Team 1296 Mecanum Bump Test

[JamesCH95]

Polymers as traction surfaces generally lose traction efficiency as normal force is increased. Search CD or any tire test data if you don't believe me. I would anticipate some specific traction (tractive force per unit weight) loss as you add on weight. You will have more total tractive force, but relatively more mass to move.

[pfreivald]

I'd be worried about repeatedly ramming my mecanum wheels into the bump... The AM HD's are quite nice, but they're not indestructible!

[wireties]

Originally Posted by pfreivald

I'd be worried about repeatedly ramming my mecanum wheels into the bump... The AM HD's are quite nice, but they're not indestructible!

The angled surface on the wheel assembly hits the bump first - it helps a little. And we don't plan to make a living going back and forth, just did not want to NOT be able to.

[pfreivald]

Originally Posted by wireties

The angled surface on the wheel assembly hits the bump first - it helps a little. And we don't plan to make a living going back and forth, just did not want to NOT be able to.

Right on -- I couldn't see that on the video!

[JohnSchneider]

Get to it Keith!

[mdiradoorian]

What I am worried about is that the wheels with lose its strength after hitting the steel bump a million times. Also you got over a wooden bump do you think there might be a difference between the steel and the wooden because they are two completely different materials?

[wireties]

Originally Posted by mdiradoorian

What I am worried about is that the wheels with lose its strength after hitting the steel bump a million times. Also you got over a wooden bump do you think there might be a difference between the steel and the wooden because they are two completely different materials?

We are thinking the wheels will hold up but the metal bump is worrisome.

[Kyler Hagler]

Man that robot looks sexy!!

[JamesCH95]

Originally Posted by wireties

Why would we not believe you?

It was meant as the 'royal you' as every time this topic of polymer load/pressure sensitivity comes up someone inevitably goes: "well that's not what my physics teacher says!" and I might be a little tired of making the same points over and over

[pfreivald]

Originally Posted by JamesCH95

It was meant as the 'royal you' as every time this topic of polymer load/pressure sensitivity comes up someone inevitably goes: "well that's not what my physics teacher says!" and I might be a little tired of making the same points over and over

As a physics teacher, I use the disclaimer that the line "...but it's really more complicated than that!" is implicit at the end of every explanation.

[wireties]

Originally Posted by JamesCH95

Polymers as traction surfaces generally lose traction efficiency as normal force is increased. Search CD or any tire test data if you don't believe me. I would anticipate some specific traction (tractive force per unit weight) loss as you add on weight. You will have more total tractive force, but relatively more mass to move.

Why would we not believe you?

Depends on the nature of BOTH surfaces of course but I'm sure you are basically correct and we have looked at the data. If we double the mass I expect a 25% reduction in the friction coefficient for urethane rubber. So I'm thinking the effect is (at worst) linear for the small increase in weight (and probably not that bad) and the bot will push its way over the bump at full weight - we'll see soon.

[Gary Dillard]

Originally Posted by JamesCH95

Polymers as traction surfaces generally lose traction efficiency as normal force is increased. Search CD or any tire test data if you don't believe me. I would anticipate some specific traction (tractive force per unit weight) loss as you add on weight. You will have more total tractive force, but relatively more mass to move.

While that may be true for "frictional traction", it's not necessarily true on carpet where a considerable amount of the tractive force is actually a normal force between projections on the wheels and fibers on the carpet. The reason brekoflex type belting and some mechanums get such good traction is the amount of embedded overlap when edges on the wheel surface dig down into the carpet, which is a function of weight but more importantly an inverse function of contact area. Our previous testing and data has shown this.

[JamesCH95]

Originally Posted by Gary Dillard

While that may be true for "frictional traction", it's not necessarily true on carpet where a considerable amount of the tractive force is actually a normal force between projections on the wheels and fibers on the carpet. The reason brekoflex type belting and some mechanums get such good traction is the amount of embedded overlap when edges on the wheel surface dig down into the carpet, which is a function of weight but more importantly an inverse function of contact area. Our previous testing and data has shown this.

Some teams have run experiments showing that wider wheels, i.e. ones with more ground contact, get better grip. A big part of this is that the 'embedded overlap' interface you talk about becomes saturated at a certain contact pressure (the carpet and tread have finite depth) and as pressure increases above this threshold the coefficient of friction drops off. Even before the interface is totally saturated the coefficient of friction generally drops off slightly with load because the carpet/tread presses together more firmly, making it more difficult for the tread to embed further.

This analysis is based on my knowledge of tire-road interfaces and what I've learned from reading many CD threads on the subject.

One thread with related discussion is here: http://www.chiefdelphi.com/forums/sh...ad.php?t=98664

[MrForbes]

Is that a normal 3.5" thick 4x4? Or is it a special 4" thick 4x4?

If I were you, I'd build a short section of "real" barrier, put it on carpet, and try it. You might find that it's a bit harder on the robot than it looks in the video with the 4x4.

[wireties]

Originally Posted by squirrel

Is that a normal 3.5" thick 4x4? Or is it a special 4" thick 4x4?

If I were you, I'd build a short section of "real" barrier, put it on carpet, and try it. You might find that it's a bit harder on the robot than it looks in the video with the 4x4.

normal, so not quite 4" tall - we are quite worried about a "real" barrier and are looking to build one, the 4x4 test was kind of impromptu - the bot has did well so hopefully things go well with the real barrier

how are teams building a real barrier?