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relationship of turn radius to chattering/skidding

post #1 of 27
Thread Starter 
This is a technical question.

1. assume a 125-140 lb woman skiing
2. with skills level 7-8, skiing dynamically, edges appropriately up, turns initiated early
3. on an empty blue groomed slope in NE in the mid-morning
4. homemade snow underfoot, last natural was 2 weeks ago
5. she is attempting to ski this slope non-stop as fast as possible
6. thus she's using medium radius turns, skiing a narrow line, lingering in the fall line for speed
7. Plan A - she's on short slalom skis, 154's, published turn radius of 12mm or so
8. Plan B - she's on gs skis, 160's, published turn radius of 16mm or so


What will be the difference in how the Plan A skis and the Plan B skis grip the snow? Will one chatter or skid out, while the other holds true?
post #2 of 27
Chattering and skidding are technique flaws not ski sidecut issues.

I'll leave the rest to the instruction debate team.
post #3 of 27

Chattering

Motor problem
post #4 of 27
Chattering and skidding are definitely technique problems, but all that really means to me is that the skier in question should check out some other skiis that would be more forgiving.

I will also say that some skiis really induce chattering more..and yes..the bigger sidecut can be a factor for sure. But that doesn't mean big sidecuts are bad. Its a complex relationship between sidecut and flex, etc.
post #5 of 27
For her to ski the same "narrow line", she may have to skid the GS ski to cheat a bit ... and if it's a true GS, she may have to skid quite a bit to stay in a narrow lane.

I see less problem on the SL and as a 165 pounder who loves my 156 SL's I don't see any problem with that at all.

Assuming (regarding the chatter), she is on a reasonable "race stock" type ski, chatter shouldn't be an issue.
post #6 of 27
Thread Starter 
Here's some further clarification of the original question:

If a slalom ski has a deep sidecut, and the skier puts it up on edge and weights it appropriately, it will want to cut a turn through the snow with a short radius, plus or minus some small amount. However, if that skier forces that slalom ski to perform a turn with a radius larger and outside this this range of mms, and he/she holds it in the fall line longer than it wants to be, it seems that the ski will grap/release, grab/release.

True? If yes, wouldn't a different ski, one with a wider waist and a larger turn radius, act better with such a turn?

And wouldn't a longer ski offer more ski/snow contact and thus more gripping power while following a narrow line that is skied fast?
post #7 of 27
LiquidFeet...

Yep, I see it as you do. Your original question seemed phrased to seek out a confirmation without spelling out the actual underlying question so I decided to wait for more info.

I've no time to peck away at spreadsheets for the moment but the basic answer is Yes - if a ski's sidecut is to 'tight' for a given turn radius at a given speed then the ski will not be able to carve the turn and must exhibit at least some scarving or skidding.

Effectively, you'd be selecting a turn-radius where the ski would need an edge-angle less than the angle required to properly 'grip' the snow/ice surface. A Geekier way to say it is that the force vector from the CM may be pointing at the Outside-ski's engaged edge but that edge is not at the 'critical angle' necessary to hold laterally - so you've got to slide sideways a bit to compensate.

On the issue of longer-edge = more gripping-power... that's more complicated.

It depends on the 'lateral cohesion' of the surface in which the edge is buried. On ice you might want a shorter edge in order to penetrate deeper. Laterally, an ice surface is pretty strong.

On soft snow you'd likely want more edge engaged. Softer snow would still permit the longer-edge sufficient depth penetration but since soft snow is weak across its surface you'd want more edge to distribute the lateral forces more widely.

In a way, this begs the question: what sidecut & length of ski is optimal for someone who wants to carve both short and medium radius turns at a variety of speeds on a variety of surfaces?

.ma
post #8 of 27
Thread Starter 
MichaelA,
Thanks for getting technical. I've never gotten the info about lateral cohesion of snow surface. It makes sense that ice or hard snow demands the skis be deeply, or at least firmly, embedded, which a shorter ski would be able to do better than a longer ski by delivering more pounds of skier's weight per inch of ski. This is exactly the opposite of what I anticipated.
post #9 of 27
You will have to skid a slalom ski to get a bigger radius out of it on the hardpack. If it's a stiff well damped ski, it should scarve without chatter. If it's too soft it will be more prone to chattering.

The stiffer a GS ski the more likely you will have to skid it to make the sl turn. I can't recall ever encountering chatter while making small turns though.
post #10 of 27
Thread Starter 
The turns I am trying to describe are incomplete turns ... narrow line ... staying in the fall line more than cutting across the slope .. fast speed ... they would be medium radius turns if completed. Done on very short slalom skis, with 12 mm turn radius. Obviously I have trouble with skidding/sometimes even chatter. I have suspected sub-par skills are the cause but am wondering if it may be partly the skis. The guys I am trying to keep up with all have longer skis with wider waists. However, they weigh more than me. Just trying to get info from the engineer-type-thinkers out there who love the physics of this thing. Thanks for the info on stiffness of ski and its relationship to chattering.
post #11 of 27
It sounds as if your tips are hooking up and trying to make a shorter turn, but then being forced out. Stiffer skis would most likely not be pliable enough to get hooked up on the short track. I don't know how well they would work for you at 130 lbs though. You would need another ski for going slower than 35, so if your getting skis for blasting, you might as well get a longer radius than this as well (SX11?). 12 mm is less than half an inch! Now that's a short turn!
post #12 of 27
I love ice and chatter. One exercise I like to do is to ski on ice and vary my angulation, feeling where the ski chatters, and then feeling where it grabs. The sound of chatter is a great coach, and doesn't charge much for private lessons.
post #13 of 27
I got rid of the chatter when I switched to a straight sidewall "slalom carver" and ditched my "off the rack" consumer SL's.

Problem may be with the ski.
post #14 of 27
Quote:
Originally Posted by LiquidFeet View Post
This is a technical question.

1. assume a 125-140 lb woman skiing
2. with skills level 7-8, skiing dynamically, edges appropriately up, turns initiated early
3. on an empty blue groomed slope in NE in the mid-morning
4. homemade snow underfoot, last natural was 2 weeks ago
5. she is attempting to ski this slope non-stop as fast as possible
6. thus she's using medium radius turns, skiing a narrow line, lingering in the fall line for speed
7. Plan A - she's on short slalom skis, 154's, published turn radius of 12mm or so
8. Plan B - she's on gs skis, 160's, published turn radius of 16mm or so


What will be the difference in how the Plan A skis and the Plan B skis grip the snow? Will one chatter or skid out, while the other holds true?
If it's not icy (and probably even if it is) it doesn't make sense that someone with skill level 7 would encounter any chatter while trying to go fast on a blue slope. If a ski were to chatter under those circumsances, I would check the tune, not the ski itself. Neither ski is likely to act very different under the circumstances you outlined. I ski Fischer RX-8' (165, so it acts like a slalom ski, and my son's 170's), AMC 76's (176), RX6's (175, 16-18 m radius), and I have some older Atomic 9.18's (18 m radius), all of which given my size should produce something like the reactions your skier gets at her size on SL and GS skis. I can't remember the last time I skidded on a blue without ice under those circumstances. With ice, I could overpower some skis (but not SL or GS) on a blue, but I weigh twice what your skier does. For real chatter take a 98mm waist ski down an icy black diamond, then you should get some chatter and maybe a skid. GS and SL skis don't skid easily, unless you flatten them. It could be rounded edges, but if the snow is not hard, even relatively dull edges shouldn't be giving way on a blue. One other possiblity, which would not occur in my skiing because of my size, is that the skis may be too stiff for your skier, and she may be using rotary skills to turn the skis when they are flat, and that could result in a skid or chatter.
post #15 of 27
My own opinion is that chatter is typically the result of trying to make a SHORTER-radius turn than the ski or the skier's technique (or both) is capable of producing. I think there's a hard-to-describe but easy-to-feel difference between chatter and skidding.

I think skidding is what she'll feel if she's trying to make a LONGER-radius turn than the ski really wants to make (12-meter ski being forced into a 20-meter turn). To me, that's a more likely result than chatter in the skiing scenario you described.

So, my response is that the gs skis are more likely to chatter IF she tries to muscle them into a tighter turn than they want to make (or her technique is capable of forcing them into). By the same token, the slalom skis are more likely to skid because she's staying in the fall line and not really making as short-radius a turn as the skis are designed for.
post #16 of 27
Just a thought- does she have boots that don't correct a Q-angle issue? She may be on opposing edges. Women with a big Q-angle often ski better with wider skis, yet both skis here were racing skis, which usually implies a narrow waist.
post #17 of 27
It's the Indian, not the Arrow
post #18 of 27
Thread Starter 
Granted, it may be the Indian not the Arrow. However, this Indian is beginning to inquire about different Arrows just in case.

I phrased the question the way I did so people would not second guess my skills, since there is no video. If both skis would perform equally under these conditions, then it's the Indian.
post #19 of 27
Does anyone know the physics behind chattering? I read a review on a ski once and most reviewers stated that the ski chattered. If it was a pure technique issue, they'd have the same problem will all of the skiis they tested.

There must be something about a ski that makes it susceptible to chatter.
post #20 of 27
I second the motion for more info on what people mean by 'Chatter' - anyone got a more precise definition for what they experience?

.ma
post #21 of 27
Chatter is often produced by 'outrigger technique'. Different edge angles, the outer ski on high angle 'carving' the inner ski under CM at a low angle, skidding. The skier feels the downhill ski begin to skid and they weight the uphill ski to 'regain control', this reduces grip even more causing the downhill ski to recamber/decamber/recamber/decamber. Chatter. Another technique that produces chatter is 'pushing the ski tails out' to create edge angle, skippity-skip-skip-skip.

A different sidecut will not teach this skier to trust their edges and committ to the turn. THAT is the problem. Trust.

To answer which ski will hold better, thats a matter of construction not really sidecut. A Head XRC 1400sw will grip better than a Head XRC 800 because of how it's made. A Volkl Allstar will grip better than a 4 Star, etc. A PSIA DTeamer would rail turns on rental gear, someone with imperfect technique will run into chatter on WC Race Stock skis. It's not the ski.
post #22 of 27
Quote:
Originally Posted by Whiteroom View Post
A PSIA DTeamer would rail turns on rental gear, someone with imperfect technique will run into chatter on WC Race Stock skis. It's not the ski.
It's easy for the rental gear not to chatter -- insufficient edge grip entirely.

What I'm talking about is a ski press test, where on a GS course at Mont-Ste-Anne, testers complained that a particular ski lacked energy AND was prone to chatter. No other ski was said to be prone to chatter -- they tested a whole bunch.

You really think it is the testers?
post #23 of 27
Quote:
Originally Posted by BigE View Post
Does anyone know the physics behind chattering? I read a review on a ski once and most reviewers stated that the ski chattered. If it was a pure technique issue, they'd have the same problem will all of the skiis they tested.

There must be something about a ski that makes it susceptible to chatter.
BigE,

I'm no expert on the matter, but my understanding of "chatter" is when the ski in insufficiently torsionally stiff to hold at a given edge angle/pressure. At this point the ski flexes/collapses, loses edge grip, comes off the snow and straightens (both camber and torsionally). The rub, however, is that the skier still has the ski tipped, and is still turning it. The ski re-engages and the whole process starts again. The "chatter" is the rapid dis-engagement/re-engagement of the ski multiple times throughout the turn.

I'm sure others can give a much more detailed explanation.

Where the "radius" comes in is as we make a tighter radius turn, were are generally putting more pressure forces on the ski (assuming we'er skiing the same speed in both turn. This would cause a ski that is prone to chatter to break loose. Open the turn radius up and you'll be less likely to cause the same ski to chatter.

L
post #24 of 27
I'm not trying to push product "A", or trash product "B" ... there is a range of difference among the skis of all manufacturers and I know that at the top of the food chain most race skis will perform quite well in the "civilian" environment.

That said, I haven't been changing brands for over five years now so this may be a bit "dated", but since the skis have not been identified the question still begs to be asked, based on my experience.

My Volkl P-40 SL's were horrible at speed under the exact conditions noted above, as were the Atomic that I demoed that year; cap "off the shelf" .... 170's. The tips would start shaking and dancing on any kind of groomed but firm surface .... yet my wood core GS (Volkl) ... were fine.

The switch to Stockli SC's and SL's .... shorter 168/156/160/, respectively and I had never experienced any similiar chatter at speed again. The 156's may squirm a bit on a straight run.

Gotta look at both the "arrow and the indian" .... IMHO .. why leave one out????
post #25 of 27
So given the info, which will chatter less A or B??

SL or GS for a mid radius turn level 7 skier on firm manmade snow.

Which arrow won't chatter?
post #26 of 27
Neither should chatter for the given incomplete turns. Chatter becomes an issue under very high loads, like those in high speed short complete turns, as pressure control becomes a problem.

I suspect that what the poster is calling chatter, is actually something else... not the entire ski lifting off the snow and hammering into the hardpack.
post #27 of 27
Quote:
Originally Posted by Yuki View Post
Gotta look at both the "arrow and the indian" .... IMHO .. why leave one out????
Well the indian is not easily examined without video. Tell us about the arrow.
We've established it has about a 12m radius, that it is doing incomplete LR turns down the fall line at high speeds and the tips appear to be "chattering", that is catch release rapidly so that they repeatedly slap the snow. Is that right?
What model and brand and how old is the ski?
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