Requesting Physists Help With Friction

[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.