Originally Posted by notaPINKtruck (Post 792130)

This is based on my understanding from freshman physics at Cal. One of the reasons explanations tend to be so poor is that (a) this is not generally taught in an engineers' physics classes and (b) physicists are just now beginning to fully understand the nano-scale forces at work between two bodies in contact.

The basic f = uN model only applies to very hard/rigid materials on an incompressible surface. Therefore, the model is broken by treads/tires in several ways.

(1) Treads are rubber, and thus compress (changing the orientation of their atoms and thus the coefficient of friction.

(2) Carpet is also "squishy" to a degree, and thus violates the model in the same manner as above.

(3) The pile (fibers) of a carpet form a small layer ABOVE the "main" surface of the carpet, meaning that your static friction is dependent on how far you have "sunk" into the carpet and thus how long you've been sitting there.

(4) The rubber that tread is made from is molecularly composed of VERY long hydrocarbon chains (think sort of like a pearl necklace). These chains then are tangled into an "elastomer" structure, like a big bundle of cords thats bunched up (and stretches out, thus the strechiness). Because of the girth of these molecules and the uneven electron density between the carbon "backbone" of the chain and its outer hydrogens, significant Van der Waals (VdW) forces occur between the molecules of the two surfaces at contact point, essentially forming very weak "temporary bonds" which require energy to break (i.e. move). See "dispersive adhesion" on Wikipedia.

We now understand VdW forces to be the reason that geckos can stick to surfaces so well. Their feet are coated in thousands of hairs (which then split into even smaller sub-hairs like a tree) called setae. These hairs' miniscule size and sheer number allow the gecko to exploit the same polar/VdW forces mentioned above, and stick to a surface using even just one toe.

I know this is long, but I hope it helped. I guess the moral is that friction is one thing but "sticktion" is another. Gratefully, this years' playing surfaces are essentially ideal as far as the "standard model" for friction goes, so you won't have to worry about making gecko wheels for your bot until next season .

Originally Posted by notaPINKtruck (Post 792377)

Already did it yesterday. The result was almost exactly the same. If I recall correctly, it took about 35lbs of impulse and 25lbs continuously to move the bot transverse. I don't have any numbers on the force required to make the wheels slip (this wasn't really easy to determine with our kit chassis + wooden board + light team member + fish scale test rig). We had them sit in between the wheels and then rest all of their weight on the back wheels. This is not surprising considering the near-ideal surfaces we're dealing with.

Originally Posted by dtengineering (Post 807716)

As much as I agree with the statement that "in the simplified theory of friction as taught in high school physics class, extra wheels should not increase traction", it is important to remember that the simple relationship between normal force and resultant force makes some assumptions that may or may not hold true in a practical environment.

For instance Formula One race cars evidently find that having a larger contact patch improves traction. This does not contradict the theory taught in high school physics, but does mean that some of the assumptions made in that theory are not valid in the race track environment.

For instance the "high school" theory assumes that neither surface deforms due to the normal force (remember that the regolith lies over top of carpet, and is a fairly thin material...), nor fails under the resultant force. Given the "white powder" produced by spinning wheels as reported on CD, there is definite surface failure during dynamic friction of highly loaded wheels. Perhaps this is less of an issue with lightly loaded wheels....

Perhaps they are on to something with this design, perhaps not. I suspect not, but if the team has tested multiple wheels and found them to be an improvement then they have my congratulations for not being bound by the assumptions of a simplfied theory.

Originally Posted by Daniel_LaFleur (Post 807997)

While I do not disagree with the jist of what you've posted here, I do want to correct a common misconception. F-1 cars use wide tires to dissipate heat, not to gain additional traction. Additional traction in F-1 is generated by the wings on the car providing additional nominal downward force. At speeds above 100MPH that additional downward force is greater than the weight of the car and the car could drive upside down on the ceiling (If it could get there).

Originally Posted by domoarigato (Post 811400)

The Fn, however, can be altered within the scope of the rules...