Shape and flex pattern
Posted 2 months, 3 weeks ago by Seth Masia
Here’s a history lesson: When Norwegian farmers began running across snowy meadows on planks of wood, they quickly discovered that where the snow drifted deep, the plank flexed. When you jumped on the middle of the plank, it bowed downward into the snow. This meant that in order to push the ski forward, you had to push it up out of the hole it had made in the snow. That took a lot of work.
Some bright soul figured out that if you steamed the plank and bent it into a gentle arch (what we call camber), the skier’s weight would spread more evenly across the surface of the snow, and the skier’s weight would straighten out the camber instead of sinking into the snow. The loaded ski now made a more-or-less straight beam and could be pushed straight ahead. Running got a lot easier. If the camber were made high enough, you could carve the tip and tail a lot thinner, making the ends lighter and more supple. The ski became even easier to run on, and floated nicely over uneven snow.
By about 1840, in the Telemark region, local woodcarvers had figured out sidecut. Making the ski narrower in the middle helped the ski turn with better agility. But the narrower waist tended to sink deeper into the snow, so to avoid the original problem of the center sinking, the middle of the ski had to be made a bit thicker and the camber a bit higher. Thus, by trial and error, ski-makers learned the art of balancing flex pattern against sidecut and camber. Change any one element and you had to change the other two.
One more element entered into the balance: torsional stiffness. In a ski meant for running across uneven natural snow, you wanted a supple tip to float over and conform to the surface. But you needed strength, too — if the tip were too thin and soft it would break. As long as a ski was made from a single piece of wood, a clever solution was the ridge-top shape, carved with a central reinforcing rib standing above the top surface. The rib reinforced the ski’s beam flex (its stiffness in bending) but allowed it to twist a bit to absorb the impact of sastrugi, suncups, and the like.
After 1935, quality skis were laminated from a variety of tough hardwoods and lighter softwoods. As I mentioned yesterday, all alpine skis were still more or less the same shape, so the adustable elements were flex and weight. Now, by choosing and aligning the laminates, a skimaker could adjust the torsional stiffness more-or-less independently of the beam flex. The ability to do this took a quantum leap with the adoption of aluminum and fiberglass structures.
It became clear that racers wanted higher torsional stiffness than recreational skiers. For grip on ice, race skis were engineered with torsional stiff around 1.9 newton-meters per degrees. This proved to be a practical limit: if the torque went much higher (some metal skis went to 2.5 nm/deg) they felt harsh and hooky, and had to be edge-bevelled pretty aggressively. Most recreational slaloms — bump skis, for instance — torqued at about 1.7 nm/deg. At 1.5 nm/deg and below you had forgiving intermediate skis. And a balance had to be found between torsion and beam flex: one factory called this the “torflex ratio.” A stiff beam flex, for a stronger, more precise skier, could use a stiffer torsion.
When “shaped” or deep-sidecut skis first appeared, it took a couple of years for engineers to figure out new flex patterns and torque ratios. Early shaped skis had a tendency to “hinge” in front of the binding, so they sort of plowed in deep snow — it was the original sinking-plank problem all over again. Compared to straight skis, it was found that shaped skis needed a longer stiff section in the middle and softer ends, and the progression of flex distribution had to be matched to the exaggeration of the sidecut. Similarly, torsion had to be softened a bit at the ends to soften the edge bite. Otherwise, the new wide shovels had a tendency to climb up the sides of moguls, and the new wide tails were reluctant to release at the end of the turn.
Balancing flex and torsion to sidecut is an art. It requires skill in adjusting core thickness, and clever choice of core laminates. Getting it wrong isn’t a disaster — a ski that feels a little harsh at the ends can usually be fixed with some smart tuning. But it’s so much more satisfying to get it right.
Seth Masia
Vail Ski School
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Seth -
If I’m at a ski shop and looking to get a pair of skis, is there an easy way for me to determine whether or not a ski has an appropriate flex for me? What about torsional rigidity?
The short answer: No. There’s no easy way to tell if the ski is too stiff or too soft. You have to trust that the factory did the design job properly and that the shop’s recommendation is savvy. A lot of skiers wind up on more ski than they need or can use — along with oversize boots, it’s one of the big reasons people struggle in challenging snow conditions.
If you’re unsure about the appropriate ski flex and torsion for the snow you want to ski, that’s a good reason to get really expert advice. Or to get a ski designed just for you.
I’ll have more to say about this when I talk about sizing skis.
There’s a lot of “rockered” camber ski designs hitting the market this season. What are the advantages and disadvantages of these rockered skis?
The rocker is a way to make a very big heavy stable ski feel reasonably agile, but the solution only works in deep snow. The rocker makes the ski inherently unstable on any kind of firm surface — and severely limits the “bite” area of climbing skins. So the rockered ski is a one-trick pony, best suited for bottomless snow.
BTW, the rockered powder ski isn’t exactly a new invention. Those of us who learned to ski powder on GS race skis — and that includes anyone who learned to ski powder before 1990 — eventually wound up skiing on a bent metal ski. Sometimes you wouldn’t figure out it was bent until the tail end of a great powder season. During the years I taught at Squaw Valley, my favorite ski for deep wet Sierra Cement was a pair of Atomic 635RS, the fluorescent-pink successor to the famous Red Sled. Several years of descents through Magoo’s Madness off Emigrant’s northeast nose bent both skis back into a smooth rocker, without affecting powder performance. But the skis were scary on the last blast across groomers to the locker room.
Another approach to this problem was pioneered by Mike Brunetto. He reasoned that in order for the shovel to float up in powder, either the tail had to sink or bend. You can make the tail sink by making it narrow, but that doesn’t help overall float or turn shape — so he built a ski with a very soft, resilient tail, and called it the Heliski. It was hugely popular among deep powder freaks in the ’80s, before fat skis appeared (the Heliski had a traditional GS shape). If you think about it, making the tail soft has the same effect as building in a rocker: you get heroic performance in powder at the price of grip and stability on hard snow.
Seth
I’ve smiled seeing the word ‘rocker’ being used inthe ski/snowboard industry. It’s a flash from the surf industry. surfboard / sailboards couldn’t be decambered so ’shape’ for turning w/ for and aft rocker. Whenthe word rocker was being used solely bythe surf industry the ski industry was building skis w/ camber. Decambering the ski, and bending it INTO the snow resulted in fore and aft rocker.
When we finally decided that carving skis was more desirable to pivoting skis, and that up and down was not as efficient as over and against shape became the issue. That was when snowboarding hadn’t received wide acceptance.
Now that snowboarding thinks that skiing is a derivative of it (rather than the opposite), and riders have part surfer in their souls the word ‘Rocker’ emerges in the sport.
Rocker helps the ski not ‘dive’ in the deep and wet (fore and aft shape)…. like on a windsurfer: inside edge and ride the shape no finesse of pressure required.
Now, what I don’t get ………. my boards all had graphite stringers so that the board would not flex…… (how many boards did I break at HO-o-KE-pa by coming over the wave and landing flat NOT riding the rocker?) is why anyone would rather be ‘ON” the ski than ‘vs” the ski ..but there again is where personal preference plays a role.
Ryan