Originally Posted by Rusty
Rebound works like a sailboat tacking in to the wind...
I think a sailboat tacking into the wind may be unrelated in mechanics. In such a case momentum alone
is used to ‘get thru’ the moments of weak (or no) thrust from the wind. Then it is the continuous deflection of wind off an angled sail that continues to drive the boat forward. Where do you see any elements of a ‘springy rebound’ in the sailboat example?
Originally Posted by Rusty
A skateboarder with both feet on the board can self propel forward on a flat surface until they get tired.
A skateboard primarily uses the same ‘acceleration turn’ technique described so well by PhysicsMan Here
. It is the muscular energy directed at moving Mass toward the inside of the turn that does the real work and not the springiness of the skateboard. (although I suspect that the ‘give and return’ of a flexible platform helps progressively distribute
the muscular ‘impulse’ over a longer time duration - thereby assisting balance and flow of the activity... Probably the same thing is true on skis.)
Originally Posted by Rusty
You want proof of the power of rebound? Place your ski tips and tails on top of a couple (of sturdy) chairs and bounce up and down on them
In the case of skis spanning two chairs - our skis once again spring back perpendicular to the top plate. How would this directly upward thrust propel you (en mass) forward? Even if you leaned forward just as you were ejected upward, the result would be a Fore/Aft rotational impulse - you’d somersault forward (assuming you didn’t cheat and add in some deliberate ‘forward thrust’ using your legs against the friction
of the chairs - on snow such useable friction would be absent.)
In the interests of full disclosure - there is
a bit of forward thrust that can result from the 'pinch' between gravity and the off-centered upward thrust of ski rebound. In the chair example the skis are pushing upward while gravity is pulling downward - and having leaned forward, our CM is no longer directly
between them. Our CM is therefore 'squished' forward a bit - just as squeezing a marble between two fingers can cause it to squirt out of the finger pinch.
The example of exiting Crud Snow by WmCraven above is a good example of using Ski Rebound effectively - he makes use of the bent ski's ‘lifting’ ability perpendicular to the top plates (as with across the chairs above) to get away from the snow.
This leads directly to the Tail Jetting thing again. On snow, we have no real forward/backward friction to push
against so any thrust forward
by the tail of a sliding ski is canceled out by its own sliding backward. We instead rotate
about our CM. We only feel a forward thrust if we also ‘jumped’ forward from against a platform. Any ‘Lever’ applied off-center to a given free-floating Mass causes a torque on that Mass - primarily causing rotation. The same idea applies to a forward sliding Mass when that lever is applied in the Fore/Aft plane (sans any other 'pinching' accomplice).
The same concept applies to 'Pulling the Feet Back' vs 'Leaning Forward' while sliding forward on relatively flat skis. Whichever technique we consciously employ, we actually
re-orient our whole body about our CM. Either way, when sliding forward we end up pushing our tails downward and lifting our tips upward - causing the skis to slip back while our upper-body rotates forward around its CM. (The skis might not actually 'go backward' - they may simply 'stall' or slow down imperceptibly as compared to the CM.)
Of course, with stationary skis static friction
holds us in place so we can actually lean forward without our feet moving back. Still, if we lean forward too quickly
our skis will still release and slide backward (having overcome their static friction).
In the case of jumping or thrusting
to the side from well-set ski edges at turn apex (which does incorporate the tiny amount of added spring from our skis) we are primarily using muscular power along with centripetal force to accomplish the burst of thrust to the side
- at which point we then twist our skis into that direction after or during the thrust
- and believe the thrusting sensation
came primarily from our skis when in reality most of it comes from the centripetal force of the skis.
Also consider that any time we bend our skis it consumes energy to do so.
Due to friction (surface scraping, snow compression and snow displacement) the mere act of bending our skis consumes energy even when carving perfectly into a turn. When the skis unbend, the un-bending also consumes energy. We never get out what we put into it because the whole cycle leaks energy.
Again, I'm not saying ski rebound doesn't exist. Only that we attribute far too much to it. ‘Releasing’ the partially stored energy in our skis to assist in un-weighting certainly works. So does the pop-under transition using ski rebound to thrust our skis across under us (so long as the 'pop' is released while still on old inside-edges).
FWIW… I too believed in the Rebound Myth for a long time. About six years ago a clinician was describing how it worked to us when I detected the same fly in the ointment as SnoWonder did. Careful on-snow testing revealed to me that the spring-back (by itself) did not actually contribute what was claimed. I spent a lot of time pecking away at mechanical investigations to find what might really be happening and my thoughts eventually evolved into the ideas above.
So, Yes - I believe ski rebound does exist and that it can be useful. Just not in the ways so commonly believed.
<By all means, launch those slings and arrows of outrage