Requesting Physists Help With Friction

[notaPINKtruck]

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 .

[Woody1458]

I'm glad to hear my instinct was correct

[Ken Leung]

You know what, guys, I dare you to do an experiment on this.

Get one of your old robot, or a cart with something heavy on it, and put on some rover wheels you are supposed to use this year. Lock the wheels, and pull the robot or the cart with a hook scale. Figure out the maximum amount of force before slipping, and the amount of force you need to keep it slipping.

Then keep everything the same, except putting more rover wheels on, and try again. I dare you to report your finding from this experiment . I dare you to let the FIRST community knows how exactly these wheels are going to work on the crater surface.

Nothing beats a quick and dirty experiment you can rig up in 30 mins.

[notaPINKtruck]

Originally Posted by Ken Leung

You know what, guys, I dare you to do an experiment on this.

Get one of your old robot, or a cart with something heavy on it, and put on some rover wheels you are supposed to use this year. Lock the wheels, and pull the robot or the cart with a hook scale. Figure out the maximum amount of force before slipping, and the amount of force you need to keep it slipping.

Then keep everything the same, except putting more rover wheels on, and try again. I dare you to report your finding from this experiment . I dare you to let the FIRST community knows how exactly these wheels are going to work on the crater surface.

Nothing beats a quick and dirty experiment you can rig up in 30 mins.

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.

[jgannon]

Though it's not exactly related to the topic at hand, this seems like a good place for "my mind is being blown by friction" questions. Can someone explain why there is a twofold difference in the friction coefficients quoted for the inline and transverse directions on the rover wheels? I'm having a lot of difficulty understanding exactly what mechanic comes into play to make that happen, since the material is smooth and appears to be essentially uniform.

[Bongle]

Originally Posted by jgannon

Though it's not exactly related to the topic at hand, this seems like a good place for "my mind is being blown by friction" questions. Can someone explain why there is a twofold difference in the friction coefficients quoted for the inline and transverse directions on the rover wheels? I'm having a lot of difficulty understanding exactly what mechanic comes into play to make that happen, since the material is smooth and appears to be essentially uniform.

My only guess is that once the rover wheels get scuffed up by spinning a lot, the grooves along the wheel (in the direction it spins) will have an effect on its grippiness sideways. So perhaps the numbers they gave us are correct only for sufficiently scuffed wheels. We ran our lightly-loaded kitbot on a somewhat dirty hardwood floor, and the wheels picked up some dirt and scuffs. I suppose with enough running with a heavy enough robot, they'll develop substantial grooves.

As for the actual question, surface area CAN affect friction if the surfaces deform. You don't see race cars with skinny tires because the bigger tires allow for more stickiness, and you don't see skates with large surface areas because it wouldn't generate enough pressure to melt the ice and create the layer of water that you actually skate on.