# Coefficient of friction

I was looking at scidkelly’s post about the static pull test on vex wheels.
http://simplerobotics.org/Compare_wheels.htm
https://vexforum.com/t/dynamic-pull-test-with-mecanum-wheels/20209/1
I noticed that the chart is in pounds. Coefficient of friction(mu) has no unit, but is related to F=uN. Both force and normal force can be in pounds. Is the chart supposed to represent coefficient of friction or one of the other two? Does anyone have coefficient of frictions for the wheels?

I rememeber these tests and got a chuckle when the double omni’s had more traction than the so called high traction wheels.

You can divide the table by the 7.5lb normal force to estimate the test u.
E.g. the 4 in double omni front would be around u = 6.75/7.5= .87

You can measure your own u for your robot by locking your wheels and putting them on a ramp. Tilt the ramp until the wheels slide and the u can be estimated at u = tan(angle) where angle is the ramp angle with respect to the horizontal. Make sure that the surface of the ramp is a field tile if that is where you are going to use the wheel.

Thanks, this is what I needed. Unfortunately, I cannot test at home since I do not have a kit. I may be able to do it at school and I might be able to get my physics teacher to lend me a force sensor and data logger.