Learn This: Friction Science

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Friction is the magic ingredient in climbing. It’s what keeps you off the ground and makes subtle weight shifts and delicate sequences successful. Understanding the how and why will make you a better climber. In simple terms, friction is the resistance that one surface encounters when moving over another. In high school physics terms, friction is independent of the contact area, but in a climbing context, friction is proportional to the contact area (more contact equals more friction). We’ll look at three materials—rubber, skin, rock—to see how each behaves.

Rubber

Climbing shoe rubber is designed to be soft enough to mold to the rock, and this softness results in increased friction because it wraps around the irregularities in the rock, upping the contact area between the two surfaces. This softness and the resulting performance vary based on temperature; this means some days you can stand on a certain foothold and others you can’t.

FrictionFig1

Temperature

Climbing shoe manufacturers design their rubber to work best in a specific temperature range—approximately 32° to 41°F. (Of course, all rubber compounds are different, but this is an average according to shoe manufacturers.) Below this range, the rubber is harder and won’t mold well to the shape of the rock. Above it, the rubber will be too soft and will deform easily, causing it to slip. The reason climbing shoes work best in the cold is because they are designed to. But why such a low temperature range? At temps above that range, most people’s hands will begin to sweat (even ever so slightly), which reduces gripping ability. So shoe rubber is designed to function best at the same time that we have optimal hand grip. The 32° to 41°F range is too cold for hands to sweat, but it’s not so cold that you can’t warm your mitts up relatively easily. The variability of rubber’s friction with temperature means that a climbing shoe that works well when bouldering in Hueco in January won’t have maximum performance sport climbing in Rifle in August. (That’s not to say climbing in Rifle in the summer is pointless; your shoes just won’t be in the absolute optimal temps for highest friction.)

When Formula 1 cars hurtle around a track at 200 mph, the tires heat up significantly, so F1 tires use rubber that works best at those high temps. This is why pit crews pre-heat their tires before a race—so they work optimally right from the start. Climbers can do the same thing with some very simple tactics. During winter when your shoes are cold and dry, try warming them up by putting them in your puffy next to your heat-radiating body, and during summer, leave them in the shade or next to a chilled water bottle after you take them off.

Edging

When standing on a small, flat edge, friction isn’t the deciding issue; this is why mountaineering boots are reasonably good at edging. Standing on an edge is mainly a mechanical act, so the best rubber for edging is actually stiff, meaning it won’t deform and roll off the edge. In contrast, smearing is most effective with soft rubber. Manufacturers compromise by choosing a rubber compound that lies somewhere in the middle, striking a reasonable balance between smearing and edging performance. Reasonably new shoes tend to be more effective on really small edges, as their more pronounced toe allows them to maximize the contact area (fig. 1).

Tread

Car tires have tread to improve grip in wet conditions because it allows the water on the surface of the road to escape as the tire presses down on it, improving the contact between the tire and the road. Without treads, the water is unable to escape, and a thin layer of water remains between the road and the tire, reducing grip.

However, if the road is dry, the tread reduces the contact area between the tire and the road, thereby reducing potential grip. This is why treadless tires are used in motor racing when the track is smooth and dry. This is the same reason climbing shoes don’t have tread while sneakers and approach shoes do.

Rock

Anyone who has climbed on a variety of rock types knows that rougher rock (limestone, unpolished sandstone) seems to have more friction. But rougher is better only up to a point. On really rough rock, the irregularities are so large that the rubber can’t adapt to them. This is the point when increasing roughness reduces friction (fig. 2). Softer rubber is particularly effective on rough rock, as it molds better. Similarly, rubber will perform better on rough rock at a slightly higher temp than the aforementioned range (32° to 41°F). Morals of the story: Smooth rock is best climbed in temps on the lower/colder end because you don’t need rubber to be at maximum softness. Rough rock is best climbed in temps on the higher/warmer end because of increased softness.

FrictionFig2

Rock factors that determine friction:

>>Grain Size The physical dimensions of individual particles of rock: Some are larger, and you can see the big crystals (coarse-grained quartz monzonite of Joshua Tree); some are smaller, and the rock seems more uniform (fine-grained granite of Yosemite).

>>Grain Shape Sharp grains will have higher friction than round ones. How the rock was weathered (wind, water, glaciers, etc.) influences the grain shape to be smoother or more jagged.

>>Porosity The proportion of the non-solid volume to the total volume (fig. 3). The less porosity (meaning denser rock, less air—image on right), the larger the surface area for rubber contact (read: more friction).

>>Sorting Rock that consists of a range of different particle sizes is less porous than rock that is well-sorted (uniform particles size), as the smaller particles fill the gaps between larger ones.

>>Composition Chemical components in the rock; e.g., granite is largely silicon dioxide and aluminum oxide. Certain compounds have more or less friction.

>>Cementation How well the rock is held together.

FrictionFig3

Skin

The skin on our fingertips gets a lot of punishment from friction, and it’s not unusual for sore or bloody tips to force a session to a premature halt. Our tips can only bear a certain amount of pressure before the skin starts to tear. This pressure (force over a given area) can consist of a low force on a small area (a tiny crimp on a slab), a high force on a large area (a big sloper on an overhang), or anything in between. Get more mileage from skin by maximizing contact area with the rock, which spreads the load and reduces the chance of damage. Contrary to what you might think, if your skin is feeling thin and damaged, avoid crimps and small holds, aiming for climbs with bigger, slopier holds that require more skin contact.

Temperature

Like rubber, skin is affected by temperature. Obviously if it’s too hot, your skin sweats and creates a layer of moisture between your skin and the rock, hence reducing friction. Also like rubber, your skin grips best at a certain temperature range. It varies by person because of the differences in the temperature at which people sweat, but generally speaking, it works best in the 32° to 41°F range mentioned previously. Cold makes the skin become harder (it reduces the fluidity of the liquids that constitute our cellular membranes) and prevents sweat. When it gets too cold, your fingers will get numb, and that will prevent you from climbing. Keeping hands warm between burns is crucial to being able to grip on super-cold days. Beyond that, very dry air in 30° and below temps can cause your tips to get smooth and hard, sometimes described as “glassy.” This condition reduces grip, especially on smoother rock types (some types of granite, polished sandstone), and it can cause your hands to slip quickly and without warning. Warm your hands and then use sandpaper lightly on the tips to roughen skin for increased friction and prevention of that glassy feeling.

Best Qualities for Skin

>>Smooth There should be no rough spots where dead skin is hanging off and can easily tear. Use sandpaper to remove this fraying skin and to smooth the tips between attempts.

>>Thick Calluses These make climbing on rough rock or sharp holds more comfortable, meaning you can bear down more and increase the contact zone. However, if they get too thick, there is a danger that the entire section will rip off in one massive chunk. Your skin will be naturally thick in some spots, but use sandpaper to smooth calluses down so they’re even with the rest of your skin. It shouldn’t stick out at all.

>>Dry Moist skin is slippery, so dry is good. Sweat, water, or any other liquid on your skin acts as a lubricant, decreasing friction. But if skin gets too dry, it becomes brittle and tears easily, so moisturize regularly.

Chalk

In 2001, researchers from the University of Birmingham came to the conclusion that chalk—wait for it—reduces friction. Participants in a study were asked to hold a flat rock in their hand while an outside force pulled it away. The coefficient of friction (basically how much friction existed between the two surfaces) was less when participants’ hands were chalked (vs. unchalked). So why do we use it? Because a chalked hand still has better friction than a sweaty, unchalked hand. If you are lucky enough to have hands that don’t sweat, then you can enjoy great grip without chalk. Otherwise, only use the bare minimum of chalk necessary. Once you dip your hands, remove the excess by blowing it off or patting your pants before you start climbing. Try one of the brands of chalk that contains an extra drying agent to increase sweat absorption.

Remember this:

Think in terms of the Goldilocks Effect. The rubber on your shoes should be soft but not too soft, your skin should be hard and dry but not too hard and dry, temperatures should be low but not too low. While we can’t manipulate many of the environmental factors that influence friction, there are a few simple things that we can do to tilt the odds in our favor.

>>Choose cold, dry days with highs in the 40s (noaa.gov has the most accurate weather reports) to attempt your projects.

>>Wear quality climbing shoes that have broken-in but not damaged (fraying or hole-filled) toes and soles.

>>Carry a small carpet or utilize the included patch of carpet on your pad to clean your shoes of any major dirt. Taking it a step further, use a damp rag to fully clean the soles, allowing a few minutes to air-dry.

>>Use the minimum amount of chalk necessary, remove any excess from your hands before you climb, and never put chalk on footholds.

>>Brush the holds between attempts to remove chalk, thereby increasing contact and friction between your skin and the rock itself.

>>Keep your skin in good condition by moisturizing consistently and sanding it as appropriate.

David FlanaganAs a climber, writer, and freelance journalist living in Dublin, David Flanagan has developed and published guidebooks for several areas in his home country. His newest book, Bouldering Essentials, has practical advice for everyone interested in bouldering—beginner or expert (threerockbooks.com). 

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