I’m trying to find two things. First off, what kind of material(I’m thinking some sort of rubber) do most teams use, and second, what would be the coefficant of kinetic friction on the carpet(Requires knowlage of what kind of carpet FIRST uses). I know I can find this out experamentaly but I don’t quite have enough information to do it.
About 2 years back our team help with the AZ regional clean up, as a reward (and they had nowhere to store it) the officials gave us among a few other teams the carpet. We divided it up among the teams. To me it seems like regular commercial carpet, your school probably has it.
Are you going to account for the friction of the drive train or just free spinning wheels?
It is pretty much commercial carpet. I think the specs have shown up a few times.
Wheel coverings: Lots of teams use conveyor belting. Others use the standard wheels. Mostly rubber, though some use urethane. Metal is not allowed.
You may also want to check on plastics and metal. HDPE and Lexan have appeared as driving surfaces; diamond plate made its appearance in 2006. (And then there was 2007…)
The coefficient of kinetic friction is a material property. It will be the same for a given wheel regardless of what it is connected to or loaded with.
The coefficient of static friction would also be very important to know. When the wheels are not slipping the friction is actually static, not kinetic. The coefficient of kinetic friction only comes into play after your wheels have started slipping, or when scrubbing during turns.
Many teams that make their own wheels cover them in Incline Conveyor belting from McMaster-Carr. I believe the ones that are typically used are the Gum Rubber Wedgetop, SBR Roughtop, Rubber Roughtop, and Nitrile Roughtop.
This was just in a thread around here somewhere, but I didn’t find it on the first search and I should be working on a paper right now :eek:
If you are looking for a rough number for the coefficient of friction, Team 494/70 did some experiments and found it to be around 1.3 (for conveyor belt “brouwn rough” on sample carpet). I believe the KOP wheel is around 0.9. Web is supposedly assymetirc with 1.1-1.2 in one direction, and 1 in the other.
If you are interested in making some scholarship money, do the experiments and write up your results. Also check out the surface area relationship. Since this grip is like tire grip, part of it is actual coefficient on friction and part is mechancial grip from fibers and tread interlocking.
On a side note :Too much Info Warning: While tire grip can be estimated by a single coefficient of friction, actual grip/weight depends on temperature, load, %slip or slip angle, and tire age…Most of these are usually small factors, but ask anyone who has driven 3 season tires below 32F, they can be huge factors.
Is it even possible to have a coefficient of friction over 1? I think mue (friction coefficient) has to between 0-1.
This is a common misconception. A value for Mu greater than one simply means that the frictional force will be greater than the normal force on the object.
Ff=Mu*Fn
This isn’t related to wheels, but it is related to friction. You may find this my spreadsheet in the whitepapers helpful http://www.chiefdelphi.com/media/papers/1741
Not to be picky, which is natural for me as a physics teacher…but the coefficient of friction of any type is related to two objects … the wheel contact material, “tread” (or any material for that matter) AND the carpet or other surface. Values of mu for the wheel tread materials found on here are, for the most part, experimentally determined based on the
use of the playing field carpet as this second material.
If you do a look up on frictional coefficients in a materials text be careful not to assume that any figure is good for the playing surface. They will give you a good general feel for the coefficient but different materials react differently with different surfaces.
good luck
That’s not picky, it is correcting my (fairly significant) error.
I realized this when I made the follow up post regarding Mu > 1 but must have forgotten to save my edit (that’s what I get for being on CD at 2 in the morning)
IFI lists the coefficient of friction for its traction wheels (which I assume is in relation to carpet). The coefficients for wedgetop and roughtop are listed as 1.2 and 1.3, respectively.
The surface constraint brings up an important point for rookies. Make sure you drive your robot on carpet. The traction goes up, and some robots that are very manueverable on concrete or other hard surfaces have a difficlut time on carpet. Some won’t turn well.