Compliance ski sidecut shape to deflection. Part 2

Ski sidecut shape determines the trajectory along which goes “pressed into the arc” ski. But how ski lay on edge is determined by the natural deflection of the ski. The first estimation revealed no correspondence between these parameters. In this note I want to share with accurate measurements of 4 pairs of skis, and with very bright result.

To begin let me remind what was discussed in the first part. In the animation it shown how loaded ski lay on the edge. And it is case of a perfect match deflection characteristics and sidecut. Firstly ski touch the slope by the widest points on the tip and tail, then force apply to the middle of the ski, which deflects the ski in accordance to its deflection characteristics. On animation the entire edge touch slope at once. It is clear that this can be so only in the case of a perfect match shapes of deflection and sidecut. Otherwise, the ski  firstly touches slope with some point, and only then under a further forces will be on the edge completely.

Typical values for such efforts for deflection to contact of slop is 20-30 kg for man slalom skiing. So all considerations about the desirability of compliance ski sidecut shape to deflection relate effort to this range.

A lot or a little, this 20 kg? Compared with the weight of the skier is not a lot. In the second half of the arc near slalom pole weight is also overload due to the additional forces. Even by the eyes it can be seen (see. photo), that to each ski force applied much more than 20 kg. But there are other parts of arc where the pressure on the ski is close to zero. At the beginning of the turn even just to put any pressure on the ski is not very easy (more info here: “Ski pressure control at first half of turn“). And this beginning of turn with minimum load is go immediately after the end of the previous turn with a maximum load.

As far as I understand, generally can be two different concepts in the design of slalom skis and sequently is technique of skiing.

The first is that the deflection of the ski and sidecut shape are independent characteristics. Each carries its own function, manufacturers can change these characteristics from model to model as independently. Skier respectively should as little as possible to work with the load of ski less than 20-30 kg. That is, if you want that ski behaved predictably, push to ski quickly and strongly. Phase of turn, where you can not push strong, you need to fly without any pressing.

The second concept, it is just the connection of the characteristics of the natural deflection of ski and sidecut. It feels when skiing as “rails”. That is a very good control at low loads. It is possible that this concept can have disadvantages as ski is likely not so responsive and active. But to me (right now) this “rail” is closer. In addition, I think that “rail ski” pass part of arc when ski not loaded faster.

Now turn to the measurements.

At stand were measured deflections under different loads (6-30 kg) of 4 pairs of skis (Atomic 2016 ans Ogasaka 2015-2017). At the same skis were measured sidecut shapes. Then measured sidecut shape was transfered to deflection shape that is the necessary for simultaneously touch the slope at different angles. Finally, the calculated deflection shapes gently superimposed on the measured sag while the angles of edge were chosen for the best possible matches.
To begin with skis Atomic Redster FIS SL 15/16, for which at the first part it was not found any match for load 20 kg.

The graph shows the measured deflection of ski under loads  5.7, 10.6, 17.0, 18.5, 23.4 and 29.8 kg (not including the weight of bindings). These curves are overlaid with animation of calculated deflection shape that defined by sidecut respectively at angles of edging 20, 34, 46, 49, 56 and 63 degrees. It is seen that the tail (from the heel of ski to bindings) have to match all the curves, whie the bow of the ski coinside only at the minimum load 5.7 kg and respectively edging angle of 20 degrees. And this coincidence is not too good. At the bottom of the chart it shown the ski with marked sections of matches. At tail of ski the plot is continuous so we can speak of the “rail” tail, but at the tip coincidence is “fragmentary”, just illustration the crossing of the two curves. The tip of the ski is noticeably softer than it should be based on shape of sidecut. I think that this is done deliberately. In addition to the concept №1, I mark another point, which is not connected with the rail and proper form. Atomic perfectly lead into the turn. That is, when edging, ski immediately begins to turn. I had “other” ski that when “just edging” continued to go in the same direction, while in order to make them turn I had to “knock” (exactly like on the animation at the begin of notes). But Atomic is not this case. I think that this feature is associated with the Atomic specifically chosen softness around the shovel (widest part of the ski tip) that may not be compatible well with total shape of deflection of ski tip. In general, I do not want to doubt the engineering thought of authors of Atomic skis. I note only that there is no concept of “tip rail” in this model.

Further data from three pairs of Ogasaka Triun SL model years 15-17. How it was described in separate notes, these skis are different. For models 15 and 16 sidecut is the same, but shapes of deflection are somewhat different, while “guest from the future” ski ’17 is remarkably different from the other two and by sidecut shape, and in deflection and stiffness.

As the stifness and shape of the deflection of ski Ogasaka 2015 differs from Atomic, then under the same loads calculated edge angles differ, namely 20, 36, 50, 53, 59, 64 degrees (in Atomic were 20, 34, 46, 49 , 56, 63). The coincidence of the “rail” in tail is about like Atomic, with all loads. In addition upto about 50 degrees angle we can talk about the rail in tip section, and then the picture becomes like Atomic, too soft tip for such shape of cut-out.

In Ogasaka 16 year, as can be seen from the comparison of deflections, deflection curve has become slightly more balanced, more rigid tip and the softer tail. This has led to a much greater alignment with the curves on the graph, the rail in tip remains to angles 53 degrees and possibly steeper, and the coincidence of the curves generally better. So, it possible to mark the movement in the development of Ogasaka ski in this direction.

Features of Ogasaka 2017 ski are simply amazing. The coincidence of the curves at the tip is just absolute over the entire range. The coincidence in the tail is a little worse, but the tail is softer (not by itself, but with respect to the calculation form). There is guess the concept of a soft exit from the turn. I’m still confused by total high stiffness of skis Ogasaka 2017. Indeed, since the vertical scale in millimeters, and the position of left support in all charts is the same, the difference is very noticeable. This ski is a lot harder then other three of this review. But apart from this is not to say anything. It is possible that because so perfect “rails” it is not necessary to fly the state of unstable parts of turn, so it will simply lead to a change style of skiing. But I would be more calm if the stiffness remained about the same 🙂

The overall conclusion is good: compliance ski sidecut shape to deflection do exists!

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Vadim Nikitin

 

 

 

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