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Why do people keep using this picture?

post #1 of 58
Thread Starter 
Why does this thing keep coming up? As discussed previously, one of the vectors is clearly flat out wrong (esp given the static snapshot nature of the diagram).

Even if he has a bunch of stuff right, the fact that LeMaster a) ever put this up and b) has not fixed it makes me not take him all that seriously. Does he not know better or does he "spin" the vector because of ideology? Or was it just bad proofing? Given this, I just assume there are lots of hidden flaws in his thinking given the clear flaw here. And the fact that people reference that picture makes me wonder.

At least stick a disclaimer on it so people who do not know any better do not take that green vector seriously...
post #2 of 58
Well, Spindrift, if the blue and yellow vectors are accurate, then the green vector would be too, as it just shows the resultant (sum) of the other two.

And I would argue that the blue and yellow vectors do, in fact, represent the forces Hermann is experiencing (from his frame of reference)--gravity (yellow) pulling straight toward the center of the earth, and centrifugal force, resulting from the turn he's making. The green vector represents the total force Hermann is applying to the snow.

Of course, if you would prefer, you could just as legitimately draw the diagram from another frame of reference, showing the upward force of the snow on Hermann's feet opposing gravity (equal and opposite the yellow vector), and the centripetal force acting on Hermann's center of mass, pushing him laterally into the turn (opposite direction of the blue vector)--the force that actually causes the turn. In that case, the resultant vector would show as equal and opposite the green vector that offends you so, representing the total motive forces acting on Hermann Maier.

Both are accurate, but involve different frames of reference. What's important is to understand the frame of reference Ron is coming from, and not to mix frames in any one analysis.

Best regards,
Bob Barnes
post #3 of 58
Thread Starter 
The green vector is wrong - and not just because you could argue "equal and opposite". It shows a fundamentally incorrect relationship between the CM and skis (or snow if you will) in the picture. It paints an incorrect picture, or impression, of strong and balanced technique. (edit: I'm not saying the skiing lacks balance or strength --- just that the impression created by the vector diagram is an incorrect interpretation )

I'm too lazy to do the search thrash to find the prior discussion. However, look at the skis & how pressure is being applied to them. It is crystal clear that the green vector shows direct transmission of all force to the outside ski --- while in fact the "real" vector lands somewhere between the skis. Otherwise you would not see the inside ski engaged the way it is. IMO , this is not nit-picking. If people are going to get into deep technical discussions (which I am neither qualified nor inclined to do) they should not miss or gloss over Physics 101 issues that are so blatantly on display.
post #4 of 58
Fascinating. Spindrift, some details are in order here. How would redraw the vectors to represent the forces as you see them?

Best regards,
post #5 of 58
Thread Starter 
Originally Posted by Bob Barnes/Colorado View Post
Fascinating. Spindrift, some details are in order here. How would redraw the vectors to represent the forces as you see them?

Best regards,
OK - I did some quick editing - so that might have put us a bit out of sync.

I think the current version of the post explains this. But to clarify a bit... Power is clearly being transmitted to both skis, or they would not be tracking and flexing as they are. So, in reality that green vector lands between the skis. Looking at the body and guess-timating, I'd figure the real thing would land 3/4 or so toward the outside ski. Obviously the green vector is the sum of the other two. And I suspect that computing or measuring these with any precision is tough. So in this context these things are schematics, if you will. I just think they should not so clearly diverge from a visible reality.

While I am firmly a member of the amateur camp and in no way present myself as a pro, it seems to me that this has several implications with respect to the discussions in this forum. First, my impression is that keeping that green vector between the skis is a fundamentally important aspect of balance given the equipment and technique I see being used by the skiers I strive to emulate - it provides a solid platform for a balanced stance and a greater margin of error recovery. Second, it provides a clean base for ILE based turn transitions and management.
post #6 of 58

Good point!

Ah--OK, well done. Now I see what your concern is. Thanks, Spindrift! It's less a fundamental disagreement with Ron's physics, and more a matter of detail--is that correct?

Ron's vectors do show Hermann Maier balanced entirely on his outside ski, as you suggest. If that's not what's happening, then the problem is the blue vector, which perhaps should not be quite as long. Shortening it (on the pointy end) would move the tip of the green resultant between the skis, as you suggest.

In my opinion, Hermann probably is balanced almost entirely on his outside ski in this image. But there is, clearly, some snow flying from his inside ski, so there is at least a little pressure on it. I used this image in another thread to illustrate that Hermann Maier is applying some pressure to the tail of his inside ski, to enable him to engage the tip of the outside ski (actually, the whole ski) by rotating his outside femur (applying torque to the ski about its very inclined "vertical" axis). So I certainly agree that there is at least minimal pressure on the inside ski.

But I don't think Ron's diagram in this case is far off, really, for practical purposes. You guestimate 75-25 distribution, and perhaps you're right, but I would think it's probably closer to 90-10. Certainly, you could find an image of a World Cup racer with both skis equally pressured, or even the inside ski more pressured than the outside ski. Accurately drawing the force vectors for those situations would, as you say, require shifting the blue vectors to the inside of the turn, which would place the tip of the green vector accurately between the skis.

And I agree--it would be unfortunate to send the message, through this diagram, that all the weight must always be entirely over the outside ski. On the other hand, I think it is equally dangerous and potentially confusing to suggest that both skis "should" always be pressured. Depending on the snow conditions, I still allow (but rarely force) all, or almost all, of the pressure to shift to my outside ski.

Thanks for the clarification! Good point, Spindrift!

Best regards,
Bob Barnes
post #7 of 58
Are those Ron's arrows?
Ron says the closest thing to carving a turn is a ball rolling around the inside lip of a bowl. That is, the bowl continually re-directs the balls attempt to go straight ahead.(we make the bowl's edge with our bent ski in the snow) There is no force in the direction of the blue arrow, only an imaginary one.
Most of Ron's diagrams that I've seen show an arrow of the snow "pushing back" against the green arrow.
post #8 of 58
I love the marble in the bowl analogy, Slatz. Thanks for reminding us.
post #9 of 58
Thread Starter 
Originally Posted by SLATZ View Post
Are those Ron's arrows?
The URL for that image is:

post #10 of 58
Ahh--be careful what you say about "imaginary" forces, Slatz! There's nothing imaginary about it--ignore it at your peril! I love this little cartoon I came across on the 'net (http://imgs.xkcd.com/comics/centrifugal_force.png):

It doesn't matter whether you believe in it or not, or what you choose to call it, or how you choose to explain it. It'll kill you. For real!

Best regards,
post #11 of 58
Bob, is this more like what you feel is going on?
Something still seems a bit odd to me here, but I can't quite see it right now.

Originally Posted by SLATZ View Post
There is no force in the direction of the blue arrow, only an imaginary one.
Most of Ron's diagrams that I've seen show an arrow of the snow "pushing back" against the green arrow.
I agree. Looking at this in terms of the acceleration of the skiers CM (easier, since Force=mass*acceleration), the blue "force" should be towards the center of the turn, the yellow (gravity) remains, there would also be a component along the path. The reaction of the slope on the skis is then in the opposite direction of the green arrow, I believe. I tried to work this graphic out, but it looked odd. Maybe the beer is clouding my logic, I'll try again later.
post #12 of 58
Nice work, KRP. Yes, I suspect that your modification will please SpinDrift.

Like I said, I think the original diagram was accurate, for all practical purposes, but if the intent is thow that Hermann is balanced on both feet (or, more accurately, over a point somewhere between them), then your diagram shows that nicely.

Best regards,
post #13 of 58
I love the cartoon Bob.

I think people use this picture to illustrate the forces (in the right frame of reference) without regard to distribution between two skis, so that people can understand things one step at a time. It's easier to just imagine all the force is on one ski, understand the forces, and then modify it for the action of the other ski. Also from the skier's perspective at this point in the turn, the weight and centripetal force of the inside leg itself is likely enough to account for the forces on the inside ski; he wouldn't need to push that foot down much if at all.
post #14 of 58
Alright, hows this:

P.S. I'm stealing that cartoon to add to my engineering comics!
post #15 of 58
I'd suggest that the blue arrow should be a "3D" one in the direction the skier would go if suddenly they lost contact (pretty much straight towards the camera) If you took away the skis he wouldn't follow the blue arrow.
The "crushing force" is your body continually slamming into the wall of the centrifuge at a high rate of speed as the centrifuge keeps getting in the way of the straight line your body keeps trying to take.
You need to be able to visualize it in three dimensions. How would you draw the arrows if you were looking down from above? Or from inside the centerr of the turn? (it's just the draftsman coming out in me. you know, the three views )
post #16 of 58
Yes, idealy it would be, but this is of course compressed into a 2d image. In reality, there are two components to the blue arrow; one perpendicular to the skiers path, and one tangent to the path (the direction the skier would go if suddenly they lost contact). The blue arrow is the resultant of these two accelerations (or forces, depending on how you look at it), not the direction that the skier travels at any instant.

Of course, with the angle of the slope and the changing location of the CM, this can be a messy dynamics problem. For the intent of this image, I think this works fairly well.

Few. I really need to stop applying my education :.
post #17 of 58
Yes, that's one way to look at it, Slatz. But any way you describe it, like I said, there's nothing imaginary about it!

KRP--your most recent diagram confuses the issue, I think. You are mixing reference frames, or showing forces that apply to two different things. The blue arrow shows the centripetal force applying to Hermann Maier, causing him to turn. The yellow arrow shows the downward force Hermann is applying to the snow, due to gravity. The green arrow shows the combination of centripetal force and the upward force of the snow on Hermann's feet--the opposite of your yellow arrow.

Good Photoshopery, though, I must say!

Best regards,
post #18 of 58
Yep, that's it! I knew something didn't look quite right, I was focused on not moving the yellow arrow for some reason. While attempting to change my previous picture my time to edit expired and the original image was lost.

I think this is further comfused by the fact that the arrows are being applied to only a point (CM), but we are trying to look at the effects on the whole body/ski system. Definatly not the ideal way from an engineering perspective, but an engineering perspective ruins the fun of skiing anyway!

Here is what the image should be:
post #19 of 58
Yep, that's better. Now just rearrange the vectors so that they all show their tendency on Maier's CM, and we have the original image only now showing the "equal and opposite" reaction forces:

(Image of Hermann Maier copyright Ron LeMaster (www.ronlemaster.com). Since LeMaster's original copyright is missing from these retouched images, they should be included here. Ron--if you object to this use of your images, please let us know.

Blue vector: centripetal force (from the lateral pressure of the snow on the skis)

Yellow vector: Upward pressure of the snow on Maier's skis; reaction force of the downward pressure Maier applies to the snow, due to gravity.

Green vector: Resultant force--sum of the other two

Best regards,
post #20 of 58

Thanks for the copyright note, I wasn't sure what to do with that.
post #21 of 58
BB, great cartoon!
("Dr. Evil: No no no, I'm going to leave them alone and not actually witness them dying, I'm just gonna assume it all went to plan. What?" - quote from Austin Powers)
post #22 of 58
It makes more sense now.
I'd still like to see the three view diagrams from drafting class. Maybe with the new computor programs that let one view an object from all angles this could become clearer.
post #23 of 58
Wow, now that was picking the knit.
From my perspective, I stand and move and the earth is the thing moving underneath me. When I look at a leaf, most people say it is green... I say it is every color but green as it absorbs all the spectrum of visible light and reflects green.
Now I gotta shut up and ski... damn... no snow.... oh well, carry on with these technical discussions. I like the adjustments to the image (shorter blue vector to draw down the green), although my guess is Hermann is probably 90ish/10ish on the outside ski, and the CM is probably not at his belly button, but slightly to the right of his "belly button".

You gotta shake your head and smile at the tech talk.

post #24 of 58
Well then snowpro - ya might get real dizzy shakin yer head at this next ...

I think it's Kinda nice when people question the nature of such well established images. I too have always disliked this particular representation of Forces sans qualification. I see it as being accurate in a general sense for rigid objects - but not sufficient to describe detailed skier dynamics (for which I suspect it was never intended).

My own particular gripe is that the arrows shown assume a Rigid Body where arms and legs are all rigidly attached to (and unmoving in relation to) the torso. If the image were to be qualified with “The arrows drawn are a generalization of summed forces” - then I think it works nicely for many discussions. Without such a qualifier, Spindrift’s observation and many other objections are likely to come about.

To explore it further: What if the skier shown (in the original image) is actually morphing their body in some way at that exact moment? Say Maier is actively lifting his Left Arm upward and/or actively articulating his waist such that his shoulders are actively being ‘leveled out’… then what?

Any such muscular induced accelerations of attached Masses will undeniably change the Magnitude and Direction of the Force Arrows drawn; maybe only a little, but probably a lot - certainly enough to skew the Green Arrow. If the skier is lifting that left arm in the moment documented by the image the skier would be accelerating that attached Mass upward and this would change the magnitude of the Yellow Arrow. The skier would momentarily experience an increase in downward Force at their CM thus skewing the Green Arrow also.

Another consideration is that the diagram might actually be accurate - but not complete. As just mentioned we are not rigid objects and each individual Arrow drawn is actually a summation (or resultant) of other Arrows that were not drawn.

For instance:
What if Maier is neither ‘lifting’ nor ‘standing on’ that Left Leg - meaning he is *not* supporting any portion of his upper-body with it? We could then consider “Maier’s Relevant Mass” to be everything except his Left Leg. In this case the only Mass relevant to the Arrows drawn would be the Mass that is actually supported by his Right Ski.

It is entirely possible that only the Mass of Maier’s Left Leg is being supported by his Left Ski in that image. That Left Leg may represent perhaps 20% of his overall Body’s Mass - certainly sufficient Mass to bend that Inside-Ski as shown - yet *not* participating in any way at bending his Right Ski. This suggests the need for a second set of arrows representing the Forces acting independently on the Mass of his Left Leg.

If this seems far-fetched - try it some time. At the end of the season I skied a great deal with no option but to do just this. In early March I pulled my right calf muscle apart while teaching. It was ugly. It’s a Very unpleasant feeling to have one’s body part tear slowly apart.

I finished out the last hour of that lesson by teaching one-ski skiing since it’s all I could do. I found it quite a strain to hold that useless right leg up off the snow all the time so I quickly learned to continually ‘rest’ that leg on the snow - but with no other weight on it besides its own weight. I also kept that foot slightly forward at all times. While my right calf still took a beating from the lumpy snow it really wasn’t all that hard to do. I managed quite a few more runs down groomed black terrain before going in. (Never say die when skiing )

A week later I had the opportunity to train with a couple of National Alpine Team members. No way I was going to pass this up so I signed up despite the still-healing injury. On my first run that morning I booted-out at abundant speed. During my gyrations to recover, my left ski was stripped off and I fell heavily onto the front of my right ski - tearing my right calf apart even more fully than before… Nauseating sensations I must say.

Unwilling to forgo this time with Demo Teamers I insisted on continuing on with the group. I stayed with them all day by keeping my body weight primarily on my left leg and continuously balancing over the left ski. Most of the time my right leg had only to support its own weight. Even so, looking at the video later on I could clearly see the right ski throwing up snow and even carving cleanly in both directions when Dynamic Parallel was employed. I got quite good at ‘feathering’ that bum leg - much like the feathering the prop of a bum engine on a multi-prop airplane. That leg didn’t contribute very much and stayed mostly out of the way.

The bottom line is that readers should not take any such diagrams as the Full and Complete All-Encompassing Literal Truth. Such diagrams are merely conceptual representations of basic ideas.

Discussions with Technical Analysis of Skier-Motion frequently break down in translation between parties because participants use ‘generalizations’ (like the Maier diagram) to support their own precise descriptions of relationships-in-isolation.

Heck, even the concept of ‘Center-of-Mass’ (CM) breaks down when we realize that multi-jointed, multi-segmented biomechanical beings have many individual CMs that add up to the oft-described skier-wide CM.

Each individual body-Joint separates two independent body-Segments. Each body-segment has its own CM that can operate independently from every other body-segment’s CM. When two such segments are momentarily held rigid with respect to each other - they effectively become a single segment with a new (temporary) CM location that includes the Mass of both. When we discuss skier motions we often imply with unintended precision the motions of the “Skier’s CM”. We do *not* generally take into account the many independent CMs and their potentially dramatic impact on our proposed idea.

I realize such infinitesimal detail is way more than most of us care to consider but it is the reality with which we are dealing in Technical Discussions whether we choose to recognize and incorporate the ‘dynamic individuality’ of body parts or not.

When Technical information is presented without consideration for the obscure details these 'unaccounted for' detais behind our generalizations leaves us with gaping holes in our proposal - holes that others are sure to try and fill or contest. Sometimes with accurate information; more often with equally incomplete (and contrary) rebuttal. This leaves the floor wide open for emotional expressions of highly dogmatic convictions in place of the comprehensive reasoning that might otherwise properly connect the dots.

post #25 of 58
Good eye, spindrift. One point to consider in relocating the ground intersection point of the green arrow. All a ski has to do to throw snow it contact the ground. It doesn't require much pressure. Take a look at Hermann's inside ski. It appears to me to still be cambered.

And Bob, move your CM placement a tad to the left,,, he's angulating.
post #26 of 58
Thread Starter 
Originally Posted by Rick View Post
Take a look at Hermann's inside ski. It appears to me to still be cambered.
You may well be right. Although I suspect those babies take a bit more force than my skis can before decambering (oh wait, my favorite skis are pre-decambered!!!). Plus, if I were gonna be me I'd say it looks like there's more pressure on the tail than the front. And I'd also ponder the decambering impact of "downward" force vs "outward force" in that context... But then I'd probably just be grasping at straws! All food for thought though...
post #27 of 58
Originally Posted by michaelA View Post
A week later I had the opportunity to train with a couple of National Alpine Team members....

Unwilling to forgo this time with Demo Teamers I insisted on continuing on with the group.
Are you talking US Ski Team or Demo Team?

Most of us consider "National Alpine Team"" to be racers.

They ski rather differently.

(In the spirit of nitpicking)
post #28 of 58
"And Bob, move your CM placement a tad to the left,,, he's angulating."
That brings to mind another thing Ron LeMaster points out, a boomerang (angulated skier?) has it's CM outside it's body somewhere in the throat of the "V". Ron's definition of CM is: "the point around which a body rotates". That's why there's a difference (however slight) between the CM and the CG. Look at the side view of the CM in the Three Turns Continued thread. (There's that "three view" thing again. Darn draftsmen )
post #29 of 58
Originally Posted by newfydog
Are you talking US Ski Team or Demo Team?
Oops, sorry - didn't know that stating 'National Demo Team' could also mean 'U.S. Ski Team'...
...Oh wait - it doesn't. (Actually, I just went with the 'Official' name I'd seen in the paperwork.)

Must say, it was a fun day despite the injury! Our group was quite large so we split up between Rick Lyon and Jeb Boyd. I did the calf damange when I snuck up behind Rick and was sync'ing just behind him at a goodly speed. He went to the right around an island of trees and I went left - directly onto a steeper slope. Knowing I had a limited range of 'tip' until boot-out I didn't 'tip' any more - but the suddenly steeper slope effectively 'tipped up towards me' and down I went.

Rick's group was 'just going to ski' and headed off to the steep off-piste. With my right ski already barely controllable I went with Jeb who planned to 'teach' and stay (mostly) on groomed terrain.

Rick is very much a race coach and runs his own race training. Not sure if Jeb does so. I found quite a lot of similarity between Rick and Jeb's skiing although Rick always seemed to be pulling away from the group which is why I jumped in behind him - to watch his technique. Probably should have been watching the road ahead instead .

post #30 of 58
Isn't that Dave Lyon who's on the D-Team? (There is a Rick Lyons from Hood Meadows/Timberline too, very good instructor but not PSIA D-Team.) I took a clinic from Dave at Big Sky in December that was excellent--he's quite the biomechanics analyst, though he's a man of few words and has a wicked Snoose grin.
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