I was looking into the plaction wheels and I was wondering what the pros and cons of wedgetop and roughtop tread were.
Thanks
THIS post: http://www.chiefdelphi.com/forums/showthread.php?t=89326&highlight=roughtop talks all about that.
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Sorry, I checked the forums but I did not see that post.
I had to go through a few pages myself, only going there because I had the question also. It’s a good question to ask.
The difference in traction from roughtop and wedgetop is going to be stronger. Roughtop has individual “fingers,” which increase the surface area in contact with the rough carpet, while the wedgetop only has the wedge pattern, which is going to have to bend as one. This will decrease the amount of plaction in contact with the ground.
What the tires look like (Surface Contact View)
Roughtop:
Wedgetop:
X
X
X
Or: Think of it this way: Which is going to have more surface area when pressed against the ground with ~120 pounds on it: A Pine tree branch, or a dowel rod?
The pine branch’s needles are going to compress and increase the surface area in contact with the surface, while the dowel rod is going to not compress as much, and have less area in contact with the ground.
Good thinking, but it’s a little more complex than that.
First of all, roughtop and wedgetop treads come in different materials. [strike]Polyurethane[/strike] Edit: The Brecoflex Supergrip I was thinking of is PVC tread on polyurethane backing], synthetic rubber, natural rubber, etc.—so it wouldn’t necessarily be reasonable to compare a soft wedgetop with a hard roughtop.
If you’re just modelling friction between smooth surfaces, friction is independent of surface area in contact (purely a function of normal force and coefficient of friction). But FIRST carpet and tread often have features that interlock. In a steady-state case (i.e. the robot is operating) traction is conceptually the combination of the surface interactions (like interlocking, adhesion and friction) as well as material properties and failures (is this strain rate sensitive, is the tread flaking off, is cracking of the tread changing the shear forces applied to the floor at a microscopic level, etc.).
In short, I don’t really know the true answer, and don’t even have a great theory for predicting the low-level interactions. But I think some teams have tried some empirical testing that might shed some light on the issue. Note that it’s common in FIRST applications (and I suspect elsewhere) to simply treat all these complications as part of the coefficient of friction. I’ve seen or used values of 0.9 to 1.3 with good results (for both tread patterns). Edit: Brecoflex has a summary of “friction” tests for robot drivetrains on carpet.]
To make things more complicated, things like wheel width and diameter have been shown to make a difference in the actual effective “coefficient of friction” you will see when running on carpet (proof that the Coulomb model of friction isn’t a great model for tread-on-carpet interactions).
Another consideration is tread wear. Brand new roughtop treads on small diameter, wide wheels really dig into the carpet, but after a couple of competitions, you will probably want to change your tread.
You can buy a selection of tread materials from McMaster-Carr if you do a search for “incline conveyor belting”. I would recommend getting a few samples of various materials and you can trade off between traction on carpet, traction on any other field surfaces, and wear. Note that McMaster only sells this belting in 3" width (minimum), so you will need to use a vertical band saw (or similar) to cut them down to wheel size.
Yes, rough top and carpet are 3 dimensional and there is a grain to carpet too.
Leads to a complex model. Some deterministic testing would be the way to go. We tested both and went with rough top for our swerve. There are other options than conveyor belting. In 2010 we had alignment problems with our first swerve bot. We had extreme wear with the conveyor belting. To deal with this we tried bike tread. There is a high end bike store near us with over a hundred different tires in the store. Found 2 exceptional tread patterns that have monster grip in the forward reverse direction and slip in the side to side. We had allot of tread carcuses laying around so we ground the tread off down to the fabric. Used thick CA to bond the bike tire to the conveyor belt. Used some carpet thread to reinforce the edges and to prevent de-lamination. Fantastic treads. wear like iron and good traction when needed and slip when turning. Never tried it but, probably would be fantastic on 6 wheel base. Note that out of all the tires in the store only 2 patterns were good. It’s allot of work but can go way beyond the performance of conveyor belting.
This is what we did in 2006 when we switched over to this type of wheel.
If you select and test your tread material properly you will find that there are roughtop materials that will not wear down on a standard carpet playing field all season. I’m not telling what it is, but there is a material that we have used for 6 years on our robots that even using them in continuous demos and driver training has never been replaced.
If you do end up purchasing the belts from mcmaster carr and end up cutting them I suggest purchasing Blue Nitrile Rough Top belt. Based on previous posts and robots (254,294,973) I’ve heard it is a very robust material experiencing little wear throughout the season compare to the rough top AM sells. I’m not too sure if using a band saw to cut to size is a good method though (haven’t tried it myself yet).
We’ve used blue nitrile. I know 233 ran all of 2008 on one set of wheels, but we never experienced the longevity that others did. It certainly didn’t wear super fast, but it was not as good as some have advertised.
I would note that you can run the heck out of them and still have tread surface left, but it’s basically smooth with dimples in it. We would lose all of our “nubs” and get to this point somewhat frequently.
Odd, we didn’t get to that in our blue nitrile ever. By comparison, our normal roughtop wore “smooth with dimples” shortly after our second competition.
Because I am a huge fan of these, have you also considered Colson wheels?
http://www.robotmarketplace.com/products/colson_wheels.html
They have good wear properties, are pretty cheap, and are fairly “grippy” on the carpet as well. Plus I hear that there will be a COTS hub available for the 1.5" wide wheels this year.
I just found this, which summarizes tests conducted by Brecoflex for their timing belts in robot applications. It includes a chart of “friction” tests on carpet. They quote a coefficient of friction of up to 1.6 on carpet with smooth natural rubber.
Some have had success with using their belts as treads; others (1114/1503/1680 in 2006) haven’t. Either way, they’re an expensive option and require some dedicated engineering to do right.
WELCOME
we find that traction is really not as effective as maneuverability, my team uses mechanum,and we run circles around tank drive teams.
And then us tank drive teams push you around.
I believe FRC has yet to see a mecanum robot actually driving circles around anything that moves. Video evidence, please!
Anyone ever have the treads sheared off while trying to do skid steering, or trying to turn a crab drive sitting still?
We’ve had that problem. The break point is not at the rivets but in the tread itself. The roughtop tread on one of our wheels sheared off the canvas core. Has anyone else had this problem? If so, any advice?
I too would like some video evidence.
And just curious, are you trolling?
Around the CD community this is basically verbatim to what is typically regarded as the stereotypical troll post.