Interesting Glossary Definition Rick. I think most of what you wrote is probably correct but this last paragraph...
Originally Posted by Rick (from Rick's document)
We all know what happens to us when ski over a bump. As we crest the top of the bump, our momentum tends to keep us moving up, while the bump decides to go down. Hello air time. The same thing happens as we go through the transition between turns. The faster we’re traveling, and the sharper we’re turning, the bigger the bump, and the more unweighting we experience as we transition from one turn to the next, cresting the top of the virtual bump.
... seems to be a misunderstanding of the Virtual Bump concept as the VB cannot cause a 'pop' of any kind. I think you've mixed up the idea of 'Vaulting' with the geometric idea of a Virtual Bump. (Not that I don't do that from time to time also... )
We can pull the VB idea into a discussion of "What we feel" during transition and that wouldn't be unreasonable (in my view) but there is no possibility of "launching" off the surface based on the VB idea itself.
For an easy proof construct an experiment as follows:
In the image I've attached a curved trough to a sloped piece of plywood in order to simulate traveling the Virtual Bump. A marble is used in place of our skier. The sides of the trough are just a bit higher than the marble's midpoint so that it stays in the trough despite minor aberrations in the otherwise smooth surface (shown by the dotted red line in the blowup-inset). The marble will follow the curved surface which deflects the marble's path just as skis deflect the skier's path.
The question is: What happens at transition - does the marble jump up off the surface and out of the trough (yellow arrow) or does it stay in the trough (green arrow)?
Ricks text clearly suggests the marble should pop up out of the trough. I don't believe that myself. Instead, I think the marble will follow the green arrow and stay in the trough (unless it hits a bump of dust or rough spot in the surface...). I further suggest that this experiment will replicate regardless of turn radius and regardless of slope angle (within reason - at 85+ degrees any grain of dust will eject the marble)
The Virtual Bump is simply an informative geometric representation that models how the skis rotate fore and aft to accommodate the change in slope-angle directly under them as they progress through a turn. We can extend that basic idea and talk about 'compression' coming into transition and the 'feeling of lightness' as we initiate a new turn downhill, but that's really more of a discussion about veering away
from the direction Gravity is pulling us (turn finish) vs. veering into
the direction Gravity is pulling us (turn initiation) along with the creation and elimination of Centrifugal force. The Virtual Bump aligns perfectly with this scenario so it's quite easy to confuse the two (I know I have!).
A better use for the Virtual Bump concept in skiing analysis is in the realm of Fore-Aft balance.
As the skier begins a new turn their skis are effectively "rotating forward" down the backside of the Virtual Bump. The skier's CM must
rotate forward in relation to the skis/feet to accommodate this change else they will find themselves in the position shown in the next image, a position quite far from being 'in balance' considering the acceleration they are likely experiencing in that moment.
Likewise, the skier must deal with the reverse situation as they come out of the turn into turn finish. Here, the skis are rotating backward as the skier comes further across the slope and the skier will find themselves doing a face-plant unless they adjust their CM accordingly.
Note that these descriptions are geometrically general - they do not take into consideration things like "Balancing for the Future" nor "Taking a Shortcut" with the CM. There are a number of complex patterns a skier can implement to get around some (or all) of this basic idea, and in many cases a skier wants
the imbalance created by not entirely accommodating the Virtual Bump's geometry.
.maEdited by michaelA - 1/10/11 at 5:36pm