How to Create the Best Sumo Wheels to Get More Friction and Torque

(Last Updated On: October 19, 2017)

Sumo wheels are important for a high-performance sumo robot. Here I explain what I learned after two years in the Spanish National Liga (LNRC).


Introduction

The goal of create your own wheels is to obtain the maximum friction, therefore more traction. In a sumo battle all the robots have the same weight, in this case 3 kg. In case of two robots are faced, he robot which have more friction win. Friction is proportional to the friction coefficient and the force to the ground. As all the robots should weight the same, we suppose that the force is also equal. Having a higher friction coefficient could in some case determine the winner.

Schema of 2 sumo robots fighting
After some research looking for the possible materials. A nice material that could bring the maximun friction is the urethane rubber 2020 or similar.  The wheels mustn’t be sticky because it goes against the competition rules. In many regulations you have to past a sticky test to participate.
The sticky test  consist of putting the robot over a piece of paper and if you pick up the robot and the paper is still stuck to the wheel, you have an illegal wheels and can not participate.
Cleaning the wheels with alcohol is very common between combats. This increase the adherence by removing the dust of the wheels.
To make a robot wheels, three part are necessary to take into account:  hubs, rubber and a mold.
The material of the rubber is, maybe, the most important decision and sometimes is difficult to choose the rubber and find a distributor. The main feature of the material is the hardness. Our rubber needs ShoreA 20 oder 30. If we use less hardness it will be too much sticky and it will wear easily, and if we use more, it will lose friction.
Some examples are
durometer
We can measure the traction of various rubber and chose the best one.(exert o’meter)
hardness sclae in plastics
We can use “any” material with the wanted shape for the Mold.  Perhaps you must use vaseline or some lubircant to take out the wheels from the mold. We mustn’t contamine the wheels surface.
I have chosen a aluminium mold to use the same material than the hubs.
The wheels have to parts: the aluminium mounting hub and the wheel, as it can be seen in the following picture.
Design idea
The mold could be also from plastic materials, for example the Brooks family autonomus sumobots and friends have made a solid mold of High Density Polyethylene (HDPE), as it can be seen in the picture. 

zoom

Manufacturing hubs is perhaps the most difficult step if you don’t have some machine like a lathe… or you can send it to a manufacturer.
Here is a 16x speed video of the dried process:

Wheel prototypes

I have realised a wood prototype of the wheels because the manufactured wheels took longer than expected.  The final wheels works nice to test some basic functionality of the robot like try the motors, chassis dimensions and general mobility. Here leave some photos of them:

Here can be seen the two parts of the wheels: the aluminium mounting hub (6 mm) from pololu and the wood part.
 To avoid that it slides on the floor, I wrapped it whit some normal balloons.
The wheels mounted on the structure:

Design

The tires have to been specially manufactured with a given dimensions to maximise the contact area with the floor. Therefore for design, the front motors have an encoder built, therefore the are bigger and the tires are narrower than the rear one.

Design idea

The specifications of the tires can be seen in the following pictures:

Front wheels
Rear wheels
Section

Rubber casting

The wheels have been realised by a casting process. First of all the mold has to be lubricated, therefore you can take it out easily after the casting process. I have used pure vaseline, bought in the farmacy.
Vaseline
Here are the mold, with cilindric shape. They are designed to get 5mm wide of rubber.
 To centre the wheel, a screw is inserted in the middle .
A big washer is put in the screw, to avoid that the rubber entry into the hub.
Some equal sized elements are added to the mold, in order to get a concentric wheel.

 

Mix the A and B product with 1:1. Before this we calculate approximately the rubber volume that we need and we divide up into 2 to get out form each storage jar.

 

We leave about 24 houres to cure. If we are in a hurry, it is possible to take it out from the mold and leave them dry in the air. But this could generate some surface imperfections.

Results

The results are a high quality robot wheel with a lot of friction with the ground. The wheels adhere to the ground surface properly without being sticky.

Do you have some question about the process? Please leave some comment about your experience

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