Biomechanical Geekery - Stack Height, Ski Width & Knee Pain

[galibier_numero_un]

It’s been a weird year with too much time off the snow and too much time to think and geek out.

Alpinord brought up the topic of knee pain vs. ski width in a current thread. In a separate thread, I commented about an immediate negative reaction I had to what I believed was a stack height increase when I swapped out a pair of OG G3 Onyxs for a pair of OG Marker Dukes (AFD adjusted for same AT boots – Dynafit Titans). The skis were 101mm OG Coombas and the Dukes had no wear on them (no slop).

I felt an immediate medial knee “strain” when making a few skating strides on Duke-mounted Coombas. I don’t have the numbers for the Onyxs and Dukes, so I can’t say for certain that the stack height increased with the change to the Dukes, but it felt that way.

I threw together a crude model to begin thinking about what’s going on. I’m not sure how to interpret these results, other than to note that stack height and (by inference) an increased tibia length seems to have a greater effect on horizontal displacement at the head of the tibia than does ski width. What this displacement means (how it relates to Q-angle and other variables) in terms of long-term knee health is beyond me.

In the attached drawing:

• The left side holds stack height constant at 20mm and compares a 120mm waist ski against an 80mm ski. The displacement difference is .074” (8.795 vs. 8.721)
• The right side holds the ski width constant (120mm) and compares a 20mm stack height against 35mm. The lateral displacement difference is .250” (9.045 vs. 8.795)
• In both cases, the ski thickness is .750”, the boot sole thickness is 19mm (.750”) and the height from the footbed to the top of the tibia is 18”

Since it would appear to me that this is a hard(er) snow analysis, the skis were rotated 25 degrees about one edge, as opposed to its center.

I’m curious about your thoughts, or any other studies you’ve found.



… Thom

[John_B]

I'm envious of your ability to feel changes to your body. And to over analyze

[John_B]

That said I've never taken a physics class but it does sound at least semi plausible.

[kootenayskier]

Don't discount the added weight of those boat anchor Dukes.

[straightlineorbust]

Like fat skis, hope you don't like your knees. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4541126/

To be honest though as you mentioned, I think stack height is the biggest problem.

When you think of the difference in forces, the increased stack height makes a big difference

[galibier_numero_un]

Like fat skis, hope you don't like your knees. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4541126/
To be honest though as you mentioned, I think stack height is the biggest problem.
When you think of the difference in forces, the increased stack height makes a big difference

I've had too much time on my hands over the past 9 weeks - time normally dedicated to skiing.

Thanks for that link, it's worth a fresh cup before a read.

Don't discount the added weight of those boat anchor Dukes.

Yes, there were lots of ways I didn't get on with those boat anchors, but the sensation of needing to "over-edge" while skating was immediate ... nothing to do with weight.

Cheers,
Thom

[XXX-er]

why do them alpine racer types use a lot of stack height off the ski if it didnt make them faster and it wasnt good?

[galibier_numero_un]

why do them alpine racer types use a lot of stack height off the ski if it didnt make them faster and it wasnt good?

I've often wondered that as well.

Of course, there's "boot-out" which they're concerned with eliminating, and I hear talk of increased leverage. Of course, we also hear about back problems in 30 year olds ;-)

... Thom

[jono]

why do them alpine racer types use a lot of stack height off the ski if it didnt make them faster and it wasnt good?

Narrow skis + wider boots in deep, icy ruts = boot out?

I have a friend that just quit skiing because he can't bend his old knees to the side anymore. Immediately made me think: steer with the core/hips more and the knees less. Easier on the quads, usually feels easier on the knees, but I don't remember to do it exclusively often enough to say for sure. But I have at least one bad knee and it seems like it helps.

[XXX-er]

3 pin binding & leather boots on a tua cirque

I put a 25mm g3 shim under the binding

Ymmv but I couldn't tell the difference

So now days I don't give the 2 fucks about stack height

Perhaps I'm just an unfeeling Bastard?

But I tell you what I do feel is weight

[anotherVTskibum]

why do them alpine racer types use a lot of stack height off the ski if it didnt make them faster and it wasnt good?

The general belief is that too much stack height is dangerous, and FIS limits snow to bootsole to 50mm and bootsole to foot to 43mm. The former limit started at 55mm, and even then required reducing stack height from the then-common ski + Derby + riser + binding setup; my DH boards are right around 54mm with just an EPB between the ski and binding, for example.

More stack height does allow greater edge angle before booting out; it also, in theory at least, offers more leverage and thus applies more force to the skier's knees.

[rod9301]

Your analysis doesn't take into account the force from your center of mass and the edge of the skis.

A higher stack will better align the centrifugal force from your com to the inside edge of the outside ski.

So you will have less torque on your outside knee.

An extreme example would be ice skates, where the forces are totally aligned.

When you were skating there were different than you are skiing.

Ron lemaster has a good analysis in his book, ultimate skiing.

All this being said i very often ski icy groomers on my 122 Katanas and except for the first couple of days of the season, I don't feel any knee pain.

I attribute the early season knee pain to muscles not yet able to react quickly to the vibration and shocks of skiing on ice.

Hope this helps.

[abraham]

Former mag nieveparasiempre could have something useful to add if he were still around here

[galibier_numero_un]

Your analysis doesn't take into account the force from your center of mass and the edge of the skis.

A higher stack will better align the centrifugal force from your com to the inside edge of the outside ski.

So you will have less torque on your outside knee.

An extreme example would be ice skates, where the forces are totally aligned.

When you were skating there were different than you are skiing.

Ron lemaster has a good analysis in his book, ultimate skiing.

All this being said i very often ski icy groomers on my 122 Katanas and except for the first couple of days of the season, I don't feel any knee pain.

I attribute the early season knee pain to muscles not yet able to react quickly to the vibration and shocks of skiing on ice.

Hope this helps.

Thanks! Given my limited expertise, I was trying to model this from a knee alignment perspective (not taking forces into account). Thinking about it more, it's obvious that countering the forces is huge element in how you align yourself. Edit: at the same time, when we're talking about soft snow (and not injected race courses), I'm not sure how much I'm counting angels on the head of a pin.

It's a bit challenging to grasp all of the relevant issues, but the good news, is that this is mainly idle time geeking out. I'm getting my gear to work for me to stay injury free (I hope).

His website has some fascinating articles and presentations. For those interested in ski history, he has a 50+ page screen capture (PDF) of a PowerPoint presentation he gave: "Goerge Joubert Remembered": http://www.ronlemaster.com/presentations.html. Lots of cool photos from Joubert's book (I found a used copy of it in a book store last year).

Relevant to this conversation, he did an analysis on 2011-2012 FIS rules changes in the articles section.

Edit: thinking about this a bit more, I think that for those of us who ski mostly on soft snow (not racers and groomer zoomers), ski width is subordinate to most every other consideration as far as knee alignment is concerned - coming after sidecut, rocker and stack height/tibia length (in no particular order). Thanks for indulging me!

... Thom