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Sidecut *and* Turning Radius Constant Across Different Lenghts?

post #1 of 22
Thread Starter 
Okay, I know that this has probably been discussed in the past, but just to confirm:

- If a ski company claims that the same model has identical sidecut figures and turning radius across all its different lengths, the company is either providing incorrect information, or doing something really drastic via varying running length or the midpoint of the sidecut or something like that, correct? (Or not so correct?)

In case it matters, what I’m specifically wondering about is last year’s Dynastar Speed Course, which supposedly has a 104-66-91 sidecut with a 21-meter radius, even down to a non-USSA legal 172cm length, which just doesn’t seem to compute.

thanks in advance for any feedback!
post #2 of 22
Quote:
Originally Posted by Jonathan Shefftz

- If a ski company claims that the same model has identical sidecut figures and turning radius across all its different lengths, the company is either providing incorrect information, or doing something really drastic via varying running length or the midpoint of the sidecut or something like that, correct? (Or not so correct?)
Corrrect.

The dimensions can stay the same, and the radii could theoretically stay the same, but they can't both stay the same.

If you were using constant width dimensions, the radii would change as the distances between the width dimensions change in each size.
post #3 of 22
Most ski companies that show a sidecut radius in their literature show it only for one length, e.g.

Available in 160, 167, 177, 185 cm
Sidecut 110-69-101
Radius 17m / 177 [meaning it's 17 meters in the 177 cm size]

Atomic has a sheet on their website that lists the (varying) sidecut dimensions and radius for all lengths. It's not clear to me whether (and which) other makers also vary the sidecut dimensions with length.

Incidentally, your 104-66-91 figures are pretty much dead-on 21 meters at 185 cm long.
post #4 of 22
There are skis which do have identical dimensions across all lengths and their radii vary substantially, approx. 2 meters/10 cm of length.
Examples from last year´s Dynastars are some Skicross models:
Skicross 8 has 155/164/173/182cm and corresponding radii 14/16/18/20 m. As you see, such skis are almost slaloms in the shortest lengths and almost GS in the longest, i.e. very different animals.
If the radius differences are smaller than that or totally non-existent you know that there must be some change in dimensions, regardless of whether they give them for all lengths (like Atomic and some others do) or not.
Also, the availability of precise dimensions is sometimes limited.

Dynastar belongs to manufacturers who don´t offer much information. They always give one sidecut per ski. That´s also the case with the Speed Course. They say "21m" for all lengths but the referential length for those 104-66-91 was 184cm. They don´t tell you more. The way they combine the lengths (172/178/184/189cm), radii 21m and only one sidecut 104-66-91 is nonsense. If they wrote "more than 21" it wouldn´t be much better but at least not misleading.
You´re right, 104-66-91/172 cm can´t produce 21m radius. I guess that about 100-66-88 considering contact length of 154 cm could.

Another problem is the tip width which is mostly given at the widest point but for the radius calculation it is needed at the contact point (or according to the FIS calculation).

I have never measured the retail Speed Courses so that I don´t know the truth.

The racestock Speed Course Comps have different dimensions and sidecuts.
post #5 of 22
Physicsman's sidecut formula is here:
http://forums.epicski.com/showthread...ormula+sidecut
post #6 of 22
Thread Starter 
Wow, great, thanks for all the informative replies - love that spreadsheet too!
And the updated answer/explanation based upon in-person inspection is:
- the waist width is wider for the shorter lengths (at least for the 172cm), which is especially entertaining since the waist width is prominently (and incorrectly for the shorter lengths) incorporated into the topskin's graphics.
post #7 of 22
Quote:
Originally Posted by sjjohnston
Most ski companies that show a sidecut radius in their literature show it only for one length, e.g...

Atomic has a sheet on their website that lists the (varying) sidecut dimensions and radius for all lengths. It's not clear to me whether (and which) other makers also vary the sidecut dimensions with length.
A lot of them do so now...Volkl, Fischer to name a couple.
post #8 of 22
Something to watch out for on P-Man's Sidecut Radius calculator...

He uses the assumption that the ski's contact length (some call this the running length) will be approximately 12% less than the ski's chord length (the number most manufacturers publish as the ski length). You'll get more accurate results if you actually measure your contact length (there are multiple methods to do this - search the forums). I've found that for my quiver the contact length actually averages about 10% less than the chord length (except for twintips that are about 13-14% less).


Regarding the Dynastars - I like the fact that Dynastar varies the ski dimensions based on the length to maintain a similar turn radius and performance across all of the available lengths (unfortunately they're not very good about publishing this information). There is one length of the Legend 8000 that has an 80mm waist and one length of the 8800 that has an 88mm waist. I believe that's probably where they derived the names for those skis. Of course, those aren't the dimensions they chose to generally publish in the ski mags or on their site.

In fact (I'm going too far with this I know) I was in CO Ski & Golf here in Denver and the idiot salesman sat there telling me that the 81mm waist printed on the Dynastar Legend 8000 I was looking at was a "misprint" - I pulled out my calipers and measured the ski to show him he was full of it. It's a pity that so many ski buyers have to depend on these jokers.
post #9 of 22
Great research, noodler! Anyone who finds a chink in the mighty Physicsman's calculations gets my respect!
However, I have been told a different story behind the Legend names - each model is named after the altitude of a legendary mountain, in those pesky Euro meters of course.
8800 = Everest
8000 = Annapurna
6200 = Mackinley/Denali
4800 = Mont Blanc
post #10 of 22
OK - I was just guessing on the naming thing. That naming stuff is much cooler. Thanks for the info.
post #11 of 22
The recent snub ski tips changed the chord vs. contact lengths ratio so that about 10% are, in my experience, appropriate for most (groomer) skis. Older skis with traditional tips had mostly -20 cm, now it´s less. You may see it in one of the German ski tests which has been precisely measuring contact lengths since the days of pre-shaped skis.

In skis with some sort of twintip it is, as Noodler writes, more, according to the length and height of the raised tail. An interesting example may be Dynastar Skicross 10 mod. 03/04 , though the ski is not declared as a twintip. A pity I don´t remember how much it was but I was surprised when I measured them.

One more remark to "the fact that Dynastar varies the ski dimensions based on the length to maintain a similar turn radius and performance across all of the available lengths" (Noodler).
It is not very consistent:
8800: 158/168/178/188cm have 21/24/25/27m
8000: 158/165/172/178/184cm have 16/18/19/20/21m.
Fact is that the most common lengths are kept within 1m radius. It might be even less, e.g. 19.4 = 19 and 19.6 = 20.

As to the names, I read something when the skis were new but don´t remember.
The "mountain story" wouldn´t surprise me. Their catalog is full of poetic language about mountains (the more poetry the less facts) and this would be precisely their style. Also, fortunately enough, the 4800 and 3800 are not waist widths...
post #12 of 22
Stockli show the radius for each length of ski they sell. It's quite handy. You want a bigger turn radius, choose a longer ski.
post #13 of 22
Quote:
Originally Posted by ant
Stockli show the radius for each length of ski they sell. It's quite handy. You want a bigger turn radius, choose a longer ski.
I'm glad you selected Stockli to point out a manufacturer that does a fairly good job at publishing their data. Not only that, they are one of the few companies that seem to be fairly accurate in the sidecut radii that they publish (at least based on P-Man's sidecut calculator). The calculations I've done to double-check manufacturers' stated sidecuts show that Stockli is right on. The same cannot be said for many other manufacturers. Here are a couple examples:

Head Supermojo (Published as 37.3m at 183cm, calculated as 34.1m)
Atomic Pimp (Published as 27m at 183cm, calculated as 30m)

More than likely though these are just cases of the manufacturer publishing a single radius for the model based on the "reference" length. So my point is that when purchasing skis you should be very careful to NOT go by what is published on the web sites or in the mags. Use what's printed on the ski for the length you're buying or use P-Man's calculator to approximate the value.
post #14 of 22
Quote:
Originally Posted by Noodler
I'm glad you selected Stockli to point out a manufacturer that does a fairly good job at publishing their data. Not only that, they are one of the few companies that seem to be fairly accurate in the sidecut radii that they publish (at least based on P-Man's sidecut calculator). .
Noodler, I think your astute observation that P-Man's formula assumes the contact length is the reason for this. Given the same "published" size, actual running lengths can vary significantly from one manufacturer to another. I believe the manufacturers publish values based upon what the actual running lengths are.

Further complicating this, many skis are built so that the widest dimension of the tip is actually above the "static" contact point, and there is no real consensus as to how this should be factored in.

What's important to remember here is that all of these radii are approximations. This is because we're trying to assign a circular radius value to a sidecut that probably isn't circular. Many high performance skis are elliptical, while some are still parabolic.
post #15 of 22
Quote:
Originally Posted by Noodler
In fact (I'm going too far with this I know) I was in CO Ski & Golf here in Denver and the idiot salesman sat there telling me that the 81mm waist printed on the Dynastar Legend 8000 I was looking at was a "misprint" - .
We have a pair of Nordica Beast 72tt which actually have a misprint. Company and my ruler both show a tip of 110 but the ski has 100 printed on it. I wonder if that's why they were so cheap.
post #16 of 22
Quote:
Originally Posted by troutman
Further complicating this, many skis are built so that the widest dimension of the tip is actually above the "static" contact point, and there is no real consensus as to how this should be factored in.
You've brought up one of the issues I'm still not clear on. Where do manufacturers decide to take the sidecut measurements? Is the waist measurement always the narrowest point on the ski? Or is it taken at the marked midsole point? Is the shovel width taken at its widest point or do some manufacturers take it at the end of the contact length of the shovel?

Stockli skis (and many other manufacturers) purposely place the widest point of the shovel past the normal contact length point (up into the bend as you point out) so that when the ski is tipped on edge the effective sidecut is higher and will progressively pull the tip into the turn quicker.

I wish the industry would at least standardize on where these measurements are taken so that they could be compared across the different manufacturers reliably.
post #17 of 22
Quote:
Originally Posted by Noodler
I pulled out my calipers and measured the ski to show him he was full of it.
Classic!

I'm willing to wager your sales guy had no idea what they were...!

I'm not going to ask why you were carrying them on a ski shopping trip. :
post #18 of 22
There's a standardized FIS method, but: (i) I don't know if it's what's used by manufacturers, except when marking actual racing skis and (ii) it's really not all that accurate.
post #19 of 22
Quote:
Originally Posted by Noodler
You've brought up one of the issues I'm still not clear on. Where do manufacturers decide to take the sidecut measurements? Is the waist measurement always the narrowest point on the ski? Or is it taken at the marked midsole point? Is the shovel width taken at its widest point or do some manufacturers take it at the end of the contact length of the shovel?

Stockli skis (and many other manufacturers) purposely place the widest point of the shovel past the normal contact length point (up into the bend as you point out) so that when the ski is tipped on edge the effective sidecut is higher and will progressively pull the tip into the turn quicker.

I wish the industry would at least standardize on where these measurements are taken so that they could be compared across the different manufacturers reliably.
You've hit on it. There are no standards for much of anything in the ski industry. Everything in the industry seems to be based on trial-and-error with a little bit of magic thrown in.

I view it as a miracle that the DIN standards actually were ratified and then followed. That is a relatively recent situation. I have some hopes that there might be an opportunity for this to change, but I think it will take a non-manufacturer (overseer?) to do it.
post #20 of 22
As usually we get from some specific question to more general issues. There are some important points taken in the recent posts. Here´s my take:

- practically all skis have their widest part of the tip outside of the static CP (contact point). It is as much as several cm. Similarly, the difference between the maximum width at the widest point and the width at the CP is often 3-5mm, at least in most groomer skis

- most (if not all) manufacturers simply give the maximum width. Using their data in calculations brings some radius which differs from that calculated in some other way

- fact is that to decide at which point exactly the edge starts to engage is impossible: it mostly won´t be exactly in the "technical", "static" CP but it will vary according to the penetration of the ski edge and the edge angle

- to insist on knowing the precise radius is silly anyway. As troutman points out, it´s an approximation whenever the sidecut isn´t precisely circular. As we know it´s a theoretical part of a circle which a not-arced and not-edged ski would have/has.

- as soon as the ski is in action, arced and edged, the nominal radius is left behind as a theory. Ain´t it funny to discuss the precision of something which changes the moment we start to really use it?

- IMO the most precise and only comparable method is (would be) to measure the exact static contact legth (CL), the widths in both static CP (and in the narrowest part of the ski regardless of the mounting point) and to calculate using these measurements. Any method using some percentage of the length is less precise

- I agree with Steve that to expect some standardized method is not very realistic.
"The level of basic research in the industry is low. It´s mostly marketing and sales that have the say." This is a private statement by a former long-year chief designer of one of the leading binding manufacturers. Need to say more?

- OTOH, official, "nominal" sidecut shouldn´t become a mantra. I have had some pathetic debates with skiers in love with short skis and small radii who ostracize skis with bigger radii because "they only manage a turn as big as a swimming pool", meaning the 50m olympic size (25m radius = 50m diameter hence a turn). I see a danger in overestimating the nominal radius and in neglecting/underestimating the other important factors.

As to Noodler´s callipers I also have a story. I quite shocked Debby Compagnoni´s serviceman back in 1997. We were on a double seat during a WC race and he was holding a pair of her "Test 3" GS skis. They were distinctly more shaped than the regular ones she was winning race after race with. I produced a tape measure from my pocket, asked him if I could (as you remember they were the wild years of experiments and company secrets) and promptly measured. 97mm in the tip...
post #21 of 22
Quote:
Originally Posted by checkracer
I quite shocked Debby Compagnoni´s serviceman back in 1997. We were on a double seat during a WC race and he was holding a pair of her "Test 3" GS skis. They were distinctly more shaped than the regular ones she was winning race after race with.
So I had always thought that the story was that her GS World Championship in 97 was the "first major event won on shaped skis" - is that actually not the case? Was she only trying shaped skis in training?
post #22 of 22
Quote:
Originally Posted by ssh
You've hit on it. There are no standards for much of anything in the ski industry. Everything in the industry seems to be based on trial-and-error with a little bit of magic thrown in.
I have often wondered whether it would ever be possible to standardise the "flex index" that some manufacturers list for their ski boots...
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