Quote:
Originally Posted by JVN
I was pretty sure... until your post. Am I missing something?
-John
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Let me prefice by saying this is a really good test, and a pretty accurrate method, but as this thread is starting to expose, like any acquired taste, there are various levels of CoF snobbery.
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I am only slightly messing with you. As Don and James have described, there really isn't a CoF number, but more of a CoF curve that depends on a lot of parameters. As having the most accurrate CoF is generally not an award at a Robotics competitoin (though it would make a great science fair project), one must assume that you are trying to get a CoF to do a calculation. If you are trying to do a calculation, the inherent question you need to ask yourself is how accurrate do I need to be? 1%, 2%, 5%, 10%? As the CoFs I have seen for non-lunacy wheels tend to range from 0.8 to 1.3 with most being in the 0.9 to 1.1 range, I would assume that a test with +/-10% accuracy is probably not sufficient.
Back to how accurrate does it need to be, within the measuring technique you showcase, If your angle accuracy is accurate to within +/-1 degree, at 45 degrees, you will come up with a CoF of 0.966 to 1.036. Or a total error band of about 7%. +/-2 degrees will get you 0.93 to 1.07. In terms of FRC wheels, that would be pneumatics fully inflated (according to AM wbsite) to fresh wedgetop. My phone has an inclinometer app on it that measures within 0.1 degrees, but on a flat surface, with me holding it steady, it varies +/-0.7 degrees.
As you said in your post, you don't believe a patch, or a single wheel will give you represenative data. This tells me that you believe that the weight and weight distribution must have an effect. If this is true, then a tilt table will shift the normal force distribution on those wheels and thus the tilt table will give you a different result than flat ground pushing. this is especially true when the board reaches high angles which just so happen to correspond to high tractions. Also, small flex in the board can throw off the angles as you will get a different contact patch than you were expecting.
Couple this information with the Slip % variation Don and James are talking with (on street tires it is often around 0.95 peak at 8-12% slip, and then dropping down to 0.8-ish past 20% slip), and you will find a lot of neat variation that adds up.
We used a similar method on the polycarbonate seeing which wheels slid first. We didn't need to know the exact number, just which had more traction.
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I may be a bit sensitive on this subject as I am coming off of a 24 hour race with a bunch of CoF super snobs. At around the 12 hour mark, there was a 1% difference in lap count between 5th and 14th place. There was only about 3% difference between 1st and 20th.