“It’s greasy,” we say on hot days when the holds feel like soap and our feet ooze down the rock. But while it’s easy to blame the stone, the fault actually lies in our own physiology—on those smarmy days, we lack friction. To find ideal conditions and increase our chances of sending, we first need to understand how we stick to the rock.
[Photo: Erik Kelly working shoe-to-rock friction on The Dorsal Fin (5.10d), Little Cottonwood Canyon, Utah]
On a microscopic level, friction works when two objects rub against each other. This is quantified by the coefficient of friction, which measures the force needed to slide one surface over another divided by the pressure between the two surfaces. In the case of climbing, it’s the rock’s rugosities—the tiny edges and wrinkles—catching our hands and feet that create friction. Slippery surfaces, like polished marble, have few edges to catch. Sandstone, on the other hand, is comprised of millions of tiny compressed granules providing a multitude of microscopic edges—and thus greater friction. For reference, the coefficient of friction between a hand and limestone is around 0.64, whereas on sandstone it’s approximately a grippier 0.74.
While some rocks provide naturally higher friction, no rock’s friction will noticeably change due to normal variances in ambient temperature. Deep in Earth’s crust, rocks reach temperatures and pressures at which their structure indeed morphs, affecting their friction, but the typical hot temperatures on the planet’s surface don’t substantially decrease friction—it’s our sweat that does us in. While sweating allows us to stay active in the heat by cooling us off, it also means we can lose 10 liters of water per day. And more than we like comes out of our hands—our palms have more sweat glands than any other area of the body. So while a rock won’t change in the midday sun, your fingers certainly will.
Water, or sweat in our case, is not necessarily bad for friction. In small amounts, it actually increases friction. The sweat on our hands, which normally evaporates and keeps us cool, becomes trapped when sandwiched against a rock. This builds up a thin liquid layer that acts as a binding glue, increasing the coefficient of friction. However, too much liquid and the effect is lost as the excess moisture lubricates the surface. By the time we can see the sweat on our fingers, we’ve already crossed that threshold.
We climbers usually combat this by ensconcing our digits in gymnastic chalk. The chalk’s primary ingredient, magnesium carbonate, acts as a prison for water molecules by trapping them in its molecular structure. Chalk with higher levels of magnesium carbonate traps more water, keeping hands drier longer. However, a build-up of chalk on a hold will also smooth out those friction-enhancing micro-edges, so brushing off excess chalk is essential.
A study published in 2012 in Sports Biomechanics found that using chalk increased friction on limestone by 18 percent and by more than 21 percent on sandstone. However, research has also found highly dry skin to be nearly as slippery as wet skin, since the rigidity means it can’t mold to those microscopic edges. Ultimately, keeping skin soft and supple will increase friction when other factors, like temperature, can’t be controlled for. A 1981 paper in the Journal of Cosmetic Science found that moisturizers gave hands better friction once the moisturizer has been absorbed, since the increased skin malleability allowed more surface-area contact. Thus it may help to keep hands moisturized between climbing sessions, with lotion or post-climb balm.
Another way to increase friction is to sweat less. Maintaining a lower body temperature, say by sitting in the shade between efforts, can reduce sweat. Some boulderers even bring along battery-powered fans to aim at their fingers. In controlling temperature, women may have a natural advantage, since on average they maintain slightly lower finger temperatures than men. A 2012 study published in Tribology Letters found women’s index fingers to be on average around 1.5 degrees Celsius cooler than men’s. However, the men’s fingers were found to be much more hydrated, which ultimately gave them the advantage in terms of friction.
While hot temps cause us to sweat off holds, cold temps can be just as trying. Just like dry skin, cold skin hardens and cannot mold to crystalline edges, reducing contact—just think of the dreaded “dry fire” on a cold, dry day. A little hydration can keep our hands supple (spit on them or stick them in your sweaty pits or against your neck) and a little warmth is necessary to keep our skin soft (use hand warmers or a heated chalk bag).
Friction at our feet is just as essential. The first modern climbing shoes hit the shelves in 1982 and have evolved remarkably since. At a basic level, rubber is composed of long strands of carbon and hydrogen. The entanglement of the strands gives rubber its flexibility and allows it to form to the rock, increasing friction. Most sole rubber today is a synthetic material made from fossil fuels with various additives, such as isoprene, butadiene, and silica, to make it stronger and more durable. Each manufacturer has its own proprietary formula, but rubbers are designed to form around tiny divots and bumps while still maintaining durability.
Just as our skin has an optimal temperature, so too does climbing rubber. At a certain low temperature, the structure of the molecular strands changes, making the rubber hard and glassy. Maximum friction is reached just before this transition, usually when the rubber is around 32 to 41 degrees Fahrenheit. Above this threshold, the rubber deforms too easily and can slip off.
The science of climbing friction is still in its infancy, and much remains to be systematically studied and tested. For now, it seems that optimizing friction means striking a meticulously charted balance. Cold is good, but too much cold is detrimental; hydrated skin is helpful, but sweaty skin hurtful; chalk can improve grip, but too much will decrease it. So keep all this in mind the next time you tackle your project—perfect friction is a recipe, with many different, ever-changing ingredients that need to be precisely blended to yield sending success.