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# Edge Angle vs. Platform Angle

I'm trying to understand the difference between these terms in LeMaster's Ultimate Skiing. Platform Angle seems clear. How is Edge Angle different? Is it base of support vs the ski's relationship to that?
The platform angle is the angle that is the result of the relationship between the base of the ski and the balance axis of the skier . An angle of 90* or less will result in good grip and one greater than 90* will result in more slip.

The skis edge angle is the angle that is the result of the relationship between the base of the ski and the surface of the snow. You could have a 40* edge angle and a 90* platform angle at the same time.

zenny
Got it. Thanks Zenny.
Quote:
Originally Posted by Lorenzzo

I'm trying to understand the difference between these terms in LeMaster's Ultimate Skiing. Platform Angle seems clear. How is Edge Angle different? Is it base of support vs the ski's relationship to that?

Just keep reading Chapter 2. Over, and over, and over. Actually goes for the whole book!

Even better, go to Colorado and get it from The Master in person...

http://www.ronlemaster.com  ( Info on front page )

Quote:
Originally Posted by jc-ski

Just keep reading Chapter 2. Over, and over, and over. Actually goes for the whole book!

Even better, go to Colorado and get it from The Master in person...

http://bvsd.org/lll/Documents/Adult-Fall-2014/Fall-Catalog.pdf    ( See page 22 )

Ha...yes, that's where I was. I'm hoping the terminology becomes more meaningful as it's applied in later chapters but I won't assume that.

Lorenzzo--Here (click on this link) is my best effort to explain "platform angle" (post #17 and subsequent, in the thread, "What is best edge angle for max grip?" from a couple years ago).

In short, "edge angle" refers to the angle of the ski base to the snow surface. Essentially, when combined with the ski's "sidecut radius," it determines the radius of the arc that the ski will bend into and carve on hard snow. Perhaps surprisingly, it has nothing to do with how well a ski holds.

"Platform angle" (and the related term, "critical edge angle," both terms coined by Ron LeMaster) refers to the angle between the ski edge (not necessarily the same as its base, due to base edge beveling) and the angle of the force applied to the ski by your body--what Zentune refers to as your "balance angle." Platform angle largely determines whether a ski grips or slips sideways.

The two are fundamentally different ways of measuring ski tipping, and they have completely different effects. Great skiing requires controlling both, constantly, and independently. Please read the thread I linked to above for (much) more. And feel free to paste text and/or images from that thread here if you want more discussion.

Best regards,

Bob Barnes

Quote:
Originally Posted by Bob Barnes

Lorenzzo--Here (click on this link) is my best effort to explain "platform angle" (post #17 and subsequent, in the thread, "What is best edge angle for max grip?" from a couple years ago).

In short, "edge angle" refers to the angle of the ski base to the snow surface. Essentially, when combined with the ski's "sidecut radius," it determines the radius of the arc that the ski will bend into and carve on hard snow. Perhaps surprisingly, it has nothing to do with how well a ski holds.

"Platform angle" (and the related term, "critical edge angle," both terms coined by Ron LeMaster) refers to the angle between the ski edge (not necessarily the same as its base, due to base edge beveling) and the angle of the force applied to the ski by your body--what Zentune refers to as your "balance angle." Platform angle largely determines whether a ski grips or slips sideways.

The two are fundamentally different ways of measuring ski tipping, and they have completely different effects. Great skiing requires controlling both, constantly, and independently. Please read the thread I linked to above for (much) more!

Best regards,
Bob Barnes

Greatly appreciated. Those concepts will need some sinking in...

Yes--some sinking in! But it will be well worth the time and effort, if you want to understand the technical side of skiing. It's a critical, but frankly still commonly misunderstood, distinction--between "edge angle" and "platform angle." Thanks for bringing it up again!

If you're still interested in reading more, after the thread I linked to above, here's a couple others from our archives that have touched on the subject (and there are more):

Let's Get Physics-cal

Best regards,

Bob

Quote:
Originally Posted by Bob Barnes

Yes--some sinking in! But it will be well worth the time and effort, if you want to understand the technical side of skiing. It's a critical, but frankly still commonly misunderstood, distinction--between "edge angle" and "platform angle." Thanks for bringing it up again!

If you're still interested in reading more, after the thread I linked to above, here's a couple others from our archives that have touched on the subject (and there are more):

Let's Get Physics-cal

Best regards,
Bob

Bob, thanks again, I will take a look at all of these resources. I'd love to gain an intuitive understanding of these areas, if that's possible.

Although it's a bit tedious to wade through a written description of these concepts, it is actually very easy to demonstrate and feel the differences on the snow--with a little guidance. That really is the best way to get that "intuitive" understanding you're looking for, Lorenzzo.

You can feel "platform angle" ("critical edge angle") very easily. Find a good slope that is moderately steep, but that you're still comfortable on--a blue run usually suffices--with firm, smooth snow. You should be able to release your edges and sideslip down it easily. Stand still with your skis across the slope, gripping the snow with no slippage. You will probably note a little ankle tension, at least in your downhill ankle, and probably a little "knee angulation" and hip angulation--but there is no need to overdue it. Now, slowly, relax your ankle(s) and reduce your edge angle, just a little at a time. At some point, your skis will suddenly "release" and start slipping. Since you are standing still, the force you're applying to your skis is always straight down, so the changing edge angle alters the platform angle, relative to that force. When the angle reaches 90 degrees or so (your skis will be "level"), that is where they'll release. That point is "critical edge angle."

Now imagine making a high-speed carved turn, if that is something you can visualize. Your body leans into the turn ("inclines") for balance, and your edge angle on the snow increases along with it. But if your skis are holding, you'll still feel the same ankle tension as before, when you were standing still. And if you relax that ankle tension, just slightly reducing your edge angle, your skis will lose their grip and slip. Even though their edge angle on the snow may be much higher than it was when you stood still, the platform angle must still remain less than 90 degrees if you want your skis to hold. Reduce the platform angle, even with very high edge angles due to inclination of your body, and your skis won't hold.

You may be able to experience the same thing without actually moving, if you find a partner to stand below you on the hill and pull on your poles while you resist. The partner simulates the turning forces (centrifugal force, if you must), allowing you to incline into the hill without losing your balance--which enables you to explore the angulation movements (ankles, knees, hips, spine) that affect platform angle.

These experiments will reveal an important technical understanding: Platform angle is influenced and controlled with angulation movements--essentially sideways bending in the ankles, knees (it's complicated), hips, and spine. Edge angle (vs. the snow surface) is influenced by all of the above, as well as inclination movements--leaning the body, or more specifically the center of mass, into the turn for balance. Inclination does not affect platform angle.

Good luck. Keep the conversation going!

Best regards,

Bob

Anyone know if LeMaster has any workshops scheduled for this fall aside from the ones at the college that last over several weeks?  Something where I can drive out to CO for a weekend and get a dose of it?

Lorenzzo, think of it like this.  when you're leaned in during a turn and creating turn forces, there is a gravity component and a turn force component which add up together into what is sometimes called a resultant force vector.  I don't have my RLM books handy to see if he used that term, but I bet he does and has a picture of it.  Here is one, the green arrow is the resultant force vector:

Now in order for the edge to hold, imagine for a minute the edge is carving out a little slot in the snow.  If you look at that slot from the position of the green arrow, its like a "Shelf".  if that shelf is 90 degrees to the green arrow then it will hold the ski on the shelf.  If the shelf is tipped outwards a bit more, then the ski will slowly slide off the shelf (ie, lose edge grip).  If its tipped outward even more then it will slide off the shelf even faster, so to speak(ie, have almost no edge grip).  So edge grip is created by having the ski tipped up such that there is an  angle of 90 degrees between that green resultant force vector and the angle of the ski (which is carving out the shelf it rides on).  If the ski is tipped INWARD more then neccessary it will be locked into grip, which is what people call edge locked carving.

note how in this picture the leg is lined up with the green arrow.  That is not always the case.  Don't let the photo deceive you into thinking the green resultant force vector always goes right down the leg.  Its a result of the other two forces, which you may or may not line your leg onto as you ski.

The leg is pretty much laterally 90 degrees to the bottom of your ski at all times, though with knee bend and knee angulation, photos can look otherwise, the shin at least will always be pretty close to 90 degrees laterally from the base of the ski.   (That is ignoring beveling for simplicity).

If your leg is above the green arrow, then slippage.  If your leg is below the green arrow, then you have edge lock.

All of the above is referring to the so called "platform angle", which is the angle between the green force vector and the base of the ski (or base edge).

But also note the angle of the snow is different from the green arrow.  The angle of the snow is approximately the blue arrow above.  The angle of the ski related to that blue arrow would be RLM's so called "edge angle".  Edge angle matters in terms of how the ski will bend and carve on the snow, in combination with whatever amount of hold its getting from the platform angle described above.  Platform angle tells you whether the ski is gonna arc or not.  Edge angle determines the carved turn size.

Of course if you reduce your angle, whichever way you want to call it, you are reducing both platform angle and edge angle at the same time.  Its one movement, but two ways of inspecting the ange, one relative to the green force vector and the other relative to the snow surface.  If you increase your tipping to get better platform angle, then your edge angle also tips in and decreases the turn size, etc

[/quote]
Quote:
Originally Posted by Bob Barnes

Although it's a bit tedious to wade through a written description of these concepts, it is actually very easy to demonstrate and feel the differences on the snow--with a little guidance. That really is the best way to get that "intuitive" understanding you're looking for, Lorenzzo.

You can feel "platform angle" ("critical edge angle") very easily. Find a good slope that is moderately steep, but that you're still comfortable on--a blue run usually suffices--with firm, smooth snow. You should be able to release your edges and sideslip down it easily. Stand still with your skis across the slope, gripping the snow with no slippage. You will probably note a little ankle tension, at least in your downhill ankle, and probably a little "knee angulation" and hip angulation--but there is no need to overdue it. Now, slowly, relax your ankle(s) and reduce your edge angle, just a little at a time. At some point, your skis will suddenly "release" and start slipping. Since you are standing still, the force you're applying to your skis is always straight down, so the changing edge angle alters the platform angle, relative to that force. When the angle reaches 90 degrees or so (your skis will be "level"), that is where they'll release. That point is "critical edge angle."

Now imagine making a high-speed carved turn, if that is something you can visualize. Your body leans into the turn ("inclines") for balance, and your edge angle on the snow increases along with it. But if your skis are holding, you'll still feel the same ankle tension as before, when you were standing still. And if you relax that ankle tension, just slightly reducing your edge angle, your skis will lose their grip and slip. Even though their edge angle on the snow may be much higher than it was when you stood still, the platform angle must still remain less than 90 degrees if you want your skis to hold. Reduce the platform angle, even with very high edge angles due to inclination of your body, and your skis won't hold.

You may be able to experience the same thing without actually moving, if you find a partner to stand below you on the hill and pull on your poles while you resist. The partner simulates the turning forces (centrifugal force, if you must), allowing you to incline into the hill without losing your balance--which enables you to explore the angulation movements (ankles, knees, hips, spine) that affect platform angle.

These experiments will reveal an important technical understanding: Platform angle is influenced and controlled with angulation movements--essentially sideways bending in the ankles, knees (it's complicated), hips, and spine. Edge angle (vs. the snow surface) is influenced by all of the above, as well as inclination movements--leaning the body, or more specifically the center of mass, into the turn for balance. Inclination does not affect platform angle.

Good luck. Keep the conversation going!

Best regards,
Bob

Thank you for the detail. I think I follow except the part about increasing Inclination. Wouldn't increasing Inclination reduce Platform Angle (the way LeMaster refers to changes) assuming no compensation through angulation? If through angulation the Resultant Force doesn't change, then I can understand how Platform Angle wouldn't change.

Yes, I'd like to get on the snow to reduce the abstraction of this.

Edit: Ok, looking at BTS's photo I think I see your point about inclination. The Resultant Force vector would change but without altering angulation the Platform Angle would not, right?
Quote:
Originally Posted by borntoski683

Anyone know if LeMaster has any workshops scheduled for this fall aside from the ones at the college that last over several weeks?  Something where I can drive out to CO for a weekend and get a dose of it?

Lorenzzo, think of it like this.  when you're leaned in during a turn and creating turn forces, there is a gravity component and a turn force component which add up together into what is sometimes called a resultant force vector.  I don't have my RLM books handy to see if he used that term, but I bet he does and has a picture of it.  Here is one, the green arrow is the resultant force vector:

Now in order for the edge to hold, imagine for a minute the edge is carving out a little slot in the snow.  If you look at that slot from the position of the green arrow, its like a "Shelf".  if that shelf is 90 degrees to the green arrow then it will hold the ski on the shelf.  If the shelf is tipped outwards a bit more, then the ski will slowly slide off the shelf (ie, lose edge grip).  If its tipped outward even more then it will slide off the shelf even faster, so to speak(ie, have almost no edge grip).  So edge grip is created by having the ski tipped up such that there is an  angle of 90 degrees between that green resultant force vector and the angle of the ski (which is carving out the shelf it rides on).  If the ski is tipped INWARD more then neccessary it will be locked into grip, which is what people call edge locked carving.

note how in this picture the leg is lined up with the green arrow.  That is not always the case.  Don't let the photo deceive you into thinking the green resultant force vector always goes right down the leg.  Its a result of the other two forces, which you may or may not line your leg onto as you ski.

The leg is pretty much laterally 90 degrees to the bottom of your ski at all times, though with knee bend and knee angulation, photos can look otherwise, the shin at least will always be pretty close to 90 degrees laterally from the base of the ski.   (That is ignoring beveling for simplicity).

If your leg is above the green arrow, then slippage.  If your leg is below the green arrow, then you have edge lock.

All of the above is referring to the so called "platform angle", which is the angle between the green force vector and the base of the ski (or base edge).

But also note the angle of the snow is different from the green arrow.  The angle of the snow is approximately the blue arrow above.  The angle of the ski related to that blue arrow would be RLM's so called "edge angle".  Edge angle matters in terms of how the ski will bend and carve on the snow, in combination with whatever amount of hold its getting from the platform angle described above.  Platform angle tells you whether the ski is gonna arc or not.  Edge angle determines the carved turn size.

Of course if you reduce your angle, whichever way you want to call it, you are reducing both platform angle and edge angle at the same time.  Its one movement, but two ways of inspecting the ange, one relative to the green force vector and the other relative to the snow surface.  If you increase your tipping to get better platform angle, then your edge angle also tips in and decreases the turn size, etc

Seems clear with the photo and your described images. I'd love to attend a workshop if there are some several days or less.
Edited by Lorenzzo - 10/6/14 at 6:55pm
Quote:
Wouldn't increasing inclination reduce Platform Angle?

No, not in itself. Only angulation can change platform angle. Don't forget that angulation and inclination are independent movements, and we control them separately, but simultaneously. Since inclination refers to the center of mass, not to any particular body part, you can think of pure inclination as moving the entire body as a unit. In other words, if you took a bronze statue of a skier and tilted it at an angle, everything would tip together, including the skis. The angle of the body to the skis--the platform angle--would not change, even as the edge angle (relative to the ground) would change.

However, since the two movements can (and do) take place simultaneously, it is possible to increase inclination while keeping the edge angle on the snow the same. To do that, you would have to decrease your angulation as you increased your inclination. That scenario would indeed result in platform angle decreasing as inclination increases--and it would probably cause your skis to slip out from underneath you!

Best regards,

Bob

Quote:
Originally Posted by Bob Barnes

No, not in itself. Only angulation can change platform angle. Don't forget that angulation and inclination are independent movements, and we control them separately, but simultaneously. Since inclination refers to the center of mass, not to any particular body part, you can think of pure inclination as moving the entire body as a unit. In other words, if you took a bronze statue of a skier and tilted it at an angle, everything would tip together, including the skis. The angle of the body to the skis--the platform angle--would not change, even as the edge angle (relative to the ground) would change.

However, since the two movements can (and do) take place simultaneously, it is possible to increase inclination while keeping the edge angle on the snow the same. To do that, you would have to decrease your angulation as you increased your inclination. That scenario would indeed result in platform angle decreasing as inclination increases--and it would probably cause your skis to slip out from underneath you!

Best regards,
Bob

I think I was writing my above edit as you were responding. Yes, I see that now. Interesting to think about how inclination/center of mass as you've described it contrasts with angulation.
Quote:
Originally Posted by Bob Barnes

However, since the two movements can (and do) take place simultaneously, it is possible to increase inclination while keeping the edge angle on the snow the same. To do that, you would have to decrease your angulation as you increased your inclination. That scenario would indeed result in platform angle decreasing as inclination increases--and it would probably cause your skis to slip out from underneath you!

Sounds like the making of a fine stivot.

Nice description, BTS (post 11).

Quote:
note how in this picture the leg is lined up with the green arrow.  That is not always the case.  Don't let the photo deceive you into thinking the green resultant force vector always goes right down the leg.  Its a result of the other two forces, which you may or may not line your leg onto as you ski.

In fact, I'll suggest that the vectors (arrows) are not quite accurately drawn in that image. Because Hermann Maier is somewhat angulated (bent sideways--shoulders and head tipped less than legs), his center of mass (where the lines meet) is not actually centered on his "belt buckle" area as shown. To be accurate, it should be slightly to the left in the image (to Maier's right). With the center of mass more accurately drawn, it would be clearer that the outside ski is at the necessary "critical" edge angle to hold, with the leg (or at least, the knee) well below the green force vector. Here's an adjusted image that may be a little more accurate, and that shows also the angles we're discussing:

(original image by Ron LeMaster)

Note that the top of the green force vector is now slightly left of the original, and the vector is also tipped at a steeper angle as a result. It forms a more acute platform angle "α" with the ski (tilted red line). Meanwhile, despite only moderate angulation, Maier is so deeply inclined into the turn that his edge angle on the snow (red angle "θ" in the illustration) is extreme!

In short, the green force vector (resultant of the other two vectors) directly represents inclination, and has nothing to do with angulation. It passes through the center of mass and the base of support--inside edge of the outside ski, if the skier is balanced 100% on the outside ski. The angle of the leg, on the other hand, reflects both inclination and angulation--and the more angulation, the greater the difference between the leg angle and the green vector. The green vector, then, represents balance--which is why Zentune accurately described platform angle as the angle of the ski relative to the "balance axis."

---

Quote:
Of course if you reduce your angle, whichever way you want to call it, you are reducing both platform angle and edge angle at the same time.  Its one movement, but two ways of inspecting the ange, one relative to the green force vector and the other relative to the snow surface.  If you increase your tipping to get better platform angle, then your edge angle also tips in and decreases the turn size, etc

I'm not sure I'm following you here, BTS--but if I am, then I think we disagree on an important point. It is critically important that we think of inclination and angulation as separate, independent movements--even as we manage them both simultaneously. We can increase one while decreasing the other, and we can increase or decrease either or both, as the need arises. And that's cool, because, as I've mentioned, while they are both edging movements, one (angulation) controls primarily platform angle (yes, it also affects edge angle), while the other (inclination) primarily affects edge angle on the snow. Of course, inclination (of the center of mass) is not something we can adjust at will--it is directly tied to balance, and for any given moment in any given turn, there is only one angle of inclination that results in balance. Because angulation describes movements of particular body parts relative to other parts--and does not necessarily affect balance or involve the center of mass--we can control it at will. (OK, I know I've opened a big can of worms here. I do not mean to imply that you can just move body parts around without affecting balance. But because you can move one part and compensate with another, you can angulate or de-angulate without disturbing your balance. That's the beauty of angulation!) Yes, platform angle and edge angle (relative to the snow) are two ways of describing tipping a ski. But they also describe two separate movements, even when they occur simultaneously. Perhaps that is what you're getting at after all--because we really cannot decrease inclination (because it affects balance) as we increase angulation, it is generally true that as we angulate for stronger platform angle, we also create higher edge angles on the snow (which then, as you describe, tightens the turn radius, requiring greater inclination for balance, which further increases edge angle and tightens the turn radius...in a vicious cycle!)

Best regards,

Bob

Quote:
Originally Posted by cgeib

Sounds like the making of a fine stivot.

Yes! That's exactly how that's done--decrease angulation (and therefore platform angle) until the ski lets go of the mountain (often adding some powerful rotary movements to twist the skis to a a new direction).

Best regards,

Bob

Quote:
Originally Posted by Bob Barnes

I'm not sure I'm following you here, BTS--but if I am, then I think we disagree on an important point. It is critically important that we think of inclination and angulation as separate, independent movements--even as we manage them both simultaneously. We can increase one while decreasing the other, and we can increase or decrease either or both, as the need arises. And that's cool, because, as I've mentioned, while they are both edging movements, one (angulation) controls primarily platform angle (yes, it also affects edge angle), while the other (inclination) primarily affects edge angle on the snow. Of course, inclination (of the center of mass) is not something we can adjust at will--it is directly tied to balance, and for any given moment in any given turn, there is only one angle of inclination that results in balance. Because angulation describes movements of particular body parts relative to other parts--and does not necessarily affect balance or involve the center of mass--we can control it at will. (OK, I know I've opened a big can of worms here. I do not mean to imply that you can just move body parts around without affecting balance. But because you can move one part and compensate with another, you can angulate or de-angulate without disturbing your balance. That's the beauty of angulation!) Yes, platform angle and edge angle (relative to the snow) are two ways of describing tipping a ski. But they also describe two separate movements, even when they occur simultaneously. Perhaps that is what you're getting at after all--because we really cannot decrease inclination (because it affects balance) as we increase angulation, it is generally true that as we angulate for stronger platform angle, we also create higher edge angles on the snow (which then, as you describe, tightens the turn radius, requiring greater inclination for balance, which further increases edge angle and tightens the turn radius...in a vicious cycle!)

Best regards,

Bob

No I don't think we disagree and I'm not sure how you got anything related to inclination or angulation out of what I said, I didn't think it was relevant to muck up the question about platform angle vs edge angle with discussions about balance.  I was only trying to show in the last paragraph above that if you tip your skis more, somehow, then both platform angle and edge angle change.  Its one movement that effects them both, but two different ways to look at the angles created, one relative to the green arrow and one relative to the blue arrow.   Questions about balance are a separate issue.  Achieving critical platform angle does not, technically speaking, require someone to be in balance.  it only requires the lower leg to be in line or below the resultant force vector.  Of course if they are out of balance they will have other problems soon.  hehe, and yes angulation is how you get the legs below the force vector without losing balance, excellent point.  Why do you think we disagree?  Lost me there.

Quote:
Originally Posted by borntoski683

... I was only trying to show in the last paragraph above that if you tip your skis more, somehow, then both platform angle and edge angle change….

Not necessarily sure that is a given, BTS.

Take the top of the turn for example. Theoretically... After you pass through transition and establish a platform angle (critical edge angle) and are upside down in the turn, would you not be able to maintain that platform angle as you continued to topple into the turn and increase edge angle as you inclined further but without the need to angulate more?

What about in the stivot example I already pointed out. If you decrease angulation to the point that your feet start to slip away from you, then you will begin to incline further like we see the racers do, but at that point there may be no need to decrease angulation anymore, so that platform angle could be held while the body continues to incline and tip the ski to a greater edge angle.

Quote:

Originally Posted by Bob Barnes

Because angulation describes movements of particular body parts relative to other parts--and does not necessarily affect balance or involve the center of mass--we can control it at will. (OK, I know I've opened a big can of worms here. I do not mean to imply that you can just move body parts around without affecting balance. But because you can move one part and compensate with another, you can angulate or de-angulate without disturbing your balance. That's the beauty of angulation!)

Great discussion going on here. To be really anal about it, the angulation affects balance because of the changing turning radius and corresponding centripetal force.

At low edge angles this effect is quite small, but at large edge angles it is significant.

Consider e.g. the two last frames of the following time-laps.

The increased angulation and corresponding edge angle between the two last frames will upset the balance and cause the rebound from hell that only Marcel and a few others can handle.

This picture also illustrates another aspect of platform angle vs edge angle. This SL turn is so tight that Marcel is not engaging edge locked (i.e. platform angle<90). The skis slip a little bit at first. If he would have been too angulated here the skis would lock too early and the forces would be too large to handle the dynamic balance. This could also be called patience in transition, although in this case everything happens in fractions of a second.

Edit: For reference, the carved turning radius is approximately given by R=sidecut radius x Cosinus(edge angle), and the centripetal force is inversely proportional to this.

Nice image, Jamt.

Also looks like he reduced edge angle 2-3 while maintaining platform angle.
Quote:
Originally Posted by Bob Barnes

Now imagine making a high-speed carved turn, if that is something you can visualize. Your body leans into the turn ("inclines") for balance, and your edge angle on the snow increases along with it. But if your skis are holding, you'll still feel the same ankle tension as before, when you were standing still. And if you relax that ankle tension, just slightly reducing your edge angle, your skis will lose their grip and slip. Even though their edge angle on the snow may be much higher than it was when you stood still, the platform angle must still remain less than 90 degrees if you want your skis to hold. Reduce the platform angle, even with very high edge angles due to inclination of your body, and your skis won't hold.

Not to be picky but, a platform angle that changes from 90 to 88 degrees would be considered a reduction in angle, no Bob? Or is this an FoR thing?

zenny

I thought it was just me, Zenny!

My wife gets really upset when she asks me to turn up the air conditioning and I make it colder! (I swear I didn't know what she meant ...again)
OK, guys. Ya got me. Mea Culpa.

How 'bout "more acute platform angles" (more small) grip better? Does that turn your freezer down? Cgeib--I'm glad to see that your cynicism continues to sink to ever higher levels!

Quote:
Originally Posted by Bob Barnes

OK, guys. Ya got me. Mea Culpa.

How 'bout "more acute platform angles" (more small) grip better? Does that turn your freezer down? Cgeib--I'm glad to see that your cynicism continues to sink to ever higher levels!

Errrr, ehhh, ummm …stay on topic…

So, Bob…

We are accustomed to seeing the line of action intersect the inside edge of the outside ski to illustrate platform angle. Is it always there? Doesn't the line of action move inside and outside of ski snow edge interface and a platform angle can still be maintained?

It's a small point, BTS--I fully agree with your definitions and descriptions of platform angle, and thank you for posting Ron's helpful image. I also agree that you do not, technically, need to be in balance to create a critical edge angle (a term closely tied to platform angle, but not yet really defined in this discussion--critical edge angle is the platform angle of 90 degrees, the point where your skis just begin to grip positively, and likewise, the point at which they begin to let go when you reduce your platform angle [less acute, larger angle--for the special help students among us ;-) ]. That is a good point--balance is indeed a separate issue, technically, although in the real world, it is always critical--and, it is also inherently tied to inclination.

In a discussion of edge angles and tipping, it is hard to avoid bringing in the movements of angulation and inclination. They are the tools of edge control, and equally important, they are, individually, the specific tools tied to controlling edge angle on the snow and edge angle to force applied (platform angle). And balance, being almost synonymous with inclination, is likewise inseparable from a discussion of edge angle.

The main point that I disagree with is your suggestion "that if you tip your skis more, somehow, then both platform angle and edge angle change." To me, this is a critical point. I can (and so can you) tip my skis without altering platform angle (with "banking"--inclination without angulation--as in the tipping bronze statue I alluded to earlier). And I can modify platform angle without changing edge angle on the snow (by adjusting both inclination and angulation, simultaneously), as Cgeib has pointed out.

It's a small point in the overall discussion. But to me, it is vitally important to think of the two "different" edge angles as being separate and independently controllable, even if, as a practical matter, we almost always do adjust one with the other as you suggest.

Best regards,

Bob

Quote:
Originally Posted by cgeib

Not necessarily sure that is a given, BTS.

Take the top of the turn for example. Theoretically... After you pass through transition and establish a platform angle (critical edge angle) and are upside down in the turn, would you not be able to maintain that platform angle as you continued to topple into the turn and increase edge angle as you inclined further but without the need to angulate more?

What about in the stivot example I already pointed out. If you decrease angulation to the point that your feet start to slip away from you, then you will begin to incline further like we see the racers do, but at that point there may be no need to decrease angulation anymore, so that platform angle could be held while the body continues to incline and tip the ski to a greater edge angle.

Actually great point, I was trying to keep the discussion simple for the sake of clarifying platform angle vs edge angle....

But as you say, its definitely possible and even probable to have a fairly constant  platform angle developed over time, even while the skier is continually tipping the skis into the turn.  This is because the centripetal forces are building up as the skier continues into the turn.  So just to maintain a critical platform angle, and to maintain balance as well against that increasing lateral force, the skier will have to continually inclinate into the turn more and more, even while theoretically maintaining approximately the same platform angle relative to that changing resultant force vector; but the edge angle relative to the snow surface is definitely changing radically during this time frame, because the snow surface angle does not change as quickly as centripetal forces do.  Or they could be changing more I suppose, as in the case of bumps, but either way.....platform angle and edge angle do not neccessarily change together....point taken!

So yes you can in theory be continually leaning into a turn through one way or another and maintaining a constant platform angle even while the edge angle is definitely changing against the snow.

Edited by borntoski683 - 10/7/14 at 9:03am
Quote:
So, Bob…

We are accustomed to seeing the line of action intersect the inside edge of the outside ski to illustrate platform angle. Is it always there? Doesn't the line of action move inside and outside of ski snow edge interface and a platform angle can still be maintained?

Glad to see you've moved past your little mental obstacle, Cgeib!  ;-)

Good question--glad you brought it up (although I know that you know the answer).

Ron's diagram, and my "adjustment" of it in post #16, both show the line of action (the green "resultant" vector, the line of balance, ... ) passing through the center of mass and through the inside edge of the outside ski. No, it does not always do that.

"Line of balance" is a good name for this vector, because a simple definition of "balance" is that it is the relationship between the center of mass and the "support point," or "point of contact") If we only had one support point--as in a unicycle, or a pogo stick--it would be simple to visualize, and the green vector would always pass through the center of mass and the point where the wheel or the stick touches the ground--no exceptions (at least when in balance).

But on skis, we have two skis, and they are long. (Am I moving too fast here?) So our potential "contact point" can be anywhere within the shape outlined by the two skis, or even outside the skis if we support some of our weight on a pole. (This is the area Larsson and Majors defined as the "polygon of sustentation" in their iconic 1979 book, "World Cup Ski Technique.")

So, the vector passes through the inside edge of the outside ski, as shown, only when the skier is 100% balanced on that single edge. Whenever the inside ski shares the load, the vector passes through a point somewhere between the skis--directly in the center between them when weighted 50:50.

It is interesting to note that moving some of your balance support to the inside ski would steepen the angle of that vector, theoretically increasing the grip of the skis by strengthening (creating a more acute angle of--damn, you're a pain!) the platform angle.

An interesting thought, but I do not suggest that is a good reason to try to ski more "two-footed."

Best regards,

Bob

Quote:
Originally Posted by borntoski683

So yes you can in theory be continually leaning into a turn through one way or another and maintaining a constant platform angle even while the edge angle is definitely changing against the snow.

Yep. Same on the exit out of a turn which I think this akin to @bud heishman's Twist-n-tip. i.e. Flow out of the turn toward the next one while keeping the ski engaged and working.
For you tuning geeks out there, this discussion helps illuminate why a larger base bevel (say the difference between a .5 and 1) can have the effect of delaying engagement a bit, or a lot...least on firmish snow anyway.

If anyone wants some info on reducing the angle formed by the side/base bevel relationship, either by increasing one bevel or both, feel free to PM me.