Hypothesizing About Roll Stability

[Horton]

I was thinking about ski stability in the shower this morning. When I write ski reviews, I talk a lot about roll stability. I think I need to better explain the impact of roll stability. Below is how I see it.

 

There is plenty to be said about roll stability from the apex to the wake but here I am going to mostly address from the wakes to the ball.

 

In the last few years skis have generally gotten wider and width is a major component of roll stability. The result is more stable skis. More must be better, Right? No. If I may paraphrase what one ski designer said to me this morning, “a 4 foot by 6 foot sheet of plywood is very stable but it will not turn very well”.

 

Skis that are under stable side to side or “loose in roll” can theoretically hold more angle and turn faster. Many of the first generation of fast skis were very loose in roll. At the apex of the turn they easily rolled to the inside and the ski turned fast and hard. These skis took a lot of angle but through the edge change and approaching the ball line they made it harder for the skier to keep from moving to the inside resulting in loss of width and a narrow path to the ball.

 

At the other end of the spectrum, an over stable ski can do a couple of odd things. Most typically an over stable ski will easily get wide but not turn well because the skier cannot roll the ski over on edge. The other possibility is the ski will get wide but will always turn very abruptly. In this case the ski resists rolling to the inside up to a point and then it basically stalls and moves to the inside all at once. In both cases the ski many not generate ample speed from the wakes to the ball because it resists rolling over on edge.

 

This concept became clearer to me recently when I moved two of my friends from older skis that are very loose in roll to skis that are more stable. On the old skis both skiers consistently leaned to the inside prematurely, sat back after the edge change and skied narrow to the ball. After changing skis I see both guys more centered side to side and front to back in the edge change. The new skis both turn slower but smoother and from wider. The old “loose” ski was turning hard and fast but making the skiers narrow and stressed.

 

Perhaps the biggest Ahh Haa moment was when I realized that these guys were more on their back foot after the edge change because it felt safer on the less stable ski. Being on their back foot was literally a defensive position on a ski that was not comfortable. In this case changing skis actually improved their stance.

 

[jimbrake]

@Horton - do you ever notice that a ski is asymmetrical in how it rolls up on edge? May roll very quickly and easily to one side (loose) and not to the other (stiff)? I'm very sensitive to this and want my ski to roll up the same on both edges or maybe a little more on my onside. I've found that this is related to the vertical angle that my binding forces my front leg (lower leg from foot to knee) into. Actually it's really about where my front knee is side to side over the ski. When I get a new binding or even a new wrap (I use D3 Leverage bindings and am very happy with them), it always changes my lower leg angle and this immediately manifests itself in the side to side roll. I know as soon as I'm getting pulled up the first time because I can feel the ski wanting to go to one side more than the other and I'll be all "damn it". What I do to remedy this is I cant (not rotate) my binding as needed with washers under one side of my front plate to lift one side of the binding a slight amount. Works great. I wish I had a more "slick" way of doing this than washers. Like various thicknesses of plastic strips or something.

 

Just thought I'd post this in case any of the readers have experienced this problem and thought it was the ski. Could be the ski, but it's probably your stance as dictated by your bindings.

 

Adjusting canting, forward (ramp) angle, height and all that is a science/art in alpine ski race boot fitting and super important to get the right stance based on your personal physiology. In water skiing its non-existent to primitive. We already know that binding position front to back is very important. I think there is a lot more we could do with individual foot position in terms of rotation, cant, and fore/aft angles, too. Just need more experimentation I guess.

[Wish]

John, after riding the AM and S2 back to back, there was a notable difference in this area. AM seemed to roll to a much much higher edge over the S2. Doesn't depth of the bottom of the ski have a great deal to do with this. In standing them up bottom to bottom, they were very close to the same width most of the way through the ski. Obviously a difference in tunnel set up.

[Horton]

@Wish

My comments about width are a generalization. The actual mechanics of roll is more than just width. Tunnel depth could be a factor as could rocker, taper, bevel, thickness, flex and who knows what else. Those two ski are pretty different.

 

@jimbrake

What you are saying makes sense but no I do not relate to it. I have been ridding the same style of bindings for 5 or 6 seasons and since they are not rubber they do not vary in the way you describe. I do know that changing the stiffness of the boots can have some radical impact on edge change. Let’s just say that a little dampening is a good thing => super stiff is not really necessarily better.

 

I do think it is always a little asymmetrical. My center of mass is farther back going into on side so the ski will be looser in roll in that direction. The farther forward your weight the more stable a ski is going to be in roll. If everything else would stay equal (impossible) you could adjust roll stability by simply moving bindings forward and backwards.

 

[MrJones]

Okay. I'll bite on this. I am coming off basically 3 seasons on a 66.25 9900SL (The normal ski. Not mid/wide/fat/whatever) and am now on a 66.25 Nano One. Despite liking the ski I am having some trouble adjusting.

 

The 9900SL was narrow in the forebody and would hammer the turn taking a lot of angle into the wakes. If I held the angle solidly through the edge change all was well. If I came off of it, not so much.

 

The N1 is much more stable, but my perception is consistently that I am not getting enough from the finish of the turn into the wakes. I say this may just be a perception as I don't have a lot of sets on the ski. Only rode it for about 3 weeks in the fall and have only been skiing again for 3-4 weeks now.

 

Oddly enough your comment about sitting back as a defensive tactic may be spot on for me. I sent some video to Skidawg for abuse and analysis. He noticed I was loosing my "stack" as I got farther down the pass causing some issues. I think I have been in this habit for a long time due to the ski type I have basically ridden since 1996.

 

I will be thinking about this the next few sets and let you know what happens. Good topic of discussion. (At least for me)

[A_B]

Not to muddy the water too much, but I also think fin setup plays a part. A shallow fin allows the ski to roll easier than a deeper one.

[DW]

@Jim brake: Before adjustable cant alpine ski boots became the norm plastic wedges were used to properly adjust the ski to boot camber setting. You might try scouring ski shops to see if they have some of them laying around (unfortunately from decades ago!) instead of the washers, although that is exactly what I use for exactly the same thing. No problems to date with that system of tuning. Side note - it also improves turn symmetry for me.

[Texas6]

John, this is what you think about in the shower? I clearly don't think enough

[gator1]

@jimbrake: The softer the rubber binding, the less important canting is, IMO. The taller and stiffer bindings need to be lined up with your bone structure, and when you get to hard shells, it has been critical to me. I struggled for two years when I got my ski stolen and had to replace it and bindings with "the latest top of the line" (insurance bought it for me). I'm bowlegged and LFF. Ski would go right after the wakes and go left as I moved to the front of it. My buddies kept telling me I looked "twitchy". Once I canted the bindings, quite a bit, everything smoothed out.

 

Using the washers (or nickels and dimes as per Jaime Beushense post somewhere on BOS) can get you confused, as some of the plates aren't stiff enough and bow under your weight plus 700lbs as you are pulling. @DW, I've tried the snow ski cants. They're from the skinny ski era, and are therefore not long enough.

 

Once I got my required angle figured out (keep stacking washers on dry ground until both bindings line up with your shins), I went down in the basement and got on the trusty Bridgeport and milled up some cants that support the whole plate. Big difference.

 

@Horton: wonder if the whole ski width, edge roll thing is shadowing snow skis: As snow skis got wider and more shaped, they had to change the boots to make them stiffer in side to side, and they had to make bindings wider to give us more leverage in roll.

[Than_Bogan]

Very insightful, John. I learned something by reading that.

[MCskiFreak]

Im in the middle of studying for my last set of finals so I cant delve into the problem in detail right now...that will have to wait until the end of next week. I did remember that I had this paper that investigates the dynamics of planing hull forms in turns and thought it may be of interest to some of the nerdier members among us. The paper topic is largely applicable to the roll stability problem of a waterski, and if nothing else is an interesting read.

[Horton]

@gator1

I understand that you have made your extra stiff bindings work for you but I would have advised you to go to a less stiff binding. I think you would have been skiing back at your level a lot faster. I strongly believe that dampening at the edge change is a good thing.

[gator1]

@horton

Well, if I'd know BOS existed, and you had advised me, and I had listened (not super duper at part 3 of that sentence) I'd have bought Wileys instead of the newest-best and I might not have torn my AT. That's what happens when you allow an insurance payment to modify a lifetime of economic conservatism.

 

You need to get all your advertisers to stamp "check out BOS!" on their warranty cards as part of your placement fees.

 

I get your soft binding = soft edge change belief. Tried it, and seems like it'd be another half a season to go back. Since each year is 20% of the remainder of my career I'm gonna stick with what's working now.

[Horton]

@gator1 I agree that if what you have works => stay with it.

 

I do think the Wileys are too far in the other direction as far as stiffness but they are pretty safe if not too tight.