Wear the Fox Hat, I apologize for the delay in responding to your thoughtful and insightful post which I thank you for. I had a busy weekend and with no time to absorb what you said.
FoX: David, Your comments yesterday got me thinking. Your issue isn’t with going straight, but with being on edge, according to what you said.
DM: To a degree. My issue is the ability to develop the most stable platform available under foot.
Fox: Now, when skiing, the edges are used to turn,
DM: More than the edges I believe we also need to apply twisting force about the long axis of the ski. But edges are definitely an issue in turning. I agree with your an analogy to the round about and your statement:
”For an object with width > 0, turning must consist of one or more of the above.”
”Likewise today, if we consider running, an athlete does not turn using 1. or 2., but only 3. To use 1. would require him stopping running to make the turn. To use 2. would twist his ankle. The only option is 3. to allow one leg to travel further than the other.”
Fox: At higher speeds, how does the runner achieve this sort of turning? Again, as far as I can see, there are three ways:
1. Change his stride pattern so the outside leg takes bigger strides, and the inside one takes smaller. Perhaps this is done at slow speeds or on tight turns, but it breaks the rhythm, and would unbalance the runner at speed
2. Put more weight on his outside foot. Now, due to centrifugal forces, this is more natural. At speed, his body will want to go straight on while he tries to turn, so there will be a greater force from his body acting through his outer foot. When this happens, it is almost as if 1. is occurring. The reduced force on the inside gives reduced grip and reduced power from that leg, so less distance is covered. This is one of the principles at work when a car turns a corner – you feel the car leaning out, and inside the car, you get pushed to the outside of the turn.
3. In a sense, the third way is “cheating”, because, instead of changing the runner, you change the bend. Put a bank on it. (and I don’t mean one with an ATM!). This allows the runner to continue around the bend without having to worry about changing weight or stride. And for this, I’ll give another car example: which is easier? Formula 1 or Indy Car? I would argue that Indy Car is easier, due to the banked circuits. The drivers can keep the speed up around a bend, while in F1, they have to back off the power to get around a flat, or slightly banked bend. If the driver doesn’t have to slow down, then there are less additional forces acting on him and the car while turning. And less forces = less stress on the frame.
DM: I think I see where you are going (please correct me if I am wrong). My approach is to develop a (relatively) stable platform under the outside foot (ski) to stand on so that when one turns the platform acts like the banked corner you refer to. But if used in the manner I propose the skier only inclines as required to balance the opposing forces. This recognizes that the optimal ‘bank’ angle is related to the speed of the skier (car) and the radius of the turn. Given the right mechanics the skier can control this aspect.
Fox: In skiing, until about 10 years ago, the technique involved a mixture of 2 and 3. There was a shift of weight and leaning onto an edge, which basically created the effect of a slightly banked turn. Then along came skis with increased side-cuts. The effect of the side cut was to give the ski more of a natural ability to turn, even before putting any pressure on it to flex it. This means the ski effectively adds to the banking of the turn. Before modern skis with their side-cuts came along, skiing was more akin to F1, requiring weight transfer as a major part of the turn, yet now it is more like Indy Car, maintaining speed through the turn by using the skis ability to carve.
DM: The pivotal issue here is the process that controls the angle of the ski in the snow. Prior to the new sidecuts skis had waist widths of 70 mm with shallow sidecuts. There were two problems in terms of platform especially on hard pack snow, 1) the ball of the foot was not far enough into the sidecut on the inside turn aspect of the waist and, 2) the sidecut was too shallow. This required too much force to get the ski on edge. And went it went on edge the sidecut was too thin to provide stability. In other words 2 critical mechanical considerations were wrong in terms of platform mechanics.
Reducing waist width while increasing sidecut closed the gap. Elevating the foot multiplies the force that can be applied with the ball of the foot into the sidecut area. Things are different on softer snow. Here a wider waist and deeper side cut are desirable to resist the penetration of the ski into the snow as it rotates onto its edge. A general rule is that a ski with a deep sidecut will assume a flatter angle on a flat surface than a ski with shallower sidecut even with the same counterflexion. Sidecut is a mechanism for progressively applying force to the snow that results in the generation of turning force.
Fox: So, as I see it, the easiest way to make a turn is to allow one side to travel further than the other, and to do this on a banked turn. Or, to put it another way, let the skis carve their way around the turn while not putting a large proportion of the weight/pressure onto one ski or the other.
DM: This is where my thinking diverges from yours. Either Fastman or Arcmeister referred to ‘the contribution of each ski to the turn'. Clearly each ski does contribute. So I prefer this term. But it is my position that the relative stability of the platform under each foot is weighted in favor of the outside foot.
In walking the balance system positions the foot under the trajectory of CM to effect balance for the new stance foot. In skiing steering has a similar effect. But as the forces increase in the 3rd phase of the turn it is my theory that CM must adjust more to the position of the stance foot than vice-versa. To do this requires that the skier be supported on a (relatively) stable platform. If it were not so then it would be exceedingly difficult for the balance system to position CM with any degree of accuracy.
Rather than pursue this train of thought how about if I start a new thread on the mechanics of the inside and outside limbs in a turn?
Thanks for your post. [img]graemlins/thumbsup.gif[/img]