Gear Physics: The ATC Is Climbing's Cheapest Piece of Gear and Its Most Crucial

I've been doing adventure sports since forever: mountain biking, road biking, extreme skiing, ski touring, rock climbing, whitewater kayaking. I'm a real fucking jock is the honest truth. So, my life has seen a continuous flux of specialized gear, technologies that enable all of the above. Some of its pretty cheap, some embarrassingly expensive—but it's all neccessary. Outdoor sports are paradoxically highly dependent on technology and technological progression, even if we don't often think of it in technological terms.

So, there is another side to gadget lust. Mostly, these gadgets don't require electricity or a network connection, but they are nonetheless the product of R&D and testing processes that software engineers would find comforting. I thought it would be fun to look at some of these gadgets as the technologies that they are: where they come from, how they work, how they will become obsolete.

So, that's what Gear Physics is: an ongoing series highlighting the hardware of shredding and sending and boofing and dropping (in). If you have a piece of gear you'd like to see featured, please give me a shout at michael.byrne@vice.com or on Twitter at @everydayelk.

Rappelling does feel a lot like landing in a jetliner. Now nearing arrival and preparing to descend, you the rock climber have spent one or perhaps many pitches nervously and with absolute deliberation warring against gravity. Like the aircraft, you are capable of this and have done it many times, but every ascent—every moment spent suspended against the acceleration of gravity is by virtue of technical ability, whether granted by aerospace engineers or years of physical and mental conditioning. It comes together, as it always does, but the experience will never not feel novel. As someone that climbs rocks and dreams about airplanes, I can give you this assurance: the metaphor is sound.

Now it's time to descend. It will be just a small portion of the total flight time, but it's where consequences start happening really, really fast. Every foot closer to the ground means one less foot to recover from … an incident. And the ground is coming so quickly until it's suddenly there. The spoilers are out, the airbrakes scrape through the now-dense air, the plane flares. And lands.

There's a flare in rappelling too. A good rappel is a controlled fall. There's a sort of contradictory sense that the faster you plummet, the more in-control you actually are. This is true in a lot of adventure sports. You push things because you know you can bring them back. At the end of my full-time/overtime freeskiing days I asked myself a lot, Why is this fun? This thing about control was definitely part of the answer.

So you fall and right before the ground, your braking hand, which has been letting the rope slip through it like a chalk-caked, calloused pulley, locks the rope down and the fall is arrested. Then, just as quickly, the brake comes back off and, pop, you're on the ground like everything is normal.

In addition to some fairly serious situational awareness, the device that enabled this control is goofy piece of aluminum tubing known as an ATC. Air Traffic Controller. Things don't get more reliable.

Properly, the ATC belay device is manufactured by Black Diamond Equipment. This is the only company that sells a product with this specific name, but the same basic design is produced by pretty much every rock climbing outfitter and sold as the generic "belay device."

That is, when a climber refers to an ATC, they are almost certainly referring to a type of device, not a specific Black Diamond product. It's kind of like a Xerox machine. The photocopy idea was too big and general to be contained within a brand, so we use the word "xerox" to refer to the technology or idea. A xerox machine could be made by any number of manufacturers. It's a concept.

In the case of the ATC, this seems pretty reasonable once you actually see the thing. It's nothing more than an aluminum tube split down the middle with with a divider. It's attached to a plastic loop, which is just there to help keep you from dropping it. One can usually be had for about $15, which is not much more than a basic locking carabiner. Indeed, aside from individual carabiners, the ATC is likely to be the cheapest piece of gear in a climber's arsenal. It's easily one of the best bangs for your buck in the whole of adventure sports.

What does it do? The ATC provides friction, basically. I'm often reminded of this when I go to grab my ATC with bare hands following a rappel, where it's used to control the speed of a climber's descent down a free-hanging rope. Aluminum is among the best heat conductors out there, and this fact plus sustained friction against a climbing rope equals an intensely hot little wad of metal.

In climbing, safety is maintained via a system known as belaying. One climber ascends while attached to one end of a rope, which is fed through some anchor point above them (or in some cases below) from which it connects to a second climber, who is usually at the bottom. The second climber is responsible for feeding the rope out as the first progresses up the face. If the first were to fall, they would fall onto the rope which would stretch dramatically and then pull taut against the first anchor point. From here, the rope would pull upwards against the second climber at the bottom, who is connected to the rope via their ATC.

So, let's imagine we're falling. Say we tried to plant a foot on an unexpectedly slick nub of rock and with no warning we're airborne. First, ahh! Next, we feel the rope catch. This probably takes like half a second.

The friction of the first anchor above us soaks up over half of the falling force. Our climbing partner below is left to manage the rest. Attached to this second climber's harness is a carabiner, a small metal oval with a locking or spring-loaded gate. The rope will run downward from the anchor above us through this carabiner and back up as a sort of mirror of the anchor above. This anchor, the one attached to our partner, will be augmented, however, by the ATC, which serves to guide the rope vertically downward through the carabiner and then immediately back upward. This setup provides a braking mechanism. To prevent the rope from traveling upward through the device (and allowing us to travel downward toward the ground), our partner has to drop their hand to "lock" the rope off via the friction provided by the ATC tube.

It takes surprisingly little effort to arrest even the biggest fall and a relatively light belayer can protect a big fall from a much larger climber safely. Again, about half (52 percent, according to a widely-cited paper by Stephen Attaway) of the downward force of the falling climber actually reaches the other climber, with the remainder being taken up by the friction against the upper anchor (a carabiner) and the rope itself. So, if a falling climber exerts 4.5 Kilonewtons of downward force (200-ish pound climber falling for five-ish feet before being caught by the rope), then the belayer needs to soak up about 2.5 Kilonewtons of upward force.

An ATC device maxes out at about 2 Kilonewtons of braking power, while the upper limit for a single human hand holding a rope is a few hundred Newtons, according to a paper prepared by the German climbing bolt manufacturer Bolt Products. Thus, given an average fall, the rope is going to move through the belayer's hand and through the ATC device. This isn't quite the problem it seems, however.

Prior to the 1930s, climbing was a rather more grim activity. Ropes then were made of hemp and were thus "static." That is, when a climber fell, the rope had little or no give. Consequently, even protected falls hurt like hell, but there was also a much greater chance of the rope breaking. This changed thanks to some Sierra Club guides who developed a technique known as dynamic belaying.

Rather than running ropes through machined tubes, belayers then wrapped them around their bodies. In a dynamic belay, should the climber fall, the partner would allow the rope to slip around them for some distance. Rather than jerking to a stop and placing a huge shock-load on the rope, the climber would be decelerated over time.

This is what happens now when the falling force exceeds that of the ATC and the belayer's grip. The rope slips for some distance through their hand. How much extra stopping power this slippage offers depends on what's known as the fall factor, which is calculated as the distance a climber falls divided by the length of rope between themselves and the belayer. So, if a climber has ascended 2 meters above their belayer (imagining the belayer belaying from some ledge as below), and then falls 4 meters to below the belay point, they've had a fall of factor 2, which is as bad as it gets. If the same climber had the same 4 meter fall from a position 10 meters above the belayer, the factor would be .4, which is much better. The difference is all in the length of rope available to stretch and absorb the impact.

In any case, if the belayer allows the rope to slip (or they just have to let it slip) for an extra 20 percent of the climber's height above them before catching, they will have reduced the impact force of the fall by up to 60 percent. It's sort of like building some extra stretch into the rope.

To be clear, there are other belay devices. An earlier variation of the ATC-type device is known as a Sticht plate, which basically swaps the tube for a plate that the rope wraps through in a similar way. This appeared in the 1970s, while an early tubular design appeared in 1983 as the Latok "Tuber." The ATC itself would appear in the 1990s and has since become far and away the most popular belay device. On most any given day I could walk around my local climbing spot checking harnesses and it would be only the rare exception where an ATC wouldn't be present.

Part of the ubiquity nowadays is its usefulness in rappelling. Here, the ATC is used by the rappelling climber to control their fall. The rope is fed through the device and a carabiner in a similar manner to belaying and the rappellee adjusts their descent by changing the angle of their brake hand to the device. It makes for a smooth and graceful trip down, particularly compared to another, more elaborate belay device known as a GriGri.

The GriGri is meant to do the same thing as an ATC, but rather than the aluminum tube set-up, the rope here is fed through a stainless steel U-shaped channel. In the center of the "U" is a movable, spring-loaded cam. When the rope moves abruptly through the channel, the cam rotates, tightly squeezing the rope between metal plates. It's very effective and has the huge benefit of being automatic. Unlike the ATC, the GriGri would still lock off the rope even if the belayer was knocked out cold. It's neat, but a GriGri will run you $100 at least and for overall smoothness, it's still hard to beat the ATC; the GriGri's cam mechanism is a bit jerky.

Rappelling is a process of continuous moderated braking, which also means continuous friction. All of those Newtons have to go somewhere, and this is heat. I've never seriously burned myself with an ATC, but the lore is there. Not only have climbers burned themselves, they've even succeeded in melting climbing ropes. Which is ominous as hell, if true.

Black Diamond decided to do some experiments to see how hot an ATC is actually likely to get and whether or not this is hot enough to seriously damage a climbing rope (or a climber's hands). In simulating a fairly quick rappel of 125 feet in 10 seconds with a 400 pound load (a climber plus a lot of gear), the testers were able to get an ATC up to about 493 °F, which is indeed hot enough to melt a climbing rope. That said, similar conditions would be pretty hard to replicate in an actual climbing scenario, intentionally or otherwise.

At an actual cliff, even with a heavier climber rappelling at Kamikaze speed, the testers couldn't get an ATC above 275 °F, which is hot enough to sizzle spit and burn skin, but not hot enough to melt rope. Good thing.

Next time in this series, we'll go a bit higher tech—maybe a wrist altimeters or avalanche beacons—but the ATC is an amazing example of a recent, ultra-simple technology that's managed to help enable an entire sport at $15 a pop. You can't get a pair of skis waxed for that.