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Residual Camber
In the run up to the 2009 Noquemanon, I was classic skiing in mid 20 degree weather.
When the gun went off for the race two days later, it was five below zero. I was in the first group out, the "touring" class. I had the wax right. The previously good kick zone seemed to get very tiny, even though I was heavier with an extra clothing layer for the cold. Taking two days off prior to the event, I did not suddenly forget how to ski.
I was on a good pair of race skis in the scenario described above. In observations since then, my touring rock skis - which are generally pretty slow with fish scales dragging- would have been the answer for the kick but with a loss of maneuverability. The temptation here is to get another race set that has a softer flex in the kick pocket for cold weather skiing when efficiently counts. Looking from the other direction, I suppose it's possible that the softer kick pocket could start out fine but then drag if the temperature went up 20 degrees over the course of six hours in a marathon event.
Is there anything to this, a softer kick pocket for a cold day on classic skis? I use a no wax set with a stiffer kick pocket on falling snow.
J, Hudson, WI
Hi J - While skis do get a bit stiffer at very cold temperatures, the difference is small, and doesn't account for your difficulty. Often, the most important determining factor has to do more with the shape of the wax pocket than the strength of the pocket.
When you put your full body weight through the ball of your foot, you compress the pocket completely in that location. In other words, the ski is closed - in contact with the snow - under the ball of your foot. But on most skis some of the pocket in front of the ball of the foot doesn't close all the way flat. I call this leftover shape in the pocket "residual camber". The shape and amount of residual camber in your wax pocket helps to determine the conditions in which your ski will perform optimally.
It's important to remember that ski tracks are not completely flat. The surface is variable - smooth, bumpy, soft, hard - and the snow crystals vary as well. In conditions where either the snow or the wax are somewhat soft and pliable (think soft tracks, or wet conditions when the wax is soft) a ski with more rather than less residual camber is good. The soft wax/snow at the interface mean that there is compliance at the kick, and it's not hard to get grip. And when the ski is gliding the track surface effectively "averages out" to flat, and the residual camber creates a little vortex which limits surface area contact. So the potential for drag is much reduced, making the ski much faster running when you're using sticky (slow) wax.
When both the tracks and the wax are hard there is less compliance at the interface, and a ski with a lot of residual camber will have limited surface area contact, resulting in poor kick. In these conditions a ski with a flatter-closing pocket will provide much better kick. However, there is a trade-off with speed, since the flat shape of the pocket means that the potential for drag when the ski is in motion is much higher. For this reason we sometimes use a stiffer ski for cold hard tracks - particularly when we anticipate abrasive snow and the potential need for a binder or basewax cushion.
So, let's reconstruct your Noque experience. You didn't mention the specific track conditions, so I'll take a stab at it: For the week prior to the race you were skiing on fine-grained snow, waxing with something in the soft blue drywax range. While they're not "sticky" wax these blues tend to be quite broad-range and forgiving, and a ski with some residual camber can work well. On race morning you woke up to freshly groomed tracks that had set-up at very cold temperatures. You didn't mention new snow, so I'll assume this snow had been skied and worked all week, so that it had a chance to set-up plenty hard. You put the appropriate hard (green) kick wax on the skis, ensuring that the snow/wax interface was quite hard. I'm going to guess that you have a fair amount of residual camber in your skis, and that you were kicking on just the wax that was under the back of your foot, and a couple of cm at the front of the pocket.
You can test my theory about pocket shape by clamping your skis together with a C-clamp positioned at the ball of the foot - about 8cm behind the binding pivot. Clean the skis of both kick and glide wax first. Then, when you have the skis full compressed, hold them up to some light. You'll see the residual camber easily enough! You can also slide a bit of paper or an index card in there to help locate the zone more accurately.
Because residual camber provides great speed, it is something that all ski companies build into the design of their skis. Understanding what it is, how to spot it, and how much you want in your skis is one of the keys to selecting appropriate classic skis.
To talk specifics, I will reference Salomon Equipe 10 skis and their DNA Ski ID stickers that come on each pair. (I choose to reference Salomon because they are the key ski supporter of SkiPost.) Salomon's DNA Ski ID allows Salomon to guarantee that skis are reaching the skier within their exacting engineering criteria. It allows ski retailers to have skis on their wall (or to custom order skis from their "custom" warehouse in Utah) that can be confidently selected for a skier's specific needs from the skis "DNA". A person using the DNA Ski ID can use one characteristic or many to consider how a ski works or fits. I like to use many.
Below is some guidelines using the Salomon DNA Ski ID system to answer your specific Noque ski question above.
With respect to residual camber there are a couple of measurements that Salomon provides on their Custom ski DNA stickers that can really help. The amount of residual camber that a ski has is determined by the location of the high-point of the camber. If you think about the camber as a spring, the high-point is just the top of the spring. If the high point is under the foot, you'll compress the camber all the way flat. If the high point is far in front of the foot, you'll always have residual camber.
The position of the high point is measured by Salomon both at half weight and at full weight. Those numbers are the L3 (half weight) and L3.3 (full weight) numbers. We'll focus on L3.3 - the full-weight measurement since we're thinking primarily of kick performance right now. The other number to pay attention to is the H3 value which is the height of the highpoint when the ski is loaded at full weight. Using the L3.3 and the H3 values you can get a very good idea of the shape of the pocket when you are kicking the ski. Negative values in the L3.3 number indicate a position in front of the balance point, and positive values indicate a position behind the balance point.
- If L3.3 is a small value (from 0 to -5cm) then the high point is near the foot, and the ski will close fairly flat. This ski will usually have a low H3 (under 0.1mm). It will be very easy to kick, even in cold hard tracks, but it will have a tendency to drag. So you often want to pick this ski with a high and strong camber to ensure good glide characteristics.
- If L3.3 is a large value (in the range of -10 to -12cm) but the H3 is low (0.1 to 0.2mm) then the ski has a long but low residual camber. This little bit of extra shape in the pocket can ensure a lot of extra speed, but these skis still tend to be easy to kick and forgiving throughout a broad range of track conditions and temperatures. A ski with this pocket shape can be picked quite a lot softer than the flat-closing example above, and will require less impulse to have secure kick.
- If L3.3 is a large value and H3 is moderate (0.15 to 0.25) you're likely looking at a ski that is best suited to high-moisture conditions - drywax in the violet to red range.
- If L3.3 is a large value and H3 is high (around and above 0.3) then the ski is best suited to klister conditions.
Of course, none of this would have helped you on the Noque morning. On race day you have to go with the skis you have, not the skis you want. So what could you have done to help your situation? Well, going back to your C-clamp set up - once you've identified the residual camber, you can mark it. You can also note the shape of the opening with some additional notations on the sidewall of the ski. On a day with cold, hard tracks you can help your cause a great deal by building up a layer of basewax in this residual camber zone to help fill-in some of the extra space and provide better surface area contact with the snow. It's not as good a solution as having the right pair of skis, but if you practice on training days, you'll get pretty good and making sure that your skis are in the game.
Zach Caldwell
Caldwell Sport
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