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  1. #1
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    Various Ski Dimensions and Weights


  2. #2
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    Hmm.. the Mo'ship is 2cm longer than Jak, but 16 grams lighter.

    Wierd.
    "Have fun, get a flyrod, and give the worm dunkers the finger when you start double hauling." ~Lumpy

  3. #3
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    2 Questions:

    Is the Length the contact length of the ski or overall length?

    Why is the Effective Length 88% of ski length?

  4. #4
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    The mothership weight data wasn't published anywhere, so it came from homerjay's home test.

    The ski length in this chart is the length that you would buy the ski in.

    The effective length being 88% I got from here. Think of it being the point from the fattest point on the tip to the fattest point on the tail.

    http://www.websurd.com/epic2004/PMSidecutRadCalc.xls

    The sidecut radius stuff I borrowed from here.
    http://www.math.utah.edu/~eyre/rsbfaq/physics.html


    This data is just a quick reference guide and by no means constitutes any legal binding.

  5. #5
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    I am assuming effective length is the amount of the ski you actually use to turn the ski. Like, you don't use the edge of the tip or tail. I think that is what that means, but I am not sure. Also, I would guess 88% is a mean amount because different ski designs may be different, like some may use 90% and some may use 86%.

    Edit: beat me to it with a FAR better answer.
    "Have fun, get a flyrod, and give the worm dunkers the finger when you start double hauling." ~Lumpy

  6. #6
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    it seems like the effective edge data should be modified slightly to accomodate a super twin tip (PR, jak) a moderate one (teledaddy) and none (tm-x)

    the effective edge on a tm-x should be way longer than that of a PR, not 1.7 cm, when the ski is like 2cm longer...

    i would propose: 88% for no (or mostly so) twin, 82% for minimal twin (1.5 x 12%), and 76% for mega twins (2 x 12%)...
    Last edited by marshalolson; 12-07-2004 at 09:46 PM.

  7. #7
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    Is sidecut really tip minus mid width?? What about the tails??
    Obviously, a ski that is 100-80-100 will turn tighter than 100-80-90, and yet by your formula they are the same sidecut.
    Perhaps (((tip plus tail) divided by two) minus mid width) would be better?

    This would give you an average width of the tips and tails, and then measure sidecut from there.

  8. #8
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    marshalolson, you are absolutely correct, that the effective edge of twin tips would be less than that of non twins. I just didn't bother adjusting for it.

    ScottG, to perform my sidecut radius I assume that the ski edge is an arc of a circle and determine the radius of that circle. A sector is found by cutting off the edge of a circle, or disc, with a straight cut that is perpendicular to the radius. The sidecut depth of the ski is the maximum height of the ski edge measured from the contact line. So in this case when you sort by Ascending side cut radius you get the following.

    1.BD Havoc
    2.Atomic TM-EX (2004)
    3.Atomic TM X
    4.Salomon Pocket Rockets
    5.Voile Carbon Surf
    6.Goode Carbon 95
    7.K2 Work Stinx
    8.Volkl T Rock Skis
    9.K2 Hippy Stinx
    10.Karhu Jak
    11.Line Mothership
    12.Atomic Tele Daddy

    Using the average calculation to measure the sidecut depth proposed by Scott and sorting by ascending sidecut radius you get the following:

    1.BD Havoc
    2.Salomon Pocket Rockets
    3.Atomic TM X
    4.Voile Carbon Surf
    5.Volkl T Rock Skis
    6.K2 Work Stinx
    7.K2 Hippy Stinx
    8.Goode Carbon 95
    9.Atomic TM-EX (2004)
    10.Karhu Jak
    11.Line Mothership
    12.Atomic Tele Daddy

    The Atomic TX-EX's position is the only ski that is significantly affected by changing the calculation of the sidecut depth. Which makes sense because there is such a large difference between it's tip (118) and it's tail (100)

  9. #9
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    My 192cm Line Motherships are actually 188 cm long.

    Anybody have any idea what a Tabla Rasa weighs in at?

  10. #10
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    Nice analysis.
    Goode skis seem a little tough to come by. The only online source I found with minimal searching is bentgatemountaineering. Their 82 waisted ski is even lighter and comes in a 176 or 186, which may be a better length for some dudes.
    Doesn't Goode play a significant role in the build of DB skis? Are Goodes of the same quality level (for a non-custom ski)? Opinions?
    another Handsome Boy graduate

  11. #11
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    goode plays a very significant role in the manufacturing of DB skis (carbon fiber expertise), but the designs are all DB.

  12. #12
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    Quote Originally Posted by bad_roo
    Anybody have any idea what a Tabla Rasa weighs in at?
    According to some obscure website called dbskis.com the 186cm versions
    weight 7.4lbs / 3.4kg pair.

    The floggings will continue until morale improves.

  13. #13
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    Just out of curiosity, where did you get the ski weights? That is the lowest weight I have seen for the work stinx.

    On K2's website they are 3400, but for a 174cm length.
    He who has the most fun wins!

  14. #14
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    The 3400g is for the weight of the 174 work stinx, from the bentgate site. I put the 181 in the ski length column because that's the length that I'm personally interested in. Apologies for any confusion this may have caused.

  15. #15
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    These sidecut radius calculations are *badly* in error

    Ireallyliketoski, there are at least three ways you can see this for yourself.

    The first way is simply to compare the numbers you obtained to those published by the mfgrs. For example, the dimensions of your very first entry are almost identical to a Volkl Explosiv. No Explosiv has a sidecut radius of 9 point something meters ... they are all at least in the mid-20's, with the longer lengths being in the low 30 meter range.

    The second way is by common sense: Have you heard of ANY fat ski with sidecut radii in the 8.5 to 12 meter range produced by your calculation.

    The third way would have been for you to simply spot check a few of your values using my sidecut radius calculator, ie, the exact one that you got the 88% from: http://www.websurd.com/epic2004/PMSidecutRadCalc.xls

    For example, if you plug in the dimensions for your first entry into my spreadsheet, it calculates a sidecut radius of 23.9 meters, far from the value of 9.56 meters that you obtain.

    As someone else pointed out earlier in this thread, I notice that you sidecut depth also differs from the standard convention. I suspect that this is where your sidecut radius error arises.

    Good luck in finding the problem.

    Tom / PM

    PS - I just noticed that the column widths in my calculator are too narrow, and some of the numbers display as #### instead of displaying the value. I'll fix it shortly.

    PS#2 - You may now consider yourself officially Techno - JONNNNGGGGG'ed

  16. #16
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    pman to the rescue!

    good to see you round these parts again.

  17. #17
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    My bad, I got the formulae for the sidecut dimensions and sidecut radius from this site. http://www.math.utah.edu/~eyre/rsbfaq/physics.html. I didn't really check my work, and I apologize for that, it's probably because I don't generally look at the sidecut radius when buying skis, it was just something I saw on that site and plugged and chugged. Fortunately for everyone there are people like you PhysicsMan looking out for the common good. Thanks dude.

  18. #18
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    > ...good to see you round these parts again...

    Thanks for the kind words. It's been crazy busy for the past month or two. I've been supervising 20 senior research projects this semester so I've hardly even had time to hang out where I usually do (ie, on Epic). I try to check what's going on over her about once a week and usually find info that hasn't yet, and may never make it to Epic. For example, the new Volkl fat lineup. Sweet sounding skis.

    The really funny part is how I found out about this thread - An older Epic guy that I know saw it, knew something was wrong with the numbers, but didn't want to let it be know that he lurks here, so didn't say anything himself and instead emailed me about it. I told him that there were lots of older guys over here as well (...they just act young...), but he still wants to remain in the shadows.

    Ireallyliketoski ... no problem, man. At least you "showed your work and cited your sources", so we could figure out where the problem crept in. BTW, thanks for finding all the weight data. I wish weight and flex numbers were consistently published by all mfgrs.

    Seasons greetings to 'yall.

    Tom / PM

  19. #19
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    Quote Originally Posted by marshalolson
    it seems like the effective edge data should be modified slightly to accomodate a super twin tip (PR, jak) a moderate one (teledaddy) and none (tm-x)

    the effective edge on a tm-x should be way longer than that of a PR, not 1.7 cm, when the ski is like 2cm longer...

    i would propose: 88% for no (or mostly so) twin, 82% for minimal twin (1.5 x 12%), and 76% for mega twins (2 x 12%)...
    The Jak has a half-tail twin-tip, whereas the Mothership has a tall/high tail-tip. Karhu and Line share a mid-fat/fat ski factory, the Mothership has two titanium plates and the Jak has one (regular, not Jak BC). Full twin-tips running surface is a lot shorter than skis with no twin-tip. The Mothership is a pretty light ski, with decent rebound, and decent performance on Western hardpack. I'm going to remount my bindings 1.5-2.5cm backwards - because I get too forward on the steeps and in deep heavy powder. I wish it didn't have as much of a sidecut and Line should make its premier fat ski like the Jak (smaller tail twin-tip). I should have gone longer on the Mothership (192cm). The Jak and the Mothership are basically the same length (190cm vs. 192cm, etc.).

  20. #20
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    Physics Man,
    the formula you use for radius is locked in your spreadsheet.
    Also, what is your formula for area?

    Judging from your comments, and pulling out the Lind/Sanders book, it seems that "Sidecut" should be treated as the average sidecut, or

    Sidecut = ((tips + tails - (2 times waist)) divided by four

    As for the sidecut radius formula, by referencing page 205, you seem to be talking about the UNFLEXED sidecut of a ski that is not on edge. (e.g., this is not a true turn radius, but only the radius of the sidecut when viewed from above the ski).

    So your formula is:
    Sidecut Radius = (contact length squared)/(8 times sidecut)

    And then the approximate turn radius or contact radius when the ski is angled theta degrees from the snow and deflected into contact with the snow is:

    Contact radius = (sidecut radius)*(cosine theta)

    Using this last formula, I get the following results for a 29 meter radius ski:

    angle contact radius
    0 degrees 29.0 meters
    5 degrees 28.9 meters
    10 degrees 28.6 meters
    15 degrees 28.0 meters
    20 degrees 27.3 meters
    25 degrees 26.3 meters
    30 degrees 25.1 meters
    35 degrees 23.8 meters
    40 degrees 22.2 meters
    45 degrees 20.5 meters
    50 degrees 18.6 meters

    Well, I guess I answered most of my questions.
    the main thing that would add to accuracy would be to actually measure the effective length of a given ski rather than "guesstimating" it as a percentage of total length. This may not help for a ski you are drooling over, but if you can get your hands on one, you can get an accurate measure to compare turn radii.

  21. #21
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    Quote Originally Posted by ScottG
    Physics Man, the formula you use for radius is locked in your spreadsheet....
    Yeah, sorry, but I had to do it. Several years ago, I sent a similar unlocked spreadsheet with no hidden cells to a guy. The dufus mucked with the formulas and passed along the modified version to other people saying it came from me. His screw-up wasn't intentional, but it wasted a lot of my time explaining to people what had happened, why there were errors, etc.


    ...Also, what is your formula for area?...
    Just Simpson's rule ( http://mathworld.wolfram.com/SimpsonsRule.html ) based on the estimated effective length, trying to approximate the load-bearing area of the ski in soft snow.


    ...Judging from your comments, and pulling out the Lind/Sanders book, it seems that "Sidecut" should be treated as the average sidecut, or

    Sidecut = ((tips + tails - (2 times waist)) divided by four ...
    That's exactly right. That's pretty much *the* standard definition.


    ...As for the sidecut radius formula, by referencing page 205, you seem to be talking about the UNFLEXED sidecut of a ski that is not on edge. (e.g., this is not a true turn radius, but only the radius of the sidecut when viewed from above the ski)....
    Yup, that's why they call it "sidecut radius" or sometimes "geometric sidecut radius", not "turn radius" or "flexed contact radius" or something similar.


    So your formula is: Sidecut Radius = (contact length squared)/(8 times sidecut), And then the approximate turn radius or contact radius when the ski is angled theta degrees from the snow and deflected into contact with the snow is:

    Contact radius = (sidecut radius)*(cosine theta) ...
    Precisely.


    ...Using this last formula, I get the following results for a 29 meter radius ski:

    angle contact radius
    0 degrees 29.0 meters
    ...snip...
    Well, I guess I answered most of my questions....
    Yup, and in fine style.

    BTW, if you do a search over on Epic under my name and some search terms like { sidecut cosine }, you'll probably come up with a bunch of posts where I discuss this stuff at more length. OTOH, you already have it down pat, so there's not much point in doing so unless you are curious.


    ...the main thing that would add to accuracy would be to actually measure the effective length of a given ski rather than "guesstimating" it as a percentage of total length. This may not help for a ski you are drooling over, but if you can get your hands on one, you can get an accurate measure to compare turn radii.
    Exactly.

    If you can actually lay your hot little hands on the ski that you are interested in, there is another improvement to the calculation that you can make. The above formula does not rely on knowing the fore-aft position of the waist of the ski. If you do know it, you can use other formulae which allow you to either (a) make a better estimate of the average sidecut radius, or (b) calculate separate sidecut radii for the forebody and aft sections of the ski. The latter is an interesting exercise, but prone to major league error if you are off by even a little bit in any dimension.

    One last point, play around with the input numbers in the above formulae (or my spreadsheet), and you will see that even minor changes in the sidecut or assumed length can change the calculated sidecut radius by several tenths of a meter. This is why I usually round sidecut radii to a half-meter. It makes no sense to be more precise.

    Cheers,

    Tom / PM
    Last edited by PhysicsMan; 01-06-2005 at 11:51 PM.

  22. #22
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    This is awesome. I've been wondering where the reported sidecut/turn radius numbers come from, for a looooong time.

    When manufacturers say "turn radius", though, do they really mean "sidecut radius"? Or is there some standardized number for angle of inclination/deformation that is used in calculating an actual turn's radius based on the sidecut, etc.?
    Last edited by Deep Days; 07-08-2008 at 01:34 AM.
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  23. #23
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    they mean sidecut radius

    at least i do when i say a radius
    they also probably mean an average radius as the tip\tail sidecut radii are different.
    k2 hints to this fact with their "progressive sidecut" or whatever they call it.

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