Coefficient of Static Friction on a Turntable

[jhawks8]

I'm doing a lab for my technical dynamics class and that lab is to determine the coefficient of static friction between a metal washer and both metal and rubber surfaces.

I did the experiment using 3 washers set at different distances from the center of the turntable as instructed, and recorded the RPM at which the washers of each distances flew off the turntable. My data is below:

Rubber Surface
r(m) |RPM |w(rad) |µ | AVG µ
0.03 |123 |12.88 |0.507|
0.03 |154 |16.13 | 0.796| 0.676
0.03 |147 |15.39 |0.724|

0.06 |100 |10.47 |0.671
0.06 |97 |10.16 |0.631 |0.607
0.06 |88 |9.22 |0.52

0.1 |71 |7.44 |0.564
0.1 |80 |8.38 |0.716 |0.647
0.1 |77 |8.06 |0.662

Metal Surface

r(m)|RPM |w(rad) |u...|AVG µ
0.06 |77 |8.06 |0.397
0.06 |67 |7.02 |0.301 |0.376
0.06 |80 |8.38 |0.43

0.08 |62 |6.49 |0.344
0.08 |62 |6.49 |0.344 |0.363
0.08 |67 |7.02 |0.402

0.1 |61 |6.39 |0.416
0.1 |61 |6.39 |0.416 |0.403
0.1 |58 |6.07 |0.376

What is not making sense to me is why the coefficient of static friction is the lowest when in the medium distances. Also, the largest coefficient for the metal surface is the largest radius, and the largest coefficient for the rubber surface is the shortest radius.

I need help understanding this, or if I'm doing something wrong and I don't know it. Any help is really appreciated.

 

[jhawks8]

Sorry for the sloppy tables.