Gear Physics: The Leg-Saving Brilliance of Skis That Let Go
Image: Alex Egorov/Shutterstock

Gear Physics: The Leg-Saving Brilliance of Skis That Let Go

Modern skiing was made possible by a 1930s invention: releasable ski bindings.
September 2, 2016, 12:00pm

Nearly every adventure sport I can think of owes itself to some technological singularity. Some new invention came around and all of sudden one sport became a new sport. Mountain biking has suspension systems, rock climbing has sticky rubber (stay tuned), mountaineering has crampons, and downhill skiing has releasable bindings. Where once a fall encountered while hurtling (or even just kind of cruising, really) down a mountain portended a very high likelihood of a broken leg, now skis release (usually, but not always) before being able to do any damage.

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The feeling is distinctive. One moment you're in a controlled careen down a steep face and the next you're airborne, sideways and then upside down, and then gravity smashes you back into the mountain. All of the kinetic energy of you-plus-gravity is now manifested as a chaotic tumble of limbs and gear. As you twist and roll log-like, your skis will resist that motion because they're heavy and long. Just imagine trying rolling your body down a hill without long things attached at a perpendicular angle to your legs and then imagine it otherwise.

Now, imagine instead of trying to roll yourself down that hill, you had instead been pitched down the hill like a bowling ball. You can feel the torque in your shins, as if someone just took your foot and twisted it like a giant wine opener.

Writing for SKI magazine in 1971, Morten Lund wrote, "A ski is a lever. It gives the snow leverage on your leg. You can get hurt without skis, of course. You can fall and twist your leg stepping off the curb, you can play tennis and get a spiral fracture or what looks like a boot-top fracture. In skiing you can fall and hit a tree, causing a fracture impact. but the common injury is the result of a ski acting as a lever—the snow twists the tip, which twists your leg."

With contemporary ski bindings, almost as soon as this spiraling pressure begins to manifest as pain, it releases and your boots detach from your skis and you are now free to ragdoll down the mountain without the threat of having your legs destroyed by the leverage of 340 or so centimeters (combined) of metal, fiberglass, wood, and polyethylene.

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I've experienced the above scenario a lot. Accordingly, I've spent a lot of time collecting the post-crash dispersal of gear known as a yard sale. I have never, however, broken a leg. This is thanks to an often taken for granted technology: releasable ski bindings.

Hjalmar Hvam

Circa 1935, ski injuries were unavoidable. It's been estimated that at the end of any given season, some 10 percent of all skiers would be out of commision thanks to an injury, most likely involving a lower-leg break, according to Seth Masia, a Colorado ski instructor and gear historian, writing for the International Skiing History Association. This was the sport as seen by Hjalmar Hvam, a champion ski racer and owner of a ski shop in Portland, Oregon.

At the time, skis were affixed via a steel clamp which held the toe of the skier's boot in place, which was then attached by cable to the heel of the boot. It was these Kandahar bindings that were largely responsible for the steep injury rate of the time. While these "bear trap"-style bindings had the advantage of firmly locking boots to skis, opening the door for faster and more daring skiing styles, this was also their downfall. What Hvam imagined was a binding that would keep a skier attached to their skis during the stresses and strains of normal ski maneuvers, but that would yield given the torque encountered in a fall.

In 1937, Hvam himself took a bad fall on Mount Hood. Following a leap from a windblown cornice into a ravine, he twisted to avoid a chunk of rotten snow and fell hard. He broke his leg in an ugly spiral. Later, during a post-surgery ether daze, Hvam had his eureka. "When I came out of the ether I called the nurse for a pencil and paper," he would later write. "I had awakened with the complete principle of a release toe iron in my mind, and was immediately able to sketch it."

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His invention held the toe of the skier's boot by a pivoting clip. When upward "normal" pressure was applied against the clip, it stayed in place. When that pressure was removed, in the event the skier leaned forward as in a fall, the clip was free to pivot from side to side, thus allowing the release of the boot.

SKI magazine announced at the time, "The Hvam bindings are designed so that when the foot is given a severe twist, the toe irons will open. The tension required to release them may be adjusted by the skier … novice or expert." The latter is likely to be putting more strain on the bindings are part of normal skiing, so the tension is increased. For the beginner, that same tension is likely to indicate a fall.

After World War II, ads pitched his Saf-Ski bindings with the slogan, "Hvoom with Hvam—and have no fear!" He sold a few thousand pairs, and watched through the next two decades as new competitors imitated and then advanced his design. Eventually, he was run out of business by skyrocketing insurance costs.

By the 1950s, some 35 companies were making ski bindings, including contemporary names like Look, Marker, and Solomon. The latter had swapped the crude pivot of the Hvam bindings, and similar designs by Marker and Look, for a toe cup whose lateral motions were guided by a steel cam aided by ball bearings. In 1952, Mitch Cubberley patented a new design that allowed the toe clamp portion of the binding to release in all directions, and not just side-to-side.

Skis and rockets

In the mid-1950s, it became clear that, despite the popularity of and apparent improvement in technology, actual injury rates weren't improving in concert, according to Masia. The technology was there, the fundamental concepts, but the physics were lacking. A releasable ski binding only matters if it knows when to properly release.

In 1961, the German rocket scientist Robert Lusser, who had helped the Americans develop the cruise missile and early jet aircraft designs, tore his Achilles while testing out some ski bindings in a hotel room in the Swiss Alps. Determined to solve the binding release problem, he conducted a thorough engineering study, among the first of its kind.

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He identified three critical and so-far unsolved problems. The first was of friction. In order for the ski to actually release from the boot, it had to be able to rotate out the toe clamp. This he solved by adding a small teflon strip to the contact point between the boot toe and the ski surface. Next, there was the problem of the boot's heel, which lacked its own release mechanism. He added one. Finally, there was the problem of recovery. Bindings at the time, once subjected to sufficient forces, gave all the way. There was little elasticity. To this end, he remade the entire toe-clamping mechanism such that the clamp, once it began to give, could return to center should the torque disappear. This was huge.

Look's Nevada line would later be based on Lusser's toe-clamping ideas:

Before passing away in 1969, Lusser managed to patent and license his new technologies to several big names. This was around the time that skiing began to feel pretty technological and modern. Leather and springs were giving way to plastic and elaborate mechanics. Costly devices were invented to test precisely the myriad forces experienced between a skier and skis. In 1969, the FDA commissioned a $70,000 study by Dr. Larry Sher intended to get to the bottom of ski-inflicted injuries and to offer theoretical guidance on the development of next-generation "phase I" ski binding technology. The results, according to Morten Lund, who covered the research for SKI, were a work of genius.

The new bindings would incorporate low-friction pads with new preset release parameters that could be determined from a skier's weight and ability level. "This will reduce injury very decidedly for the great percentage of skiers," Lund wrote.

According to Masia, the low-friction binding systems developed during this period would eventually result in a 75 percent decrease in injuries across the industry. Most remaining Ski Patrol sled rides were, moreover, the result of upper-body injuries rather than leg injuries. Much of the credit would go to Mitch Cubberley and his briefly-ubiquitous Cubco bindings. At one point, Cubco was selling 200,000 pairs of bindings annually, dominating the global ski rental market.

The ACL stalemate

Ski binding technology is far from done. In fact, modern ski bindings utterly fail to address a chronic, even epidemic, problem: ACL injuries.

The anterior cruciate ligament (ACL) is one of a few key connections within a knee joint, providing rotational stability from the knee and keeping the tibia from popping out in front of the femur. You need it, and it doesn't repair itself. An ACL injury can really only be repaired surgically. ACL injuries, which account for 20 percent of all ski-related injuries, have been identified as a major impediment to the growth of the sport.

In skiing, it happens in awkward slow-speed backward falls, aka the phantom foot scenario. A 2014 paper explains: "The phantom foot situation, also known as the backward twisting fall, usually occurs when a skier falls backward and catches an edge as he puts all his weight on the inside of the downhill ski. As the skier falls, his body twists in the downhill direction. At the point of injury, the hips are below the knees (which are flexed over 90 degrees) and the tail of the ski, in combination with the stiff back of the boot, creates an abnormal, unnatural twisting load on the tibia. This results in internal rotation of the tibia and valgus, blowing out the ACL."

The force is applied internally and never has a chance to reach the binding where it can apply the torque and force a release. It's debatable as to whether it's possible at all for a force-driven ski binding to prevent ACL injuries. A company called KneeBinding has been pushing a new sort of binding with a heel release designed to detect the signature motions of an ACL-tearing fall, but it's both expensive and has gotten mixed reviews. It at least doesn't seem like a general solution a la the early-days advances of releasable bindings.

Somehow I managed to never pop an ACL. At the same time, I'll never really know how many of my own personal yard sales would have resulted in a lower-leg break. I can think of a few where I'm surprised enough to still be alive, where I'd wind up at the bottom of some pitch not just missing skis but poles and gloves and hats. I've had days and even weeks wrecked by crashes like that. Just being able to tame the physics of an epic ski wreck at all is pretty amazing.