Maximizing Friction Coefficient for Mini-Sumo Robot Tires: Tips and Tricks

[milo_ec]

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.
Thanks!
:P

 

[gleem]

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.

 

[Rive]

the force pushing them together contribute to.the frictional force.

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...

 

[jrmichler]

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.

 

[milo_ec]

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...

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.

 

[milo_ec]

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.

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.

 

[milo_ec]

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.

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.
Thanks!

 

[Rive]

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

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?

 

[CWatters]

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

In case it's not obvious it should be all wheel drive.

 

[milo_ec]

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.

 

[Tom.G]

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,
Tom

 

[NTL2009]

Could you use square or hexagon tires? That would give a flat surface and lots of traction against being pushed.

 

[Tom.G]

Could you use square or hexagon tires? That would give a flat surface and lots of traction against being pushed.

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!

 

[NTL2009]

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!

Square/hex tires definitely would have other problems. Like all things, it's a trade-off.

I was thinking of the tracks used by Earth moving machines ('Cats'), but I guess that only applies to rough surfaces. The "contact force and the coefficient-of-friction" comes into play on very smooth surfaces. But nothing is perfectly smooth, I'm curious if there is some real world effect.

OK, another crazy idea (or brilliant idea, if it works! :) ) - suction cup/s? Use a vacuum pump or mechanical push/release?

 

[gleem]

OK, another crazy idea (or brilliant idea, if it works! :) ) - suction cup/s? Use a vacuum pump or mechanical push/release?

Maybe. Remember TARS in the movie Interstellar. Put the suction cups on the feet keeping one set anchored at all times.

 

[NTL2009]

Maybe. Remember TARS in the movie Interstellar. Put the suction cups on the feet keeping one set anchored at all times.

Or just one large suction cup in the center. Lift it when moving, drop it and pull a vacuum when being attacked.

 

[Baluncore]

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.

You must keep low while preventing others getting below you. One way is to have light wedge shaped radial fingers, like an outer skirt, that rest on the floor and wears to conform to the floor surface. Very little vertical movement is required, they could be hinged under their own weight, or lightly sprung. That minimises the height at your outer edge, while an enemy blade passing under a finger and over another will lock before you are lifted. You only need sufficient fingers to block or obstruct a blade, or to pass under the enemy. Less fingers give less floor pressure. You are looking for a finger material with a low coefficient of friction to the floor.

 

[milo_ec]

suction cups are actually an interesting idea, inother news, I just got the tires, and also silicon rubber to cast tires and experiment a little. It is worth noting that these robots can move fast, very fast. I¨ll try to get some footage and pics of waht i got so far.

 

[some bloke]

remember that small wheels are better than big ones for this sort of thing - whilst tractors and industrial stuff have large wheels, this is for ground clearance. a small diameter wheel will increase the torque you can put down onto the track. Watch some videos of Razer from Robot Wars pushing the other robots around - tiny wheels, damn near unstoppable.

Can you steer? if so then the ideal approach will be to get alongside the opponent and push them sideways.

On a smooth surface like you describe, a walking suction bot could be pretty awesome. The problem is that if the opponent is fast, it will always get away.

I was working on something else when I came across this:
https://www.inventables.com/technologies/suction-cup-tape
it's essentially sticky tape but without the glue, it uses little dimples as suction cups to stick to a smooth surface. I think it could be a brilliant thing to put on the wheels! and it would certainly work wonders on a set of tracks.

Are you allowed any sort of grappling device? IE can you pick up the enemy and then drive to the edge and put them down? or are you limited to the skirt?

if you have the space, having a skirt which slowly revolves will do wonders, as it would turn the opponent sideways when you attack, and turn them away when they attack. not spinning like a weapon, but just to move the opponent around a bit.

 

[sysprog]

Hey @milo_ec -- brilliant move coming to PF to let the membership be part of your advisory set in your robot war campaign. Do you have a link whereby we could look at all the rules/ constraints/ parameters/ conditions? I for one am intrigued.