact patch with a larger wheel? I mean, clearly you do with a a pneumatic tire that can squish, but what if we're talking something like an AM Plaction wheel?

And great point about the low CG not always being a good thing! I hadn't thought of that.

[Chris is me]

Originally Posted by JamesCH95

The advantage of larger wheels is a larger contact patch, which generally means more grip. Remember that real coefficients of friction are generally inversely related to contact pressure.

Not in any qualitative tests I've done. Granted, with Vex wheels... but carpet / tread interactions can't be THAT different from foam / rubber ones. (Both wheels had been modified to use the small wheel's tread)

[LLogan]

I do not think that "more torque" is a justifiable advantage of smaller diameter wheels. It's possible to manipulate gear/sprocket ratios to cause a smaller wheel and a larger wheel to have the exact same torque and speed.

I don't think a larger contact patch is correct either, because wheels are circles and technically circles are tangent at only one infinitesimally point.... Of course, it's the real world, so the contact point is more than just an infinitesimally small point, but in order to have a larger contact patch, the wheel would have to compress a bit. Rigid metal/plastic wheels do not compress very well.

Thanks for input elsewhere, guys!

[AdamHeard]

You guys can have your larger contact patch and higher CG. We'll take the 10+ lbs we save and make good use of it.

[rwood359]

Originally Posted by LLogan

Rigid metal/plastic wheels do not compress very well.

The wheel doesn't but the carpet probably does. It seems like the larger wheel will have more contact area with the carpet.

[LLogan]

Well, a robot's acceleration (and thus pushing force) is caused by the force of friction between its wheels and the surface that they're on. The larger contact patch (larger surface area) shouldn't have anything to do with the torque because surface area doesn't matter for friction.

But, your team is the one who has tested this, and you have said that it helps, so I'm going to believe you.

[Lil' Lavery]

Originally Posted by LLogan

I do not think that "more torque" is a justifiable advantage of smaller diameter wheels. It's possible to manipulate gear/sprocket ratios to cause a smaller wheel and a larger wheel to have the exact same torque and speed.

That's exactly the point. Both sizes of wheels are going to require a reduction to get the desired speed. However, you're going to have to have a greater reduction for the bigger wheels than you would for the smaller wheels. The greater reduction means you're going to (somewhere in the system) need to use either a larger gear/sprocket/pully or another stage of reduction (both of which obviously have higher costs in weight, space, and money).


Wheel diameter is not a simple thing to pick. It's highly dependent on your entire drive system and how it is designed. There are situations where I'd want a larger wheel and situations where I'd want a smaller one. It's an engineering trade-off, like everything else.

[Jared Russell]

If you are really convinced that contact area significantly affects traction with carpet and roughtop/wedgetop belting, then give me a 2" wide 4" diameter wheel over a 1" wide 8" diameter wheel any day.

Speaking of which: I think it's about time that an enterprising team conducts an experiment to prove/disprove the contact area theory once and for all...

[Joe Johnson]

There are many things that are the same or can be made the same (like speed or torque -- this can be made the same using ratio) with small wheels but others cannot be so easily made the same.

There are two that come to mind.

1) the radius that hits the floor is one difference that is mostly a negative (even with "rigid" wheels there is some deflection and the smaller the radius the smaller the patch on the ground -- generally speaking)

2) smaller tires are ... smaller. That matters in several ways. It means you can put contact point closer to the frame perimeter. It means that the wheels can be below something that a larger wheel would hit. It means that there is the potential for less weight. etc.

The best engineers I know (in FIRST or out of it) are not dogmatic about things like this. In some cases smaller is better in others larger is better. What "better" means varies from game to game and from robot to robot.

Joe J.

[IKE]

We did a larger wheel (8") for less intrusion into the chassis when going over the bump. The cost of this was not being able (initially) to do the style of collector we wanted. We then found a work around for this. The larger wheels do have larger wheel torques for the same tractive effort and thus could actually cause you to flip over easier. Do the free body diagrams, and you will find many compelling reasons to go small. We did large to have a smoother motion over the bump.

[JamesCH95]

Originally Posted by rwood359

The wheel doesn't but the carpet probably does. It seems like the larger wheel will have more contact area with the carpet.

Exactly, though Jared341's point is very valid. One can always use a wider wheel to get the same contact area with a smaller diameter wheel.

Originally Posted by LLogan

Well, a robot's acceleration (and thus pushing force) is caused by the force of friction between its wheels and the surface that they're on. The larger contact patch (larger surface area) shouldn't have anything to do with the torque because surface area doesn't matter for friction.

But, your team is the one who has tested this, and you have said that it helps, so I'm going to believe you.

In a simple HS and even College physics sense the contact pressure has nothing to do with the coefficient of friction because physics teachers like to keep everything nice a orderly, which is good when one is learning the basics.

HOWEVER, from my experience in race-car design I have had the opportunity to look at things like tire performance tests which clearly show an degradation in the friction coefficient as normal force is increased. Granted it wasn't a Skyway or Plaction wheel that was tested, but it was still a polymer tread surface and I believe it demonstrates the principle quite well.

[Peter Matteson]

2 of my reasons for favoring small wheels haven't even been brought up.

1.) Lower rotating mass.
2.) Lower rotational intertia.

Also Dr. Joe is as usual correct. The game determines the wheels to use. We would change from 4" wheels given the right reasons.

[Gary Dillard]

For anyone who is interested in data regarding contact area and traction, my daughter April posted a paper (do a CD-media search in papers on "traction") several years ago where she took measurements using Brecoflex belts and carpet. Basically, the higher the pressure (lower contact area for a given weight) the higher the traction, because the wheel digs deeper into the carpet and gets a contribution from the normal force that contributes to the sliding resistance.

[kevin.li.rit]

Originally Posted by gorrilla

Well,
It really isn't a disadvantage unless your in a drage race

Is the difference in torque even noticeable between a 6' and a 4' wheel?

Coming off the same gear box, Isn't there no difference in torque, but a difference in the applied force against the ground?

[,4lex S.]

Originally Posted by Coffeeism

Coming off the same gear box, Isn't there no difference in torque, but a difference in the applied force against the ground?

The torque on the axel of the wheel would be the same, but they are speaking of the torque at the contact point due to the greater moment arm (4" vs 6"). So yes, basically R cross F, F being the applied force.