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Grip physics?



 
 
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  #51  
Old April 8th 06, 09:09 PM
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Zach,

If you shorties (I'd typically advise you a mountain bike size Small or
Medium, I ride X-Large or XX-Large) are on 192's, by rules of proportion
(194cm dicided by 170cm, multiplied with 192 ski's), for height I'd be
on.....hold on now people...217cm ski's!
And then, I may still have relatively longer legs so I could "handle" even
longer still? This really sucks, I though at first that my body style might
actually be super suited for skate-skiing, and now I hear I may have to do
with ski's 10% shorter than ideal. Exactly as in bikes, until 29" wheels hit
that market and made me from a very so-so technical rider to one that drops
anyone anytimes when it comes to technique.

From your explanation, I get the impression that wider ski's might actually
be just the ticket after all, to make longer ones that are still stiff
enough for heavier/stronger legged athletes. I'm no phisicist, but I would
guess that if you add 10mm to the 40mm width of a ski, that immediately adds
25% of stiffness. Yes, also weight, but not a full 25% of what sits on the
foot (binding, boot, etc). Skiers that can put more pressure on a ski can
also pull on it harder, I would say.

Assuming a whopping 25% wider ski, longer skiers like your friend and myself
might be able to have 220cm ski's (10% longer than his rare Madshus') that
close at appopriate pressure, and with a huge surface area, be it good or
bad for glide. If width is such a small factor in a ski's performance that
it's even disregarded lately, at least it wouldn't kill the advantages of
the perfectly fitting-length and flex of the ski's?

I didn't quite understand your "torque on edge" argument against wider ski's
(language barrier and technically under educated, I'm afraid). And I
appreciate that wider as well as narrower-than-standard have been tried over
the years, and rejected. But wouldn't a couple decades of ski development
have come up with some tricks to make it work in 2006?

I hope you (and others) won't confuse my fresh look at the technical side of
this sport being the total newbie that I am, with being cocky. I get really
cocky, when I actually know what I'm talking about. Not there yet with
ski's, just trying to learn.

Looks like that if I would try biathlon, the extra 4kg of the rifle would
make my future ski's even less desirable. My current 81kg is in pretty bad
shape, I might actually weigh 85kg in true race shape, adding some upper
body muscle this summer rollerskiing. And I actually thought being a strong
guy made it an advantage to carry the same 4kg rifle as people weighing much
less themselves. That would make me 89kg already...

Thanks for your insight.

J

"Zach Caldwell" schreef in bericht
oups.com...
Hi Jan - you're definitely not heavy, given your height. But at 81kg
(regardless of height) you're certainly in the category that will be
best served by a full-sized ski. From any manufacturer you'll be on the
stiffest flex category of the longest ski. You won't be looking for the
stiffest pair in that category, by any means. But that's where you
belong.

In general when you get close to 90kg it gets quite difficult to find
an appropriate skate ski for an agressive and technically competent
racer in general inventory. The skis exist, but they're not nearly as
common, and they're on the very outside of the design spec for the
skis. Interestingly nobody makes anything longer than a 195 (Madshus)
for skate skis. That's been the case for several years. Anyway, I had
one 88kg customer this year who wanted four pairs of well matched skate
skis. He's a strong and aggressive skater, and it took me a lot of
tries to find skis for him.

A lot of this depends on what brand and model of skis your're looking
at. A Fischer 610 gets very stiff near the close and can fit a very
broad range of skier weights. I'm 170cm and 65kg and my prefered
Fischer ski is a 192 stiff - the same size you'd be looking for. My
wife is the same height as me (actually, probably a CM or so taller)
and closer to 58kg and she prefers my skis to her own 187s. The skis I
use as test skis are a 96kg factory flex (145% of my body weight).
That's generally a ski fit for somebody closer to 73-75kg, but it's
what I like to ski on. For you I'd want to find something like a
105-110kg ski, depending on a bunch of specifics that aren't important
here. Madshus, on the other hand, makes skis with a very soft and
supple finish. A ski that is well fit for a 75kg guy would be
pretty-much unskiable for me. And my skis would turn inside out under
you. A Madshus (or similar design) has to be fit much more carefully.
But it's also possible to fine-tune the feel and feedback that the
skier will get to a much higher degree. And you'd be on a ski that
closes at something more like 85kg than 105kg.

I have a good friend who helps me grind skis from time to time when I
get very busy, and who is almost exactly your size. Really tall and
skinny with a background as a ski racer and cyclist. We actually found
a couple of old pairs of 200cm Madshus skate skis from Peter Hale - a
long-standing Madshus rep in this country - to set up for him this
year. Then we ran out of snow and never got them onto the grinder. So
next year. Anyway, this guy has been frustrated by the lack of longer
skis on the market. He can find skis that are stiff enough, but it's
difficult, and he feels that he can definitely handle longer ski.

Best of luck getting well set-up. I've never felt it was a great
advantage to be short - but it sure beats being too tall!

Zach



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  #52  
Old April 9th 06, 05:02 PM
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Hey Jan - this is turning into a two-way conversation, which I'd be
happy to continue offline if you'd like. Interesting that you note 29
inch mt bike tires. I like those as well! As a short guy it's hard to
find a bike that can fit the larger wheels gracefully, but they sure do
roll over things nicely. my opinion would be that advantages of larger
wheels on a mt bike might be analagous to the advantages of longer ski.
Smoother transitions, lower "rolling" resistance, plain and simple. The
ergonomic consideration is how to build a bike frame to fit the
rider...

Anyway, my e-mail address if you want to continue to conversation is


Zach

  #53  
Old April 9th 06, 05:24 PM
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Cheers Zach, you obviously put serious time and efforts in these replies,
and although I'd love to pick your brain, it seems unfair. Other people's
thought would be great here too obviously, if only to answer original
question with personal experiences.
Most I know about bikes I learned lurking forums, I don't even buy mags.

If you ever need a design for a small 29" bike that fits and handles like
you want it do, do drop me a line, it's sorta my gig to figure that sort of
thing out. what you do with ski's, on a simpler level I do with 29" bikes.
Sometimes manufacturers actually listen to me and give me carte blanche and
a proto to try, way cool.

Happy trails,

J

"Zach Caldwell" schreef in bericht
oups.com...
Hey Jan - this is turning into a two-way conversation, which I'd be
happy to continue offline if you'd like. Interesting that you note 29
inch mt bike tires. I like those as well! As a short guy it's hard to
find a bike that can fit the larger wheels gracefully, but they sure do
roll over things nicely. my opinion would be that advantages of larger
wheels on a mt bike might be analagous to the advantages of longer ski.
Smoother transitions, lower "rolling" resistance, plain and simple. The
ergonomic consideration is how to build a bike frame to fit the
rider...

Anyway, my e-mail address if you want to continue to conversation is


Zach



  #54  
Old April 9th 06, 07:36 PM
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Zach

Too pity you can't find those valuable measurements of kicking forces
along the footbed. You said the forces were predominantly going through
the ball of the foot. All of the time? I mean, they didn't register the
resulting force move along the footbed with the time of the kick?
By the way, you have the opportunity to talk to Kris and Justin. What
do they say, do they initiate the kick with the heel pressed against
the ski (and I as I have said this doesn't mean that the center of mass
is above the heel), or do they kick with the toe only?
I may seem too pushy asking these questions, but the fact that good
skiers have such different views at this crucial point of ski technic
surpises me.

Vladimir

  #55  
Old April 10th 06, 12:35 PM
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Those guys don't spend much time thinking about their heels. My feeling
is that the differences between what top skiers talk about are
primarily differences between what they talk about, not so much the way
they ski. I know of no skiers who actually initiate a kick without the
foot flat on the ski (or heel pressed against the ski, as you say).
Whether they focus on loading the ski through the heel or through the
ball of the foot is of relatively little importance because we're
talking about mental imagery much more than actual biomechanics. I've
seen coaches nearly come to blows over this issue, and other similar
ones. And in fact I wouldn't call the difference crucial at all. There
are very few crucial differences in technique among the best skiers.
There are stylistic differences and personal strengths. The truly
crucial differences occur and lower levels of the sport and have to do
with body position. A skier who is trying to initiate a forceful kick
with the kicking foot well in front of the center of mass will struggle
compared to a skier who initiates a kick with the center of mass
forward over the kicking foot. But good luck finding that first skier
on the world cup!

Once you refine the population of skiers being examined to those top
level skiers who all initiate a kick with their center of mass forward,
the range of actual loading positions narrows a great deal. There is
still a range, and there are ski selection implications to be sure. But
the range is fairly small and skewed well toward the agressive
(forward) end of things compared to the general population of non
world-cup ski racers.

Zach

  #56  
Old April 10th 06, 05:22 PM
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Zach Caldwell wrote:
....

we're
talking about mental imagery much more than actual biomechanics. I've
seen coaches nearly come to blows over this issue, and other similar
ones. And in fact I wouldn't call the difference crucial at all. There
are very few crucial differences in technique among the best skiers.
There are stylistic differences and personal strengths.


Excellent point that we can't be reminded of enough! Thanks.

  #57  
Old April 16th 06, 07:30 AM
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Zach Caldwell wrote
. . . A skier who is trying to initiate a forceful kick with
the kicking foot well in front of the center of mass will
struggle compared to a skier who initiates a kick with
the center of mass forward over the kicking foot.


This seems very helpful -- it's going to change how I ski -- but also rather
complicated. Thanks a lot for sharing it, Zach.

It's complicated because:
(a) a key bottleneck which limits power in classic diagonal striding
(compared with skating) is the short _length_ of the leg-push while the grip
zone is stopped against the snow, so I've long thought that by lengthening
the leg-push out in front should add power.
(b) elite racers (including Kris Freeman) in side-view videos do move
their ankle joint significantly out in _front_ of both the hip joint and the
knee joint before starting their leg-push.
(c) "center of mass" is a "virtual" concept. Though precisely defined in
physics, it's not located in any specific location of the body -- its
virtual position moves with different body configurations, might even be
outside the body -- pretty difficult to tell where the skier's
center-of-mass is located from video analysis.

Here's my response to those complexities:
(a) starting the propulsive leg-push with ankle-joint out in front is
usually not effective for elite racers on moderate
terrain -- because they've got another more important limiting bottleneck to
deal with, namely reducing the "dead spot" in time (thus increasing stroke
turnover frequency).
(a) non-racers who don't want to try to "rush" their turnover frequency
might benefit from pushing with the ankle joint out in
front.
(a) serious racers in special terrain or snow situations might sometimes
push from out in front
(b) elite racers have two physical reasons for first moving the ankle joint
out front before they push, even though they do not start their actual
propulsive push until the ankle joint is closer to underneath their hip
joint.
(c) "center of mass" is not very important to the key physical arguments
here, so let's talk instead about locations relative to the hip joint of the
currently-pushing leg, which _is_ readily observable in side-view video.

Details:
(a) power bottlenecks for serious racers.

I'm thinking now that the reason elite racers do not push with the ankle
joint out in front is because they would first have to wait for their whole
body to stop relative to the snow before they started their leg-push. That's
because in order for the grip zone of a classic ski to grip with static
friction to transmit pushing into the snow, the ski must be stopped relative
to the snow. So that's what some (many?) non-racers do: after making the
previous leg-push, then Wait and slow down until the ski roughly stops, so
they can push again with the other leg, then Wait . . .

The "wait" part doesn't work for winning elite racers, because speed depends
on power, and physical Power can be thought of as having three key physical
drivers: pushing Force multipled by pushing Distance divided by Time. So
the racers need to manage the Time aspect, which implies looking for ways to
reduce the Wait. The trick is to start the _ski_ moving backward relative to
the hip while the rest of the skier's body mass is still moving forward --
so the net resulting speed of the ski relative to the _ground_ goes to
zero -- and static friction happens and the skier can start the propulsive
leg-push which transmits force into the ground.

(b) why elite racers move the ankle joint out in front

Getting the ski moving backward requires some distance, and that's the first
reason that elite racers move their ankle joint (and ski) out in front of
the knee joint. If they start accelerating the mass of the ski and lower leg
backward from out there, then by around the time the ankle joint passes
underneath the knee joint and/or hip joint, the ski will have achieved
enough relative backward speed to be temporarily stopped against the snow,
in a good configuration to get lots of propulsive work from the main
leg-push.

Of course elite racers get very good at unconsciously sensing and
controlling the instant and leg-joint-configuration when the ski stops and
the neural impulses for the main leg-push muscle-push starts. And there's
trade-offs between the Force - Distance - Time drivers of physical Power: If
a racer got in the habit of starting the main push with the ankle joint back
farther behind the hip joint, the backward ankle speed would be faster, so
the wait Time could be cut further -- but the pushing Distance driver in the
numerator of Power would be reduced, so the net impact might be to reduce
overall stroke-cycle Power (and thus overall forward skiing speed). Each
racer learns to make their own trade-offs based on their own specific
muscular capabilities and different terrain and snow conditions.

The second physical reason racers and many other skiers can add power by
moving the ankle joint out in front of the knee joint is more fun in the
skiing, but takes too much time to explain here (unbalancing the
forward-backward reactive force pair in the leg-recovery move).

I think it's still valuable for speed to move the ankle-joint way out front,
and valuable for fun to "kick" the foot explosively forward -- even though
in most situations like Zach says that's not where the propulsive push
should start. What I want to learn now is how to start my leg moving back
_early_, and learning how to "sense" the transition to static friction --
without the obvious signal of waiting for my body to stop.

(x) more . . .

* I think elite ski racers _can_ train their leg muscles (notably the
knee-flexor "hamstrings") to push strongly with the ankle-joint starting out
in front of the knee-joint. (Especially if they apply some help from the
pole-push.)

* When doing hill-bounding up steep hills on snow, I've seen videos where
elite racers actually _do_ make their main leg-push with their ankle-joint
in front of the knee-joint. They've learned to employ a different trade-off
of Time versus Distance than on moderate terrain, because the Time cannot be
reduced much anyway in bounding, because once both skis are up off the
ground, there's no choice but to wait for gravity to do its thing. And
unlike runners, skiers (and "nordic walkers") can use their poles to _help_
their leg muscles push through the ankle-joint out front phase.

* Has anyone put force-sensors in cross-country ski bindings, with radio
digital output synchronized with side-view video frames, so we could
actually _measure_ from what ankle - knee - hip configurations different
skiers stop the ski against the ground and start their main leg-push? And
_measure_ how the starting leg-joint configuration might change in different
snow and hill situations -- including steep hill-bounding? How is it
different if the skiers do not use poles?

Thanks for the insight, Zach. It will definitely change how I ski classic.

Ken
____________________________________
Zach Caldwell wrote
[ in response to a question about toe-ball versus heel pushing
+ pressure under the topic of "Grip physics"]
Those guys don't spend much time thinking about their heels. My feeling
is that the differences between what top skiers talk about are
primarily differences between what they talk about, not so much the way
they ski. I know of no skiers who actually initiate a kick without the
foot flat on the ski (or heel pressed against the ski, as you say).
Whether they focus on loading the ski through the heel or through the
ball of the foot is of relatively little importance because we're
talking about mental imagery much more than actual biomechanics. I've
seen coaches nearly come to blows over this issue, and other similar
ones. And in fact I wouldn't call the difference crucial at all. There
are very few crucial differences in technique among the best skiers.
There are stylistic differences and personal strengths. The truly
crucial differences occur and lower levels of the sport and have to do
with body position. A skier who is trying to initiate a forceful kick
with the kicking foot well in front of the center of mass will struggle
compared to a skier who initiates a kick with the center of mass
forward over the kicking foot. But good luck finding that first skier
on the world cup!

Once you refine the population of skiers being examined to those top
level skiers who all initiate a kick with their center of mass forward,
the range of actual loading positions narrows a great deal. There is
still a range, and there are ski selection implications to be sure. But
the range is fairly small and skewed well toward the agressive
(forward) end of things compared to the general population of non
world-cup ski racers.

Zach


____________________________________





 




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