HDPE or ICE FLOOR

[Bongle]

Originally Posted by EricH

Namely, surface area doesn't matter.

This isn't _entirely_ true. In an ideal physics world, yes, but there are cases where surface area is proportional (or inversely proportional) to grip.

-Racing cars have very large, wide tires (even though this increases unsprung weight) because it means that if the tire hits an imperfection in the track, it doesn't lose grip. Since the track is always imperfect, this has the effect of effectively very slightly raising the car's coefficient of friction (although the ideal rubber-on-ashphalt coefficient remains the same)
-Skates have very little surface area because it takes a lot of pressure at the skate-ice interface to create the microscopic layer of ice that the skate glides on.

Likewise, I'm not sure the rubber-on-carpet case is a cut-and-dry Ff = uFn case. It might be like Velcro: if you have two big sheets together, it is much harder to pull one off sideways than if you have two small sheets together. Despite the normal force being the same, it takes much more force to move the two sheets relative to each other when there is more surface area in contact.

There have probably been some teams that have done tests, it would be interesting to see the results.

[JesseK]

If they do a mostly-HDPE field, I think we'll see alot more tank tread attempts for drive trains this year. The successful ones will have the right tread pattern.

However, I suspect that if we see any HDPE, it will only be on part of the field, or a small field element. That stuff isn't exactly the cheapest material around...

[EricH]

Originally Posted by Bongle

This isn't _entirely_ true. In an ideal physics world, yes, but there are cases where surface area is proportional (or inversely proportional) to grip.

-Racing cars have very large, wide tires (even though this increases unsprung weight) because it means that if the tire hits an imperfection in the track, it doesn't lose grip. Since the track is always imperfect, this has the effect of effectively very slightly raising the car's coefficient of friction (although the ideal rubber-on-ashphalt coefficient remains the same)
-Skates have very little surface area because it takes a lot of pressure at the skate-ice interface to create the microscopic layer of ice that the skate glides on.

Likewise, I'm not sure the rubber-on-carpet case is a cut-and-dry Ff = uFn case. It might be like Velcro: if you have two big sheets together, it is much harder to pull one off sideways than if you have two small sheets together. Despite the normal force being the same, it takes much more force to move the two sheets relative to each other when there is more surface area in contact.

Sort of true. However, your Velcro analogy isn't exactly a wise one. Velcro is designed to lock together. It's designed not to move in any direction. The more locks you have on a door, the harder it is to open, right? The same for Velcro. Rubber on carpet isn't designed that way, and neither is rubber on HDPE. And, with velcro, if it's on some smooth surface, it won't stick. There's a piece of HDPE in every field box because of this fact.

Skates are designed to create semi-frictionless motion (or to use friction to create that!). They need to melt the ice and have it immediately re-freeze. Ever wonder why ice is considered frictionless?

As for the racing car analogy, that's more the right track, but you're still off. I'll link to a thread shortly that will help. (There's also a quote from the spotlights--"A drag racer can have all the output torque in the world, but it does no good on ice."--Paul Copioli) I also remember reading that the wide tires are actually to dissipate heat.

http://www.chiefdelphi.com/forums/sh...surfa ce+area
This thread is the most recent in a series asking if you get more traction (aka friction) for a wider wheel. If you want to argue that with Copioli and Baker, go right ahead. Oh, and Baker and the TechnoKats did tests on tank treads vs. wheels. See linked thread.

[Bongle]

Note that my response was entirely in response to what I quoted, and was admittedly off-topic in an HPDE thread. You said "surface area doesn't matter" in a paragraph that didn't make any mention of HPDE, giving the impression you were referring to all traction-related cases. Hence my giving of examples where surface area DOES matter.

skates and velcro

I was just pointing out examples where the frictional forces between two surfaces was decided by phenomena beyond Ff = u*Fn.

Oh, and Baker and the TechnoKats did tests on tank treads vs. wheels. See linked thread.

And they found that even on a slippery surface like HPDE, surface area makes a (very slight) difference. On a surface like a carpet where interlocking between wheel rubber and the surface will be more likely, I'll bet that relationship is greater. However, since this thread is about HPDE, I'll stop posting and we can keep discussing via PM if you want.

The message I'm trying to drive home is: Designing by idealized physics equations is not always the best approach. There are cases where it isn't. Use them as very strong suggestions, but do some empirical tests to see if other factors come into play as well.

[EricH]

Originally Posted by Bongle

And they found that even on a slippery surface like HPDE, surface area makes a (very slight) difference. On a surface like a carpet where interlocking between wheel rubber and the surface will be more likely, I'll bet that relationship is greater.

It's not, according to Baker and 45. It's about the same. And the gains there are lost elsewhere (same source).

You're right, designing to ideal situations is not the best way to go. It can provide a pretty good approximation, however. If it didn't, we wouldn't use ideal cases at all, now would we? It'll give you an idea of what will happen, so you can definitely reject ideas. If you have a bunch of ideas that should work, you can mock them up and see if there's something you missed in the "ideal case" that should have been there.

[dtengineering]

Originally Posted by Bongle

The message I'm trying to drive home is: Designing by idealized physics equations is not always the best approach.

Darn reality... always getting in the way of those beautiful equations! Actually I would suggest that the real test as to how well one understands an equation is by how well one understands the limitations on the equation.

Were it not for the limitation on damaging the playing field, a very small, but hard, contact area would provide dramatically greater friction than a very large soft one. I believe someone previously mentioned using saw blades as tires. Of course it might be even better than that to use Gecko Toe Pads as a traction material.

Suction cups would also stick nicely to a smooth surface.

Personally, I think it might also be interesting to see a section of the playing field filled with pea gravel to a depth of a few inches.

Jason

[gorrilla]

i just dont want to slip while im carrying the robot

[EricH]

Originally Posted by gorrilla

i just dont want to slip while im carrying the robot

From experience, if it's HDPE, you probably won't slip unless you try to.

[DarkFlame145]

Originally Posted by rc_cola1323

I've been around for a while and I know quite a bit of FIRST history. I was wondering if this years game had a ice or HDPE floor. What wheels would you use: tread, wheel diameter and why? Also, would teams move away from omni's because too much traction wouldn't be that BIG of deal like the past couple years. Imput would be great.

Preseason Preparation, the game hints always make me curious.

gooey, pneumatic tires that are just about flat. Or maybe tank treads.

[gorrilla]

Originally Posted by EricH

From experience, if it's HDPE, you probably won't slip unless you try to.

ill probobly find a way

we are very clumsy

[AdamHeard]

I can only think of one recent year, 2003, where HDPE was a surface you HAD to interact with to be competitive, and even then, the most dominant team just used roughtop tread. 111 didn't win because they could push, they won becuase they employed a superior strategy for controlling the top; ramps and a crab drive.

Since most FIRST fields are mostly carpet, and if HDPE is used as a floor it will probably be minimal, I would suggest you really think twice before designing a base for HDPE. As, I imagine the base optimized for HDPE would not be optimized for carpet, and would require a good deal of new design and research. I believe this time, and possible weight/complexity, would be better spent on other aspects of your robot design.

Bah, what I'm really trying to say is, don't just look at a problem at face value and say, I'm going to solve you for what you appear to be at face value. In 2004, teams thought they had to climb onto the HDPE platforms to hang from the bar, some made radically complex drivetrains to achieve this. When I saw 60/67/254 hang from the ground (and 60/254 at AMAZING speed as they used their drive motors to power their winch) and completely ignore the platform, I was inspired. Not only did they not have to design any radical crazy drivetrain to climb onto the platform, allowing them to stick with a simple 6 wheel, they were able hang faster than anyone.

I highly doubt that, "Lets push", "No, lets push HARDER!", will ever be a good strategy in FIRST. You can't mention 71 in 2002 as a counterexample, as their extreme pushing ability was just one piece of the puzzle that made their robot dominant that year.

[Brandon Holley]

Originally Posted by AdamHeard

I highly doubt that, "Lets push", "No, lets push HARDER!", will ever be a good strategy in FIRST. You can't mention 71 in 2002 as a counterexample, as their extreme pushing ability was just one piece of the puzzle that made their robot dominant that year.

Adam,
I wholeheartedly agree with you except for one other variable. What if the game doesn't allow you to interact with anything but HDPE (or some other kind of slick surface) ?? The teams that have adapted to a different kind of surface and gone ahead to make a robot that thrives on a slick surface will most certainly have an advantage..

Natural selection right? Survival of the fittest...the teams that will thrive are the teams that will be able to adjust quickly to a change of environment..

Just my 2 cents.

-brando

[Akash Rastogi]

This is actually a serious question:

What do you guys think of teams drifting around on the field during a match?

Controlled drifts, of course.

[Elgin Clock]

Originally Posted by lilstogi11

This is actually a serious question:

What do you guys think of teams drifting around on the field during a match?

Controlled drifts, of course.

I'm thinking that if it's drifting on carpet, the carpet will bunch up even more on the ends of the field than it already does, but I would like to see that (drifting) happen on a game one of these years for low (height-wise) robots only - for tipping safety concerns of course.

[AdamHeard]

Originally Posted by Brandon Holley

Adam,
I wholeheartedly agree with you except for one other variable. What if the game doesn't allow you to interact with anything but HDPE (or some other kind of slick surface) ?? The teams that have adapted to a different kind of surface and gone ahead to make a robot that thrives on a slick surface will most certainly have an advantage..

Natural selection right? Survival of the fittest...the teams that will thrive are the teams that will be able to adjust quickly to a change of environment..

Just my 2 cents.

-brando

It's not impossible, but it's rather unlikely we'll get a field like that.