Skiing on fat skis. When are they too wide?

[mdf]

I'm also not seeing the stack height argument. I think the geometry goes the other way, a longer lever arm working with you, vs ski width is a longer lever arm working against you.

 

[Choucas]

Skiing mostly in Vermont, most days are hard snow days. I am very comfortable on narrower skis (high 60mm to low 70mm waists) with no knee pain. Soft snow on 95 to 100mm waisted skis, no problem. Getting high edge angles on the wider skis on hard snow is a recipe for knee and back pain. Trying to rail them like I do with the skinnier skis on firm stuff invites some lingering pain issues for this senior skier. I love skiing at higher speeds on hard snow with frontside skis/non-fis race skis. You can do things on them that you just can’t manage with wider, rockered skis with side cut radii in the mid 20’s. At least, you can’t do them all day, day after day. younger skiers can ignore this for now.

 

[François Pugh]

I'm also not seeing the stack height argument. I think the geometry goes the other way, a longer lever arm working with you, vs ski width is a longer lever arm working against you.

Torque is force times distance. The force is applied by the snow or ice at the edge beneath the foot. The farther away from this edge your knee is, the more torque it will fell. Use a short tire iron and a long breaker bar on a wheel lug nut and you will easily see what applies more torque to the nut.

 

[mdf]

Torque is force times distance. The force is applied by the snow or ice at the edge beneath the foot. The farther away from this edge your knee is, the more torque it will fell. Use a short tire iron and a long breaker bar on a wheel lug nut and you will easily see what applies more torque to the nut.

But that's my point. There are two lever arms here, one working with you and one against.

 

[Eleeski]

Heavy skis hurt my knees. Regardless of width.

Lots of powder skis are both wide and long. This takes more material and adds more weight. The longer skis have more swing weight and lever arm - and will stress you more.

I took my wife's skis to Japan by mistake. Wide short lightweight skis (Praxis Backcountry 160). I was surprised how well the skis worked for me. I guess there was enough area to support me in powder and the edge authority was fine on the ice. They didn't like to go fast but they turned like magic in the powder's tight trees. My knees held up well through many consecutive days of mixed conditions.

Hard snow hurts my knees. Regardless of what skis I ride. Even short narrow SL skis. The jarring vibrations that transfer to the knees can have more force than lever arm forces.

Knee pain is a complicated issue that defies a simple response. Well, maybe one simple response "try a lot of skis to see what works for you - and doesn't hurt too much."

Eric

 

[Mike King]

A more direct summary of the data.

Why Wide Skis Aren’t Good for Your Knees - Realskiers

In his introduction to an overview of current research on the effects of wide skis, Prof. John Seifert of Montana State University explains two fundamental ways that wide skis expose the knee to injury. (We’ll get into the mysteries of just how wide is “wide” and other variables in a minute.)...

realskiers.com

 

[François Pugh]

But that's my point. There are two lever arms here, one working with you and one against.

The "torque" working with you is used to twist the skis up onto an edge angle; it's actually a sideways force at the knee that becomes a torque at the ski. It's resisted more by a wider ski so wider ski (with it's bigger lever arm from edge to centre) so need you to apply more torque(wide ski bad), but more stack height gives you more leverage (stack height not a problem here). In the grand scheme of things, compared to the torque working against you, not much required to tip a ski, but for wide skis being tipped against a hard snow surface many times a day it's still a consideration. If the ski sinks in to its midline, then it doesn't rotate about the edge, and all is good.

The torque working against you (well against your knees) is due to the upward reaction force needed at the edge (once the ski is twisted to a tipping angle) to counteract gravity and keep you on the surface and not accelerating into the centre of the Earth; at the knee it's a very significant torque, unless your line of action of the net force acting through the ski is straight up the leg (e.g. skiing slowly so that you are far within the critical angle required for no slip).

 

[JFB]

Fat skis are too wide when the snow is firm and 70 mm skis are more fun.

 

[David Chaus]

Fat skis are too wide when the snow is firm and 70 mm skis are more fun.

Everything you posted is completely subjective.

And yet, I don't disagree.

 

[Fuller]

The obvious solution is to take at least 2 of the 4 pairs of skis you own to the mountain every day. If you misjudge your first choice and it starts dumping after lunch, you know what to do and when to do it.

 

[mdf]

The "torque" working with you is used to twist the skis up onto an edge angle; it's actually a sideways force at the knee that becomes a torque at the ski. It's resisted more by a wider ski so wider ski (with it's bigger lever arm from edge to centre) so need you to apply more torque(wide ski bad), but more stack height gives you more leverage (stack height not a problem here). In the grand scheme of things, compared to the torque working against you, not much required to tip a ski, but for wide skis being tipped against a hard snow surface many times a day it's still a consideration. If the ski sinks in to its midline, then it doesn't rotate about the edge, and all is good.

The torque working against you (well against your knees) is due to the upward reaction force needed at the edge (once the ski is twisted to a tipping angle) to counteract gravity and keep you on the surface and not accelerating into the centre of the Earth; at the knee it's a very significant torque, unless your line of action of the net force acting through the ski is straight up the leg (e.g. skiing slowly so that you are far within the critical angle required for no slip).

We seem to be talking past each other. I understand and agree with what you wrote. But the question was about binding lifters (stack height), not ski width.

 

[François Pugh]

^^Answer is Binding lifters (more stack height) help with "torque" working with you to twist the ski (lifters are good. However, binding lifters add to the sideways torque needed to resist gravity (lifters are bad). It won't be a problem if you are carving a turn at the limit of grip at the critical angle, with force aligned perpendicular to the bases, but it will be a problem at any turn size bigger than that critical situation or at any speed slower than that critical situation. In the end it's a balancing act: Narrow skis and high performance arc-2-arc carved turns favour more stack height but add some risk of injury and require more skill to control skis. Wider skis on harder snow and lower performance turns all favour less stack height.

 

[Paul Lutes]

The edge.

So a longer lever mean less energy to get on edge i.e. less push on your knees, right?? Still not seeing why stack height is inherently bad for your knees, as long as the binding and boot connection is solid. I'm good with the increasing width being a problem because it creates additional force at 90 degrees to the line of your leg and knee, but stack height just extends that same line of foot - calf - knee- thigh. I'm a biologist/petty bureaucrat by training, so physics confuses me

 

[Johnny V.]

The obvious solution is to take at least 2 of the 4 pairs of skis you own to the mountain every day. If you misjudge your first choice and it starts dumping after lunch, you know what to do and when to do it.

That's what lockers are made for................................

 

[Ken_R]

If you cant ski then Phat Skis are too fat.

 

[kayco53]

As many here have mentioned, snow conditions are the critical factor in a ski being too wide or not especially if you have sensitive knees. Also find wide skis more revealing of boot fitting and ski tune issues as they will compound the extra effort of wider skis in the wrong conditions.

I find that if the snow is soft and the edges can easily dig into/get below the surface, then my 50 year old knees can easily daily a 105mm or less ski. If it’s firm all day, then I’d want something narrower for sure. I’d want at least several inches of fresh snow to pull out the 115mm plus skis like you demoed.

You kinda answered “are the conditions appropriate for skis this wide” with your “not bad on a few icy spots” comment as it must have been too firm for 115mm skis that day. Your friends might have also hit much different snow conditions on the same run with better tuned skis wearing tighter fitting boots so tough to compare.

Many of the design aspects of wider powder skis can be found in a more suitable width for conditions like you faced that day so you might have LOVED their narrower model. So don’t give up on wider skis just yet as you may have just have chosen too big of a tool for the job that day.

You are right there was no need for those skis that day. Only skied them because they had their demos there. Was using my 84s that day but I knew of some powder stashes to try them in.