|Originally posted by Prosper:
I have a counterintuitive question. I've read on this forum that, given the same ski length and dimensions the stiffer a ski, the worse the flotation in powder (ie softer skis float better). Also, given the same ski, the shorter the ski the worse the flotation (ie longer skis float better). Additionally, given the exact same ski in the exact same length a lighter skier will flex a ski less than a heavier skier (ie lighter skiers generally should be on a softer ski).
Given the above reasoning, it seems to me that if a lighter and a heavier skier are on the exact same ski (relatively stiff flex) with the exact same length skiing powder, the lighter skier will experience worse flotation due to his or her inability to flex the ski. If you're still with me and your head doesn't hurt, is this reasoning correct? I ask because it's counterintuitive and doesn't seem to make sense. Any thoughts? Maybe a good one for Physicsman to tackle.
Then, there are at least three separate factors that people always tend to lump together when they are comparing the "float" of different skis in powder:
1) The average depth of the flexed ski under the surface of the snow;
2) The depth to which the skier (or at least his boots) is immersed in the pow; and,
3) The tendency of the ski to nose dive and auger in to bottomless powder.
You asked whether a lighter skier on the same skis will experience worse flotation due to their inability to flex their skis.
What will happen is that for your hypothetical lighter skier, #1 will be less, #2 will also be less, but #3 will be more. So, for some light people, the most noticible thing about a particular ski (in bottomless powder) may be its propensity for un-nerving tip-diving, so they will simply report that this ski "doesn't float well", even if most of the time, it was only a few inches under the surface because of their light weight.
Basically, the apparent paradox which you found only arose because of the vagueness (multiple-meanings) in the commonly used term "float".
I could go on for pages about what design parameters influences each of the above three observables, but I think the most important points for deep powder (ie, not in stratified or partially compacted conditions) are:
1) A flat ski tends to dive for exactly the same reason that a flattened hand held out of a car window at 60 mph tends to get pushed down if it is at all tipped downward. Ball up your hand (ie, let the ski flex a bit) and this effect goes away. If a stiff ski is always tip-up, it will give you lots of support in bottomless conditions, but make even a little fore-aft error in such conditions, and you will go over the handlebars.
2) The average depth of the flexed ski under the surface of the snow (#1, above) is determined pretty accurately by the total load bearing area. A simple and surprisingly accurate formulat to calculate this is just A = Length * average_width, where average_width = 0.25*tip_width + 0.5*mid_width + 0.25*tail_width.
3) The depth to which the skier feels immersed in powder (#2, above) is not the same as the average depth of immersion of the ski because in bottomless conditions on soft skis, due to the flex of the skis, your boots may be many inches below the average depth of your ski. With stiff skis (relative to the skier's weight), there will be almost no difference between #1 and #2, but with soft skis, the middle of the ski can easily be 6 inches further immersed than the tip (even if the ski is level), and this will contribute to an overall feeling of being more "in the powder" or "less float" for a soft ski of the same surface area as a stiff ski.
Tom / PM[ September 05, 2003, 09:35 AM: Message edited by: PhysicsMan ]