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Has anybody selected underfoot size based on foot width instead of resort conditions?

MikeHunt

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Was just watching this video about fat ski syndrome and there's bits in it showing underfoot size is biomechanically influenced by how wide your foot is.


The normal recommendation is to choose ski underfoot based on the conditions of the resort (ie narrower for east coast and wider for west coast conditions).

Does choosing underfoot size based on foot width make sense though?

That is, go skinny underfoot if you have a narrow foot and wider ski if you have wider foot?
 
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François Pugh

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A wider ski to prevent boot out is very tempting for me, maybe something around 80 mm with a lot of lift would work for me. As it is, hard conditions mean no big angles. Soft conditions mean railroad tracks are quad tracks (two skis and two boot toes). Boot-out Hurts.
 

Philpug

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"Narrow skis are designed to be on the snow, wide skis are designed to be in the snow"

This is the first I heard of ski width being in relation ot foot width, I always related it more to skier height and the leverage the skier creates. Some of this IS translating into ski design with more and more brands scaling ski widths so as the skier gets taller (or shorter) the waist width changes. It is not uncommon to have a ski model with up to a 6-7mm difference in waist from the shortest size to the longest.
 

Big J

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I watched the video and I think it applies to me. I have a 106mm last and think that my foot is more suitable for wider skis as my fore foot approaches the ski edges more on a wider ski. I ski up to 108mm underfoot in bounds and have not noticed any problems skiing any of them on firmer snow. I am able to carve my Kastle BMX108 well. I have noticed that the narrower waists do turn/carve faster. I have many skis in the 84-108mm underfoot. My daily driver is a Kastle MX89. My powder skis are 116mm. I learned quite a bit from watching this video.
 

Lauren

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Was just watching this video about fat ski syndrome and there's bits in it showing underfoot size is biomechanically influenced by how wide your foot is.

I can understand this standpoint, however, I think for the most part, your foot width is negligible. I think the binding width is more important, as it's the part that actually interfaces with the ski. Your boot is going to be wider than the binding in every scenario I can think of. Certain bindings have a wider platform because they're intended to be used on wider skis (Attack and Wardens come to mind as ones that actually advertise their widths and marketing totes them to have better energy transmission for wider skis). People with vastly different foot widths all use the same bindings, thus will get a very similar result in ski control.
 

Big J

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The way I took the video is that the wider the foot the better it will have direct downward pressure on a wide under foot ski edge. In regard to bindings the width of the foot and the corresponding downward pressure does not change as the width of the binding increases. To me the boot sole and foot width have more of an effect on the downward pressure than the binding width does. This does not include ramp angles and other binding specs effects on edge pressure. To me a wider binding has the same effect as a wider ski in regards to down pressure.
 

AmyPJ

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Speaking from experience, a narrow foot sucks on wider skis. I came to the realization this season that my narrow foot/narrow boot absolutely can't get the lateral leverage that someone with wider feet/boots can. It's biomechanically impossible. I have found that the widest that I can ski happily on is 88 under foot, which corresponds with the width of my boot shell. (Approximately.) I'm planning to go narrower for my all-mountain ski next year. What ski that will be remains to be seen...
 

Magi

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Was just watching this video about fat ski syndrome and there's bits in it showing underfoot size is biomechanically influenced by how wide your foot is.

The normal recommendation is to choose ski underfoot based on the conditions of the resort (ie narrower for east coast and wider for west coast conditions).

Does choosing underfoot size based on foot width make sense though?

That is, go skinny underfoot if you have a narrow foot and wider ski if you have wider foot?

Ding Ding Ding.

I've maintained that the first parameter of importance relating to ski width is foot width for a few years now.

Roughly three classes of ski:
Narrower than your foot. (65ish) [these are "easy" to tip, and relatively sensitive to small angle changes in the leg shaft]
About as wide as your foot. (80ish) [These are a great balance of easy to tip but not overly sensitive]
Wider than your foot. (95+) [Relatively hard to tip, these skis also don't punish leaning/banking into the turn with the upper body nearly as much.]


The reason for the above is that moving the point of ski/snow contact in and out (relative to your centerline) changes how much force is required to tip the ski, and the resulting change in the "critical angle" of the ski to your CoM for a given amount of tipping. In softer snow (where the snow easily deforms to match the ski) this process is dramatically lessened (which is why wider skis feel better in softer/deeper snow).

Pretty much the only reason* to pick a ski that's wider than your foot (all other shape parameters held constant)** is to float higher in the snow so you push less snow, so you go faster. That's it. Other reasons pretty much come down to an inability to use the ski as designed, and various parameters of the ski design papering over the skier's lack of ability to point the ski where they want to go and go where it's pointed.



*[A wider ski also tends to ride over / not get "hung up" when skidding sideways through things (because the edge on the other side is higher off the ground). I also sometimes choose a wider ski in gnarly conditions (death cookies/refrozen crud) where ability to sideslip without catching the downhill edge is paramount over all other considerations.]

** [You can ABSOLUTELY change how a ski will interact with the snow by changing its shape, and wider skis tend to make different design decisions. You may want a "wider" ski for those design decisions. My point is that all the "width" does is change the area underneath the ski, and the only thing that increased area does is make you go faster, by decreasing how much snow your leg shafts push out of the way (by floating you higher).]
 

Magi

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I can understand this standpoint, however, I think for the most part, your foot width is negligible. I think the binding width is more important, as it's the part that actually interfaces with the ski. Your boot is going to be wider than the binding in every scenario I can think of. Certain bindings have a wider platform because they're intended to be used on wider skis (Attack and Wardens come to mind as ones that actually advertise their widths and marketing totes them to have better energy transmission for wider skis). People with vastly different foot widths all use the same bindings, thus will get a very similar result in ski control.

Go a step further - the DIN interface is the exact same for every boot. So how could foot width matter at all?

The answer is that it's how far inside/outside your foot the ski is transmitting force from (the point of ski/snow contact) that determines the torque/leverage on the boot/leg shaft, and the ratio of change in leg shaft angle to change in critical angle (angle of the ski to the CoM). As that binding gets wider - the force your foot/boot system applies is transmitted closer to the edge of the ski, but that mostly helps improve power transmission when a ski isn't that torsionally rigid. If you had a ski made of infinitely rigid material - the width of the binding doesn't matter a bit.

The further away and outside from the edge of your foot the force is transmitting - the less mechanical advantage you have for tipping.
As you bring the line of force inside the foot, you gain mechanical advantage, and amplify the grippyness (because smaller changes in leg position result in larger changes to the angle of the ski to your CoM [which controls grip]).
 

Lauren

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Speaking from experience, a narrow foot sucks on wider skis. I came to the realization this season that my narrow foot/narrow boot absolutely can't get the lateral leverage that someone with wider feet/boots can.

As devil's advocate...How do you know it's the boot width that is presenting the challenges with getting lateral leverage? How do you know that someone with wider feet can get more leverage?
 

Lauren

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The answer is that it's how far inside/outside your foot the ski is transmitting force from (the point of ski/snow contact) that determines the torque/leverage on the boot/leg shaft, and the ratio of change in leg shaft angle to change in critical angle (angle of the ski to the CoM). As that binding gets wider - the force your foot/boot system applies is transmitted closer to the edge of the ski, but that mostly helps improve power transmission when a ski isn't that torsionally rigid. If you had a ski made of infinitely rigid material - the width of the binding doesn't matter a bit.

The further away and outside from the edge of your foot the force is transmitting - the less mechanical advantage you have for tipping.
As you bring the line of force inside the foot, you gain mechanical advantage, and amplify the grippyness (because smaller changes in leg position result in larger changes to the angle of the ski to your CoM [which controls grip]).

Bringing this back to the original question. Do you take your boot width into account when choosing your skis due to your explanation above? Or does the boot width have a negligible affect on the performance? I'm completely on board with the fact that wider skis take more leverage to get onto edge; but I don't think your boot width has a large enough impact on that leverage to constitute considering your boot width for your ski width choice. However, I 100% consider my binding choice based on the width of the skis it's going on...a wider ski performs better with a wider binding IMO.
 

Saintsman

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As devil's advocate...How do you know it's the boot width that is presenting the challenges with getting lateral leverage? How do you know that someone with wider feet can get more leverage?

I think I can see this - it's all about leverage and the turning moment. Someone with a wider foot is applying pressure further away from the pivot point than someone with narrow feet. Although the difference is small, in relative terms it's on the order of 10-15%. As the Moment is a straightforwrd weight x distance from pivot, 2 people of equal weight but narrow vs wide feet will see effectively all of that difference. So a wide footed person has to exert that percentage less pressure/leverage to get on edge

(Forgive the extremely poor GCSE science explanation from dredged up from 25-odd years ago)
 

AmyPJ

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As devil's advocate...How do you know it's the boot width that is presenting the challenges with getting lateral leverage? How do you know that someone with wider feet can get more leverage?
See what @Magi said above.
 

Lauren

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I think I can see this - it's all about leverage and the turning moment. Someone with a wider foot is applying pressure further away from the pivot point than someone with narrow feet. Although the difference is small, in relative terms it's on the order of 10-15%. As the Moment is a straightforwrd weight x distance from pivot, 2 people of equal weight but narrow vs wide feet will see effectively all of that difference. So a wide footed person has to exert that percentage less pressure/leverage to get on edge

Personally, I think this is oversimplifying the leverage, pressure and pivot points of your ski. The pressure applied to a ski is not just on the edge of your foot, it's distributed across the entire thing (albeit not necessarily evenly). So it's not just a force times distance equation, like in high school physics. In order for that 10-15% difference in width to make a non-negligible difference in leverage, all of your weight would have to be located at that pivot point. In reality, probably 80-90% of a person's weight is located somewhere along the 70-ish millimeters under the boot lug (not at the very edges of their boot width). I do agree that there would be SOME difference, but I believe it is completely negligible when you are choosing the width of your skis.
 

Saintsman

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Personally, I think this is oversimplifying the leverage, pressure and pivot points of your ski. The pressure applied to a ski is not just on the edge of your foot, it's distributed across the entire thing (albeit not necessarily evenly). So it's not just a force times distance equation, like in high school physics. In order for that 10-15% difference in width to make a non-negligible difference in leverage, all of your weight would have to be located at that pivot point. In reality, probably 80-90% of a person's weight is located somewhere along the 70-ish millimeters under the boot lug (not at the very edges of their boot width). I do agree that there would be SOME difference, but I believe it is completely negligible when you are choosing the width of your skis.

From memory (and this is a long time ago) it works roughly as follows. Once the width of the boot exceeds the width of the lug (I'd love to see a boot that narrow) the pivot point becomes the centre of the binding. And from a physics perspective the enormous majority of the weight is transmitted through the outside point of the foot. So, for quick and dirty maths, you can work it out as everything going through the outside point and be in the ballpark. It;s close enough that by the time you reach 10 runs, everything else being equal, the wider footed skier probably has enough left over to go the extra run.

Of course, in the real world everything else isn't equal.
 

Magi

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Bringing this back to the original question. Do you take your boot width into account when choosing your skis due to your explanation above? Or does the boot width have a negligible affect on the performance? I'm completely on board with the fact that wider skis take more leverage to get onto edge; but I don't think your boot width has a large enough impact on that leverage to constitute considering your boot width for your ski width choice.

It's the width of your foot, not the width of your boot (these correlate, but are not the same), and yes.

A ski that's the same width as your foot (aka the point where power transmits from the boot to your body) feels different than a ski that's narrower or wider. The bigger the difference in relative width between the ski and the foot - the bigger the amount of mechanical advantage you gain/lose.

Someone with a 82mm wide foot on an 86mm wide ski has lost mechanical advantage. This is a "wide" ski *for that foot*.
Put someone with a 90mm wide foot on that 86mm ski and they will gain mechanical advantage over the ski. This is a "Narrow" ski *for that foot*.
I am explicitly *not* arguing for an absolute amount of difference that a millimeter makes. I'm trying to demonstrate that there *is* a difference in leverage and it comes from the ratio of foot width to ski width.

I can personally feel a dramatic difference between 2mm of ski width difference (78 vs 80 mm, a 2.5% difference in ski width), your mileage may vary depending on your kinesthetic awareness.


However, I 100% consider my binding choice based on the width of the skis it's going on...a wider ski performs better with a wider binding IMO.

You ski real skis that are not infinitely rigid from side to side. That is why you are completely correct that a wide binding skis better on a wide ski.


Personally, I think this is oversimplifying the leverage, pressure and pivot points of your ski.

@Saintsman is correct and his is the right model to use when trying to analyze leverage between where the snow applies force (at the ski edge) and where your body resists that force (in your foot, and *technically* along the length of your leg shaft inside the boot but that is effectively constant since we're holding the leg in question constant and only varying foot width in our model.).

The pressure applied to a ski is not just on the edge of your foot, it's distributed across the entire thing (albeit not necessarily evenly). So it's not just a force times distance equation, like in high school physics. In order for that 10-15% difference in width to make a non-negligible difference in leverage, all of your weight would have to be located at that pivot point. In reality, probably 80-90% of a person's weight is located somewhere along the 70-ish millimeters under the boot lug (not at the very edges of their boot width). I do agree that there would be SOME difference, but I believe it is completely negligible when you are choosing the width of your skis.

The foot stands on the bootboard, which is connected to the boot lug, which touches the binding, which attaches to the ski. When modeling the question of leverage, all of this can be pretty darn accurately modeled as a rigid system with two interface points (ski to snow, and boot to foot). That forms a triangle, and that triangle's shape/size tells you whether you have advantage over the ski or the ski has advantage over you.

None of what you're referencing (which is maybe necessary to simulate a ski moving in three dimensions) applies to the question of "Do I have leverage over it, or does it have leverage over me). We don't care about the fore/aft balance here (we're explicitly ignoring it, because it'd be constant for all situations). We *do* care about where their center of mass is relative to the ski edge when we go from the static to the dynamic - but again, you're drawing a triangle from the CoM Straight down, then over to the ski/snow interface.

If this is a foot: __||__
And this is space so things format correctly: ---

And this is a foot over a wider Ski:
------__||__
_________________

And this is a foot over a Narrower ski:

__||__
--_

Surely you can see at a glance that there's a change in leverage?

If your argument is simply "no one can tell the difference between those two scenarios", then I can only respond - I have never met someone who can't feel the difference between a 22m radius cheater GS ski that's 65 underfoot and a 22m radius 95 underfoot ski while making a railroad track turn.
 
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