Ski fitting question

"Jrw" wrote in message
news:wse4g.28055$HC3.24018@trnddc07...
How can one put 120% of body weight on a ski? One can jump up and come
down on a ski but one cannot push down harder than their weight since they
have nothing from which to push?

If you accelerate your body mass upward you have to exert a downward force
of more than 100% of your body weight. Also, the kick is started with a
"pre load" in which the ankles and knees flex so the body mass moves
downward. You exert more the 100% of your body mass to stop the downward
motion. The force required to stop the downward motion and start the upward
motion plants the ski.

Stand on a scale, have your partner note your static weight, then jump
(spring) upward and have your assistant note the peak force registered
by your scale. It's physics (Newton's 2nd and 3rd laws).

If you spring upward from your scale (i.e. the upward hitch as you
completely shift weight to the kick or grip ski as the trailing ski
passes the weighted ski), the upward acceleration, if 2 m/sec/sec will
add 0.2 times your mass as on opposite reaction to your upward implusle
move. If you look at classic skiing body position, as you transition
from glide with your ski forward of neutral to the "kick", your glide
(forward) leg begins to straighten. That motion is the little upward
hitch that applies a downward force to the ski (the equal and opposite
reaction of Newton's third law).

Edgar

Edgar wrote:
Stand on a scale, have your partner note your static weight, then jump
(spring) upward and have your assistant note the peak force registered
by your scale. It's physics (Newton's 2nd and 3rd laws).

If you spring upward from your scale (i.e. the upward hitch as you
completely shift weight to the kick or grip ski as the trailing ski
passes the weighted ski), the upward acceleration, if 2 m/sec/sec will
add 0.2 times your mass as on opposite reaction to your upward implusle
move. If you look at classic skiing body position, as you transition
from glide with your ski forward of neutral to the "kick", your glide
(forward) leg begins to straighten. That motion is the little upward
hitch that applies a downward force to the ski (the equal and opposite
reaction of Newton's third law).

Edgar

Jumping up and then coming down will exert more force, but I don't see
how flexing your muscles will help it will just cause you to lift some
of your weight. If you bound that might help and that is a reason
classic skiing in bad wax/kick condition expends more energy with
additional lifting.

do it on a scale, I bet you'll see it happen: Stand on a scale
balanced on one foot. Squat and flex your knee so you can balance on
that foot and hold this positionat least long enough to confirm that
this is 100% of your weight. Then quickly and forcefully straighten
both the hip and the knee (keep your balance!). You're pushing down on
the scale to *move* your body weight upwards - and I think you'll see
at least a momentary weight reading on the scale than your body
weight. I don't know if I could personally achieve 120%, I might try
it this weekend out of curiousity. On a ski the amount more than 100 is
dependent on your ability to begin with 100% of your weight truely
balanced on that one ski, and also the quickness and strength you use
to force that foot down, move your body upwards as you straighten your
hip and knee before you shift to the other ski.

Camilo wrote:
do it on a scale, I bet you'll see it happen: Stand on a scale
balanced on one foot. Squat and flex your knee so you can balance on
that foot and hold this positionat least long enough to confirm that
this is 100% of your weight. Then quickly and forcefully straighten
both the hip and the knee (keep your balance!). You're pushing down on
the scale to *move* your body weight upwards - and I think you'll see
at least a momentary weight reading on the scale than your body
weight. I don't know if I could personally achieve 120%, I might try
it this weekend out of curiousity. On a ski the amount more than 100 is
dependent on your ability to begin with 100% of your weight truely
balanced on that one ski, and also the quickness and strength you use
to force that foot down, move your body upwards as you straighten your
hip and knee before you shift to the other ski.

You can surely apply force to the ski beyond your weight, but it isn't a
very efficient process, it must be very impulsive and a good deal of
the effort uncoiling your muscles goes into lifting your body up. If the
ski is flattened only for the millisecond when the greatest impulse
pushes this overweight, then it might not really help in grip since one
really wants grip/friction in the horizontal direction not the up and
down which is wasted motion and energy. Running is a style that lifts
your weight more than one has to and presses down hard surely isn't
efficient and I guess that is why double poling comes in so much in
classic skiing. The questions of how quickly one can apply impulse are
important I am sure that impacts the different feel of the ski I lot
more than some simple measure of flatness with 120% body weight since in
real skiing this is very much different than a static measurement. I
guess that means Zach can try to find more useful measures of ski
performance. I guess trying the ski under different conditions is the
only real test.

Impulsive loading of the ski happens. I'm not trying to prescribe a
certain level of impulsive overload as being "correct" or anything like
that. But overload at some level is a fact of skiing. I'll leave to all
you technique experts to figure out how much/little might be just
right.

The thing about measuring skis is that, at best, we can establish
benchmarks. We don't make any attempt to actualy replicate what happens
to a ski when you're actually skiing. There are NO static loads in
skiing. The reason we check to see what happens to the ski at a number
of different loads is that it gives as some idea of the rate of change,
and a sort of indication or prediction of what we might expect from the
more subtle loading that occurs during skiing.

A good example is testing the "closing" flex of a classic ski. It
doesn't actually matter whether a ski closes at 55% or 65% of your body
weight - you don't spend any time skiing with that much weight on the
ski. But it gives us a reasonable prediction of how the ski will
respond to real-world loading.

You're right to expect, Jeremy, that trying the ski on snow is the only
"real" test. But it's a test that we usually don't have the luxury to
perform as completely as we'd like when we're selecting skis. So, when
we've got an opportunity to go back and measure known good skis, and
known bad skis, and see what characteristics they share, then we stand
a chance of narrowing the field a bit prior to deciding which skis to
put on the snow. And when we get the chance to measure several thousand
pairs of skis we get a pretty good sense of the range of available
values in those measurements.

It's taken me several years of testing skis to understand how useful
that 120% measurement can be (simply in the context of indicating the
action of the ski beyond full weight). So, while I continue to try to
find more useful measurements, I'm guessing that your dismissal of the
idea might be a bit premature.

Zach