Velcro tread

[Michael Hill]

Originally Posted by cgmv123

It's their job to make those kinds of calls. Are they supposed to pass every mechanism that looks unsafe because it hasn't hurt anyone yet?

This isn't an issue of safety though...it's a completely different thing.

[cgmv123]

Originally Posted by Michael Hill

This isn't an issue of safety though...it's a completely different thing.

Same concept applies. The LRI is the final arbitrator.

[Michael Hill]

Originally Posted by cgmv123

Same concept applies. The LRI is the final arbitrator.

This shouldn't be the LRI's call. It's the Head Ref's field, let him/her determine if the team is damaging it.

[Al Skierkiewicz]

Originally Posted by Michael Hill

This shouldn't be the LRI's call. It's the Head Ref's field, let him/her determine if the team is damaging it.

As Jon pointed out above, the LRI is the first contact in the chain. However, everyone should remember that the LRI, the Head Ref and the FTA work together to insure that the event runs on a level playing field. The Head Ref enforces the rules on the field, the LRI enforces the rules in the pit and the FTA (as the FIRST rep at the event) sees that everything runs smoothly. I train all LRIs to regularly check in with the Head Ref and FTA throughout the day and to make themselves available during finals on the field. As far as field damage in particular, all of us will take input from the field supervisor and will rectify the problems as they arise. I worked with at least 20 teams this year alone, whose robot was damaging the field as reported by the field super, not including Champs.

[FrankJ]

You can consume multiple bowls of popcorn discussing the difference between attaching & friction. You will find the LRI, Head Ref, & FTA don't have a lot of time for popcorn during a competition. . Their concern will be does it meet their understanding of the rules? Will it potently damage the field.

Anyway in simplistic terms, Friction can be thought of as little ridges on one piece interacting with little ridges on the other piece. With a tangent force, the ridges try to push the pieces away from each other. This counteracted by the normal force pushing the pieces together. At some percentage of the normal force, COF, the pieces will begin to move. Whether or not there is damage to the material depends on the strength of the material, geometry of the rigdes, etc. With Velcro, the tangent force is largely independent of the normal force so clearly something is going on besides friction. All of this is done in a theoretical frictionless physics lab of course.

[EricH]

Originally Posted by Michael Hill

This isn't an issue of safety though...it's a completely different thing.

Just to run this to ridiculous...

Suppose two higher-CG robots are in a pushing match. Neither has Velcro tread--but in a previous match, a robot with Velcro tread got into a pushing match and shredded the carpet near that area--but, due to the nature of Velcro damage, it wasn't spotted and covered. Now, suppose that one robot is winning already... and then the other hits the slick area left by the Velcro-bot and loses traction. The pushing robot now takes a huge power advantage, slams the pushee right through the slick area and to the wall, inadvertently getting under their bumpers and sending them over the wall into the nearest volunteer who doesn't have a chance to get out of the way. Is it still not a safety issue?


Part of any competition environment has to be that the playing area has to be kept as consistent as possible (unless the inconsistencies are specifically noted in the rules, etc., or otherwise accounted for). If the field (generic) is constantly getting extra wear due to one player, all matches after that player are getting an inconsistent playing area unless the grounds crew got in.



And you still haven't answered this: Velcro attaches to the field, which is specifically disallowed in the game rules. The head ref just sent the robot back to the LRI to get that fixed (even with NO damage to the field whatsoever, as determined by the Field Supervisor, the FTA, and the Head Referee). Can the LRI send the robot back out with no changes? Yes. Can the head ref immediately disable the robot prior to the match? Yes, and he most likely will.

[Max Boord]

Originally Posted by EricH

Just to run this to ridiculous...

Suppose two higher-CG robots are in a pushing match. Neither has Velcro tread--but in a previous match, a robot with Velcro tread got into a pushing match and shredded the carpet near that area--but, due to the nature of Velcro damage, it wasn't spotted and covered. Now, suppose that one robot is winning already... and then the other hits the slick area left by the Velcro-bot and loses traction. The pushing robot now takes a huge power advantage, slams the pushee right through the slick area and to the wall, inadvertently getting under their bumpers and sending them over the wall into the nearest volunteer who doesn't have a chance to get out of the way. Is it still not a safety issue?

No but you may want to ban all types of wheels as this could happen just by having a robot with traction wheels and a robot with slick wheels getting into a pushing match.

Originally Posted by EricH

And you still haven't answered this: Velcro attaches to the field, which is specifically disallowed in the game rules.

How exactly do you get traction without attaching to the field carpet?

[cgmv123]

Originally Posted by Max Boord

How exactly do you get traction without attaching to the field carpet?

Really?

[EricH]

Originally Posted by Max Boord

How exactly do you get traction without attaching to the field carpet?

Friction.

Here's an example of the difference between traction by friction and traction by attaching. Go outside to your back yard (if you have one--if not, try the front yard). Push your hand down flat on the grass and try to slide it--it'll probably move (though not necessarily easily). Now, instead of a flat hand, use a hand in a hook shape, digging in (or just grab a handful of grass). Doesn't move, does it? Now, just for good measure... grab that handful of grass and yank hard on it.

Now, that's a bit simplistic. But it's kind of what we're getting at here. Most robots will be using friction (yes, there's microscale interactions with the carpet and all, but for the most part that's counted as a slightly higher mu value). Maybe a robot uses attaching--see what happens when you grab and pull grass. That's what the difference is.

[Max Boord]

Still confused as to how friction can happen without any level of attachment. I am also reasonably sure that carpet will neatly fit between the ridges of wheels like this which is hardly a "microscale" interaction with the carpet.

[T^2]

Originally Posted by Max Boord

Still confused as to how friction can happen without any level of attachment. I am also reasonably sure that carpet will neatly fit between the ridges of wheels like this which is hardly a "microscale" interaction with the carpet.

You're correct, but typical treads only apply lateral force at the carpet interaction point, while velcro hooks grab onto carpet fibers and pull upwards. There's no legitimate argument that this won't damage carpet to some degree over time; as has been said already, the "loop" side of velcro loses its stickiness eventually due to damage to the fibers. Whether or not the damage caused by velcro is enough to cause its ban from use as a traction material under the FRC manual is the point under question here.

[Jon Stratis]

Michael -

First off, the question of damage to the field is one of intent of the rules, not letter of the rules. The intent of R6 and G10 combined is to prevent damage through a specific set of actions, and velcro falls into that category of actions.

The LRI's are there to make sure that every team has a successful event. In part this means we make sure everyone follows the rules so no one has an unfair advantage over anyone else. In another part, this means we make sure the robots and environment are as safe as possible. In yet another part, this means we work with teams to ensure they can compete on the field as much as possible.

The way the rules are written, R6 is, in my opinion, unclear with regards to velcro. As such, it's entirely possible for a team to get different rulings at different events - one inspector may interpret it as legal, while another sees it as illegal. However, G10 is very clear - velcro treads cause you to attach yourself to the field, which is illegal. You may very well have an LRI pass you based on the robot rules, only to be disabled on the field. Personally, knowing the game rules almost as well as I know the robot rules, I see it as part of my job to help such a team fix a problem that will cause them to be disabled, even if it's not technically illegal from a robot rules standpoint, before it actually causes them to have a problem on the field.

At the end of such a discussion with a team, for any issue, there are four possible outcomes. First, the team agrees and makes a change. Second, I have to tell the team I can't pass them because of a specific robot rule and they have to make a change. Third, the team convinces me what they did is OK and we all walk away happy. And fourth (my least favorite), the team won't make a change, I don't have any robot rule to point to, and I have to go tell the head ref what to watch out for from the team (This brings back memories of robots that could extend past the legal distance from the frame perimeter, or ones that had trouble getting in the starting configuration while loaded with game pieces...).

[JamesCH95]

Originally Posted by nuclearnerd

MamaSpoldi - that's incredible! I don't understand how any thoughtful judge could rule that a rubber pad were any different than a stationary rubber wheel.

We ran into the same issue with our brakes. We wound up being able to use them after carefully recessing the rivets that held our rough-top brake tread in place, far deeper than any riveted traction wheel I've ever seen. We were using wheel material, attached in the same way that many wheel treads are attached, and it was still a big issue.

Being that the burden of proof rests with the team, not the judge, we had to do what we were told to do despite no sound argument (IMO) that what we were doing was any different than other teams, or any evidence of damage to the field. I don't fault the LRI for taking a very conservative stance, but it was quite frustrating. Perhaps once there is more precedent with teams using brakes (or at least we have a longer history with them) people will feel more comfortable with it.

[Jon Stratis]

regarding pop rivets and tread material... at a recent off season in Minnesota a team's tread wore down enough that, when they got into a pushing match situation the pop rivets that held the material in place actually ripped not only through the carpet but also into the gym floor underneath! It's probably a good thing when inspectors lean toward the conservative side when looking at those!

[cfair]

Since this thread has the ear of the inspectors, can I ask if there's any history of teams using low pressure (a vacuum cleaner) to increase the normal force of their wheels? It would do a service to the event by cleaning the carpet! It could be in the form of a wide, non-contacting nozzle that essentially sucks up the carpet (it would need a pretty small gap all around to hold any pressure difference), or in the form of smaller feet that actually contact and (dare I say it?) attach. Think of air-bearing pucks on CMMs: with a positive pressure they are bearings; with a negative pressure they are brakes. I know in a very early game (2003?) some teams used suction devices to attach to a HDPE platform in the endgame.

I'm kinda with Max Boord here in that the nature of the game is to attach to the carpet. Would a single 24" diameter "foot" coated with roughtop be considered illegal - it probably won't damage the carpet, but the other robots trying to push you around probably will.