Has 6 wheeled designs surpased tank treads

[Manoel]

Originally Posted by Pat Roche

Paul, the kinematical physics say that surface area does not matter. A wheel and a track will slip once the coefficient of friction is broken regardless of surface area.
-Pat

Yes, but that is assuming ideal conditions. As others have said, you can have materials like Velcro, or rubber, that will "stick" to the surface. If you were to measure mu between rubber and asphalt, and used two blocks with the same weight but a different surface area, you'd get a slightly different result for mu (would it be linear with area?). Try it with Velcro and carpet and the difference would be huge.
For instance, extrapolate and loctite a block to a table. Loctite 158184 will hold 1300 N per square centimeter. Take a 1x1x1 (cm) block that weighs 100 g and pull it till you manage to rip it apart from the table. Would you consider your mu to be 1300 (Force/Normal)? Obviously not, because friction is not the only issue here. Same with the situations I discussed above.

Now I have two questions I'm not sure of the answer:
a) If surface area really doesn't matter, why is it much harder for a treaded robot to turn on a dime than it is for a robot with wheels? Also, why does it get much easier if you add an idler pulley? (I haven't put a lot of thought into this one yet, but I think it's not entirely dependent on the surface area, but also because the treads dig into the carpet and you have to overcome the fibers in order to skid laterally)

and

b) (not that it really makes much difference, but...) We all know that a rolling wheel has v=0 on the contacting point and v=2wr at the diametrically opposed point. Now, do we have static of dynamic friction? I would say static, considering if you "break traction" the force you apply to the floor will be reduced, and as the wheel won't be translating anymore, the relationship I mentioned above won't hold true.

[jimfortytwo]

I'm not prepared to say whether or not an increased surface area given the same normal force yields more traction. However, anyone who is arguing that it makes no difference whatsoever has both not actually tried it for themselves, and has not read the whitepaper on the subject.

[Paul Copioli]

jimFortytwo,

Was that aimed at me? If so, then I am ready to put my money where my mouth is. Are you?

-Paul

[Tom Bottiglieri]

I would trust what the UFH Engineer has to say about this.

Thats just me though.

[jimfortytwo]

Originally Posted by Paul Copioli

jimFortytwo,

Was that aimed at me? If so, then I am ready to put my money where my mouth is. Are you?

-Paul

No, I wasn't trying to single you out. Though I imagine you have not yet, in fact, read the white paper in question, and have not done the experiments yourself. Or are you holding out on us?

I suggest reading the whitepaper. Following your challenge, I reread it, and the results still somewhat surprise me. I won't bother summarizing, as it would spoil the surprise.

the paper:
http://www.chiefdelphi.com/forums/sh...threadid=15719

I guess the other part of "putting my money where my mouth is" should be to mention the traction tests I did for my team last year. We were looking at 8" hard rubber tires with an aggressive crosswise tread pattern. I did tests for kinetic friction with 30 pounds and 20 pounds of pressure on the tire. I was working with the risky approximation that in a 4-wheel drive robot each tire sees 30 pounds (30*4 =120), and in a 6-wheel design tires see 20 pounds (20*6 = 120). I can't remember the exact numbers, but my recollection is that I extrapolated a 10-20% benefit in the case of distributed load. Obviously a better procedure is needed to give any meaningful results, however.

I remain convinced that the traction advantage of treads over wheels is so highly dependent on the carpet and material that you can not generalize it. If I can put together materials to do a proper investigation of the subject, I will. Unfortunately we're on winter break, so that might have to wait until January.