4" vs. 6" vs. 8" wheels. Advantages?

[Taylor]

We found that double-wheel sets in the rear of our 2009 robot did significantly improve "traction"; double-wheel sets in the front yielded negligible results.
We used a wide-style 2-ToughBox 4WD drive base (6 actual wheels in a 4wd configuration); our weight was evenly distributed front-to-back and side-to-side; the improved traction was amplified by the trailer.

[billbo911]

What I find interesting is, my biggest question for this thread was in regard to the diameter of wheels, thus the thread title. I knew that gearing could nominalize torque and speed differences so I believed there had to be other reasons for making a size selection. Asking the question about width was actually a side note, yet it has become the main topic of discussion, and I welcome that!

Please keep the conversation going, there has been a HUGE amount of good information presented.

I really would love to see some data that supports the anecdotal evidence because that is where there "appears" to be discrepancies with physics. My guess is that actual data will support the physics, when ALL variables are accounted for.

[thefro526]

Originally Posted by Kevin Sevcik

I'll pluralize that anecdote. 57 went with 4 slicks at our first Lunacy regional and driving performance was lackluster. I argued loudly that just doubling up the wheels was completely pointless, physics! etc. But we had a little weight and a lotta desperation, so we gave it a shot. I subsequently had to eat my words when 8 wheels DID perform better than 4. I still have no adequate explanation for it.

The only explanation I can come up with is that the Regolith was not "Rigid".

In an ideal world, the Regolith would not have depressed or deformed at all when weight was put upon it, but at every competition I went to and drove it did. We spent build season practicing on field where the FRP Regolith was placed directly over a tile floor and found minimal if any difference between the number of wheels and traction - and the floor felt much more slippery than any Regional Field or The Curie Championship Field. I'd venture to say that the Deformation of the Regolith Playing Surface due to the Carpet Underneath was probably the cause for the strange friction differences - though not knowing enough about the physics behind it I can't really give a conclusive answer.

There was also the "breaking in" effect that I still don't completely understand. It seemed that Fresh Fields were more slippery than a field that had about 40 or so matches played on it. I'm not sure if the coating of the FRP began to break down dude to the heat of wheels spinning over it or if there was some other thing at play here.

...


On the topic of the original post, We prefer to use 6" wheels when the game allows. We've found that 6" wheels are a nice compromise between the Small-Low COG benefits of a 4" wheel and the Higher Speed and Obstacle climbing abilities of 8" wheels.

[billbo911]

So now for an odd addition to the question(s):

Since wider wheels do seem to improve traction so much, and smaller diameter wheels seem to have a decent advantage over larger diameter wheels, what happens if we go to extremes a bit? Say, a 3" wide X 3.5" dia. wheel with rough-top. Would there be enough variation in the speed of the wheel, from it's inner edge to it's outer edge, while turning the robot, to be a problem?

[Kevin Sevcik]

Originally Posted by billbo911

So now for an odd addition to the question(s):

Since wider wheels do seem to improve traction so much, and smaller diameter wheels seem to have a decent advantage over larger diameter wheels, what happens if we go to extremes a bit? Say, a 3" wide X 3.5" dia. wheel with rough-top. Would there be enough variation in the speed of the wheel, from it's inner edge to it's outer edge, while turning the robot, to be a problem?

That's going to depend on the width of your wheelbase, but at first glance, I'm doubting it will matter. The narrower your wheelbase the worse it will be, but the narrower your wheel base, the worse problem you'll have simply scrubbing the wheels sideways when turning. So I think if you had things narrow enough for that differential velocity to matter, it'd still be dominated by the much larger force required to scrub your wheels. It's certainly an interesting problem to consider though. I might try throwing some physics at it tonight to help me sleep before kickoff.

[Chris is me]

Originally Posted by EricH

Have you ever actually observed that F=m*a?

Yeah, several labs in physics, and it's also a scientific law so I'm going to give it more credence than any other idea.

Have you ever measured the coefficient of friction without using F=u*N?

Yes. You can determine a coefficient of static friction by placing your traction material on the surface it's measured against (so a square of carpet), then rotating the surface with one side against the ground until the material slips. The tangent of the angle of your material is the coefficient of friction.

Every time that we as engineers use an equation we've learned, we're parroting back stuff we heard from some guy.

True, but "some guy" should generally be things determined by science that are easily verifiable!

The only difference is that for these established equations, "some guy" is whoever wrote the textbook and whoever taught the classes. Sure, they've been backed up by decades or centuries of testing.

This is the only reason they are given credence and you can't just brush that aside! It's the very basis of science as a whole

But only a few tests have actually been run on wheel width vs. traction at the FRC level. I can only think of 2, and only 1 has actually been finished and put out there so far.

So a good general policy is if we don't know, don't state things as fact, which was my original point.

[548swimmer]

So I got time to run the test, and the double width strip of tread had 20N of static friction force, and the single wide tread had 18 N of frictional force maximum. I videotaped the force gauge, and I'll get a formal report posted as a whitepaper soon.

[EricH]

Originally Posted by Chris is me

So a good general policy is if we don't know, don't state things as fact, which was my original point.

Correct, as far as it goes.

But in the case here in front of us, there is data, widely observed. It's just not quantified. Because it's not quantified, all we really have is what has been generally observed. We know, but we don't know exactly, therefore we generalize.

[Chris is me]

Originally Posted by EricH

But in the case here in front of us, there is data, widely observed.

Anecdotal evidence does NOT equal data. You're comparing apples to oranges half the time, and non-equal test cases ("oh that team had treads and could push anyone around!") are useless.

[548swimmer]

Originally Posted by Chris is me

Anecdotal evidence does NOT equal data. You're comparing apples to oranges half the time, and non-equal test cases ("oh that team had treads and could push anyone around!") are useless.

As of right now I have empirical proof that surface area affects static friction. A video of this proof will be uploaded to YouTube within the week, but for now you just have to take my word for it.

[Chris is me]

Originally Posted by 548swimmer

As of right now I have empirical proof that surface area affects static friction. A video of this proof will be uploaded to YouTube within the week, but for now you just have to take my word for it.

The double wide strip weighed twice as much - that's pretty important!

[EricH]

Originally Posted by Chris is me

The double wide strip weighed twice as much - that's pretty important!

But, with that noted, it shouldn't be terribly hard to rerun the test with a few fishing weights duct-taped on top of the single-wide strip so the strips are equal within a few grams or so. Then, put up both videos.

[548swimmer]

Originally Posted by Chris is me

The double wide strip weighed twice as much - that's pretty important!

Actually, it didn't because I used two segments of 2x4 for the weight on the double wide strip and stacked two 2x4 segments of the same length for the weight on the single wide tread. The only weight difference is the weight of 3.5"x6" of rougtop tread and 3.5" of two-inch aluminum angle iron, the total of which could hardly combine to cause an additional 2N of frictional force.

This would be one of those "if you don't know, don't state something as fact" moments.

EDIT:
I had one of our animators edit the video, and I just got it sent to my email unfortunately, it's a download link so I am unable to view it on my iPod . However, I trust he did a good job and so here's the link: http://dl.dropbox.com/u/4749895/Comp.mp4

[Chris is me]

If that's 1/4" thick AL, that's a third of a pound of material. Assuming those 2x4s weigh about 3 pounds and your traction material is 1.0 CoF (rough guesses here), the third of a pound would indeed make the difference.

That being said, I think it's VERY cool that you tested that with demonstrable result. If doubling thickness does increases your traction by 11%, that could be something.

[EagleEngineer]

4 inch the smaller the better. For this game you also need a low center of gravity and their are no obstacles... h