October 15, 2007
Ski flex can be as simple or as complicated as you wish to make it. As simple as finding a ski to fit you or as complicated as finding a fast ski.
Several years ago I met a Finnish ski technician named Harri Altonen. Harri introduced me to the concept of measuring the pressure areas or contact zones with the snow to figure out if and when a pair of skis will be fast. Since that day I’ve become basically obsessed with finding really fast skis, flexing every pair of skis I could get my hands on. Going as far as the Atomic factory in Austria to flex more skis. That was like a dream come true. Hundreds of skis all to myself. After a couple of years of flexing and testing skis, I think I am ready to say that there is a measurable flex difference between fast and slow skis.
When a ski glides on snow, it has to deal with resistance from friction (dry snow) or suction (wet snow) as the obstacles to speed. Gravity can be a real hassle as well. Friction and suction are always present. The issue is finding a ski that minimizes one or the other to match the conditions. Ever skied on really cold snow, so cold that your skis crunch? That’s friction. Your skis are not melting enough snow and they crunch because they are gliding on snow not a thin layer of water. It’s like rubbing a ski against styrofoam. Friction is dependent on the coefficient of friction from snow (wax decreases the coefficient of friction) and the normal force which is basically your weight. F=uN. Remember the “friction is fun formula” from physics class? I bet a lot of people would disagree with that statement. I personally don’t think friction is fun at all when your skis are slow. Notice that friction does not depend on surface area. The same amount of friction spread over a larger area means less friction per square inch and therefore faster glide in dry snow. Longer contact zones are faster in dry snow.
In wet snow, suction is a big problem. Ever come out of the shade and hit a sunny spot and nearly fallen forward because your skis almost stopped? That’s suction. Suction does depend on surface area. Suction is caused by a thick layer of water acting as glue. In warm temperatures you need to rill your skis to break up the suction. Decrease the surface area and so decrease the suction. Friction would increase but suction is a far bigger problem in wet snow. Shorter contact zones are faster in wet snow.
So, the theory is pretty simple. Long pressure areas fight friction and short pressure areas fight suction. I should point out that wax and stonegrinding effect glide much more but those are factors you can change. Compressing a pair of skis and leaving them in the sauna does not always produce the best results.
The problem with flex is figuring out how short is short, how long is long and how do you measure it? Well, precision is very important. When you compress a pair of skate skis at half body weight (glide) and full body weight (push), the pressure areas or contact zones are different among skis with similar weight ranges. Also stiffer skis tend to have shorter pressure areas and softer skis tend to have longer pressure areas but this is by no means set in stone. My dream is to find a really stiff and therefore responsive ski for cold snow and a soft ski that floats well for wet snow. Notice that I haven’t talked about stiff skis for hard track and soft skis for powder, the reason is while that consideration is important for the feel of a ski which is related to speed, I think that pressure areas have a more direct correlation to speed than stiffness.
Compress a pair of skate skis at 7-8cm behind the balance point (that is about the ball of the foot) to half body weight and slide a piece of paper from the middle out and from the tip and tail in. Mark those spots. Then compress to full body weight and mark those spots. You should have marked four contact zones of different lengths (use X’s for half weight and O’s for full weight if you like). Two contact zones for half body weight, one in front and one behind the balance point. And two contact zones for full body weight, one in front and one behind the balance point. If you are bored, for further precision you could compress a pair of skis to 30, 60, and 90 kilograms to see how the contact zones react. In general measuring from the middle out to the tip and tail, it is very easy to find the right place for the paper to stop. These are the important marks for fit to the skier. But from the tip and tail in, it is much harder and these are the contact zone marks.
It would be very easy to make any ski look like a wet snow ski by sliding the piece of paper farther in or a cold ski by not sliding the piece of paper in far enough. Precision is key. Another issue is at half weight the pressure areas can be much shorter than at full weight. They can also be much shorter in front of the ski than behind the heel. This is where you have to develop your skill in identifying what is short and what is long and where it is most important. I think some level of consistency between half and full body weight is important. A ski should not look very different at half weight than it does at full weight. This can happen sometimes and basically what you have is a wet snow ski at half weight and a cold snow ski at full weight. If the difference is really big, this ski could be good for a much heavier or a much lighter person but likely I would choose a different ski.
I once had a pair of skate skis that were labeled warm. At half weight they looked warm and at full weight they looked cold, but if you have a heavier person on those skis (like me, remember I said gravity can be a hassle? I meant that in more than one way) then all of a sudden the half and full weights look cold. And wouldn’t you know it, they were great cold skis. My friend, Adam Swank tried them and said they were some of the fastest skis he’d been on. So he protested when I wanted to get rid of them.
Another important thing to keep in mind is approximately two thirds of your weight is behind the foot and one third is in front of the foot when you ski. Therefore I believe that the pressure areas behind the foot are two thirds important. This can be annoying because the pressure areas back there can be really long. My advice is strive for consistency between half and full weight and measure more than once to be sure of your marks. With practice you will develop a standard of what is short and what is long for each of the contact zones.
Next time I will write about classic flex. Theoretically, classic flex should be easier to figure out because you only have half weight to deal with. At full weight the ski should be fully compressed to get good kick. However the margin of error on classic skis is tiny compared to skate skis because it is so important to get good kick. I believe the softest ski without dragging is best. The thickness of klister makes that difficult but feeler gages with all the different thicknesses of metal can provide hours and hours of entertainment. And finally, I think that stronger skiers should not have stiffer skis. Good kick is most important unless you are ok with double poling.
In conclusion, be precise, look for consistency between half and full body weight and if you are uncertain measure again and pick the best pair. Flex can be really complicated if you want it to be, or you can just enjoy skiing. If you want to talk about flex, write me at nikolai248@yahoo.com or call or come visit me at The Ski Hut, (218)724-8525.
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October 18, 2007 at 1:35 pm |
Thanks for the write-up. I’m going to evaluate my own skiis and start paying better attention to which ones are better dry or wet.
I have to take exception to your comment about dry friction. After stating that friction doesn’t depend on surface area, you go on to say that less friction per square inch leads to faster glide. This is only true if the surface area does not increase by the same proportion! Glide is affected by the total force, not the force per unit area by itself. I don’t disagree that you’ve had faster skiis with a longer contact zone, but it’s because the total friction force is decreasing. And since your weight is staying the same, that means that you are getting a lower coefficient of friction with a longer contact zone.
October 20, 2007 at 6:12 pm |
I guess all I can say is friction depends on weight, if you spread out the weight over a larger surface area then the friction gets spread out therefor decreasing the force of friction per square inch. I don’t think the coefficient of friction changes. Thats determined by how smooth the surface is at the microscopic level.
October 21, 2007 at 8:37 pm |
As I remember it (and it’s been 25 years since I’ve had a physic course), the basic F = uN eq. says that friction is independent of surface area. It’s a first order eq. and as you dive more deeply into the question of glide and friction, I think you’re finding that the coefficient of friction changes with ski flex and surface area. So when you say, “The same amount of friction spread over a larger area means less friction per square inch and therefore faster glide in dry snow.” That statement would make sense if the coef. of friction was decreasing depending.
Jay
October 21, 2007 at 9:25 pm |
Nikolai,
Thanks for the article. There are some quibbles, but it’s a good conversation starter.
For one, I find itobjectionable to hear the word “suction” used to describe the impedance in wet conditions. You’re much closer to the truth when you compare it to glue. It’s adhesion/cohesion (the adhesivity of water to the ski, and the cohesion of water to itself), and is primarily a mix of VanDerWaal’s interactions. But not “suction”. Suction requires a partial vacuum, and it’s just not happening with your skis.
There are a variety of strategies for minimizing adhesion in wet conditions. Structure, hydrophobic base treatments, and proper ski selection are primary. It’s important to note that wet conditions can be in conjunction with hard consolidated platform, or it can be wet with an unconsolidated soft and loose skating lane. The choice of skis and flex will need to take the track conditions into account.
Thanks again for starting the discussion…
-Mark Waechter
October 22, 2007 at 7:34 am |
Mark,
If wet conditions present adhesion/cohesion problems due to vDW, then rilling a ski should increase the surface area of the ski and should increase the opportunity for adhesion/cohesion and slower skis. (We know the opposite is true.) I’ve always heard that surface tension (a spin off of cohesion as you use it) as the reasoning for slow skis in wet snow, but I’ve never heard a convincing explanation of the molecular level attraction between ski and water. Why not suction…the type with a partial vacuum? If you pry apart two wet pieces of glass, there is suction, and it seems plausible this is occurring with skis.
(Hopefully I’m typing better this morning.)
Jay
October 22, 2007 at 1:33 pm |
Jay,
Structure inhibits the capillary propagation of the adhesive boundary layer. The ridges create discontinuities that impede continuous capillary flow, and the structure increases the path distance for the capillary propagation, which helps resist the expansion of the adhesive area.
Also, structure, at a microscopic level, changes the intimate contact area (contact at the peaks).
When you glue two sheets of plastic together, is that suction? No, of course not. …there are other things happening (if you want to start a fight, ask two chemists how glue works…), but part of the deal with glues is adhesion and cohesion. When you break apart something that’s glued together it fails at the dissimilar junction (adhesive failure) or the glue breaks (cohesive failure).
It’s typically not a vacuum when you pull apart two pieces of wet glass… Where would the vacuum come from? There’s nothing to limit the flow of water… …except the adhesion and cohesion. You can get the same result if you have microscopic holes drilled in the plates (which would release any vacuum). The wet glass surfaces have adhesion. Similarly, if skis had suction problems, you could solve it by using a very porous ski base… …but that doesn’t work. The adhesion is still there. AS a thought exercise, consider the wet glass plates, but think of what would happen with progressively more sticky fluids (or less sticky).
Also, with 2 pieces of wetted glass, try sliding them apart (sheer), and it still results in difficult stickiness.
feel free to email (xcgrind@ultratune.net) to further discuss…
-mark
October 24, 2007 at 5:34 pm |
Hey Fellas:
Another thing to keep in mind, more related to ski flex than physics lessons. There are many ski companies, and thankfully for us retailers, they all do different things well. Each company has its own philosophies on how a camber profile should be built into the ski. Each ski company has its own molds. Some have VERY stiff tails, while others have much softer tails. So, when shopping for skis, we have to recognize each companies’ design philosophies, so as not to confuse a “soft snow” Rossignol with a “soft snow” Salomon ski.
Yeah! More food for thought!
Best-
Eric
November 12, 2007 at 10:03 am |
Actually, Jay is correct – there is suction between the ski and snow in the presence of water. As Mark points out, a pressure drop is required – it is related to the surface tension of the water. But there is a real mechanism for this.
http://en.wikipedia.org/wiki/Young-Laplace_equation
For the same reason that water will wick up the inside of a small tube, your skis will get sucked down to the surface of the snow.
The example with a couple pieces of glass is a good one. If there’s a liquid film, it is suction that pulls them together. The reason why they don’t slide is primarily because the surfaces are touching at a number of different spots on the surface, generating a force to balance the suction. At those junctions, you’re trying to slide two pieces of glass past each other, and if the water is squeezed out, the situation at the start is similar to dry sliding.
December 6, 2007 at 4:15 pm |
Interesting stuff; trying to solve the mystery of the ski. (Misery) for those of us who rarely find a fast pair. No one yet has mentioned air entrainment within the snow. Fastest conditions occur when the bottom of the ski has the fewest contact points with any snow condition. This is why grooming machines have combs, & why skis should rilled for wet conditons; the rills insure that there is always some air between the bottom surface of the ski & the track, that breaks the adhesion somewhat.
Friction coefficiant will rise substantially with fewer contact points. Snow or ice at these points is more apt to melt for better glide, and the more air entrainment in the snow, the more area for this water to flow into, hence there will be greatly reduced adhesion. “Corn snow” is the best example of this condidtion.
Now if we could only find a way to reduce friction with this cursed fine grained cold cold stuff we would really be getting somwhere
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