16 December 2008

How sticky are your rock boots?

Is it possible to determine experimentally which company makes the best rock boots? Spadout.com [1] ran an article where they tested the coefficient of friction for various climbing shoe rubbers. The experimenter cut out pieces of boot rubber, put a small weight on top of the rubber and put the rubber on an inclined granite plane. The angle of the plane was then increased until sliding occurred; the sliding angle was used to give each rubber a ‘sticky rating’ – where a steeper angle required means stickier rubber.

The experimental setup used by spadout. Image from [1].

The 9 different rubbers showed a 6.5° range – which seems to suggest that there is not much difference between them (the results were later scaled to give more pronounced differences in the sticky rankings). Although in the realms of gnarly slab climbing a little extra friction might go a long way. However, rubber friction is not straightforward and there is more to rock boots than just the sticky factor.

The coefficeint of friction is not affected by normal force for most materials, but for rubber it can vary. Image from [2].

The friction coefficient for rubber can vary with normal force [2], in other words, unless you apply a force that is similar to the weight of a climber, the coefficient you obtain will not be representative. Of course, this will only disrupt the rankings if the different rubbers show different relationships between normal force and friction coefficient. Also, if each type of rubber has a different temperature at which it is designed to be optimal [3], testing the boots at the same temperature will mean that the results will be biased by how far you are from each boot’s preferred temperature.

Perhaps more important, climbing requires two seemingly contradictory properties; high stiffness for edging and soft sticky rubber for smearing on smooth slabs. The test may be indicative of smearing performance – but is that the most important aspect?

[1] http://www.spadout.com/r/climbing-rubber-test/

[2] M. Lindner, Experimental and analytical investigation of rubber friction, University of Hannover, Pdf here: http://fluid.ippt.gov.pl/ictam04/text/sessions/docs/SM2/12086/SM2_12086.pdf

[3] An interesting discussion of climbing friction: http://www.theshortspan.com/features/friction.htm

No comments: