The Science of Friction

The known fact about friction

The common understanding is that getting as much rubber as possible on the rock is good and will work against you slipping. This is pure intuition, everybody knows this! I have seen this fact argued by professional climbers and I too have argued this point.

Now... what are the actual facts of friction?

The actual science of friction

Static friction, the friction before starting to slip, is what we care the most about in climbing. The friction force works against the force that pushes towards making you slip... until it no longer can equal that force, and you do slip.

The formula for Static Friction Force (Fs) is:

Fs = μs * N

Where μs is the coefficient of static friction and N is the Normal Force.

To dig a bit further

• the coefficient of static friction is all about how one material responds to the other. Generally, rough and sticky surfaces yields a high coefficient and polished smooth surfaces yields a low coefficient. Note that a material does not have a coefficient, the coefficient is between two surfaces.
• the Normal Force is the force acting perpendicular to the surface plane (between the two surfaces).

Do you see the drawing on the wall?

Before we dig further down the rabbit hole... did you see the area of the surfaces anywhere in the formula? Are you sure?

Yes, it's a fact... the surface area has nothing to do with the friction force!
It truly does NOT matter if you smear that rock with all your toes straight in a big shoe or if you touch the rock with a sliver of an edge of a tight shoe (given that the rubber and the rock is the same).*

It might be worth having a look at this lecture on friction by Dr. Walter Lewin at MIT, you should really stick with it for the first 12 minutes to have your reality adjusted.

The mechanics of you slipping is either down to the surfaces or the force you apply, there are no two ways about it! So... you need to be particular about cleaning your shoes and the rock before getting on. Dirt, sand, chalk, moisture... most anything will lessen the coefficient of static friction, including temperature. Other than that, you can pick your shoes with care and switch shoes according to rock and conditions to get the optimal rubber blend / rock type combo. The big picture is that this factor is pretty much out of your control.

Now all that's left is the Normal Force, and that is the subject of most of my analysis and conclusions on advanced climbing technique. How do you control the Normal Force?

Please stay tuned for more posts on rubber friction analysis.

May The Normal Force Be With You!

*This is taught at most universities etc with little respect for the finer details of friction and how this formula came about. If you have Phd in rubber friction analysis you will know that this formula is not entirely accurate and is deducted from friction between smooth metals, but for practical application in climbing I will stipulate that for now... it is close enough :-) Please stay tuned for more scientific posts on rubber friction analysis.