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Misc. Highest friction coefficient?

  1. Feb 13, 2019 at 9:51 AM #1
    Hi, I was recently tasked to build a mini-sumo robot (10x10cm. 500g. the objective is to push your opponent out of a 75cm. diameter arena or dohyo), and I was wondering about the tires that I'm going to use. I want to increase the friction coefficient as much as I can, so I was wondering how to do so. The track or dohyo, is made of wood with black paint, very smooth surface. Everyone uses silicon slick tires 2.5cm. diameter, and about 3cm wide. Can you help me to get the coefficient up? Does the area of contact make any difference? Maybe not use slick tires? any material better than silicon for these?
    I order some of these tires to try out, and get the coefficient with the track to have a reference point.
    PS. sorry if my grammar is all wrong, I´m not a native english speaker.
  2. jcsd
  3. Feb 13, 2019 at 11:10 AM #2


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    There are anti slip sprays commercially available from Krylon or Rustoleum. They do not specify the coefficient of friction but at about $10/can you could give them a try. Surface area has no effect on the frictional force only materials in contact ( their texture) and the force pushing them together contribute to.the frictional force.
  4. Feb 13, 2019 at 12:08 PM #3
    Actually, if everyone uses the same tyre then this might be the most important. With pushing the opponent slightly upward your robot will gain some extra foothold, while the other one will lose some. If such thing is allowed by the rules...
  5. Feb 13, 2019 at 8:03 PM #4


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    You will have to experiment to find if wide or narrow tires get better total friction. Also, sanding the tires could remove any surface film and increase friction.

    Sorbothane (www.sorbothane.com) makes a range of soft elastomers that they sell for vibration isolation and damping. Those elastomers have extremely high friction coefficients when they are new. One source lists a static friction coefficient of 10.4 for the 50 Shore 00 durometer Sorbothane. I have some Sorbothane samples on my desk. The surface of my samples is so sticky that the samples stick to a vertical surface. I suspect that tires made of Sorbothane would not last long, but would get really good traction while they lasted. McMaster-Carr (www.mcmaster.com) sells Sorbothane in the U.S.
  6. Feb 14, 2019 at 7:25 AM #5
    It is allowed to have a "scraper" (don´t have a better way to describe it) on front to get the other robot a bit on top of you, that will get the effect you refer to. I design my robot to take advantage of these and get better traction, but one with a sharper scraper could get these effect to his favor, so I´m trying to compensate for these with better tires.
  7. Feb 14, 2019 at 7:29 AM #6
    Well that is something defiantly worth looking in to, but I haven´t seen those in my country. Also, I don´t think I mention these, but no adhesives are not allowed and you get penalized if you damage the dohyo in any way.
  8. Feb 14, 2019 at 7:33 AM #7
    Imports are a slow here, but I will definitely buy some samples to try. You are allowed to make repairs, so even if they only last one match, with that friction, they would be perfect.
  9. Feb 14, 2019 at 7:42 AM #8
    In such case who has the 'scraper' hanging the lowest (without touching the floor) will win. Friction is just invalid if the tire does not touch the floor at all... To handle this to your advantage will be the key I think.
    Will it be two- or four wheel drive?
  10. Feb 14, 2019 at 7:44 AM #9


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    Add adjustable ballast so it's only just under the weight limit.

    In case it's not obvious it should be all wheel drive.
  11. Feb 14, 2019 at 9:00 AM #10
    I´m aiming for a 499.5g robot, I´m at 447, but no tires or controlboard, it is two wheel drive, with some weight on the front, but mostly on the back (75% to the rear, more or less). People tend to add weight to the front to ensure that they will lift the other robot (more weight, the blade gets a tighter fit to the ground), but I´m countering that by getting better grip and a sharper blade.
  12. Feb 14, 2019 at 8:29 PM #11


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    You mention that others add weight to the front of their vehicles. If you then get your blade under them they will have a lever arm on your front, the pivot point at your front wheels, and tending to lift your rear wheels.

    All this points to either all-wheel drive as @CWatters stated, two-wheel drive on the front wheels, or at least spring suspension on your front wheels. The spring suspension decreases the lever action tending to raise drive wheels.

    Let us know how you do in the competiton!

    Cheers and good luck,
  13. Feb 15, 2019 at 11:37 PM #12
    Could you use square or hexagon tires? That would give a flat surface and lots of traction against being pushed.
  14. Feb 16, 2019 at 2:37 AM #13


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    Unfortunately that would yield less traction. Friction force depends only on contact force and the coefficient-of-friction. If you increase the contact area the force per unit area decreases, giving the same final result.

    Additionally, there will be a decrease in traction coming off the high spots of the wheels. In the limit, consider if the wheels are spinning fast enough, that the vehicle will be airborne until the next high spot comes around. No traction while it is floating in the air!
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