Regarding Mark Waechter’s Remarks in 'Dissecting Kuzmin'

Wed, Nov  29, 2006 - By Leonid Kuzmin

First of all, it is a good manner to always publish a complete reference to analyzed (dissected) original article. In this case it is http://epubl.ltu.se/1402-1757/2006/03/LTU-LIC-0603-SE.pdf. Secondly; the article was published in January 2006.

Page 2

“His conclusions were:

1. There is higher contact angle of water on an unstructured ski than a structured ski
2. There is more dirt adherence on a waxed and structured ski than a bare unwaxed and unstructured ski.”

If some one carefully read the original Licentiate Thesis, he can not find in the whole Thesis such tremens as “unstructured ski” and “structured ski”. I assume, Mark Waechter calls HSS scraped (steel scraped) ski as “unstructured ski” and stone ground (SG) ski as “structured ski”.

“In Kuzmin’s study of contact angle of water (hydrophobicity) on structured vs. contact angle of water on unstructured skis, it is important to clarify that there was not a correlation to ski speed.” 
It is not correct, it is a fundamental mistake. Ski running surface hydrophobicity and ski glide have a direct correlation [1], [2], [3].

“A smooth and unstructured ski might be more hydrophobic than a structured ski…” 
The HSS scraped ski base is not a smooth. In the Thesis, Part II, Paper A, page 4, Table 1 presented the ski base roughness of SG skis and of HSS scraped skis. From this table we can even see that SG ski base is less rough than HSS scraped ski base. Stone grinding was done by one of the most skilled expert - Lars Svensson on Tazzari RP13.2 machine.

”In moist conditions an unstructured ski has less ability to inhibit propagation of boundary layer… Adhesion-Cohesion is a strong stabilizing force, and can hold two surfaces together.” 
“Adhesion-Cohesion” in this context is a capillarity drag. First we may decrease capillarity drag by increase the ski running surface hydrophobicity. By the HSS scraping we make bristle free surface of pure UHMWPE (very hydrophobic material, more hydrophobic than any ordinary glide wax [3]) with a random micro patterns, this surface is much more hydrophobic than SG surface (see the Licentiate Thesis, Part II, Paper A, page 5, 4.2.). Secondly we may decrease capillarity drag by making coarse macro patterns with manual tool as Swix Super Riller T401. This operation is recommended by overwhelming majority of SG gurus for the wet snow conditions.

“Likewise, in colder, dryer conditions with transformed snow, a structured ski has less contact area for Van der Waal’s adhesion.”
Van der Waals adhesion (Van der Waals forces) is negligibly small in our case. Snow grains usually slide on dissimilar materials such as polyethylene, ski wax or a mixture thereof. Normally, adhesion is an important part of friction when the sliding materials are similar in their molecular structure, are macroscopically smooth, and have time to bond.

In addition, it is necessary to remember, all SG machines have two very important limitations:

• They are not able to produce a very long longitudinal structure. That is a disadvantage on wet snow;
• They are not able to produce a genuine transverse structure. That is a disadvantage on cold dry snow (see the Thesis, Part I, pages 23-24);

Page 3

“As Kuzmin points out, a smooth ski has less total surface area for accumulation of dirt and sludge.” Yes that right, but the major conclusion of this research is: It is important to understand that the two interacting surfaces, the base and the snow do not need any additional lubricant than that which is always present – namely water. The optimal roughness, high hydrophobicity and dirt-repellent ability are sufficient for a perfect glide. Stone grinding decrease the hydrophobicity of the ski running surface and contributes significantly to grime attraction as explained by [4], which is not a good way to create roughness on the ski running surface. Perfluorocarbon increase the hydrophobicity of the ski running surface, but perfluorocarbon is much softer than UHMWPE and has high tenacity. Obviously, the augmentation of softness and tenacity of the outer layer of the ski base increases dirt absorption on the ski running surface. See the Licentiate Thesis, Part II, Paper B, pages 20-21.

References

1. Colbeck, S.C., A review of the processes that control snow friction. CRREL monograph, 92-2. 1992, Hanover, New Hampshire: US Army Cold Regions Research and Engineering Laboratory. 40.
    
2. Glenne, B., Sliding friction and boundary lubrification of snow. Journal of tribology, 1987. 109(4): p. 614-617.   
    
3. Karlöf, L., L. Torgersen, and D. Slotfeldt-Ellingsen, Why is ice and snow slippery? The Tribo-physics of skiing. 2005, Swix Sport AS: Oslo. p. 35. 
    
4. Kuzmin, L. and M. Tinnsten, Contact angles on the running surfaces of cross-country skis. The Impact of Tecnology on Sport, ed. A. Subic and S. Ujihashi. 2005, Asia- Pacific Congress on Sports Technology - APCST 2005, Tokyo, September 2005: Australasian Sports Technology Alliance Pty Ltd. 318-323.