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Ski Physics

post #1 of 21
Thread Starter 
Thanks to direct imput from outstanding epic ski posters, and follow up "search" research on this forum and other ski tests, I am currently focused on chosing a pair of Head skis from among 3 candidates: IM 70, IM 75 and IC 180. At first blush, it seems simple... demo, demo, demo. But the number of demo candidates is much too high...

As the waist gets wider from 65mm IC 180 to 69mm IM 70 to 75mm IM 75, edge to edge gets slower and lift in powder gets better. But what happens if one varies the length of the skis at the same time as one varies the waist? For example, if I skied the IM 75 at 163 cm, the IM 70 at 170 cm and the IC 180 at 177 cm, which pair is quickest edge to edge and which floats better? Then there is the question as to the appropriate lengths on each ski. Based on the test they gave on the website, Head chose the Monsters as my ski and suggested I should ski them at 163 cm length based on my height, but they never asked my weight. I am 5'5" height and 168 weight). (I am an strong level 8 skier on a pair of 190 cm pair straight Olins. After a few demos I am starting to think I am alot better skier with the current technology on my side.)

On the surface it seems that if I can ski the IC 180 at 170mm and my second choice is the IM 70 at 163. The IC 180 might just have both better carving potential, better quick turns and possibly better float, but I can't calculate whether that is true. Can someone else enlightment me on how they think the variations in waist, and length affect performance. Thanks in advance for your suggestions.
post #2 of 21
I'll leave it to the experts to comment on your underlying question regarding width vs. length. But if it helps I can comment on the specific skis you mentioned... I am about your weight although a bit taller (not sure height is relevant) and a level 7-ish New England skier who up until two years ago skied for many years exclusively on 190cm K2 Comp 610's. I have skied the IM75 chip and the IM70 in 170cm and didn't feel like I wanted to go shorter on either one. (FWIW I preferred the 75-chip.) Haven't skied the IC180 but if it is a cruiser/carver type ski you may want to think about a 177cm as I am guessing you are a faster more powerful skier than I.

I suspect Oboe may have some useful input on this matter [img]smile.gif[/img] HTH

[ January 07, 2004, 07:35 PM: Message edited by: steve1321 ]
post #3 of 21
PhysicsMan left a very interesting post regarding length and amount of float that you get from a ski for your given weight a few months back. You might want to try searching for it. I think the name was float as a function of width, or possibly as a function of weight - something to that effect anyways. That thread wont answer your question that youre asking right now, but it might provide some useful input into how you make your final decision.

Being that you are nearly 170lbs, i would start looking towards skis that are around 170cm. I think that each ski you are considering comes in that length. Another thing you must consider is what type of terrain you are spending the majority of your time on, as well as what kind of snow you are spending the majority of your time on. If you are skiing mostly in NY, i would say that you probably shouldnt go with a waist that is over 70mm for your everday ski. If you are skiing mostly groomed snow, the ski with the 65mm waist may make more sense, especially if youare looking for ease and control on ice. The narrower waisted ski will offer a much better platform for making turns on very hard snow. Once you pass the 70mm waist size, you start to feel more detached from the edge of the ski when you are in you turn on hard snow.

It is possible that a 70mm waisted ski can offer plenty of float for you needs, and still provide the hardpack and groomed snow performance that you will want for groomed conditions. Also, youre going to want to take flex into consideration. Most people only consider length and sidecut when they are looking into a ski, and considering its turning radius, but flex plays a bigger role than you might think. Particularly, you will notice that each ski will flex differently - meaning not only will one be stiffer overall, but you may find that one has a stiff tip and soft tail, while the other ski is just the opposite. I have only skied the IM70 so i cannot comment on the flex of all of the other skis that you are looking at, but as a recall, the IM70 was rather soft, and actually flexed more in the tip than in the tail (correct me if im wrong - its been almost a year since i have skied it). This type of flex pattern will offer very easy turn initiation, and can be punnishing if you rely on the tail to turn. The reason the tail is stiffer, makes the ski kick out of the turn, and allows you to turn the ski from the back seat if you have to - much like the flex of a Salomon Xscream Series.

Bottom line, for skiing in NY, i would chose the IC180, or the IM70, in a 170. The factor that will make your decision final decision, is how much time do you spend on groomed snow versus softer crud or powder. Another factor that you may want to bring into consideration, is how often do you make it to a big mountain that has alot of snow to ski. If your answer to this question is often, then you may want to look into the IM75 - but again, this is only if youre skiing a lot of crud and fresh powder - and not very many groomers. If you mostly ski on groomed snow - go for the IC180 and dont look back. It will perform just fine in crud, powder, and trees. If you are skiing on and off piste but have a preference to groomed snow - go with the IM70. It will provide enough float for you in most conditions, while still offering exceptional groomed snow performance. Figure out where you sit in relation to each of those categories, and make your decision based on that.

Later

GREG
post #4 of 21
Thread Starter 
Quote:
Originally posted by steve1321:
I have skied the IM75 chip and the IM70 in 170cm and didn't feel like I wanted to go shorter on either one. (FWIW I preferred the 75-chip.)
Steve what were the virtues of the IM 75 that you caused you to prefer it over the IM 70? I would have otherwise assumed that a New England skiier would have found the IM 70 more practical in the groomed slopes that prevail in places like Vermont.

Thanks
post #5 of 21
Thread Starter 
[quote]Originally posted by HeluvaSkier:
[QB] If you mostly ski on groomed snow - go for the IC180 and dont look back. It will perform just fine in crud, powder, and trees. If you are skiing on and off piste but have a preference to groomed snow - go with the IM70. It will provide enough float for you in most conditions, while still offering exceptional groomed snow performance. [QB]

I was primarily interested in the interplay between length and ski geometry in tuning skis for their best average performance. Btw, at the moment, my East/West formula is 60% east-40% West. I thought the lengths I might want to demo at were 163 cms for the IM 75, (to make it quicker edge to edge in the east), 170 cm for the IM 70, and maybe 177 CM for the IC 180 (to improve its Western performance). PM skis the IC 200 at 184 cm and likely has an opinion on the subject of whether someone of stature (5'5", 168 pounds) could handle the IC 180 at 177 cm. Using PM's excellent formula for average width, including the extra length, the IC 180 would seem to actually be a superior western ski for all but the heavy powder days at 177 cm. It doesn't seem to be a problem handling them on groomed slopes at that length, but I haven't tried it. Wondering what people think about the above?
post #6 of 21
Quote:
Originally posted by rvwink:
...But what happens if one varies the length of the skis at the same time as one varies the waist? For example, if I skied the IM 75 at 163 cm, the IM 70 at 170 cm and the IC 180 at 177 cm, which pair is quickest edge to edge and which floats better?...
If skis didn't flex at all, "float" would simply increase with total base area, and, exactly as you suggest, one could decrease the width and increase the length proportionally to get the same "float".

Unfortunately, skis do flex, and most people seem to perceive "float" as how deep *they* are immersed in the snow, which is the equivalent to how deep the center of the ski is in the snow. This is not the same as the average depth of the ski below the surface, the quantity discussed in the previous paragraph. Because of this perception, I usually find it more accurate to consider "float" primarily a function of the width of the center section of the ski (ie, the "waist width"), with less influence from ski length and the widths of the tip and tail than one might expect.

I should also point out that "float" also has other meanings when describing the soft snow performance of a ski. For example, if the tips of a given ski tend to dive in deep powder (ie, if you momentarily get your weight a bit forward), then, no matter how large the total surface area of the ski, many people would insist that ski "doesn't float well".

With respect to your second parameter, "edge-to-edge quickness", I hate to avoid your question, but I don't think this is a particularly well-defined concept even among the tech-heads of EpicSki, and so, I wouldn't want to discuss it without a whole bunch of preparatory discussion (that we haven't had yet).

Just to give you a flavor of such a discussion, I would claim that if I quickly changed the angle of my lower leg by X degrees, then, with almost any modern boot/binding combination that is in good shape, the ski will follow along and change edge angle almost instantly, and the magnitude of the angular movement will be very close to X degrees as well. I think the phrase, "poor edge-to-edge quickness" conjures up the image of the edge angle responding much more slowly or weakly to skier input, and this is not the case at all. What may be slow is not the edge angle itself, but the speed at which the ski starts to come around once edged, but this is dependent of a huge number of other parameters including flex, sidecut, binding mounting position, etc..

If you want a single, simple parameter that characterizes "how wide" a given ski feels, IMHO, the "average width" is a good, easy-to-calculate place to start:

average_width = 0.25*tip_width + 0.5*waist + 0.25*tail_width.

Finally, I would caution that even if deeply sidecut skis float well (in the sense of having lots of base surface area), this doesn't mean that they will necessarily be good powder boards (which I believe is what you are hoping for). In the opinion of many regular powder skiers, deeply sidecut skis with wide tips and tails get too twitchy in cut up powder to be as useful as one might think. Been there, done that (and reported it in previous posts).

With respect to the three specific skis you mentioned, I think Helluva gave you some really good advice, and I don't see any need for further elaboration except to wonder why you are so concerned about float if you mostly ski the East. Personally, I would be more concerned with sidecut radius and flex for the types of snow we usually see.

Gotta run.

Cheers,

Tom / PM

[ January 08, 2004, 01:03 AM: Message edited by: PhysicsMan ]
post #7 of 21
Thread Starter 
Using Physic man's forumula, I did the calculations and was surprised by the results. The surface area difference between the IM 70 and the iC 180 is 1.1%. At the same length, the iC 180 shouldn't logically be significantly different from the IM 70 in western conditions. The surface area difference between the IM 70 and the IM 75 is 4%.

I also heard back from an iC 180 user who cautioned me to stay down at the 170 cm range and forget about the 177 cm length. I then compared surface calculations for the IM 75 in 163 cm and the iC 180 in 170 cm, using PM's formula. Their surface areas will be within 1% surface area to one another. So that helps answer the question of which will be better out west. At the same surface area, logically their flotation will be similar. Now who has a guess as to which ski would be quicker edge to edge, the shorter IM 75 or the iC 180. My guess would be the iC 180.

I am pleased that as the result of the information gained, I have a better understanding of the relationship between ski length and waist width. Special thanks to PM for his help.
post #8 of 21
The ski with the narrower waist will have the quickest edge to edge performance (typically). You will also find that a ski that is overall stiffer than another will have superior edge to edge "snap" if you will. Check the turning radius and sidecut of both skis. Make sure there are no dimension changes per length in both models. From experience, i can tell you that wider waisted skis roll much slower than high energy carving skis with narrow waists.
Later
GREG
post #9 of 21
Thread Starter 
Thanks Greg. The technology of providing a radical side cut offers substantial skier benefits. It seems to be a way of creating a win win situation. Skiers get to enjoy the float of higher surface area, along with the quickness of a narrow waist. Pretty damn good trick it seems to me. Perhaps the downside is that the iC 180 is less comfortable in the powder than the IM 70 because the uneven surface area created by the radical sidecut might not be as efficient as the more rectangular shape of the IM 70. But it appears the quickness benefits of the 65 mm waist outweight the mildly inferior powder performance.
post #10 of 21
Rvwink, my preference for the 75-chip over the im70 was based on

(1) incredible edge-grip, best of any ski I have tried, this includes 5-star, sceneo 400, rx-8, r:11 puls, fischer race sc...

(2) unshakeable stability in that one can go from ice to crud to whatever without really having to adjust one's skiing

The downside is a somewhat heavy feeling, very damp ski and for that reason I think I slightly prefer the '04 R:11. If Head ever decides to put the chip system in the IM70, that would be a no-brainer for our conditions and terrain.
post #11 of 21
If Head ever put the Chip in the iM 70, I'd hock the leftmost of my family jewels to own that in 170 cm.

The 75 Chip also had for the me the most phenomenal edge hold I've ever experienced. It made even rough surfaces feel like smooth butter. It was very slow edge to edge - meaning, it was slow in the speed with which it allowed me to make two consecutive turns in opposite directions. Even turning one way, it was slow and did not permit shorter turns to be made easily. That's why I really, really enjoyed the sensations I experienced from skiing on it, but did not choose to buy it.

Say what you will about 69mm waist versus 75mm waist, the Head iM 70 bulled through tough snow for me as well as the Rossi Bandit XX. At the same time, it has the edge grip of the Bandit [single] X - which did not bull through that same tough snow very well at all. Bear in mind that, with the Super Railflex binding, the binding position can be moved forward, making the iM 70 quicker in short turns. For me, however, after the initial pleasure at the quickness of short turns that afforded, I've put my binding placement back in the middle, where it will stay.

When I go to the Gathering at Jackson Hole, I'll bring only my Head iM 70's - the versatility of the Leatherman multi-purpose tool with the excitement and power of a Harley. WARNING: The foregoing sentence is intended to be hyperbolic analogy for the purposes of illustration about the writers feelings. Those who insist upon understanding the sentence only in a literal sense must do so only at their own risk. Skier discretion is advised. Pregnant or nursing mothers, infants, and physicists under the age of eighty should not read that sentence.
post #12 of 21
So Oboe does That mean ya kinda like that ski? Or is it just that Harley is now making a leatherman tool? Or is Leatherman now makeing Motorcycles. Since this is a ski forum I'll just go with ya like dem Boards. Since a lot of people are talking about the Heads I might just have to give them a try. BUT I Can't afford new skis!
post #13 of 21
OK, well, I guess I should clarify: Literal minded physicists over the age of eighty ain't no infants and should not rely upon Leatherman tools to become pregant while skiing, but getting Head is ok unless it's on a Harley.

[ January 09, 2004, 06:25 PM: Message edited by: oboe ]
post #14 of 21
I like Harleys. Harleys are my friends. Some things, however, work out better not on a Harley.
post #15 of 21
Oboe that was a rather sketchy sentence - i liked it, but whats wrong with Harley's? I take it youre not much into the "biker" image...

Anyhow, the reason for my post: It was stated above that the IM75 has better edgehold than all of the skis mentioned - a very long list of supposed "ice" skis and very high end expert boards - even a race board was in there. The bump in the road that im going to bring up is thr tune that was on each ski, and the flex of each ski. The tune part is easy, are you confident that each ski was tuned well, properly, and similar in sharpness from one ski to the other? I would guess that they were not. I have skied very dull heads, and very sharp heads, as well as many other skis out there. I assure you that a dull head, skis just like a dull Bandit XX, and a sharp head skis just like a sharp XX. They key to comparing edgehold if putting a very sharp tune on each ski - or a horrible tune on each ski, and seeing how much edgehold there is. As far as flex of the ski goes. If a ski was too stiff for the given snow conditions, or too stiff for the person who was piloting it (not pointing fingers, but often this happens in a demo situation which leads a ski to get a poor review by any particular skier), the skier may have the impression that the edgehold was in fact not there, when in fact the "tool" was just not being used in a way that would lend itself to showing its true advantages and weaknesses.

I would trust that with a tuned edge an RX8, and SC, would smoke an IM75 on any ice - not matter how you tune the IM75 (unless of course we are talking slushy cut up ice that requires a bulldozer to ski through). I have skied the Head chip skis in the form of the i.XRC - if you are not skiing the ski on true "ice" it does in fact feel like no other ski out there. They are absolutely glued to the snow, and connot be shaken - this is where oboe's slow edge to edge feel comes in, because they feel almost sluggish, because they lock you into the turn. This feel however is not in fact true edgehold, because i have skied that ski on ice, and it lost most of that feel as soon as little or none of the base was touching the snow - and the edgehold felt just like a normal ski. I do not know what it is about the Head chip skis that gives them that kind of feel - possibly the chip and fibers do something afterall... but i do know that it is not edgehold in the sense of planting an edge into bulletproof ice. They rather have an incredible stability on their edge, and unmatched edgehold, or rather just hold, on "groomed snow" where you are riding on more of the ski than just 2mm of edge and 1mm or 2mm of p-tex (think the ski being sunk a bit into the groomed snow because of edge pressure). The IM75 and i.XRC are great skis. I have not skied the 75, but if it skis like the XRC, head is really onto something, although im not exactly sure what it is - but the skis have a VERY unique feel.

Anyhow, the point to all that was that a great ski will have great edgehold without any kind of tune on it at all - it will simply work - the tune will just enhance the performance even further. A more mediocre ski will have edgehold with a tune, but as soon as that tune is gone, the ski will no long hold an edge on anything harder than packed powder. By personal experience i have found this to be the case with just about every midfat i have ever skied - IM70 included. Even my own Xscreams i need to have a VERY sharp edge to even come close to the hard snow performance that ANY race ski out there has - race stock skis excluded (retail models are the focus here - stocks are another whole league of their own). An example of a great ski is my room mate's Fischer WC SL's - he took them out on our first day on the snow with no tune. He was skiing on solid ice, and didnt even realize he had not tuned them yet... granted the skis took a little more work to set the edge, but once it was set they took off and skied like normal.

Later

GREG
post #16 of 21
Quote:
Originally posted by Utah49:
So Oboe does That mean ya kinda like that ski?
Liking skis is fine, but - again - boots and technique are WAAAAAAY more important. Although I continue to express my thoughts regarding equipment, these days, my heart and attention are more devoted to my skiing and instructional education.

That's why I really want our friend to choose his skis, get alligned, have great fitting boots - and then get on to the real story: SKIING!
post #17 of 21
Thread Starter 
Have just a little more patience Oboe. My mind is finally "right". . There isn't a shred of doubt as to what ski to purchase. To begin with you moved me away from Rossi to Head. Once I started searching for the ideal answer among Head's skis, I remembered PM's posts about his love affair with two pairs of XP 100 skis. Also, his ski surface formula explained how a 65 cm waisted ski could possibly be good in the West. Greg helped by pronouncing the iC 180 as the best eastern ski of the trio I was considering. Mac and Frenchie helped me decide that the 170 cm height was right and their prior posts about the iC 180 being a good western ski was also quite helpful.

Now, the acquisition process has formally started. Well before the end of January, I expect to "dancing feet" join my "dancing mind" on the slopes.

Thanks guys!!
post #18 of 21
hu-ZZAH! Mozeltov!

Let us know when the marriage has been consumated!
post #19 of 21
Quote:
Originally posted by rvwink:
...I remembered PM's posts about his love affair with two pairs of XP 100 skis. Also, his ski surface formula explained how a 65 cm waisted ski could possibly be good in the West.
Hi RVW - I just got in and noticed this post/thread. Er ... just be sure we are on the same page, exactly what base area formula for skis are you talking about? Unless my memory has failed me, I don't think I have posted an area formula in at least a year. The only formula that I posted in the past week or so was one for average ski width.

For the record, if I am heading out west and can only take one pair of skis, I take my 83 mm wide 10ex's as the best compromise / guestimated conditions ski. I don't take my 62 mm wide 9.16's, my 65 mm P40's, or my 68 mm xp100's.

Finally, did you happen to read my message in this thread dated January 08, 2004 12:09 AM?

In it, I argued that I stated that I feel that "float" (or at least the perception of it) is more directly correlated with waist width than with total base area:

"...I usually find it more accurate to consider "float" primarily a function of the width of the center section of the ski (ie, the "waist width"), with less influence from ski length and the widths of the tip and tail than one might expect..."

Finally, with respect to the suitability of deeply sidecut skis in soft snow, in the next to the last paragraph of the same message, I also stated:

"..Finally, I would caution that even if deeply sidecut skis float well (in the sense of having lots of base surface area), this doesn't mean that they will necessarily be particularly good powder boards (which I believe is what you are hoping for). In the opinion of many regular powder skiers, deeply sidecut skis with wide tips and tails get too twitchy in cut up powder to be as useful as one might think. Been there, done that (and reported it in previous posts)..."

Tom / PM

[ January 10, 2004, 11:56 PM: Message edited by: PhysicsMan ]
post #20 of 21
Thread Starter 
[quote]Originally posted by PhysicsMan:
Quote:
Originally posted by rvwink:
[qb]...I just got in and noticed this post/thread. Er ... just be sure we are on the same page, exactly what base area formula for skis are you talking about? Unless my memory has failed me, I don't think I have posted an area formula in at least a year. The only formula that I posted in the past week or so was one for average ski width.

In it, I argued that I stated that I feel that "float" (or at least the perception of it) is more directly correlated with waist width than with total base area:[Tom / PM
Your recent post related to average width, but the average width formula was the key to allow me to calculate the surface area. I also figured out that there wasn't a linear relationship between "average width" and powder float. I expressed that in the following quote.

"Perhaps the downside is that the iC 180 is less comfortable in the powder than the IM 70 because the uneven surface area created by the radical sidecut might not be as efficient as the more rectangular shape of the IM 70. But it appears the quickness benefits of the 65 mm waist outweight the mildly inferior powder performance."

I am not too greedy PM. If they are "comfortable" out West, the fact that they are not great powder skis is easily accepted.
post #21 of 21
Thread Starter 
Quote:
Originally posted by craigr:
The tecnical manual in the head site has some interesting data. They have allocated an F-index to their sklis which is some sort of index relating to surface contact. The im75 is 125, the ic200 is 119 and the ic180 is 115. URL=http://www.head.com/ski/files/pdf/ATechnicalManual2003.pdf]http://www.head.com/ski/files/pdf/ATechnicalManual2003.pdf[/url]
BTW my favorite was the ic180.
Cheers
Thanks for pointing the surface area calculation supplied by Head out Craig. I looked at that technical data, but didn't catch their surface area calculations. The average width formula that I used may work for regular skis, but perhaps it isn't entirely accurate with radical sidecut skis. The surface difference between the IM 75 and iC 180 appears to be closer to 10 percent than the 5% I calculated. I guess I need to increase the percentage of the center area, and cut back on the tip and tail percentages a bit. But average surface area is great, because it allows one to calculate what happens if you use a shorter ski of one type, and a longer ski of another type.

Delighted to hear you reached the same conclusion that I did among the same "head" contestants after actively demoing them. My data was not as accurate as I demoed them through the "dancing minds" of others. But I had confidence that Oboe, Tom, Greg, Mac, and Frencie would not lead me astray.

This brings up another point. There is no way that I end up chosing the iC 180s without my uncovering this forum through the recommendation of sing sing. The reason the iC 180 hasn't been a successful ski for Head doesn't relate to its merit as a ski, but rather to Head's lack of coverage among many ski shops and with the ski press. Head is finally winning over the minds of some skiers with their Monster series, and even the iC 300 and n iC 200 have scored successfully in some tests. But the iC 180 is still largely invisible to most skiers, and hasn't sold well in the past. Intriguingly, demoing was the key to finding the ski. I wouldn't have known I would like the head skis, unless someone whose judgment I respected, pointed them out. I trusted Oboe because he liked the Rossi's that I liked as well and that only works if I demo.
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