Skate Ski Flex for Dummies (aka the rest of us)

By: CXC Team Member, Andy Brown

I would like to preface this article by saying that having me writing about ski flex is similar to a freshman physics student lecturing about quantum electrodynamics. It’s going to be crude, and will probably cause vomiting, hair loss, and general malaise in anyone who knows better. If I can pass on a tidbit of knowledge, and give some general rules of thumb, I’ve more than exceeded my pay grade. Also most of this information was learned via osmosis from my roommate and flex guru Josh Doebbert.

Countless years have been spent collectively by the ski community worrying about which wax to choose for the next upcoming citizen race. Skiers will argue back and forth about brands, hardness, mixing, layers, graphite, fluoro, et cetera ad infinitum. I know, I’ve done it too. You just can’t help it. I once shouted to my friend across a parking lot before a race which fluoro I corked in and had 20+ master racers stare open eyed like I had just prophesied buying stock in Apple in 2005. While it is true the wax does matter, the flex characteristic of your ski is a more important and often overlooked parameter in determining overall performance. The fastest 130% pure-fluoro-holy-water-unicorn-blood uber wax in the world won’t save you when you take a squirrely powder ski out on an ice rink and fly face first into a pine tree. Do you want to fly face first into a pine tree? Didn’t think so; better learn how to choose a ski appropriate for the conditions. Moving on. We agree that we want a ski that will provide the best performance for a given day, What is the best performance? Obviously the ski that helps us ski a certain course in the minimum amount of time is the best. To do this we need a combination of stability, floatation, and moisture control. Since the memory (or lack there) of our recent encounter with a conifer is fresh, let’s begin with stability.

Ski stability is achieved by edge force and contact zone spacing. Unstable skis want to rotate under your foot like the hand of a clock. To counter this an opposing torque needs to be generated to prevent the rotation. Sideways force is generated when the ski’s edges dig into the snow near the tip and tail. Since where these edges contact the snow are spaced apart from the foot, this creates a lever (moment) arm and a stabilizing torque which we need to avoid running into another tree. The spacing of these contact zones is often referred to the ski’s wheelbase. For the edges to dig in, the ski needs enough torsional rigidity to prevent twisting of the tip and tail of the ski. Grab the tip of your ski and try twisting it clockwise, if it twists a lot it will make it harder for a ski to hold an edge in icy conditions. The good news is most higher end skis have good stiffness in this dimension and there is relatively little harm in having “too much” torsional rigidity if you ride your skis flat like your coach was yelling at you to do. Shamless plug, Rossignol skis are known for having some of the best torsional rigidity on the market. High school skiers take note; this is an excellent way to talk your parents into buying you a more expensive model of ski, “I really need this ski so the added torsional rigidity will generate more edge force and prevent me from crashing into a pine tree. You wouldn’t want me to crash into a pine tree would you?” When things get icy the force generated at the contact zone edges goes down. To counter this, hardtrack skis have longer contact zones (increases edge force) that are spaced further from the foot (increases moment arms) and thus restore the torque that was lost by the harder snow. Great, so to not hit the tree we just need good torsional rigidity and two long contact zones way the heck out near the tips and the tails and everything will be hunky-dory. Well not quite. Notice how the optimal answer to any problem is never an extrema (politicians I’m talking to you). The same is true here. Stability is often traded off with our next performance attribute, floatation.

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Floatation is a pretty easy concept. Floating over the snow takes less energy than sinking in and plowing through. To float over soft snow we need as low of base pressure as possible and a soft tip and tail to conform to the snow. Problem is when we were optimizing our ski for tree avoidance we intentionally limited the contact to near the tips and tails reducing the total contact area and increasing the pressure. Worse yet in making the ski bridge stiff enough to hold those distance contact zones, we unwittingly stiffened the tip and tail making the ski less compliant. Sure we might not hit that pine tree anymore, but the skis are stiff and have high pressure. The hardtrack skis are going to dig into soft snow faster than a ice auger running on nitromethane. Also the stiff tip will make climbing about as enjoyable as a root canal. Okay, so we really still don’t want to hit the tree, but we’d rather not punch into level 5 every time there’s an uphill or fresh snow. We decide to compromise a bit. We keep the long contact zones which will give good edge force and low base pressure. We soften up the camber and tip little bit so it doesn’t plow but still has enough contact zone spacing to give stabilizing torque. There, we’ve made the one ski quiver killer, and we ski happily off into the sunset avoiding trees the whole way…and then it gets warm.

In the Midwest the stable-floaty ski we built is actually pretty darn good for most days. It can do a lot of conditions well and coupled with a fine grind is great for typical conditions. When it’s -15 we tend to forget that snow is actually water and when it finally gets warm enough that you can distinguish the gender of a skier from more than three feet, that snow starts act more like water. When a ski exerts pressure on snow that is warm enough, it causes partial melting. This is a good thing most of the time which is why skate skiing is more fun when it’s warm enough that you only need one pair of wind briefs. Too much water however creates a suction effect. A coarser grind can help with moisture management, but at a certain point our ski just can’t keep from getting sucked down. To beat the moisture we need small contact points and a camber that rapidly rises from the snow to break the suction. Well that stinks. If we do build a ski for stability and flotation, the low riding camber and long contact zones will literally suck in moisture. If we build a hardtrack-wet ski, the high tip and tail pressure and short contact zones will kill us in cold soft snow, and finally if we build a floaty-wet ski we lose our contact zone spacing and stabilizing torque and we run into that stupid tree again. It’s an impossible problem to solve…Wait why are all those world cup skiers traveling around with vans full of skis?

No ski can do everything perfectly. There are great skis out there that can do many things quite well but adding skis improves performance especially in extreme conditions. Two or three well chosen pairs can do almost everything (if you structure appropriately). Sure skiers on the world cup have 40+ pairs, but a small well thought out quiver will serve 99.9% of skiers just fine.

Rossignol makes this really easy with three condition specific models. The S1 for colder soft conditions, the S2 for universal conditions and more hardtrack, and the S3 for wet.
Knowing all this the next time on the trail we look at the snow before we pull our skis out of the bag. We grab a ski with enough flotation that we don’t plow, enough moisture management to prevent suction, and enough stability so we don’t run into that damn tree again.

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