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

[548swimmer]

Originally Posted by Chris is me

If you're going to assert reality contradicts physics, please at least cite a source of test results or other evidence. Statements of fact without backing like this are how misconceptions are made.

Unfortunately, in the case of polymers, this is not a misconception. On a microscopic level, polymer-based materials (such as the rubber on wheel treads) have "knobs" that protrude and grab on to the microscopic "hooks" of fibrous materials (such as carpet). For this reason, increasing the contact pad increases the number of "knob" to "hook" interactions, which in turn provides an increase of traction. Idealistic entry-level physics states that F=uN, and so as long as the normal force and coefficient of friction remain the same, (it is commonplace to assume that surface area has no affect on the coefficient for basic physics because the other equations are far to complex for classroom labs) no change in frictional force will occur, regardless of the surface area.

Think of it like Velcro. What's harder to pull straight off, one hook grabbing on to one loop, or one inch of hooks on one inch of loops.

"F= uN" is physics

To build on what Ether said, this is only a rough approximation of the resistant force felt by an object, and lumps together everything from microscopic surface roughness to how deep one object sinks into another (e.g. a marble on carpet). There are, however, many other equations that these factors into account.

Sorry if this repeats any of the information on the linked threads, it's just nice to have all the important information in one place.

EDIT:

big1boom got to it first

[Chris is me]

Do any of you have measured data of how this phenomena applies to an FRC application? Anyone?

Do any of you whom are convinced about this fact with long explanations know how much surface area comes into play? Numbers and rates, not "a little" or "a lot".

[EricH]

Originally Posted by Chris is me

Do any of you have data of how this phenomena applies to an FRC application? Anyone?

Chris, see the links that Andrew gave.

Wider wheels (or tank treads) = larger contact patch. Larger contact patch = more grip than smaller contact patch. More grip = more traction for the same surface. Therefore, more drivetrain contact patch = more traction, under carpet conditions.

[AdamHeard]

Originally Posted by Chris is me

Do any of you have measured data of how this phenomena applies to an FRC application? Anyone?

Do any of you whom are convinced about this fact with long explanations know how much surface area comes into play? Numbers and rates, not "a little" or "a lot".

234 did some testing recently, I imagine they're planning on publishing the results.

My wager is the effect is larger than people would think.

[MrForbes]

If wider wheels give more grip, would larger diameter wheels also, because they increase the length of the contact patch?

[AdamHeard]

Originally Posted by squirrel

If wider wheels give more grip, would larger diameter wheels also, because they increase the length of the contact patch?

For roughtop tread, I am going to make an assumption that smaller diameter wheels get more grip as they form more of a sharp cleat than larger diameter wheels do. This cleat is a better shape for interlocking with the rows of carpet fibers.

Making it wider still retains this shape, but increases the width of the cleat.

This is purely based on the anecdotal evidence of our 2008 krab drive running 2" wide wheels.

[Chris is me]

Yes, I understand that more = more, that's not what I was asking at all.

My point was that accepting a statement as truth without understanding the magnitude of the effect, or knowing how much the effect applies, is not a good idea. Especially when "conventional wisdom" has all of those things.

It seems like a lot of people are parroting back stuff they heard from some guy instead of relying on observations and data. How is that good engineering?

[Andrew Schreiber]

Originally Posted by Chris is me

Yes, I understand that more = more, that's not what I was asking at all.

My point was that accepting a statement as truth without understanding the magnitude of the effect, or knowing how much the effect applies, is not a good idea. Especially when "conventional wisdom" has all of those things.

It seems like a lot of people are parroting back stuff they heard from some guy instead of relying on observations and data. How is that good engineering?

You are correct of course, doing the experiment yourself/having well documented reports is the proper way. I will try to find the thread I recall reading it in, it was from a few years ago. (I want to say 2003)

[BHS_STopping]

On a smaller wheel, the tread will wear down faster than a larger wheel. If we compare a 4" and 8" wheel, we can see that if both wheels travel the same distance, a point on the 4" wheel experiences twice as much "wear" as a patch of tread on the 8" wheel. In the same vein, a wider wheel experiences less wear since the load is spread out more over the tread.

From team experience, we used 4" wheels in 08 to some success, but we did have to change the tread more often than I remember using 6" or 8" wheels. This might also be because 08 called for lots of movement around a track as well. The small wheels were great for lowering the CG of our robot, and that's one tangible benefit you'll see from using smaller wheels. Small wheels also weigh less (obviously) than others and are (I believe) less expensive.

Personally, I like the 6" IFI wheels that we used in 07. They were pretty easy to deal with and gave us pretty good results.

[EricH]

Originally Posted by Chris is me

Yes, I understand that more = more, that's not what I was asking at all.

My point was that accepting a statement as truth without understanding the magnitude of the effect, or knowing how much the effect applies, is not a good idea. Especially when "conventional wisdom" has all of those things.

It seems like a lot of people are parroting back stuff they heard from some guy instead of relying on observations and data. How is that good engineering?

Maybe, when first-hand observations are scarce, you have to go by hearsay or not go at all. Have you ever actually observed that F=m*a? Have you ever measured the coefficient of friction without using F=u*N? How do you know, then, that what you've heard is correct? Maybe F=m*a^0.9999999999999993, or F=u^0.999999995*N measured experimentally. Every time that we as engineers use an equation we've learned, we're parroting back stuff we heard from some guy.

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. 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.

[Andrew Schreiber]

Ok, I continued my searches and reading and actually found some interesting threads.

http://www.chiefdelphi.com/forums/sh...traction+width was my starting point. It is quite a long thread discussing Friction as a function of area. This lead to a pair of white papers http://www.chiefdelphi.com/media/papers/1381 and http://www.chiefdelphi.com/media/papers/1382 which have some interesting conclusions.

I can keep searching or you could wait until you get into the shop and do some tests yourself. Whichever.

[548swimmer]

Originally Posted by Andrew Schreiber

Ok, I continued my searches and reading and actually found some interesting threads.

http://www.chiefdelphi.com/forums/sh...traction+width was my starting point. It is quite a long thread discussing Friction as a function of area. This lead to a pair of white papers http://www.chiefdelphi.com/media/papers/1381 and http://www.chiefdelphi.com/media/papers/1382 which have some interesting conclusions.

I can keep searching or you could wait until you get into the shop and do some tests yourself. Whichever.

Those threads are a good starting point, but after reading the write-up it does not appear that the total weight was evenly distributed, which makes the results sketchy at best.

An interesting experiment (and one I'll do tomorrow at school if I have time) would be to keep pressure constant while changing the surface area.

The issue with a drag experiment is that the knobs on the tread are interacting with the hooks radially instead of linearly, so that could affect actual performance.

If I get a chance to run the experiment I'll have the results up by 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.