The Influence of Ski Waist-Width and Fatigue on Knee-Joint Stability and Skier Balance

[wallstreetoneil]

I could "buy" all this if it weren't for the simple fact that we don't ski in a boot that's only the height and width of a foot. The ski boot itself changes the physics involved in the interaction between our foot/leg and the ski (and of course how we "feel" the width). The additional girth and height of the boot becomes an "extension" of our body parts and provides additional power over the skis that are under our feet.

Yes it becomes an extension, but it doesn't change physics, and the fact that leverage is being applied to the body - otherwise the 2000yr old physics of Archimedes and the Principle of the Lever wouldn't be true - this would be completely obvious if you compare standing in hockey skates and comparing the force required to tip it over vs going to Home Depot and strapping a 1x8 to your feet and comparing the force required to tip that over. It's the same reason soccer players will wear tight-fitting shoes with studs internal to the edges of the feet as well as track athletes that run around the bend on a track. If there were any mechanical advantage to having a wider platform to bank a turn/change directions, that wouldn't outweigh the increased strain on their knees & hips, then they would use it.

 

[Wendy]

For me, the 85 instead of 95. Actually I picked up a Blossom AM77, so it'll be interesting to see how well that works as an all mountain ski compared to my Renoun Z90.

We’ve got the same 2 skis. I’m interested to compare the 2 as well. (I nearly sold my Z90’s last year to replace with something with less side cut, but SO GLAD I didn’t; those Z90’s are special). Anyways, I’m thinking my Siderals (AM77) will get the most use here in the East, with the Z90 being my crud buster. Of course, now I have a tele setup in the mix which complicates things.

 

[Noodler]

Yes it becomes an extension, but it doesn't change physics, and the fact that leverage is being applied to the body - otherwise the 2000yr old physics of Archimedes and the Principle of the Lever wouldn't be true - this would be completely obvious if you compare standing in hockey skates and comparing the force required to tip it over vs going to Home Depot and strapping a 1x8 to your feet and comparing the force required to tip that over. It's the same reason soccer players will wear tight-fitting shoes with studs internal to the edges of the feet as well as track athletes that run around the bend on a track. If there were any mechanical advantage to having a wider platform to bank a turn/change directions, that wouldn't outweigh the increased strain on their knees & hips, then they would use it.

I never stated that there would not be a difference in the force required to tip a wider ski than a narrower ski. I'm stating that it takes less effort to tip the same ski when you're in a ski boot versus a bare foot strapped to a ski. You provided all kinds of dimensions of the foot as if it were relevant to the physics of tipping a ski over, and I countered that the dimensions of the foot are overridden by the hard plastic ski boots we're wearing when we ski. The ski boot absolutely changes the physics.

 

[wallstreetoneil]

I never stated that there would not be a difference in the force required to tip a wider ski than a narrower ski. I'm stating that it takes less effort to tip the same ski when you're in a ski boot versus a bare foot strapped to a ski. You provided all kinds of dimensions of the foot as if it were relevant to the physics of tipping a ski over, and I countered that the dimensions of the foot are overridden by the hard plastic ski boots we're wearing when we ski. The ski boot absolutely changes the physics.

If a ski boot changes physics, then there is a Nobel Prize awaiting you if you can prove it - unfortunately, it does not. What you are implying by the change of physics idea is that a Ski Boot acts as a 'free' lever. Nothing is free in physics - the 1st Law of Thermodynamics is that energy can neither be created nor destroyed - i.e. the further the ski edge is away from the weight you are trying to move (i.e. the further the fulcrum is away from the weight) the more Force that is required. Using the simple Physics Lever Mechanical Advantage Formula, and comparing the middle of your foot being 32.5mm away (half the width of race ski's 65mm waist) from the fulcrum (ski edge) of a race ski to that of a 100mm width ski, which would be 50mm away from the fulcrum (ski edge), then slightly over 50% more Force is needed on the wider ski. 50% more Force, over and over and over, while you ski, is why on firm groomers, you will get tired more quickly on wider 100mm vs 65mm waisted skis, all things being equal (which they never are but that's the math). Now, force works both ways, once you use the energy to tip the ski on edge of a wider ski, you will have a mechanical advantage potential if you can position your body weight to exploit the longer lever for increased edge grip force - for that, you would obviously need an equally stiff, if not stiffer ski (unlikely to find) so that the ski can transmit this force without flexing and losing the mechanical advantage you gained by using the increased energy to get the ski on edge. As an example for which I'm personally experienced with, the very stiff Nordica Enforcer 88, with 2 sheets of metal, in my opinion, skiing them back to back against my (Volkl Racetigers SL with 68mm waist, Volkl Racetiger GS with 70mm waist or my Blizzard GS with 65mm waist), I would say that on the iciest hardest slopes, that I can actually get more edge grip with the Enforcers - but you absolutely need more energy/ force to get them on edge (formula says approx 1/3rd more force and that is what it actually feels like when skiing).

 

[Noodler]

If a ski boot changes physics, then there is a Nobel Prize awaiting you if you can prove it - unfortunately, it does not. What you are implying by the change of physics idea is that a Ski Boot acts as a 'free' lever. Nothing is free in physics - the 1st Law of Thermodynamics is that energy can neither be created nor destroyed - i.e. the further the ski edge is away from the weight you are trying to move (i.e. the further the fulcrum is away from the weight) the more Force that is required. Using the simple Physics Lever Mechanical Advantage Formula, and comparing the middle of your foot being 32.5mm away (half the width of race ski's 65mm waist) from the fulcrum (ski edge) of a race ski to that of a 100mm width ski, which would be 50mm away from the fulcrum (ski edge), then slightly over 50% more Force is needed on the wider ski. 50% more Force, over and over and over, while you ski, is why on firm groomers, you will get tired more quickly on wider 100mm vs 65mm waisted skis, all things being equal (which they never are but that's the math). Now, force works both ways, once you use the energy to tip the ski on edge of a wider ski, you will have a mechanical advantage potential if you can position your body weight to exploit the longer lever for increased edge grip force - for that, you would obviously need an equally stiff, if not stiffer ski (unlikely to find) so that the ski can transmit this force without flexing and losing the mechanical advantage you gained by using the increased energy to get the ski on edge. As an example for which I'm personally experienced with, the very stiff Nordica Enforcer 88, with 2 sheets of metal, in my opinion, skiing them back to back against my (Volkl Racetigers SL with 68mm waist, Volkl Racetiger GS with 70mm waist or my Blizzard GS with 65mm waist), I would say that on the iciest hardest slopes, that I can actually get more edge grip with the Enforcers - but you absolutely need more energy/ force to get them on edge (formula says approx 1/3rd more force and that is what it actually feels like when skiing).

Thanks. Just so you know why I won't be replying further to your posts, I now have you on ignore.

 

[Tony S]

If a ski boot changes physics, then there is a Nobel Prize awaiting you if you can prove it - unfortunately, it does not. What you are implying by the change of physics idea is that a Ski Boot acts as a 'free' lever. Nothing is free in physics - the 1st Law of Thermodynamics is that energy can neither be created nor destroyed - i.e. the further the ski edge is away from the weight you are trying to move (i.e. the further the fulcrum is away from the weight) the more Force that is required. Using the simple Physics Lever Mechanical Advantage Formula, and comparing the middle of your foot being 32.5mm away (half the width of race ski's 65mm waist) from the fulcrum (ski edge) of a race ski to that of a 100mm width ski, which would be 50mm away from the fulcrum (ski edge), then slightly over 50% more Force is needed on the wider ski. 50% more Force, over and over and over, while you ski, is why on firm groomers, you will get tired more quickly on wider 100mm vs 65mm waisted skis, all things being equal (which they never are but that's the math). Now, force works both ways, once you use the energy to tip the ski on edge of a wider ski, you will have a mechanical advantage potential if you can position your body weight to exploit the longer lever for increased edge grip force - for that, you would obviously need an equally stiff, if not stiffer ski (unlikely to find) so that the ski can transmit this force without flexing and losing the mechanical advantage you gained by using the increased energy to get the ski on edge. As an example for which I'm personally experienced with, the very stiff Nordica Enforcer 88, with 2 sheets of metal, in my opinion, skiing them back to back against my (Volkl Racetigers SL with 68mm waist, Volkl Racetiger GS with 70mm waist or my Blizzard GS with 65mm waist), I would say that on the iciest hardest slopes, that I can actually get more edge grip with the Enforcers - but you absolutely need more energy/ force to get them on edge (formula says approx 1/3rd more force and that is what it actually feels like when skiing).

Thanks. Just so you know why I won't be replying further to your posts, I now have you on ignore.

@wallstreetoneil , welcome to the forum. This is one of those situations where social dynamics matter, and paying attention to them would smooth out the conversation a lot.

As of this writing you are on your fifth post, having joined two months ago. Does that make you a second-class citizen? No. Does that make your posts any less worthy of consideration? No. But we're all humans here, and so we are operating in a social context, where there is a lot of history over a long period of time, and where many of us know each others' strengths, weaknesses, backgrounds, professions, etc.

The topic in question has been discussed ad nauseam on this forum and its predecessors for years. There are lots of other posters here who have backgrounds in physics and engineering. You are not opening a new or unfamiliar topic. Yet there is an implicit "Here Comes The Expert To Save The Day" quality to your posts that is perhaps a little strong if you want people to be receptive to your thoughts as a new arrival at the party.

You might consider starting out with something like, "Hey, I was thinking about this and am wondering if it has been discussed here before, and if anyone can point me to some of those posts, or recommend a good way to use the search engine to find them." My two cents. And God knows MY social skills shouldn't be a model for anyone, so there is that.

 

[Bruno Schull]

Of course the physics doesn't change. I think what Noodler is saying is that the heights and stiffness of a ski boot change the force involved in tipping a ski.

Handy diagram below.

If you keep the boot height (BH) the same, and increase the ski width (SW) by 1 cm, how much will the force (F) change?

And...

If you keep the ski width the same, and increase the BH by 1cm, how much will the force change?

The height of the boot will change the numbers involved.

 

[Seldomski]

@wallstreetoneil - I think all @Noodler was suggesting is that the ski boot changes the forces/torque on everything contained within the boot. The plastic boot takes a lot of the loading and serves as an exoskeleton. Further up the chain, say at the knee, the torque/forces would be (mostly) the same.

 

[cantunamunch]

I could "buy" all this if it weren't for the simple fact that we don't ski in a boot that's only the height and width of a foot. The ski boot itself changes the physics involved in the interaction between our foot/leg and the ski (and of course how we "feel" the width). The additional girth and height of the boot becomes an "extension" of our body parts and provides additional power over the skis that are under our feet.

It is entirely possible to take a 3mm hockey edge and put it under a ski-boot like contraption.

It is also entirely too easy make the hockey-edged ski boot contraption fail, simply by moving the balance line to a portion of the transverse arch that makes you fight to engage the edge . Harder than balancing on a 100+ mm ski edge.

(In the early 2000s, K2 had just such a failure with a tall-ish inline skate boot design, a marathon height bootie biased to the outside edge, on the mistaken concept that it's the factory's job to put intermediate speed skaters into default hockey edging position. One of the most returned skates ever - only extremely bowlegged skaters made it work out of the box, for the rest it took 6degree+ cant shims).

@wallstreetoneil - I think all @Noodler was suggesting is that the ski boot changes the forces/torque on everything contained within the boot. The plastic boot takes a lot of the loading and serves as an exoskeleton. Further up the chain, say at the knee, the torque/forces would be (mostly) the same.

No one's disputing that the ski boot takes a bit of loading and serves as an exoskeleton.

What I am saying is that above the boot the torque/forces are *not* the same as without the boot if the edge is *not* balanced under the transverse arch.

In context of the OP, I am saying that: The ability of the ski boot to transmit knee loads to the ski without going through the arch *enables* higher levels of knee pain.

 

[Seldomski]

 

[Tony Storaro]

Man, I love these forums SO MUCH!!!

Science, everywhere you look...

 

[Lorenzzo]

The problem is that no one is being honest here and admitting that they're mapping optimum pleasure or 'fun' - completely unquantifiable - onto a field with sloppily defined edge cases.

And so everyone gets up in arms when their unquantifiable optimum of fun happens to be a failure mode for someone else *shrug*.




As I said - fun is unquantifiable - and so we have marketing and we have review sites. Fun is not testable.

We might as well leave physics and engineering out of it, they can only tell us about the edge cases of actual failure.

An ex-gf who employed a numeric rating scale for O-s is of a different opinion. Occasionally she'd carry it to the first decimal place.

Sorry that's all I have add to this thread but if a collision analysis, red/gray helmet thread comes up I might be all over it.

 

[Tony Storaro]

An ex-gf who employed a numeric rating scale for O-s is of a different opinion. Occasionally she'd carry it to the first decimal place.

 

[Decreed_It]

Damn y'all it needs to snow and all of us go play in it . . .

 

[Lauren]

Damn y'all it needs to snow and all of us go play in it . . .

Can't argue with science.

 

[James]

Sorry that's all I have add to this thread but if a collision analysis, red/gray helmet thread comes up I might be all over it.

A classic. I took the opposite stance, then convinced myself of it. Still convinced btw.
I got some “people like you…” responses.

Back to the issue-
So what if we made the foot centered in a boot that was much wider, say add 2cm to each side.

And another, what if the current contact patch on the boot sole was wider? What effects would it have?

 

[cantunamunch]

Back to the issue-
So what if we made the foot centered in a boot that was much wider, say add 2cm to each side.

And another, what if the current contact patch on the boot sole was wider? What effects would it have?

Isn't that what "wide footprint" bindings were all about?

 

[LiquidFeet]

A classic. I took the opposite stance, then convinced myself of it. Still convinced btw.
I got some “people like you…” responses.

Oh.
So that was what was going on.
Your certainty was so unyielding.
I'll never forget that thread.
I'm not alone in that.

 

[mdf]

Isn't that what "wide footprint" bindings were all about?

No, that is about giving the clamps a longer lever arm so there is less play in the interface when it is transmitting torque (wherever it originated) from the boot to the ski. If you had perfectly rigid materials and mechanisms it wouldn't matter.

If you had perfect rigidity all that would matter is where the external force enters the system and where it leaves. The path it takes inside wouldn't matter.

 

[cantunamunch]

No, that is about giving the clamps a longer lever arm so there is less play in the interface when it is transmitting torque (wherever it originated) from the boot to the ski. If you had perfectly rigid materials and mechanisms it wouldn't matter.

If you had perfect rigidity all that would matter is where the external force enters the system and where it leaves. The path it takes inside wouldn't matter.

I'm not disputing your argument, but I don't see how that fits the initial 'No'. If anything, you're repeating my reasoning.


A hypothetical boot with a wider-than-ISO-5355 sole would also present a longer lever arm against the ski. If the boot had perfect rigidity, it wouldn't matter whether the external force entered at the footbed or the boot cuff.

Same argument.