Blog Posts for October, 2008

Here’s an issue you rarely hear the big factories talk about: Bearing surface. That’s the area of base plastic in contact with the snow, and it has a direct effect on the way a ski feels in powder.

A typical narrow straight ski (take for example the 1995 Rossignol 4SV, 203cm, 64mm waist) had a bearing surface of about 1150 square centimeters. If a 165 lb. skier stood on one ski, the pressure underneath would be about .14 lb per square centimeter.

Doesn’t sound like a lot. But the 1990 Atomic Powder Plus — the first of the superfat powder skis — had a bearing surface of about 2000 cm2. A 165 lb. skier put about .08 lb./cm2 on that ski.

Neither of these skis is very versatile. They lie at either end of the soft-snow performance spectrum: the Rossi slalom ski was designed for hard snow and in deep snow liked to go to the bottom and stay there. The Atomic was famous for bobbing around on top, denying the skier one of the great pleasures of powder skiing, the face shot.

If you live to porpoise in and out of deep snow, you want something midway between these extremes. For resort skiing, with a firm surface under the new snow, you may be comfortable with a pressure pattern of about .11 or .12 lb/cm2; for backcountry and bottomless powder, .9 or .10 lb/cm2. For that 165 lb skier, this would imply a bearing surface around 1400 or 1500 cm2 for the resort ski, or 1600 to 1700 cm2 for bottomless snow.

There’s a set of simple formulas for figuring out bearing surface based on length and sidecut dimensions, but as a rule of thumb, with today’s sidecuts, at a length of 165cm, a 70mm waist will give you about 1200 cm2, 74mm is about 1300cm2, 80mm is about 1400cm2, and 90mm is about 1500cm2. If you need more surface, you’ll need to go longer, Big Guy.

Seth Masia
Vail Ski School

Pete mentioned that some folks have adapted to the smooth, stable feel of Volkls. Some people like very lively, energetic skis. Some skis just feel steady and predictable in funky weird snow, and some skis feel bouncy as if they can’t wait to get a new turn going. The difference usually lies in the nature of the materials. Some materials are “quiet,” some are lively.

The ski is a spring, built to flex and rebound. Like any spring, it likes to vibrate at a characteristic rate based on its length, weight, stiffness and internal friction. Think of guitar strings: Heavy strings vibrate slowly, light strings vibrate fast. Shorten the string and it vibrates faster still. Switch from steel strings to nylon, and it vibrates with a softer tone and fades off more quickly. The fade-off is called damping. The rate at which your ski rebounds — its general energy level — depends a lot on the same things that govern the quality of sound in a guitar string: length, weight, and damping rate.

Big ski factories like to talk about damping, as if it’s a good thing. When you ski on a lot of ice, you do in fact want to dampen out vibration along the ski’s edge so it doesn’t chatter out of its own track. But snow itself is the world’s best vibration damper: It compresses, but doesn’t spring back, so the compression energy is removed from the system. Because of that, it’s easy to build an overdamped ski. Put too much viscoelastic foolishness inside a ski and it begins to feel like a long rubber eraser. It grows slow to rebound when flexed in powder, seems to take forever to return to the surface — and in any kind of wet snow the glide speed goes to hell.

To get a lively ski that rebounds quickly in bumps and comes arcing up in powder, you want hard structural layers, like steel guitar strings, that vibrate at a very high frequency and low amplitude. Carbon fiber will do this, and certain very high-quality grades of fiberglass/epoxy matrix. (High-zinc aluminum skis, made of Zicral or Titanal alloys, have great vibration characteristics, until the metal fatigues with age, or bends from overflexing in bumps.) Because these materials are quite stiff (an engineer would say they have high modulus of elasticity) they have to be used in very thin layers.

To get a damp ski for tracking on ice, you can use thicker layers of “softer” glass mat in a more viscous epoxy blend, or build rubber layers into the ski. In general, when a ski has a lot of layers that “shear” against each other, the effect is to soak up vibration energy. It’s converted to tiny quantities of heat and lost to the snow and air.

The core material can also contribute to a ski’s energy level. Vibration travels nicely along the grain of a tough hardwood, and dampens out in shear between the lignin fibers in softer woods. Most polyurethane foam cores are natural damping materials, while some very light, expensive acrylic foam cores don’t have enough mass to absorb much energy.

Another mode of energy control is called mass damping. If a ski has a heavy tip (some skis actually have weighted tips) it tends to behave a bit like the bob-weight at the end of a pendulum: It takes some energy to divert it from its trajectory, and then it’s slower to reverse direction and come back to neutral than a lighter tip would be. When the combination of materials don’t give the ski the vibrational feel the designer wanted, one fix can be to tune a ski’s vibration rate by putting metal weights or thin strips of rubber at strategic points on or under the topskin.

A properly designed ski made of top-quality materials shouldn’t need external vibration dampers.

Seth Masia
Vail Ski School

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

Pete has asked me to contribute some thoughts on the art and science of ski design. Over the next few weeks I’ll post some advice, some data and some considerations regarding the design of custom skis.

Buying skis has become both easier and tougher than it was 20 years ago. Before 1990, there were about 35 ski factories around the world, and they all made essentially the same product. The design of skis had been more or less frozen for several decades. The classic slalom ski was 205cm for men and 190cm for women, shaped 85-65-75mm. This gave roughly a 40-meter sidecut radius and a bearing surface of about 1300 square centimeters. The classic giant slalom ski was 210cm for men and 200cm for women, shaped 87-68-77mm — roughly 50 meters radius and a bearing surface of 1400 square centimeters. The big differences in skis were not in shape and size, but in flex and materials. Slalom skis were of fiberglass, GS skis of aluminum. Recreational skis were thinner (therefore softer) and made of less-expensive materials. Buying skis required a lot of trial and error to find the flex pattern that worked for your weight, strength, skill and snow conditions.

Today most of that trial-and-error is gone. You can choose a ski based on matching width and turn radius to the kind of snow you like.

For hard snow, get a ski with a narrow waist: 67mm or narrower.

For soft groomers or general western front-of-mountain skiing, get a moderate waist — 68 to 74mm.

For resort powder (with a firm surface underneath) get a mid-fat waist, 75 to 80mm.

For deep snow (with an unpredictable base beneath) get a fat waist, over 80mm. If you’re big and heavy or carry a heavy pack, go even bigger: 90mm and up.

What remains is length and shape. Most men skiing at resorts can get along very nicely on a 165cm ski, most women on 155cm. If you’re stronger than average, go a bit longer but nowadays it won’t buy you a lot of additional stability. If you’re much lighter than average, go a bit shorter — it will pay off in improved agility.

Shape means sidecut. A deeper sidecut with a shorter radius carves a shorter turn. In general, this helps best on groomers. It won’t help in bumps, where you want the tail to release to avoid hanging up at the end of the turn. A good mid-fat nowadays has a shape close to 112-71-100, which gives a theoretical radius of about 12 meters and a bearing surface of 1300. Note that the bearing surface — the ski’s ability to “float” on soft snow — is similar to the classic straight slalom ski. So is the ski’s weight. But the agility — the ability to bend easily into a turn — is vastly improved due to a turn radius roughly 25% of the old long, straight ski.

So what should shape should you buy? Start with that “generic” 112-71-100mm shape at your length, then blow the waist up wider if you’re going to ski a lot of soft snow, and pull it in narrower if you’re going to ski a lot of hard snow. If you’re an expert who loves to carve, go for a shorter radius. If you want to be able to slide the tail a bit in bumps and tight woods (or if you have to skid a bit when you teach intermediates) opt for a bit narrower tail.

That’s the basics. Next time, I’ll consider the relationship between shape and flex pattern.

–Seth Masia
Vail Ski School

About 7 years ago I purchased a pair of skis that seemed perfect for me.  They received great reviews in the ski buyers guides.  I talked to people at ski shops who recommended them.  They were made by a respectable ski company.  I flexed them at a ski shop and thought they would be ideal for me.

Day one was a sunny February day and I got bucked around on my new planks.  It often takes some time to adjust to a ski and there is commonly a break-in period with skis.  After a week, I still wasn’t skiing my best, but was adapting to the skis.  After 30 days on them, I didn’t really think about them.  I was having fun skiing and my boards had been to great spots in the Colorado backcountry, as well as some world-class resorts.    I finished the season with close to 50 days on my skis.  It had been a good season.

The next December I demoed a pair of skis and was amazed.  I could ski with greater speed, on more aggressive lines, with greater agility, using less energy.  I found a pair of skis that fit me correctly and the results were clear and impressive.  My fitness level and skiing potential had been unleashed.

I had spent several years working in the golf industry as a product designer and engineer.  One aspect of my job was developing fitting systems for matching a person with their optimal equipment.  These fitting systems and custom-made clubs worked for golfers.  In the cycling world, people were also realizing great performance benefits of custom-fit bikes.  I realized that a scientific-based approach to fitting people with their optimal ski equipment could also create performance benefits for skiers.  The top world-cup racers get custom skis so they can ski their absolute best.  Why not create a system so that recreational skiers like myself won’t make the mistake of buying the wrong set of skis?    I liked the idea of simplifying the ski buying process.  So, I began studying ski design.  Several years later I founded Wagner Custom and I’m still studying ski design.

One thing that I’ve learned is that every skier is different and that one ski design won’t be perfect for everyone.   So, how does a skier identify what product is right for him or her?    There are many paths.  I suggest starting with information about ski design and ski construction.   To help with this process, I’d like to present some information on these topics.

Seth Masia, a ski equipment gear guru, has offered to help demystify ski equipment.  Seth began skiing on the glaciers above Chamonix in 1968. After ski-bumming in Colorado, he joined the staff of SKI Magazine in 1974 and was technical editor there for two decades. He served briefly as product manager for alpine skis at K2 Corp., where he helped to design the first generation of deep-sidecut skis. He has taught skiing for 25 years at Squaw Valley, Beaver Creek and, currently, in the Vail Village ski school. Seth is managing editor of Solar Today magazine, in Boulder, Colo. He skis on two pairs of Wagner Customs, both 166cm, 119-72-104mm — one pair fiberglass with a maple-ash core and one pair aluminum with a maple-aspen core.

In the coming weeks, Seth will be posting some guidelines about ski design, ski construction, and ski materials.  Thanks in advance, Seth, for sharing some of your insight.

Keep watching the Wagner Custom ski blog….