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

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

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

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?

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

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.

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

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)

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

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.

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

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.

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

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.

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

Quite right, my mistake. I believe the correct term would perhaps be contact patch? In reguard to my example the dragster has more tire contacting the ground thus interacting with the ground. Like someone else pointed out if the ground is slick (or otherwise does not allow the wheel tread to "lock" togeather with the ground) the added width is mostly useless.

As to theoretical happenings, I don't think those take into account the way tread (say roughtop) locks togeather with the carpet besides their CoF. Correct me if I'm wrong though.

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

If anyone happens to have a strip of carpet, a couple of weights and a section of tread you can test this in two seconds.

Cut two sections of tread, lets say a 2x1 and 4x1 section. Attach the piece of carpet to the board. Put the weighted tread sections (weight them both equally!) on the carpet board (evenly distributing the weight would be best, like a 1/2" piece of steel flatbar on one and 1/4" thick piece of flatbar behind the other. Tilt the board. Which one slides first? Argument solved.

Okay, well not solved, but better than a lot of the "he-said she-said" arguing going on here. :)

I'm very curious to see the "real" answer, since Cyberblue seems to be puzzled by their data.

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

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!

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

The fact that they are puzzled by their data leads me to infer that it's a puzzling subject. There's probably more to it than we've thought of.

btw the drag racing tire analog is probably not very good, because different things are going on (mainly involving temperature of the tire tread) that most likely don't directly translate to robot wheels on carpet. My drag radials "hook" a lot better when I spin them enough to make them smoke for a while first....

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

Interaction of wheels and tread with carpet is a very complex subject. Coulomb friction (F=mu*N, with fixed mu) becomes a pretty poor model for the interaction when you get into macro-scale effects like roughtop digging into carpet fibers. Add in the fact that both tread and carpet are compliant (they compress under load), and I am not surprised that Cyberblue has made some "puzzling" findings.

Factors that I would expect to be involved in the final determination of maximum tractive force (traction-limited assumption):

* Tread material
* Tread pattern
* Tread wear
* Tread orientation relative to carpet grain (the carpet is not rotationally symmetric)
* Carpet wear
* Contact patch size
* Normal force (note: will not necessarily be uniform across all contact patches, such as in a drop center 6WD)
* Temperature (can affect the compliance of many types of rubber)
* The sides of the wheel (in our testing, with worn tread, the plastic sides of a Plaction wheel start making contact with the carpet)

This identifies 9 different independent variables. There might be more (or less...some of these may not turn out to be big factors). For any single team, doing a 9-dimensional study across all of these factors would be pretty daunting.

Would someone want to make a test bed to bring to Championships? Just a patch of carpet and a scale to measure pushing force (and robot weight) - along with a system to log the data - would go a long ways. Invite teams to come test their pushing capabilities, then roll up the data into a report on CD. Maybe someone could even make a trophy for the "Highest CoF in FRC 2011". :)

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

This wasn't entirely true. There were a few teams that I drove against that year (25 and 2753 come to mind) that had 12 wheels in their drive trains versus our 6 and they had noticeably more pushing power than we did, and could also out accelerate us. I'm not sure what caused this, I always theorized that the carpet under the Regolith may have had some effect on the amount of Friction, but never cared much to test it once the season was over.

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

If you really enjoy the topic of CoF of wheels and tires, I would highly recommend:
Fundamentals of Vehicle Dynamics (R114) [Hardcover]
Thomas D. Gillespie

He has a lot of great documentation of test results from tire companies, and can explain some of the issues you guys are arguing over. The CoF of a rubber component on a surface is an extremely complex interaction. Contact pressure, temperature (both tire and surface), slip angle, slip ratio, and the tires ability to dissapate heat all play into this variable. These minor deltas usually are not necessary for engineering approximations where you only need to be within a few percent. However, they become a bigger deal when you are looking at performance applications.

As far as surface area effecting grip, probably within 10% (for the size differences we are discussing). Keep in mind that static versus dynamic friction can also often be on the order of 10% (or more). That means that if I had 10% more traction than you, and start pushing you and my wheels are close to the static grip level, and you are spinning yours, I may no have 20% more traction (all things being equal). This is a very frequent occurrence in FRC.

If someone did a truly down-town experiment on this, they really should document it well and enter it into a science fair. They could probably get some pretty big scholarship money.

*********************
One of the coolest projects I have worked on in my career dealt with the %slip vs. traction and its role in a phenomenon called Power-Hop or Wheel-hop. This problem also dealt heavily with a systems vibration issue. (Vibes and Physics are often looked at as two horibly boring classes, but put the two together and you get to do burnouts in muscle cars for a few months).:p

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

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.