Assessing the contact patch problem

Here is a fairly simple way to find out if all the wheels on your micromouse are making proper contact with the ground.

I have know for a while that Decimus 4 does not behave consistently but was unable to think of a particularly good reason why. In a previous post, I showed that the tyre wear was uneven and that led me to thing that the tyres were not making proper contact with the ground.

With a multi-wheel micromouse, it is important that the wheels all make contact evenly with the ground. If they do not, it is likely that the mouse will not behave in an unreliable manner and may show different behaviours for left and right turns for example.

Perhaps you have noticed that objects that touch a glass surface cause a discontinuity in the reflective/refractive properties of the glass at that point. You can see this when you hold a glass of water and see your fingerprints clearly from the inside of the glass. If the micromouse sits on a flat, level sheet of glass, each tyre contact patch will show the same effect.

The tricky part is seeing those contact patches from underneath the glass. You will not be surprised to learn that it is all done with mirrors. All that is required is to raise a sheet of glass and place a mirror at an angle underneath it. When you look into the mirror, you can see straight up at the underside of the mouse.

I used a small picture frame and a bathroom mirror along with some hot glue and foamboard to make a simple frame to hold the parts in place. This is far from sturdy or durable but I don’t expect to need it very often and it is easily rebuild if the need arises.

micromouse tyre contact patch viewer micromouse tyre contact patch viewer

Lighting can be critical and you will need fresh new tyres in order to see the contact patches. Worn tyres do not work unless you wet them. When you get it all right though, the results are clear.

micromouse tyre contact patch

I am going to have to shim the motor mounts to get these patches even. The two mounts are slightly twisted in relation to each other and it should be a simple matter of raising the front of one mount or the rear of another.

Now – did I glue them down? I forget.

No – it was easy. Undo one screw and place a paper shim under the mount, then re-tighten.

I am not sure it can be made obviously better. It is hard to check the patches for actual size. It is certainly an improvement though. You can just make out that the mouse is a little nose-heavy with the front wheels having slightly larger patches than the rear ones.

Micromouse tyre contact patch fixed

Clearly, the wheels are still not straight though.

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