Having a COF over 1 is possible. I don't know why that breaks some physical laws. FIRST even gave you some ideas for high-friction materials in the kit of parts.

Hmm.., let's think here. One newton down--force required to move left or right, two newtons. That doesn't make sense--it's more than just macroscopic differences in the elevation of the surface.
Now TRACTION is different all together. If you can some how "dig in" to the carpet or mesh, then yes. Or possibly if you've gone to the incredible effort of having small suction cup studs on your wheels--well no, there you still have the friction of the suction cup. Not friction, but traction, I suppose. And a ramp dominator could deffinately use being able to gear down, I suppose (but again, with the only simple fix, it would have to stop first)--but I'm sure you would've asked already if it were that crucial to the design. I suppose if you have an extra few man hours to spare, then you could try it out. We're so tight and heavy we couldn't even dream of it.

I'm not a physicst but its obvious you have no clue about friction. The coefficent of friction is F = µ R. where F is if friction, R is the weight of the object and and mu is the COF.

You're not thinking about this right. A robot weighs 130lbs but if the coef of friction equals 1.1 it would take you 143lbs of force laterally to move him, but you could lift him with just 130 lbs. There is a difference in how you apply the force.

Some COF:
rubber on concrete 0.8
glass on glass 0.9
steel on steel 0.74
aluminum on aluminum is 1.1

Also your analysis of traction vs friction is wrong. Friction is the resistance to movement. Now aluminum on aluminum is due to cold welding same with steel on steel. Its more due to chemical properties of the material. Where rubber on carpet there is more interlocking going on so its more due to the physical properties.

I would define traction is the weight of the bot time the coefficent of friction.

If you don't believe me do a search and on the internet and it will confirm everything I've said. IM me if your still confused.

the coefficient of friction can also be greater than one in cases dealing with circular motion. IE if the robot is making a turn.

$0.02

$@#$@#$@#$@#--I'm wrong again. Just isn't my week I guess. Although I don't count sticking as friction (which cold welding basically is). I would say that you'd better have a low to the ground center of mass if you're planning on more than 1.0 being you're total friction (since I'm still not convinced that you could get more than that without actually sticking on to the carpet somehow). And of course, then the bot would be lifted clear off the ground before its total friction were reached.
And remember, the easiest thing to do to get your traction up at least a little is to be as close to the weight limit as possible.

not neccessarily, lets say that you have wings that come down, these wings have file cards on the end to anchor them to carpet. if those wings only weigh fifteen pounds, youve got even less than 15lbs of downforce on those cards. If you have file cards on the bottom of your robot however, you would have much much more downward force on the cards, giving you better traction.

The Point: unless you have weight over your traction material, youre not gonna have much traction

Cory

uhm... bono, i don't mean to be rude or anything, but i'm curious: have you ever taken a physics course?
if not, quit while you're ahead :(

File cards are a bad example. The amount of traction you get with file cards has very little to do with friction. Instead, the file cards mesh with the carpet which is a force completely unrelated to friction.

Matt

It may not technically be friction but for the purposes of calculations it is. Also i doubt filecards will do much on hdpe.

We have a Continuously Variable Transmission up and running nicely. Our drivetrain consists of 6 motors (3 compose each left / right assembly.) with many custom made gears and belts. We get a very nice and well balanced blend of speed AND torque without the need to "change gears" so to speak, since it's continuously variable.

I don't believe that we "live" in a machine shop any more than most teams with shop access, but the point is that a decent transmission certainly is possible. Could we produce a CVT without shop access? Probably not. However, last year we did put a block of HDPE on two pins. The HDPE fit over the drill transmission switch and it was very easy to actuate with a small servo. Teams can and do produce transmissions such as this without shop access every year.

Why not push, zoom AND hope to do it better than everyone else? (Not that we will of course... Just *hope*)

haha.
 

It is clear that you guys are either uninformed on the subject of friction or confused.

You have to specify whether you are talking about static friction or kinetic friction.

In static friction, coefficients over 1 are easily possible.
Static friction in meaning means still, unmoving, not slipping. In kinetic friction, moving friction, a coefficient over 1 is practically impossible.

Both static and kinetic friction is applied in the case of robots, but you have to make sure you are using the right one.

Traction, also called adhesive friction is the same thing. The most dependant variable in traction (lets use this word because it puts more of a gripping image in ones mind) is the maximum static friction.

To determine max static friction between two surfaces, one must multiply the static coefficient of friction between the surfaces by the normal force (also the weight). The result of this is the maximum forward force that a robot can apply on a vertical surface (another robot or wall) before the wheels start slipping.

When the wheels slip this changes traction from static friction to kinetic friction and greatly lowers the amount of force that a robot can apply to actually push forward.

Its a little hard to explain in words. Wish I knew the coefficient of the wire mesh and hdpe. But you can determine that through testing (determining torque off wheels through motor and gearing/Fn). Havent done that yet.

Steve

transmissions very handy
 

In my eight years we've yet to try a multi-speed transmission......but I'd love to try.

We've designed our machines to be mostly offensive in the past.(we don't usually concern ourselves with torque because we try to avoid the hard pushing match-ups). This year was especially hard for us because we are using every motor and all the pneumatics........we don't use the drill transmissions so servo mounted gear switching is probably out of the question.

I love the CVT...that would be a great project, but for reliability sake, I'd probably just duplicate one of the three systems already developed by ultra-seasoned teams like 45.(their on-the-fly tranny was super sweet last year.)

Good luck to all those ramp bots this year, we'll see you under the bar!

The "object" need not be something vertically upright that the robot makes contact with either... For example, when a robot is driving up the ramp, you can essentially think of an imaginary rope attached to the back of the robot, and gravity "pulling" on this rope in the exact opposite direction that is "up the ramp."

This imaginary rope exerts a force on your robot, which may or may not (may not hopefully) be enough in and of itself to overcome the force required to break the wheels free and spin.

Remember, you're really multiplying the coefficient of friction times the NORMAL force. The normal force happens to be exactly the weight of the robot when situated on a level surface, but this normal force decreases as incline increases.

This means that if the ramp and floor were the same surface, less lateral force (relative to the robots plane) would be required to break the wheels free on the ramp than on the level ground. This makes perfect sense, since if you imagine the extreme case of a 90 degree incline, NO force is required to break the wheels free from the surface, since there is no normal force.

...
 

exactly.

i just with these people understood it.

so... I wasn't entirely incorrect...