Now, from my Honors Physics class that surface area does not effect friction at least directly Ff= µ(Fn) . However I also know from experience that dragsters use larger wheels on their drive wheels because they want more friction. Hoping that an entire sport is not living a lie, what role does surface area play in friction? I was thinking that it had something to do with only squishy surfaces.
I’ll second that - I’ve taken three classes on Mechanics and I still don’t fully understand how to explain the applicable physics to my team.
Its 11:30 in the Bay Area and we’re all on CD lol
I don’t think the dragster analogy is a good one.
Dragster tires get incredibly hot. If they were small and skiny they would explode or catch fire. Bigger tires can dissipate heat and survive longer.
I have no idea, but that’s my best guess…
Thanks I did not realize. However still there are other situations that represent using a larger surface area for more traction.
http://www.ifirobotics.com/robot-traction-wheels.shtml
Inside FIRST robotics it self, IFI sells traction wheels and advertises the wider ones to have more traction. Im also noticing that all my surface area example are wheels, maybe wheels have special friction properties that include surface area, just not that I remember.
Where else should we be at 11:45 on a Sunday night?
But seriously, I hope to go find a nice physics professor with some office hours this week to sort this out.
I also believe they are sticky, gripping to the pavement. This is a different property than friction, and a larger surface area does benefit. This is the same reason larger treads might help on carpet. However, in cases like this, there is no grip, and we will rely on friction to move our bots. Friction is not dependent on surface area, and a larger surface area won’t increase it.
As I understand it this years game should behave close to the ideal model of friction we are all familiar with. Both the floor (with the exception of the carpet) and the wheels are quite hard so the ideal model should be pretty accurate.
Between the soft rubber traction wheels and their patterned tread and the carpet both surfaces are fairly soft and have surfaces that can “grip” each other and not directly slide across each other. This is the source of the non-idealities that draw surface area into play. An extreme example of these “grip” interactions would be velcro.
On carpet using roughtop surface texture, an interlock actually happens between the two throwing off the whole “surface area has no effect on static friction” deal. this is why wider wheels usually pushed better. But this year its a flat profile wheel, and a slightly bumpy playing field which makes the traditional equations more relavant.
A larger area does not increase friction.
However, it does distribute the force of friction over a larger area. Too much force on too small an area can damage a surface.
I’m pretty sure that’s the principle behind ice skating. Your motion along the ice doesn’t experience much friction because of its tiny surface area; the small amount of ice the skate blade is moving into simply breaks off the ice instead. However, when you are pushing your skates to the side or at an angle, the ice experiences less pressure and thus you are able to propel or stop.
In the case of skidding wheels, a larger area dissipates the heat generated better, preserving the wheel.
Thanks for this well explained… explanation. The velcro example really helped.
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 
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I’m glad to hear my instinct was correct
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.
Ice skates are a bad example. They work by concentrating the force so much that the ice melts. The bottom of the blade is concave to capture the water and create sharper edges (to help increase force and melt ice). You actually skate on water.
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.
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.