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The Science of Sticky Rubber

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Alex Megos trusts his rubber while cragging in the Frankenjura of Germany. Photo: Andrew Burr

What’s the right shoe for balancing on miniscule granite nubs in Yosemite? Polished limestone in Rifle? Plastic at your local gym?

Every year, Climbing’s gear testers spend hundreds of hours parsing the differences between each rock shoe to find the best kicks for each discipline. But long before our testers crammed their grubby climber toes into the newest shoes, chemists and engineers were developing the rubber compounds that make climbing shoes the one thing they need to be: sticky.

“Rubber” refers broadly to a class of tough, elastic materials so ubiquitous it is difficult to define. About 40 percent of the rubber we use is latex, made naturally by rubber trees. On an atomic level, latex and other rubbers are made up of long strands of carbon atoms decorated with hydrogen atoms. Each string is a single molecule that’s 40,000 to 50,000 carbons long.

Each chain is floppy on its own, but together they are strong along the axis, like a section of rope. They tangle and stick to one another to form flexible, sticky rubber that can bend with your foot and mold itself around sandstone grains, monzonite crystals, or the patterned plastic of gym holds. The more contact it makes with a surface, the more friction it generates, keeping your foot where it needs to stick.

Rubber has another trick, too: It’s called strain-induced crystallization. Put enough pressure on natural rubber—say shifting most of your weight onto a shallow toehold—and the rubber changes from a tangle of strands to a highly ordered crystal form. That gives rubber extra strength and durability, explains Matthew Yang, a professor at Ferris State University in Michigan, one of the last dedicated U.S. programs training rubber scientists.

The first evidence of latex use comes from the Olmec society that inhabited the Amazon jungle about 3,000 years ago. The Olmec extracted the white, sticky latex sap from rubber trees and used it to waterproof boots and clothing. Europeans saw the substance for the first time in the early 1700s. Joseph Priestley, a British chemist, described one of its first uses in the West: rubbing away errant pencil marks, hence the word “rubber.” A British explorer smuggled tens of thousands of rubber seeds out of Brazil, which banned their export to protect its monopoly. Saplings ended up in India and Southeast Asia, where most rubber now comes from.

These days, natural rubber is a wonder, but it isn’t perfect. It’s too gummy when hot and too brittle when cold. Those long hydrocarbon chains slip too much at high temperatures and not enough at low ones. The first big improvement was vulcanization, a process developed in the 1800s that hardens the rubber with sulfur at a high temperature. Vulcanized latex stays strong and flexible at a range of temperatures and keeps its elasticity after repeated stretching. This discovery gave us rubber tires, conveyer belts, and shoe soles.

Most rubbers used now are synthetic, made from fossil fuels with additives by chemists. Carbon black is a filler that makes rubber stiffer, stronger, and more durable. Other additives speed up the manufacturing process or give a splash of color.

Most sneakers you buy use standard, mass-produced rubber formulas. Companies like Vibram, a manufacturer that produces much of the rubber found on climbing shoes, try to thrive in the niches. Vibram makes soles for smokejumpers who parachute into wildfires and for soldiers operating in both scorching Middle Eastern deserts and above the Arctic Circle. They make rubber for climbing-shoe companies like Scarpa and La Sportiva. Climbing-shoe rubbers XS Edge and XS Grip 2 are two of Vibram’s best-selling compounds.

So what exactly goes into your favorite shoe rubber? Sole makers usually won’t say because the specific formulas are as closely guarded as the recipe for Coca-Cola. At a minimum, climbing-shoe soles contain fillers, like carbon and clay, and the rubber itself is not vulcanized as much as other rubbers. This keeps them soft and sticky. One key to a good rubber is controlling—and being able to repeat—the chemical reactions between ingredients when the soles are made.

The general idea behind “sticky rubber” is that it’s soft so it forms around the smallest of divots and bumps, creating more surface contact between rubber and rock and thus offering greater friction. Sticky rubber deforms in this manner hundreds of times on any given climbing day and bounces back to the original shape, but the softness also means it will wear out faster than the rubber on hiking boots or sneakers.

Five Ten’s high-friction offering Mi6 is softer than the classic C4 rubber, and thus great for overhangs where you need maximum purchase with a small amount of rubber contact. C4 is made to stick without giving too much. On small edges where a lot of your weight is on your feet, C4 soles won’t deform and slide out from under you.

Vibram has a small plant in Quabag, Massachusetts, a rural town where the factory feels like its beating heart. Some of Vibram’s chemists are based here, others in Milan, Italy. Vibram USA’s VP of Innovation and Operations, Chris Favreau, and chemist Weilin Peng are two people well-versed in the complex world of rubber, from how to make a new rubber compound that’s a little bit harder to something that is designed to smear well.

Essentially, every sole compound is a mixture of different kinds of rubber: latex, butadiene, chloroprene, and others. To make a rubber that would hold its edge better—something a little harder—they might start by adding more butadiene. But tweaking that one factor might throw off some other aspect of the rubber, something as fundamental as how long the soles take to manufacture.

“It ends up being a lot of art and science together to create a rubber formulation,” says Favreau.

Very few new compounds are wholly invented and created from scratch. Most are variations on existing formulas, optimized for this application or that preference. Favreau and Peng both rely on the rubber chemist’s bible, The Vanderbilt Rubber Handbook, a technical manual about rubber compounding and processing now in its 14th edition. But their process isn’t something they can fully explain to someone who isn’t in the business.

Their work is about the end user’s priorities, often thinking through the process out loud. For a climbing shoe, grip on rock is paramount. Favreau and Peng can exert incredible control over a variable like grip. They can make a rubber that sticks and slides, or one that sticks and then breaks cleanly. Durability would be farther down the list for a high-performance rock shoe.

Then there are manufacturing concerns. The softer rubber that wraps over your toes for techy toe hooks and heel-toe cams needs to stay attached to the stiffer rubber underfoot and to the leather upper—and both rubbers have to play well with the glue.

And of course there’s cost. The Vibram guys laugh about a compound they made recently—“best in the world,” says Favreau—that would have more than doubled the price of their customer’s product. That one didn’t make it out of the lab.

Rubber chemists have equipment that prods, pokes, squeezes, and stretches the compounds, and some of the labs have small climbing walls to get a basic idea about how a shoe will perform. They try to dial in all those elements in the lab, but nothing goes to market before testers get their hands on—or rather feet into—it.

High up on a wall somewhere, two-time USA sport-climbing champ Carlo Traversi tests new rubber formulas for his sponsor Five Ten. He’s wearing a different compound on each foot, not knowing which is which. The lab sent him boxes of numbered prototypes. He’ll wear them for a few weeks, first in the gym, then pushing their limits outdoors to see what they can do.

“It takes a long time to feel out and learn to appreciate a new rubber compound,” says Traversi. “It’s a relationship based on trust.”

One of Traversi’s shoes is performing about as well as he expected. It might even be a compound he’s worn before. But the other one keeps surprising him, sticking where he didn’t think it would. He tests it further on different holds and rock types, trying to find its limits. These are the kinds of details he’ll report back to Five Ten’s scientists. He takes notes after every session with a prototype, comparing the shoe to previous versions they’ve sent him and detailing any big positives or negatives he’s noticed. The scientists will mull over his notes, tinker with their compounds, and send him a new shoe to put through the paces. He’s just one of hundreds of testers around the world.

Traversi meets with chemists and shoe designers a couple times a year at Five Ten headquarters in Redlands, California. In between these meetings, he’s emailing, calling, and sending pictures as he tests the shoes they send him. He says the designers usually keep him in the dark about what they’re doing. The less he knows, the more objective he can be.

“Sometimes, I get them telling me, ‘Hey, you should go test it more on this stuff,’ or, ‘Hmm, that’s interesting,’” he says. They might ask him to test a shoe more on one surface or another, trying to push the boundaries. And sometimes, Traversi says, he can find creative ways to use new rubbers that maybe the designers didn’t envision.

“That can be a really good way of designing later models,” he says.

Making a new rubber compound—from design to testing to store shelves—can take years. Five Ten rubber specialist Jason Jackman explains how in this way, the process is a bit like climbing itself.

“There’s a lot of work and a lot of incremental steps between the idea and the final result,” Jackman says. “You have small, tangible wins that eventually equal a giant win.”