Smaller Diameter Wheels - What's the advantage?
 

This year, I saw many veteran teams using wheels with smaller diameters than what I'm used to seeing and what my team has used in the past. I've seen lots of 4 inch wheels and some 5 inch wheels, as compared to the 6 inch wheels that my team uses and some of the KOP and Plaction 8 inch wheels that I've seen lots of rookie teams use.

What are the advantages of using these smaller diameter wheels? Of course, you can change the speed/torque ratios through chaining and gearing, so I don't think that's the issue. There is an obvious weight difference too, but I'm not sure if the weight loss is justifiable. The only other thing I can think of is the decreased amounts of rotational moments of inertia (smaller radius, less mass), leading to less strain on the motors when the robot accelerates or decelerates. Though, I am unsure if these are reasons at all for a team to choose smaller diameter wheels.

Are there any reasons as to why smaller wheels are advantageous over larger diameter wheels? If your team uses them, or if you know any good reasons, please share some insight.

Thanks!
-Logan

Re: Smaller Diameter Wheels - What's the advantage?
 

One of the biggest advantages of smaller wheels is that they allow your drive base to sit lower, which in turn allows for a lower Center of Gravity. Also a smaller wheel is, smaller - which allows everything to be more compact or allows for more room for other mechanisms.

Re: Smaller Diameter Wheels - What's the advantage?
 

Smaller wheels have a huge number of advantages.

They force you to make a lower frame, which helps you put a lot of components real low.

They require less torque to travel the same speed, so you need less gear reduction, and there is less load in your gearing; this allows you to make it all lighter, resulting in weight savings.

They are physically smaller, and since volume of a wheel is related to the radius squared, the weight savings are pretty substantial. Our wheels were .23 lbs each this year. This also allows them to be machined quicker and with less material, this is a cost and/or time savings depending on if you make or buy them.

Anyone who says you can't climb with small wheels is just ignorant as well. Both 254 and us had small wheels, and could climb the bump no problem (with a decent battery).

Re: Smaller Diameter Wheels - What's the advantage?
 

Agreed. We could even climb with 6 inch MECANUM wheels. Climbing ability was not at all the reason for our change to 8 inch.

Re: Smaller Diameter Wheels - What's the advantage?
 

It also allows the wheels at the corners to be 1 inch further toward the end of the robot, making it more stable on sloped surfaces. Effectively with a 4-inch wheeled robot you have a 2 inch longer wheel-base than with 6 inch wheels

Re: Smaller Diameter Wheels - What's the advantage?
 

The only disadvantage I can see would be if two robots had the same gearing(motors+transmissions+chain reductions)
The robot with smaller wheels would have a lower top speed,
But it would accelerate faster.

Re: Smaller Diameter Wheels - What's the advantage?
 

how is this a disadvantage?

When we design, we calculate a gear ratio and wheel size based on desired speeds.

Re: Smaller Diameter Wheels - What's the advantage?
 

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

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

Re: Smaller Diameter Wheels - What's the advantage?
 

At the same gearing? Heck yes! 150% larger radius means 2/3 the torque. That is a pretty substantial reduction.

The point is that wheel size should be part of your gearing calculations, so there is no reason to compare robots with the same gearing and different wheel sizes. If you want to go faster with smaller wheels, just gear appropriately.

Re: Smaller Diameter Wheels - What's the advantage?
 

A smaller wheel saves weight in the wheel. You also need less of a reduction to get your wheel to an acceptable speed / torque / whatever, which itself saves weight. A lighter wheel allows you to accelerate a little better too. Really, I would rather see this thread posted asking of the advantage of bigger wheels.

Re: Smaller Diameter Wheels - What's the advantage?
 

Smaller wheels are lighter, and extend the wheelbase (can be set further apart) for stability.

Re: Smaller Diameter Wheels - What's the advantage?
 

Compared to our older robots, everything was the same except going from 6 6" to 8 8" wheels. Without getting into gear ratios, we are getting a hell of a lot more torque and pushing power.
As far as CG goes, we adjusted for it, by placing everything a lot lower this year as it was well-suited for the game. We like it so much that I think we are sticking to 8" wheels hopefully for next year.

Re: Smaller Diameter Wheels - What's the advantage?
 

It's a disadvantage when you buy your gearboxes on the internet and don't do any work to figure out what's going to happen when you slap the thing together. Duh.

Re: Smaller Diameter Wheels - What's the advantage?
 

I'm pretty sure you meant 2/3 the radius means3/2 the tractive force and 2/3 the speed, :rolleyes: no worries, simple typo.

Smaller wheels mean less gear reduction for the same speed which does mean lower rotating mass (which is important) and it could mean fewer reduction stages which means a higher overall efficiency.

All the CG arguments are moot for two reasons:

1) A large wheel size does not mean you have to have a high CG

2) A "low" CG is not necessarily a good thing. If the robots CG is above or below the bumper zone there will be a moment on the robot if it contacts another robot. However, if the CG is in the height of the bumper zone then when the robot gets hit there is no moment and it can't be flipped, it's a simple bit of physics that is often overlooked that I didn't notice until I saw several "low cg" robots get flipped.

Edit:

@ Chris_is_me

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.

Re: Smaller Diameter Wheels - What's the advantage?
 

:eek: Thanks, fixed it.

Re: Smaller Diameter Wheels - What's the advantage?
 

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.

Re: Smaller Diameter Wheels - What's the advantage?
 

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)

Re: Smaller Diameter Wheels - What's the advantage?
 

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...:P. 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!

Re: Smaller Diameter Wheels - What's the advantage?
 

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.

Re: Smaller Diameter Wheels - What's the advantage?
 

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

Re: Smaller Diameter Wheels - What's the advantage?
 

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.

Re: Smaller Diameter Wheels - What's the advantage?
 

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.

Re: Smaller Diameter Wheels - What's the advantage?
 

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...

Re: Smaller Diameter Wheels - What's the advantage?
 

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.

Re: Smaller Diameter Wheels - What's the advantage?
 

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.

Re: Smaller Diameter Wheels - What's the advantage?
 

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.

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.

Re: Smaller Diameter Wheels - What's the advantage?
 

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.

Re: Smaller Diameter Wheels - What's the advantage?
 

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.

Re: Smaller Diameter Wheels - What's the advantage?
 

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

Re: Smaller Diameter Wheels - What's the advantage?
 

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.

Re: Smaller Diameter Wheels - What's the advantage?
 

A couple years ago, 968 did a test with 4" wheels, and roughtop tread of two widths, I believe about 0.875" and 1.625". The wider wheels were able to push with approximately 30-40% more force than the 1" wide wheels. However, we determined this was not in our strategy of game play, and not worth the added weight/space.

Anyhow, we have increased our tread width slightly over the years, from 0.75 back in 2005 and 2006, to 1" in the present season. Primarily, this was done for wear reasons, but also for a slight gain in traction. I suspect there is a limit to the benefit of "digging in" to the carpet, and some benefit to be gained by interacting with a greater quantity of fibers.

As mentioned above, in physics class they teach you surface area plays no role in friction, which is correct, for [perfectly rigid surfaces (tire included). The same cannot be said for real world scenarios. This is why you don't see dragsters running down the strip on a pair of skinny Honda Civic spare tires.