Originally Posted by michaelA
Late to the party - and I see it's degrading into another snowball fight... but wanted to get back to something posted earlier.
I’ll probably get into trouble for it - but I’m with Hobbit on the Flexing Causes Crossover
In a general sense
the skier who Flexes BOTH legs will not fall directly down the Resultant Force Vector line - instead, they'll initiate a crossover (if they do nothing else).
Looking carefully at the LeMaster image we see a clue to what really happens - and the Bright Green arrow points right at it.http://Ronlemaster.com/images...maier
The Resultant Force Vector (Green arrow) is a Result
of the two other Vectors (Blue and Yellow). If we take away the skier’s entire structural support with a sudden and complete retraction of both legs - we lose the Green Arrow. We’ve no Blue Arrow since the centripetal/centrifugal force thing is now gone, so there's no Green Arrow needed. The Yellow Arrow wins by default and takes over (gravity).
In this case the skier’s CM will ‘plummet’ to the snow via Gravity alone. They’ll do so along a Ballistic path dictated by the instantaneous tangent to the existing turn at the moment of ski/snow release. The fall of their CM will be in the Plane
of the Yellow Arrow and will be totally unrelated to the Green Arrow. (The CM's path will curve downward in that plane based on the forward speed of the skier)
In real life the typical skier uses ‘retraction’ but not usually to the extent described above.
The typical skier who Flexes aggressively (retraction) generally keeps their skis on the snow surface. If they do, then mere Flexion
does indeed initiate a crossover. The Green Arrow in the image points directly at the cause of this crossover: The still-engaged ski.
When we Flex during a turn we relieve support of our CM against both Gravity and Centripetal Force. Meanwhile, our ski (still supported by the snow against both Gravity and Centripetal Force) continues to turn under us. Essentially, our feet (BoS) will continue to move (say left) while our CM takes a slightly straighter path (to the right) than before.
Even with a large retraction effort the ski (however slightly weighted) will still diverge its own path in relation to the path of the CM - causing at least a minimal imbalance - which starts the crossover. The slower we Flex, the more the imbalance we create (the ski has more time to deviate under our CM).
This occurs regardless what speed we are traveling - but at slower speeds (and on two legs of support) it’s far harder to detect/perceive.
PS: Nice posts Hobbit! When people present a thoughtful and well-reasoned dissent to a generally accepted idea it’s always reason enough for me to re-evaluate the idea using their reasoning
to see if I can reach their same conclusions. If I can
… then it’s time to rebuild my own thinking on the idea.
This image shows half the forces acting on the skier in a frame of reference that moves with the skier. It shows a centrifugal force and a gravity force, the combination of which adds up to the green arrow. The blue line is the x-component of that total force and the yellow one is the Y-component of that force. Yes if the snow is flat, all of the blue line comes from gravity, but it need not be so.
The other half is the reaction force of the snow acting on the skier at the ski. It is exactly equal to the green force, but in the opposite direction. This reaction force makes the net force on the skier zero, and keeps the skier where he is with respect to the frame of reference (F=Ma=0).
Remove the support, and you remove the force of the snow acting on the skier, and the skier is no longer stable, but now accelerates in the direction of the green arrow (a=F/M)
Really all this talk about OLR, ILE, etcetera is fine for figuring out what's going on, but when I ski, I use both.
Retracting both legs completely causes the cm to follow the green arrow.
Retracting the old outside leg while allowing a bit of force to go through the inside ski causes the path of the cm to more horizontal. How much force to put on the inside leg and when depends on where you want the cm to be and when you want it to be there.
The skis and the cm need not follow the same path. You can retract the legs enough to have you cm almost free, but enough weight still on the skis for them to maintain their arc a little longer. You can keep enough weight on them to allow them to cut cleanly right up to, and right after transition. The amount of pressure pushing down on the snow can also be balanced between the two skis in a blend (bad word to PMTS
skier) old outside leg retraction and old inside leg extension to manage your rotation about an axis pointing in the forward direction.
PS. I sometimes unweight without pivoting too
: (though I'm too much of an old wreck to pivot without unweighting).