From ScottishClimbs

Jump to: navigation, search

Friction is the secret ingredient in solutions to many climbing problems. It enables us to stand on footholds which clearly don't exist, and hang off flat vertical holds on roofs. Here, a description of friction is given along with an explanation of how to use it.

f = μn

This is the key formula, so remember it! It says that the friction, f, equals a constant, μ (known as the coefficient of friction), multiplied by n, the normal force.

Coefficient of Friction


Being a constant, μ only changes when materials change, so there's a different value of μ for friction between climbing shoes and rock, skin and rock, cheese and cheese grater, and so on. We want to maximise this, and the consensus seems to be to wear rubber climbing shoes and use chalk on the hands. My physics text book tells me that μ = 1.0 between rubber and concrete, but it didn't state whether that was 5.10 or Evolv rubber. There is a difference between static (not moving) and kinetic (moving) friction. Kinetic friction is less, which basically means once you start to slip, you're probably not going to stop. Friction is related to the roughness of materials, but at a fundamental level, friction arises from electrons being shared between the atoms of the two materials, and they like this.

If the coefficient of friction decreases, say by sweating, the amount of friction we can get also decreases. If decreases far enough that we pass a threshold value (figure b)) and there is nothing else we can do but fail.

Normal and Friction Force

Let's first think about the simplest case: one arm holding one sloper, hanging with all body weight on that. When you hang footless on a sloper, your weight, g acts directly down to the centre of the Earth. To hang on, your weight (which is a force) need to be balanced. This means that the sloper now has to push back with a force equal size but opposite direction to your weight (the dashed arrow in a). See how this arrow has the same length, but opposite direct to the arrow for g - the forces are balanced). This reaction force has two parts, the normal force, n and the friction force, f, each with a size and direction of it's own, and it's by adding these two arrows, head to tail, that the dashed arrow is calculated. The normal force always points directly away from the surface. So for deadhanging a sloper, the direction of the normal force arrow swings out as the sloper increases in slopeyness. It's important to note here that as the angle increases, the length of the normal arrow decreases. This has important consequences as, in order to maintain the forces balanced (equilibrium), we need the friction arrow to increase to compensate (this points in the opposite direction to movement, typically at right angles to the normal arrow, away from the direction gravity is making the hand slip). The friction arrow has a reservoir of size that it can use up until it the dam bursts and we're off. However the size of this reservoir depends on the size of the normal force arrow. We're hit with a double whammy, the normal arrow shrinks as the slopeyness increases, so the we need more of the friction arrow. However the maximum friction we can get decrease as the normal arrow decreases.

If only there was some way to increase the size of the normal arrow when holds get slopier... That would solve both issues. The bigger the normal force, the more of that lovely friction, and the worse holds we could use! But how to get it? The answer is opposition and uses more than one limb.

Getting more out of the holds


Opposition is the key. Other than chalking up, you can’t change the coefficient of friction, so changing the normal force is the only real option. Without opposition, increasing the normal force will result in an unbalanced net force pushing you up, but out from the wall. Here are some techniques:

  • Smearing: You can increase the amount you can push down on the foothold by pushing into the foothold, so stand straight up on slabs. If you have a good hand hold that you can pull out on, use it to push into the wall with you’re feet. This is of critical importance in so many climbing situations so make sure you can do it. You should easily be able to smear on no footholds what-so-ever on a vertical wall given a good handhold.
  • Grabbing: The opposite of smearing, more common on steep ground. Using a good foothold to pull out, push into a sloper handhold.
  • Bridging: Pushing feet into inwardly opposing foothold, which may not individually be good footholds.
  • Fridge hugging: Using both hands to squeeze in on outwardly opposing sidepulls.
  • Pressing: Aka Gaston. Using both hands to press against inwardly opposing sidepulls.
  • Clamping/Bicycle: Getting opposition off outwards facing footholds, maybe using a heel hook or toehook.
  • Pinching: Utilising opposition between fingers and thumb.

There’s lots of other techniques and (importantly) combinations of techniques, the possibilities are as varied as every move ever done on rock. But it’s the idea of opposition, allowing the increase in normal force, which is a fundamental building block to climbing technique. There are also dynamic techniques to increasing normal force for fleeting seconds, called "momentary grip" but I can't explain the physics too well. Be aware that opposition happens into and out of the wall and left to right but mainly it’s against gravity. Good technique is no reason to eat a kebab!

Personal tools