About Friction

Take a quick look at the picture to the right. Really, not too long a look. You’re going to get confused.

Now, let me explain.

When I’m designing grinds I have to think about the friction that I’m trying to counteract and manage in specific terms. I’m certain that a real snow scientist could make this more complicated, but in my mind, I’m dealing with two different sources of friction, and with two different frictional concerns. The two sources act in combination, sometimes counteracting each other. The same is true of the frictional concerns. Finding the right skis and creating the right structure and wax solution is a question of finding balance between these frictional considerations.

Sources of Friction
Mechanical friction is one source. That is the mechanical interaction of particles – say snow crystals and the ski base, or snow crystals and more snow crystals. In general, the smoother the base surface, the better the ski will manage mechanical friction.

The other source of friction that we deal with is adhesion-cohesion, commonly referred to as “suction”. Adhesion is the tendency for water molecules to stick to the ski base, and cohesion is the tendency for water molecules to stick together. In combination adhesion & cohesion create that awful sensation we describe as suction (but which has nothing to do with actual suction). In general, the rougher the base surface, the better it will manage adhesion-cohesion.

For the sake of simplicity, I have relabeled mechanical friction as “Dry” and adhesion-cohesion as “Wet” on my friction map. In reality, mechanical friction between the ski and liquid moisture is a factor to be considered under the “dry” friction heading. Adhesion-cohesion depends on the presence of large quantities of free moisture, and that moisture also acts as a lubricant. In general though, dry conditions are dominated by mechanical frictional sources, and adhesion-cohesion becomes a factor in wet conditions. So we’re using “dry” and “wet” as stand-ins, because they’re nice, short, three-letter words that capture most of what we need to keep in mind.

Frictional Concerns
Leaving aside the sources of friction, there are two different kinds of friction that we deal with in varying proportion. Static friction is the friction between two stationary objects. If you are standing still on a hill, and your skis don’t want to start sliding, that is static friction. Technically, static friction is broken once the skis are in motion, but I use the term to describe the low speed friction that makes your skis feel like they want to stop, before you break free into our other frictional regime, kinetic friction.

Kinetic friction is the resistance caused by an interaction between objects in motion. In general, kinetic friction is our biggest concern, and we’re always trying to conceive of the interaction between the snow and the ski as a system in motion.

The break from static friction (think of it as stiction) to kinetic friction usually occurs at a relatively low speed. However, when the snow is slow and static friction is high, that break occurs at a speed that you might be going back and forth across continually, especially when you’re climbing. While kinetic friction is the larger concern, we’re always talking about a combination of factors in the balance between static and kinetic frictional concerns, just as we are with our frictional sources.

Back to the image… Now take another look at that picture. The axes are representative of the interplay between the two sources and friction, and the two types of friction that we’re always trying to balance. I have taken the opportunity to try to map the various snow conditions categories that I have identified onto this chart in order to indicate the relative range of the respective conditions, and the areas of overlap. This way of thinking may be a bit too mind-bending for most, but you’ve made it this far, so give it a try. You’ll find reference to static friction and kinetic friction, as well as mechanical friction and adhesion-cohesion elsewhere on the site. This chart might help to orient your thinking.