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Tag Archives: tire
Pinion interfering with the tire
When I applied power to the motors on Zeetah VI (ZVI) for the first time, the motors seemed to struggle and the PWM value to get the wheels to rotate seemed higher than usual. Since this was my first four … Continue reading
Posted in Hardware, Micromouse
Tagged file, friction, interefere, interference, pinion, sand, tire
5 Comments
Tyre stretcher
These new soft tyres are so soft that it is difficult to get them onto the wheels without stretching them way out of shape. No matter how hard I try, they still run unevenly. Some of that is due to the tyre itself but mostly it is just a question of trying to spread the tyre evenly. After several attempts, I gave up and decided to make a small tool to help me with the job…
A conical piece of aluminium was turned with the small end small enough to let the tyre slip fairly easily over it without much of a stretch. I ended up with a small diameter of 18.5mm but 15mm would have been better if I had a longer piece of aluminium to hand.
The large end is the same diameter as the wheel – 23mm for Decimus. A step cut in the end allows it to sit exactly inside the wheel rim.
The tyre can easily be pushed evenly up the cone and then slipped onto the wheel. Small, even movements of the tyre on the cone should ensure that the tyre ends up in place with minimum distortion.
New tyres
That was quick. One of the new sets of tyres just arrived. They are the PN racing 8 degree slicks. I ordered them 3 days ago from Kenon Hobby in the US. These tyres certainly are soft and sticky so I put a pair straight on the test rig and compared them to the tyres I have now. They seem to have about 20% more grip than the current ones. This was encouraging so I mounted them on the micromouse. That is when the first little issue came up…
They are so soft that getting them neatly arranged on the wheels is a bit tricky. They stretch unevenly and need a fair bit of massaging before they look like they are even all the way around the wheel. Once that is done they seem just fine and in a simple pull test, they certainly perform better than the existing tyres so I sent the mouse around for a few circuits. Distance calibration is out but that is not too surprising.
After a couple of trips around the maze however, the stickiness of the tyres reveals another issue. They pick up more dust that I knew was there. in the short time the mouse was in the maze (about 100 squares of travelling) there was so much dust on the tyres that they now had significantly less grip than the existing set.
Now I start to understand why the top competitors are so keen on cleaning the track and tyres. Using tyres this sticky will mean a lot of extra care in the cleaning routine. The UK rules don’t really allow for the mouse to be taken out and cleaned. For now I have put on the old tyres.
That raises the third issue. All these tyres look identical and have no markings so it is really important to keep them in clearly labelled bags.
Tyres and traction
It is true, I am losing my grip. A while ago I posted a link to a Japanese site demonstrating the grip available from a particular micromouse. A common way of doing this is to lock the wheels and place the mouse on a tilting surface. As the slope of the surface is increased, the downforce available to the tyres is progressively reduced until the mouse slips. Part of the downforce is lost due to the angle alone and is proportional to the tangent of the slope.
Thus, at 45 degrees, the coefficient of friction needed to prevent slip is 1. There is another factor at play though. As the lope increases, more of the weight of the mouse is transferred to the skid on the downward side of the mouse. This reflects what happens when the mouse is under braking or acceleration. Reaction torque transfers some force to the skid. A long mouse with a low centre of mass fares better here.
With Decimus I have been having trouble when the acceleration gets much above 3m/s/s. This is disappointing as I was aiming for at least 4m/s/s. The tyres are the most obvious reason for problems at high accelerations so I built a little test rig to see how much grip was available:
I am using Mini-z tyres. They are a bit hard to find in the UK and come in a bewildering variety of materials and patterns. these are 8mm wide slick intended for the front wheels of mini-z 1/18th scale electric racers. I have a few of these and they all appear pretty similar so the rig is intended to find the best set I have. A pair of mini-z wheels are bolted to a small meccano frame so that they cannot turn. batteries add some weight and a harness allows me to attach a spring balance. The whole assembly weights 100g and has the weight almost exactly over the wheels.
The spring balance lets me measure the force needed to start the wheels slipping. Before the test, a section of maze is cleaned and the tyres are cleaned of any dust and dirt. Several trials are needed to even out any variations.
It turns out that my very best tyres begin to slip at about 43g. My worst tyres, which do not feel much different to the good ones start to slip at about 16g! If I take into account weight transfer when the mouse is running and the possibility of dust on the maze floor, it seem I am best limiting my acceleration to 3m/s/s until I can get some better tyres.
I just ordered some replacements. These are a set of GPM slicks said to be 8 shore hardness and a set of Shimizu slicks at 15 shore hardness. I chose the GPM because they were the softest I could find and the Shimizu because several folk in micro racing forums rated them as particularly good.
Derek Hall, owner of MouseX, is fond of balloon rubber over the outside of his tyres. I tried that over the top of my ‘good’ set and found no significant difference.
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