Ski Design Dynamics: Energy, vibration and “pop”
Posted 1 year, 9 months ago by Seth Masia
Pete mentioned that some folks have adapted to the smooth, stable feel of Volkls. Some people like very lively, energetic skis. Some skis just feel steady and predictable in funky weird snow, and some skis feel bouncy as if they can’t wait to get a new turn going. The difference usually lies in the nature of the materials. Some materials are “quiet,” some are lively.
The ski is a spring, built to flex and rebound. Like any spring, it likes to vibrate at a characteristic rate based on its length, weight, stiffness and internal friction. Think of guitar strings: Heavy strings vibrate slowly, light strings vibrate fast. Shorten the string and it vibrates faster still. Switch from steel strings to nylon, and it vibrates with a softer tone and fades off more quickly. The fade-off is called damping. The rate at which your ski rebounds — its general energy level — depends a lot on the same things that govern the quality of sound in a guitar string: length, weight, and damping rate.
Big ski factories like to talk about damping, as if it’s a good thing. When you ski on a lot of ice, you do in fact want to dampen out vibration along the ski’s edge so it doesn’t chatter out of its own track. But snow itself is the world’s best vibration damper: It compresses, but doesn’t spring back, so the compression energy is removed from the system. Because of that, it’s easy to build an overdamped ski. Put too much viscoelastic foolishness inside a ski and it begins to feel like a long rubber eraser. It grows slow to rebound when flexed in powder, seems to take forever to return to the surface — and in any kind of wet snow the glide speed goes to hell.
To get a lively ski that rebounds quickly in bumps and comes arcing up in powder, you want hard structural layers, like steel guitar strings, that vibrate at a very high frequency and low amplitude. Carbon fiber will do this, and certain very high-quality grades of fiberglass/epoxy matrix. (High-zinc aluminum skis, made of Zicral or Titanal alloys, have great vibration characteristics, until the metal fatigues with age, or bends from overflexing in bumps.) Because these materials are quite stiff (an engineer would say they have high modulus of elasticity) they have to be used in very thin layers.
To get a damp ski for tracking on ice, you can use thicker layers of “softer” glass mat in a more viscous epoxy blend, or build rubber layers into the ski. In general, when a ski has a lot of layers that “shear” against each other, the effect is to soak up vibration energy. It’s converted to tiny quantities of heat and lost to the snow and air.
The core material can also contribute to a ski’s energy level. Vibration travels nicely along the grain of a tough hardwood, and dampens out in shear between the lignin fibers in softer woods. Most polyurethane foam cores are natural damping materials, while some very light, expensive acrylic foam cores don’t have enough mass to absorb much energy.
Another mode of energy control is called mass damping. If a ski has a heavy tip (some skis actually have weighted tips) it tends to behave a bit like the bob-weight at the end of a pendulum: It takes some energy to divert it from its trajectory, and then it’s slower to reverse direction and come back to neutral than a lighter tip would be. When the combination of materials don’t give the ski the vibrational feel the designer wanted, one fix can be to tune a ski’s vibration rate by putting metal weights or thin strips of rubber at strategic points on or under the topskin.
A properly designed ski made of top-quality materials shouldn’t need external vibration dampers.
Seth Masia
Vail Ski School
Permalink · Posted by Seth Masia · October 14th, 2008 · · Read 11 Comments
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October 14th, 2008 at 10:47 pm Pete Wagner
Dampening technologies seem to be frequently advertised on ski topsheets.
I’ve seen piezo-electric dampeners, X-Link bars, metal buttons, and other “external dampeners” on skis over the years. What are some other dampening tricks that have appeared on skis over the years?
October 14th, 2008 at 11:13 pm Seth Masia
There’ve been all kinds of damping devices. In the early days of metal skis it was found that a thin layer of neoprene rubber between the aluminum layer and the core would help the ski stay together — it was really a glued shear layer but some manufacturers sold it as a damping system.
Many damping systems are based on the flexibility of rubber. If you sandwich rubber between two layers of metal, when the structure flexes the rubber stretches and recovers, and some energy is lost in molecular friction (hysteresis) in the rubber (well, it’s not lost — it’s converted to heat energy). Rubber and many plastics are “viscoelastic” which means they can distort and rebound slowly.
One of the worst systems was the Olin CVL (constrained viscoelastic layer), a thin strip of aluminum in a plastic sleeve, running the full length of the ski in the lower fiberglass layer. It was used only in one model, the Mark VI slalom. The ski was waaaaay overdamped, and lay on the snow like a slug. It took real strength and determination to wake it up and get it pointed in a new direction.
One of the best systems used the same principle in a better-targeted way: the Rossignol VAS (vibration absorbing system) of the 80s. This was a piece of rubber with steel wire embedded in it — a bit of tire cord in effect. It worked better than the Olin system because, instead of damping the entire length of the ski, the VAS strips were short — just a couple of inches long — and placed only at two or three key “nodes” along the length of the ski to damp out just the unwanted vibration modes or frequencies, the ones that can be felt by the skier as instability or chatter.
During the ’70s some crazy Austrian came up with a cylinder about the size of a soup can. It fastened to the shovel of the ski. Inside was a spring attached to the top and bottom of the can, with a weight in the middle that yoinged up and down as the ski flexed. I’m not sure this device did anything at all but the manufacturer sold a few thousand units.
A variation on this idea was the Dynastar “heart,” a thick shiny red button inserted through the ski tip. Inside was a steel weight sandwiched between two thin layers of foam. Inevitably one of the hearts got knocked loose and lost in the snow. The two skis in the pair — one with a heart, one without — continued to perform as a matched pair.
For a year or two Spalding had a plastic cartridge attached to the tip of the ski, filled with steel filings. And a guy in South Lake Tahoe built short hollow skis filled with oil and BBs. They were made of clear polycarbonate so you could see the stuff slosh around inside. He also sold plastic bags of oil and BBs that could be glued to the top of any conventional ski.
The best “damper” of all time was the cracked edge. It was invented by Paul Michal of Dynamic — a continuous steel edge sawed partway through with very thin slots. The theory was that a fiberglass slalom ski so equipped would not transmit vibration along the steel edge. It had some additional benefits: because the edge was very soft in flex, the ski could be made thicker, and therefore torsionally stiffer, and naturally damper. Besides, the segments appeared to function like a serrated knife blade on ice. For over a decade, the world’s best slalom race skis, from Dynamic, Volkl, K2, Fischer, Atomic, Lacroix, Olin, Blizzard etc — had cracked edges. The edges were fragile, and very expensive to manufacture, so they’re now gone.
October 21st, 2008 at 8:36 am Pete Wagner
Seth – Have you checked out the mass dampeners that K2 incorporates in in a number of their 2008-2009 skis?
How about the Volkl Tigershark spring dampening system with power switch?
October 21st, 2008 at 5:08 pm Seth Masia
Those zinc weights have been screwed to the shovels of some K2 skis for several years now. Curiously, some Volkl models have a screw without a weight in precisely the same place. And guess what? Marker sells some bindings with a plastic-and-metal extension rod poking out from under the toe unit that runs forward and screws to the ski at just that point. The rod is part of the binding’s “Piston Control” system, damping the ski’s rebound from full decamber.
On the K2s, the distance from ski center for the zinc “Mod Monic” weight varies by length. Not only that, but some of the women’s skis use a 15-gram weight while the men’s skis use a 25-gram weight. This appears to be a way to damp out a specific vibration mode. It appears that metal skis get the 25-gram weight while lighter fiberglass skis us the 15-gram weight — the underlying vibration modes are quite different.
Time to do some testing. I’ll let you know what I find out.
October 21st, 2008 at 5:10 pm Seth Masia
And . . . I haven’t skied on the Tigershark yet, so I won’t comment. I’m sure the Powerswitch works as advertised, but I also suspect I’d figure out where I like it and would just leave it there.
October 25th, 2008 at 4:53 am Jonathan Histed
Hi!
Interesting discussion.
I am more of a cynic when it comes to vibration damping: I suspect that the manufacturers of mainline skis are into producing similar products, and have never produced really dmaped skis; that also exhibit good elastic properties.
When you twang skis in a ski shop; they all appear much of a muchness.
John Howe with his Claw skis seems to think that very damped skis ae desirable: and he clearly has a good history in ski design: and has gone as far as patenting the design to protect it.
Historically; have manufacturers not been able to combine the charecteristics of : good elsticity; cheapness of manufactuer; consistency of manufacture; and also longevity of the product (i.e. that it doesn’t delaminate in use), with good damping?
I’m thinking here about car suspension: you want it to glide over bumps; but once over a bump not to continue “boinging” down the hill.
If a very damped ski, as you assert, causes difficulties to show up in other aspects of the design: e.g. tuning; or geometry: that isn’t the fault of damping: more that these other areas need further refinement, but in the meantime put a limit on how much damping can usefully be employed currently… so whould not be used as justification for stating lots of damping is per se a bad thing: just in current designs cannot be used…
Similalry though a little off topic; your comments about not wanting a too torsionally stiff ski: again; can only be the result of overly torisionaly stiff reveals other wekanesses in the design at the moment… The patent literature is littered with patents for torsion control techniques: so the big manufacturers clearly have put a lot of effort into acheiving it; but only gone so far…
October 25th, 2008 at 8:15 am Seth Masia
Hey, Jonathan –
Thanks for the comments. My remarks about skis that are overdamped or too stiff in torsion aren’t just speculation — I skied on those skis, and bench-tested them, too.
John Howe’s CLAW ski is a brilliant design and it works beautifully on ice. It’s manageable in soft snow, too. Let’s just say you’re right in one sense: if a ski has a proper modern shape, so that its average width gives it better float than an old straight ski, you can get away with more damping. You have, in effect, a more versatile ski thanks to the better shape.
Regarding torsional stiffness: a number of skis reached the market with too much torsional stiffness. The original foam-core aluminum skis, like Rossignol’s Roc Comp, torqued at about 2.5 nm/degree, measured using the ASTM standard technique. They could be very harsh. In later models manufacturers made the aluminum layers harder and thinner, and moved the lower aluminum layer between the edges — all steps to make the skis thinner and therefore softer in torsion. No amount of fiddling with sidecut or other characteristics would have solved this problem. 2.5 is simply too stiff, and any patented device designed to boost torsional stiffness above that level is likely to fail. The early Hexcel race skis, thanks to their unusual cores, were too stiff in torsion and felt over-reactive and harsh except to the strongest, most precise racers. The original Volant steel-cap skis, same thing: to help them behave, factory tech reps often bevelled them three degrees, and the factory took accurate base-beveling so seriously that they invented a hand-held laser device to measure edge bevel to one-hundredth of a degree.
When I taught at Squaw Valley, a very smart engineer from the Bay Area hired a couple of ski instructors to test his new invention: a fiberglass tube that attached to the top of the ski with a mechanical linkage that allowed the ski to flex freely in beam flex but made it very rigid in torsion. It worked as advertised, but made the ski almost unskiable. In any kind of turn, skidded or carved, the turn shape sort of pulsed — you could feel the edge hook up and release on its own, in a rhythmic way that reminded me of driving with out-of-true brakes, or cycling with a bent wheel. The problem was that the glass tubes were twisting and untwisting, functioning like a torsion bar spring, and each cycle was transferred to the steel edge. It was a perfect illustration of the law of unintended consequences, and an object lesson about balancing torsion against beam flex.
November 9th, 2008 at 7:04 am Jonathan Histed
Mmm thought provoking…
So if the experimental ski you were describing was undegoing a torsional “whip”– dare I say: undamped: wouldn’t damping of the torsional mode have improved things ?
not that I have any idea at all how that would have been acieved.
I can’t quite envisage what you describe for the thought experiment; but I’m kinda guessing that the underlying ski was very soft; almost like a fireman’s hose; attached to a fiber glass hinged linkage ? If so then I also guess; if a torsional impulse were injected in a periodic way by the mechanical linkage; all sorts of dynamic nasties would have been excited within the “fireman’s hose” running base.
From what I imagine; the device constructed didn’t deliver uniform torsional stiffenss along the ski? if it had; and indeed dynamic torsional modes had been damped; wouldn’t that have cured the unintended consequences?
I wonder what skiing on a very damped (in all dynamic modes) soft noodle; that is torsionally stiff would feel like. There’d be no hint of compromise. “odd” and “novel” are probably the primary adjectives. maybe “impossible in your dreams” also lol
Given what you describe with the racing ski designs, and indeed retail skis designed to be exceptionally torsionally stiff, wasn’t the problem the designers were wrestling with was the one of ultimate grip in the body of a turn in a racing situation being determined by torsional rigidity. Traditionally to increase torsional regidity involves increasing the longituduanal stiffness; which leads to a ski that is almost impossibloe to ski? i.e. all the skis trumpeted as being torsionally stiff had the unintended consequence they couldn’t design out (and indeed didn’t want to trumpet as an “advantage” – because it wasn’t) of being longituduannly stiff? and it was the longitudanal stiffness that made them planks, rather than the torsional characteristics ?
Unfortunately the prototype ski you tried, had a different unintentional consequence: having nailed the torsional aim; it did weird dynamic things.
So: perhaps we just have to wait till some more “unobtainium” is back in store at the parts bin; ? lol so that we can compare notes on a torsionally stiff noodle, with optimised damping, with no unintended consequences ? I’m guessing from your thoughts that you do not beleive such a thing could be constructed; and that at some deep level; however the thing is bolted together there will be some weird unintended conseqeunces, so it is simply which set the designer likes best?
Jonathan
November 9th, 2008 at 5:54 pm Seth Masia
You’re making this sound too complex.
A designer can build anything he wants. He can make it as stiff or as soft as he’d like, in torsion or beam flex or both. Any combination is possible. But on-snow experience shows there’s a certain relationship of torsion to beam flex that’s comfortably skiable. Stray too far from that relationship and the ski feels either inadequate or harsh.
Simple.
Seth
January 2nd, 2009 at 9:02 am rob
this thread asks about the VOLKL Tigershark skis. i bought them last month and can give a report. they are the Tigershark 12 w/ “Power Switch” (PS). last season i demoed them in 175 w/out switch, at Hunter Mtn NY. i found them to track very well but difficult to turn. i liked the skis but they were too long. so i bought 168. i am 6′ tall, weigh 210 so this is pretty short. my prior skis were NORDICA Ultra Wave 180, the green topsheet model; and K2 Mach GS Race 186.
i started w/ the PS on the weakest setting. the skis were very easy to turn and slow in response. this is called the “cruise” setting, which is accurate. if you want to kick back w/ a BUD, it is ok but not for expert skiing. not on Hunter Mtn that is. it may be good for powder conditions since it permits the ski to flex the most. we do not have such conditions here, only 6 – 12″ max. i will hope to try these skis in powder next month at vail. i like soft skis in powder, so they can bend into a turn, so i think this would be the setting to select in powder.
then i went to the other extreme, so-called “power” setting. this immediately increased the skis’ response speed, made them much more suitable for fast slalom turns. the ultimate bite on ice was also improved. this is clearly the setting for east coast skiing. the grip is comparable to what i get w/ K2 Mach GS race skis 186cm, or better.
finally, i tried the middle setting called dynamic. i was skeptical that this would be a good setting, since the way the system works there are 2 channels along the top edges that are hollow, in the hollow is a long pushrod. the front of the pushrod rests on a spring. the back of the rod is moved forward or aft by the PS, to preload the spring or not. i was thinking it would be hard to ski with the pushrods alternating intermittently from loose to tight in the channel. however, i was pleasantly surprised by the middle setting. what happens is that the turn initiation is easy, since the tip is easy to bend. but once the tip bends and bites, the spring bottoms out pretty fast and the ski hardens up to grip the ice to finish the turn. so it is not a bad setting, you do not have to watch out so much for unintended tip grabbing. sort of the same effect as beveling the bottom of the tip, but without sacrificing the ability of the tip to grip ice once the rods bottom out.
i have left the edges as ground by the factory, which they say on their website is 1 base 2 side.
i would not agree that it is a good idea to just find a setting and leave it alone. if you practice on the different settings, you can adapt the ski to different snow.
btw, you can also adjust the fore/aft mount of the binding on these skis, although not on the slope like w/ ATOMIC 614. i set it w/ the BOF at CRS method, my boots are 299mm which is between the available binding settings 295 and 305, i went for 305 and do not feel a need to change it. i have the feeling that the binding rail-mount design used on these skis pressures the heel more than the toe, so rather err on the side of toe forward a bit. anyway you can change it later at home if yr first position was inaccurate.
i do not yet know the top speed of these skis, i have not found a point when the tips chatter and the tracking fails. even at 168 they probably can go faster than i ever care to ski, provided you keep them at least slightly on edge since they have substantial sidecut. probably the limiting factor on speed is the need to ski on edge and the width under foot. hence, the acceleration to high speed is more gradual than the K2 Mach race skis, no surprise as those are some slippery fast skis.
the thread is about damping, so let me mention that. i do not consider the PS to be a damping mechanism, it is a stiffness adjustment. these skis are not particularly damp. i had them resting on their tails with the tips leaning against the door of a taxicab, with the bindings suspended in the air and the skis base to base w/ brakes engaged to each other. (this is ny, remember). when the cab went over a pothole, the skis visibly vibrated a few inches in the middle like a guitar string. the vibration persisted, it did not damp out fast.
the overall look and feel of the skis is not as nice as top-end handmade skis like WAGNER or LACROIX France, and they are not likely as durable in hard use, either, since the top channels for the pushrods are thin plastic. these are not for skiers who abuse their gear, or put themselves in situations where failure is not an option. the main point is versatility in a wide range of conditions.
October 8th, 2009 at 1:16 am Seth Simonds
I’d like to thank seth and pete for putting on such thoughtful discussions. It’s truly a treat to stubble upon information of such a high calibre. I really enjoyed the dialog format: pete you ask excellent questions. Seth you are a wealth of information. I need to build skis so I can learn enough to ask a good question. In the mean time, please find something else to write about.