Hi Gonzo -
I totally agree that the phrase "folds up" vastly exaggerates the phenomena that VailSP is describing. The phrase stinks. "Folds up" suggests a really sharp fold like on an envelope, whereas I would prefer a description similar to what I said in an earlier post in this thread:
>"...he obviously doesn't mean "folding" literally, he just
>means that most of the flex is occurring in the middle of the ski
>not at the ends or distributed over the length of the ski."
I have heard other people use the phrase "fold up", and knew exactly what VailSP meant when he said it, but, like you, would prefer a more accurate term.
If you want to see a photo of a ski that is doing this, and if you happen to have access to a copy of "The Skier's Edge" by LeMaster, take a look at Fig. 3.8 (p.24) and compare the shape of the downhill ski in that photo with Fig. 2.2 on p.7 (lowest image in the montage). In the former photo, both the forebody and aft section of the ski are almost perfectly straight (ie, without flex), but clearly meet in a wide-mouthed "V" under the skier's boot. OTOH, in the latter photo, most of the flex occurs near the tip and tail with hardly any deviation from a straight line underfoot.
You tend not to find many skis that "fold" (ie, are much softer underfoot than at other points along the length of the ski), because the vee-shape that results when such a ski is loaded doesn't conform to a smooth carved arc, leads to abrupt transitions in performance between weight-forward and weight-back situations, has an unusual ammt of drag, and generally feels odd.
I should also point out that the greater the overall stiffness, the more difficult it is to feel (or even measure) differences in flex patterns (at a fixed value of overall stiffness).
An example of a ski that is fairly stiff overall, but has an unusually large fraction of its overall flex concentrated underfoot is the first generation (silver and black) Stockli Stormrider. This is quite apparent if you put this ski in a testing apparatus, support the tip and tail, load the middle, and measure the deflection at various points along the length of the ski with a machinist's dial indicator and plot the results. This is extremely easy to do in a machine shop using a Bridgeport or other milling machine (holding the dial indicator in the spindle/collet of the mill), but can even be done on one's floor if one is sufficiently careful to minimize or correct for systematic errors like bumps in the floor, small deflections of the end supports, etc..
BTW, for completeness, I should reiterate that I have never skied or measured the Axis-X or X-pro, and have only skied (but not measured) the Axis (= 7/8) (as I mentioned above).
Solamente mi dos pesos,
Tom / PM
[ May 29, 2002, 09:35 AM: Message edited by: PhysicsMan ]