measuring the coefficient of friction

test method

how to measure the coefficient of friction

* A series of sensors with a fixed and known spacing capture the times at which a test sled passes by

* The sensor data provides the velocity of the test sled and the rate at which it is slowing down

* The rate at which it is slowing down is primarily due to the friction of the ski on the snow (and a little air drag)

​* Measured this way, a consistent database of ski/wax performance over temperature, ski speed, snow types, etc can be assembled to give a complete view of performance

A typical test results shown to the left:

3 test skis run, 2 with a hard wax (Swix CH4) and one with a mid-range wax (Rex Blue).  Multiple test runs performed at different velocities.  In this example friction increases with velocity.   The friction increase is not due to air drag, friction will increase or decrease with velocity depending on the temperature.

​The repeatability of this test method is excellent, the confidence interval on the mean result is much less than 0.001.

Because friction can change with velocity a standard velocity is chosen for assembling a large database of results, in our case, we used a velocity of 5 km/hr.

why measure the coefficient of friction

* The coefficient of friction (COF) is the only physical parameter that describes the gliding behavior of a ski on snow

* Glide out tests and speed traps are useful but incomplete.  Friction can change with velocity.  If the velocity isn't known then it is very difficult to assemble a truly useful database of results over time and over a wide range of conditions