Brakes
 

We're a rookie team, so bear with me on this. We've read and reread the rules, but there always seem to be interpretations we don't think of.

Is there anything in the rules that would prevent using wheels as brakes? Our thinking is that we can attach wheels that are frozen in place which can then be lowered for braking purposes.

Re: Brakes
 

Nothing is illegal in the rules about doing that. However, you would need to use the slippery wheels.

Re: Brakes
 

I agree that this is totally legal. You would want to use the wheels in a Transverse direction to double the dynamic coefficient of friction.

Re: Brakes
 

Legal as long as they're the rover wheels. If it's any other wheel, you guys are not going to get past inspection.

The fun part is going to be locking them in position and getting them down.

Re: Brakes
 

There is nothing illegal about this plan. Do realize though, that this will most likely not produce results different than simply stopping your drive system. According to Physics, the surface area should make no difference on traction on an ideal surface. As you increase the surface area, you decrease the weight per unit area, thus canceling any gain in traction. The field this year is REALLY close to an ideal surface, so you would get no "velcro effect" from the wheels simply resting on the field like with carpet. Usually on carpet, this velcro effect will increase with the number of wheels and offset the friction lost to weight spread.

P.S. I tried to explain it the best I could, but it is still probably confusing. Let me know if you have questions.

Re: Brakes
 

You will need to consider if this will damage the floor, if so I would suggest thinking of something else.

Re: Brakes
 

if im not mistaken you can as long as only 2009 rover wheels are touching the floor

Re: Brakes
 

Whats the maximum amount of rover wheels you can have? because you could lower the wheels as they say. but what if you have a bar of 6 wheels that comes down. and has a lot of resistance but still spins on a spring or something. i am a rookie myself but my team is veteran so don't dis our team for my dumb ideas but just

Re: Brakes
 

No limit to how many wheels you can have, unless you count that they all have to fit in the sizing box in the "normal" orientation.

Re: Brakes
 

While this is true, according to FIRST and my team's own tests, the transverse coefficient of friction is different that then inline. This means that if you mount the "brake" wheels perpendicularly (rotating around the y axis), they will produce more friction than otherwise. This is because the frictional force is the product of the coefficient of friction and the normal force (weight) on each wheel. Now you're dividing the weight of the robot over more wheels (unless you pick up the other wheels, which is technically an option), but you've increased the coefficient of friction. You'll have to decide for yourself whether this is what you want. Make sense?

For all of you running your coefficient of friction tests, make sure you do them loaded. Yes, I know the physics, but we all know reality. Just trust me on this.

Re: Brakes
 

something you might also want to consider is setting the speed controllers to brake and not coast, although it probably won't help much on this floor.

Re: Brakes
 

As has been explained to me (thanks Raul! and I hope I am stating this right) adding wheels does nothing to increase the friction between the robot and the floor. As the surface of the floor and the wheels are both extremely hard and flat, they approach ideal surfaces. Theoretically, if the weight of the robot remains constant, the amount of friction applied between the robot and the floor would be the same with one wheel, four wheels or a flat plate as large as the robot.
As to using the Brake mode on the speed controllers, this works best when their is some amount of friction between the floor and wheels. The braking action is then a function of the rotational speed of a motor. The greater the speed, the more current flows into the controller and a greater back EMF is produced at the motor. At low speeds there is very little current and hence very little back EMF. I am prediciting that in this game, supreme control over wheel rotation, moving and braking, will prove to be the single greatest factor to stable driving.

Re: Brakes
 

I agree that surface area has nothing to do with friction. Now, yes the lateral friction is higher...by almost a tenth if im not mistaken.

Personal opinion...not worth the time or space. again PERSONAL opinion:) .........great now some teams gonna do it and show me up at comp :p

Re: Brakes
 

Would the above hold true if the coefficient of friction suddenly increased? i.e., locking the wheels so that they are 90* to the direction of motion, then lowering them and raising them as needed?

It wouldn't apply for wheels locking while traveling straight ahead, but maybe for that transverse friction...

Re: Brakes
 

According to our tests, inline and transverse friction is either exactly the same or very slightly different (say less than 5%). Whatever it is, it definitely isn't double transverse than it is inline.

Re: Brakes
 

Was this test loaded or unloaded?

Re: Brakes
 

Eric,
The published coefficients were .1 for inline and .12 for transverse as I remember. So there is a little difference when the tires are 90 degrees to the direction of travel. So if a team were to design the assembly to drop for braking and was able to raise the inline assy so that the number of tires remained the same, the robot would have slightly better stopping. This comes at the sacrifice of steering though as the transverse wheels are not moving and contribute nothing to the steering stability.