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Ski length as a factor of speed - Page 2

post #31 of 43

Additionally, what actually happens to a ski at high speed is that the load spreads out across the ski. Skis of a certain length can only contain so much energy and then they begin to "spill" that energy outward, which is when SL skis begin to vibrate and become unstable. The longer the ski, the more energy absorbed and held and therefore the higher the speed which can be attained. 

post #32 of 43

In general longer skis are not faster then shorter. Ok let's try not to compare 215cm DH skis and 165cm SL skis now. But fact is, SG skis are faster then DH skis, even though they are normally at least 5cm shorter. But with longer skis you get stability, and extra stability means a whole lot more then a bit slower ski. So reason why you can sometimes see top WC racers using DH skis in SG (Walchofer this year in Val Gardena SG for example) is stability, not speed. But sure, course, gate settings etc, must allow bigger radius skis to be used in disciplines, where they were not meant to be used.

And my own experience... I can ski just as fast with SL ski, then I can ski with GS ski. You can let SL ski run in GS turns at speed around 90 or 100km/h, but even though I have extremely hard SL skis (165cm Fischer Worldcup SL which were last year used as inspection skis by one of top 15 WC racers), there's no way I would feel comfortable doing this. With speed like that, ski is extremely nervous, it vibrates like crazy as soon as track is not perfectly flat etc. On the other side, GS (193cm Fischer Worldcup GS from same racer) ski just glides smoothly at same speed. And same goes for comparison of GS vs. SG, or SG vs. DH skis, of course at higher speed.

post #33 of 43

I think the length and stability question has more to do with the sidecut and the resonance of the ski.   The heft of the larger ski will dampen oscillations providing for a more stable ski, as will the reduced sidecut.  Like primoz said, SL skis feels nervous and scary at speed unless totally flat and on smooth prepared surfaces or when constantly on edge.  Theoretically I would day that a shorter ski would have the advantage, less contact area with the snow means less friction, but being able to control that shorter ski is another story.


post #34 of 43

The faster ski through the course will be the one that lets you make the cleanest arcs through the turns by matching the turn radius dialed up at the tipping angles required to balance lateral and vertical forces to stay in the groove.


If you were just to see which would go fastest down a hill without a course to stay on, the air friction would be such a major determiner of maximum speed on most hills that it  be very hard to show a significant difference using length as a variable.


I tried a little experiment with 208 SGs and 165 SCs, on a small hill.  (edit: I see I already mentioned this experiment)  Both pair maxed out around 60 mph.  The exact course taken made more difference than the skis, but as has been said, the longer ski will be better at making a good LR turn, so loose less speed on a path where you have to make some long radius high speed turns.

Edited by Ghost - 1/26/11 at 4:00pm
post #35 of 43

I race on a 186cm (21m) GS ski.  I am 5'3 and weigh 124lbs.  I am also a 40 year old female.  People are always trying to convince me to ski on a shorter GS ski.  I don't want to because I get better race results on a longer ski.  I also feel that they are more stable at high speeds.  When I went to a longer GS ski, I hadn't really improved my technical ability, so I feel that a longer ski is faster. 

post #36 of 43

It depends..


All other things equal, flex, sidecut, etc.....   A shorter ski is going to be moving you faster in a course when skating at the start is a big factor.  Longer skis tend to be faster in softer snow though.

post #37 of 43


Originally Posted by hamletcat View Post
..so I feel that a longer ski is faster. 

It is ;) I didn't do any physics calculations, but I do believe shorter skis would most like do better in (calculated) glide tests, since there's less drag, but difference in those 10 or 20cm is so small, that this doesn't matter much. For your "feel" there's so much more to take into account, not just that drag factor.  You feel more stable, so you feel more comfortable going faster and pushing your limits further on longer skis, and this plays bigger role then little a bit of extra speed shorter ski would give you. So on the end it is what you said... you feel longer skis are faster, and most likely you even get faster time on course with longer skis.

post #38 of 43

It's just the right ski for whatever situation you're in and what works for you.  For guys, except for lightweights, 165 is the default length for SL.  Nobody's going to stop you from using a 10 year old, 180 cm ski with 15 meters of sidecut, but you're not going to be competitive, because you won't be able to carve turns and ski the line that you can on a 165.


In GS, it varies a lot.  If you're not constrained by FIS regs (in Masters, you can use anything you want), it's still the ski that you can make the cleanest turns in the course.  For me, (I'm 5. 8", 185 lbs.), it's an Atomic 184 D2, 26.2 radius.  Works like a charm for me.  My teammate, who is 6.2" 195 lbs, skis on a 186 Atomic, 27.2 radius.


Above GS, it's whatever works.  I have two pairs of 201 Atomic SGs that I use for "turny" Super Gs, two pairs of 205 SGs I use for "straight" SGs, and a pair of 210 SGs I use for DH;  I had my choice of a women's DH 210, with a 39 meter sidecut, or a Men's SG 210, with a 36.2 meter sidecut.  I went with the SGs because I figured they had all the glide I needed, but I wanted a little more radius because I'm not a 25 year old Rambo lookalike. 


Notice that I haven't said a thing about length as it relates to speed.  I want a ski that lets me ski technically and tactically well in whatever event I'm in.  Once I've chosen the right length and sidecut, the speed comes from me executing to plan and making sure I have the tuning setup, including wax, that will optimize whatever gliding I can do...



post #39 of 43

The margin for error with regard to fore-aft balance becomes smaller at such high speeds, which is one reason why longer skis are used in speed events.

post #40 of 43
Originally Posted by skier219 View Post

I have been tinkering with the physics of this for the last couple days, and it's quite interesting.  The drag of the ski on that film of water goes something like mu*U*W*L/h, where mu is the viscosity of water, U is the velocity of the ski, W is the width of the ski, L is the length of the ski, and h is the height of the film of water.  As expected, the drag (aka friction) would increase with the surface area of the ski (W*L), so one might rightly expect longer skis to be slower.  But wait! Drag also decreases as the film height increases, and that's what happens with longer skis because of weight distribution.  So the real answer comes down to L/h for any given ski, and that is the last part I am working out now.  It looks like I can follow lubrication theory to model "h", and then come up with L/h.  That will provide the real answer as to the effect of ski length.  The faster ski will be the one that minimizes L/h, all else equal.  I would not be surprised to find that there is an optimum "L" in there, where going longer helps until you hit "L" and then the drag starts going back up.  Many fluid dynamic drag problems have some sort of "drag bucket" with an optimum minimum drag somewhere in the middle.

 It's nice to have a fluid dynamics expert here smile.gif.  Ski drag on snow seems like an interesting and subtle theoretical problem.  I imagine the kind and depth of structure on the base, the kind and age (hence shape and hardness) of snow crystals, and the kind of wax and how it is applied/finished would come in to it as well.  I wonder how much of an effect all those factors have?  I suppose, for a given fixed choice of those factors, your mu*U*W*L/h would still hold, hence would still be valid for the purpose of this thread, which was about the dependence on L.  Robert

post #41 of 43
Originally Posted by MastersRacer View Post

I race DH. I use 215s.

I race SG. I use 209s or 201s.

I race GS. I use 188s.

I race SL. I use 161s.

For racing what really matters is making the turns with as much arc as possible. Everyone is on the same length ski in an event, give or take 10 - 15 cm. Differences due to physics don't come into play. Turn quality does. Wax does. No racer is going to sacrifice turn shape for the theoretical advantage the physics of length may factor in.

If you want to talk all out fastest without turns: http://www.speedski.com/speed/equipment.htm

 As a matter of qualifying my understanding of speed on skis, I have been clocked at 80 mph in a race course. I know what would want on my feet at that speed.

As a masters racer, maybe that means you have been around for a while. Can you explain to me what has happened to race skis in the past 25 or so years? I used to use 223s for DH, 210s for GS and 205s (yes 205s) for SL. Then I kind of drifted away from skiing in general for a couple of decades, but have lately started to get back into it. I knew of course that recreational skis had gotten short and fat with bigger sidecuts, but was surprised that anyone would take anything as short as a 188 through a GS course. What has changed so dramatically to make that short of a ski stable at GS speeds? The new GS boards still look pretty narrow and straight to me (although maybe that is just in comparison to the new rec skis-- I don't know) so it doesn't seem to be a radical revolution in shape. Is it materials/construction, or did we always just use skis that were way too long?

post #42 of 43

I raced in the '70s on the ski lengths you mention although I had 207 SLs, :-)


Shape happened. Bode Miller helped chang the racing scene when he began winning on recreational skis with pronounced side cut. He was able to carve more, skid less thus being faster.


Faster bases and the technology that went into skis also changed. The newer race skis demand body strength as they are more effiecient at gripping the snow and ice, thus building huge forces that the body must contend with. In the tech events, you are on your edges a lot and not riding a flat ski; the shaped skis are inherently stable when on edge. They get twitchy when flat. As the skis are being edged a lot, they have the stability required to deal with the speeds of the course.


Courses have also changed. They are measured by tape, laser sights and ski lengths. Gone are the days of simply eyeballing a set. They are tighter in general and more across the fall line than courses of our youth.


GS has changed significantly this season on the FIS circuit with new (old) side cuts. Those that have figured out the new skis have excelled. Others will strive to catch up. I don't think that Ligety could have done quite so well on old construction and old sidecut. The differences in construction technology have permited a ski of similar shape to the old ones to behave quite differently.

post #43 of 43

The longest ski you can manage, the greater speed you can reach.

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