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# PhysicsMan's ski sidecut radius calculator

PhysicsMan's Sidecut Radius Calculator

Click on the above link to estimate the sidecut, average width, load bearing area (footprint), and geometric sidecut radius of a ski given just the tip, waist and tail dimensions:

The formula I use is the one derived on p.205 of Lind & Sanders, The Physics of Skiing :

R_sidecut = ( Effective_edge_length^2 ) / (8 * sidecut) .

Click on the above link, and if you have a version of Excel 2000 or later, you should see the spreadsheet come up in its own window. If someone else is already using it at the same time you are trying to access it, you may be asked, "Do you want to open a read-only copy?". Always answer, "Yes". Type in your length, tip, waist and tail dimensions (ie, overwrite the existing numbers), hit "Enter", and you should see the results immediately.

Unfortunately, if you don't have a copy of Excel on your own machine, you wonâ€™t be able to use my program. However, if you do have Excel and have problems with my spreadsheet, pls. let me know.

When you attempt to close the window after using the spreadsheet, you will be asked if you want to save your changes. Unless you want to save a copy to your own computer, you should answer, "No". I have set up the public version so that it can't be accidentally modified.

I should point out that the only variation from the Lind & Sanders formula is that I had to use an "approximate conversion factor" to estimate the effective edge length of the ski from its published chord length, given that the former number is never available.

I calculate the effective edge length using a simple multiplicative conversion factor (usually around 0.88) so that the results of the sidecut formula agree on average with the sidecut radius for a variety of skis where the manufacturer actually published BOTH the sidecut radius and the three width dimensions. Because this conversion factor (ie, the ratio of published chord length to effective edge length) isn't the same for all skis, you will likely notice small differences (a few percent) between the results of my program and numbers printed on the ski, the result of other calculator programs, etc. If you think my numbers are consistently off by more than a few percent over dozens of different ski models for which you have accurate area or radius data, pls. let me know. To be sure that the widths you use are the correct ones for the particular length ski you have, probably the best approach is to measure them yourself.

Recently, I've had a number of requests for copies of this program. I will likely be making changes to it over time, and don't really want a bunch of different versions of it floating around, perhaps some modified by other people but still being attributed to me. So, instead of emailing copies to anyone who asks, I decided to have one "official" copy always available for public use. If someone really wants to get into the guts of the program, save a copy of it on your local hard drive and then use the password, "sidecut" to unlock your copy to view the hidden cells, modify it, etc.

Enjoy,

Tom / PM

PS - FWIW, I looked at another sidecut radius calculator, http://www.math.chalmers.se/~olahe/Fri/skiradius.html , that was mentioned in another thread. In order to be used, his approach requires knowledge of the fore-aft location of the waist. The Lind formula that I use makes a couple of very minor approximations to avoid requiring this rarely available extra number. When the fore-aft location actually is known, it's likely they will get a slightly more accurate estimate of R than we do, but the differences between the two numbers are likely to be small compared to other sources of inaccuracy. The comparison between the two programs is a bit muddied because when you are entering data, don't forget that their calculator appears to require the actual effective edge length as input, whereas you input the published (ie, chord) length into my spreadsheet.

PS#2 (4/26/2004) - I just noticed that if you type in a tail dimension that is over 100 mm, that column is too narrow to display it properly, so you see something like "#####". It's only a display problem - the calculator still works fine. I'll fix it next time I update the file.

[ April 26, 2004, 08:19 AM: Message edited by: PhysicsMan ]
The sidecut radius calculator is now up and running. I was having some problems with the site I was previously using to host the file. DC came to the rescue and offered to host it.

THANKS LOADS, DAVE !!!!!

Tom / PM

[ April 22, 2004, 04:21 AM: Message edited by: PhysicsMan ]
: Thanks.
Physics Man it's great to know that you have applied your tremendous science knowledge to help the rest of us ski obsessed dummies maximise our fun on the snow.
Many kudos to you for your time and contributions.I still may get to ski in a couple of weeks out in Utah. In the interim if you can assist me in simplifying the mechanical complexities of the golf swing I might be able to endure the summer.
You're welcome.

dchan
I can't use the spreadsheet. A window sez its protected, and selecting un-protect doesn't work. I tried from work and home.
Arby - the only place in the spreadsheet you are allowed to make changes are in the first four cells - length, tip, mid and tail. If you try to make changes anywhere else, you will get the msg you saw. Could this be the problem?

Tom / PM
OK, got it now. I was using the cells under the posted numbers. Thanks.
Roundturns - I just saw your post. I'm just another ski obsessed geek like everyone else on Epic. I just happen to to push numbers around for a living.

Thanks for the kudos, tho. I hope more people get to use the calculator now that it's publicly available.

Tom / PM

PS - I know absolutely NOTHING about golf.
PM,

In the public version of the sheet,

Can you broaden column 'E' so that 3 digit values are visible as properly entered? I see that you've noticed the problem already- just a reminder.

Spatula and Zag numbers are absurd, as you might expect.

I am somewhat interested in your computation of effective float. Is this with or without the 12% effective edge reduction? (I am really asking about those waistline approximations.)

Would it then be reasonable to have a TT flag (Y/N) and so do the 12% reduction symmetrically or off the front only, respectively?
Is 69.8 a little long for a SG ski?
Physicsman, is there a way to create a second calculation line for twin-tip skis?? Seems that they are more common, even for folks that don't ski switch ever.

Also, as written, you have the effective contact length as 88% of total ski length. Does it make sense to have this expressed as a percentage???
My point being that I am not sure that ski tips or tails change length as skis get shorter.
If the average ski tip and tail is 22cm, why not just subtract 22 cm, rather than multiplying by 0.88?

For example, by your method, a 195cm ski has 172cm running length (implying a 23cm tip and tail), but a 150cm ski has a 132cm running length (implying a 18cm tip and tail).

Perhaps your calculator might be more useful by saying:
Old school ski tip and tail = 23cm
Low profile (non-twin) = 20cm
Slight twin tip = 28cm
Full twin tip = 35cm
(with the above values being derived empirically for typical modern skis)

(note - I made up the above values - they are just guesstimates)

-
Here's a more accurate and detailed caclulator that requires that you measure the distance from tip wide point to tail widepoint and waist midpoint to tail widepoint. It should work for all skis, including progressive sidecuts and twin tips.
Quote:
 Originally Posted by gapic skier Physicsman, is there a way to create a second calculation line for twin-tip skis?? Seems that they are more common, even for folks that don't ski switch ever...Also, as written, you have the effective contact length as 88% of total ski length. Does it make sense to have this expressed as a percentage???...
All good points, but the problem is that including all the nice bell and whistle options would give a surprising number of people problems in using the spreadsheet and I don't get paid for customer support - .

By far, the easiest thing for you to do is adjust the overall length to obtain whatever running length you need for the particular ski under consideration, no matter whether it is a TT, a swallowtail or a ski with a conventional tail.

Thus, if you want the sidecut radius for some ski with a running length of 150 cm, simply enter an overall length of 150 / 0.88 into the spreadsheet.

HTH,

Tom
Physicsman,
After a few years away from epic it's great to see your skiing disease has progressed admirably.

cheers...
Quote:
 Originally Posted by Tog Physicsman, After a few years away from epic it's great to see your skiing disease has progressed admirably. cheers...

Physicman's sidecut radius calculator is a neat tool to to have on this site.

On another matter; Tog, it's great that you're back here posting again.
Physicsman
any way to get some sort of edge/bevel calculator to find the optimal edge dimensions based on ski dimensions/weight/height etc. This could calculate to a certain score, and this score could rate on a scale of edge dimension/type of skiier? Make sense? Want to give it a shot- I know you can figure it out.

### A 2-Year Old Pun Everyoe Missed:

Quote:
 Originally Posted by PhysicsMan The sidecut radius calculator is now up and running. I was having some problems with the site I was previously using to host the file. DC came to the rescue and offered to host it. THANKS LOADS, DAVE !!!!! Tom / PM[ April 22, 2004, 04:21 AM: Message edited by: PhysicsMan ]
==================================================

I'm sure Tom meant this pun, when in thanking the PhysicsMan "loads," he was referring indirectly to the weights and centrifigal forces applied in his ski physics calculator....("Loading" the skis, loading the flex, coiling the ski for explosive release of energy to leap into the next turn, etc.)

Good Spirit, Guys & Gals!
Brock, MtnGuide - Sorry I didn't see your posts in this thread till just now.

Brock - Estimating the optimal edge/base bevel involves a lot of personal preferences and details of the mechanical response of the ski that hardly are available to even the folks in the factory, e.g., torsional stiffness at all points along the length of the ski, micromovement of the edge relative to the core when up on edge and under load, etc. Sorry, but trying to figure this out on a simple spreadsheet is way above my pay grade.

MtnGuide - (I'm not telling.)

Tom / PM
I can't get either of the links to radius calculators to work. I saved an excel file from the link to PhysicsMan's Sidecut Radius Calculator that I can't get to open. I also could not open it in a browser. I have the latest version of excel. The http://www.math.chalmers.se/~olahe/Fri/skiradius.html one appears to be down. Since this is a sticked thread, can anyone offer any other resources or advice?
PM's one works ok for me with Excel 2K/Mozilla.
Great program, only problem is that it doesn't seem to like a tail above 99.9mm -for example the Crossmax V12 in 178 cm is 117-69-101
Hi Dr. Will. - I know about this problem, and Comprex also pointed it out in post #10 in this thread (2003). The problem is not serious. The column width is just a tad too narrow to accept more characters. Just type in the number you want for that dimension. Even though it won't display properly, the spreadsheet will still run and should give you the correct sidecut and area answers.

Unfortunately, the spreadsheet that you see is not on a server that I have access to. I've asked the mods to correct this column width issue years ago, but that's when DC was the only mod, and he was swamped. I think that people have just been ignoring this quirk. However, now that this bug has been reported again, maybe one of the current mod squad will widen that column and resave the file to their server.

As a temporary fix, just use the password given in post #1, and fix the width on your own copy of the spreadsheet.

Cheers,

Tom / PM
PhysicsMan
I just used the side cut radius calculator. It works great. Thanks for your effort in getting it going.
The calculator works great but it seems to underestimate radius on GS skis, on which a snub-nose profile seems to be the norm, where the listed tip width is beyond the contact point.

Does your calculator use the tip width at the contact point or the largest tip width, even beyond the contact point?

Also, which tip width does FIS use when calculating radius?
D(C), you can plug any dimension into the calculator -- whichever is relevant to the turn. Try bending and edging the ski on a workbench surface and see which point engages the surface. On several skis I have studied, the max-width point stays 7-10mm off the surface when the ski edge is fully engaged. Thus, I do not consider that "width" when computing the turning radius -- I use the width at the contact point instead. It depends on the ski -- other skis may be different.
If you do not have Excel it will also work with Open Office available free for Windows Linux and Mac http://www.openoffice.org/
Thanks for posting this info Tom. As for folks that want revisions, you don't even need the password, you can simply copy and paste the cells of the worksheet A1 thru L3 in to a blank excel workbook of your own and edit the formulas in row 2 as you like (or widen column E). The online version is good just the way it is for most general references, and bulletproof!

Thanks again Tom
Here is another calculator that fine tunes a bit more... http://members.fortunecity.com/hhitme/skiradius.html

### sidecut...

It seems that the postings are not so recent but I don't understand the need for any estimate at all. Why not actually measure the length of the functional edge with a string along the edge from tip to tail ends but the wide contact point to the floor. ALSO, I would not even begin to use the numbers posted on the skis. There is no standard there. I've seen skis from the same company with identical footprints that have different numbers posted on them from one season to the next.

One can hold a ski that says 114 mm for the tip nest to a ski that says 118 for the tip and see that they might be the same...

I would be interested in the surface area of the ski for a measure of floatation. It seems that feature might be possible. While the turn is, as everyone knows, determined by the relative width distances from tip to center and tail. Width is something else.
This is a cool deal and dynamic thinking.

EJ
Quote:
 Originally Posted by EJL ... but I don't understand the need for any estimate at all. Why not actually measure the length of the functional edge with a string along the edge from tip to tail ends but the wide contact point to the floor...
There is a need for estimate. I don't have in my hands skis from catalog in all lengths to measure them. There is a radius given only for one length. If I want radius for another length, I have to calculate it.
I'm thinking about radius only when making decision what to buy. I don't care about radius of skis I already have.

Take a look on this calculator: http://www.geocities.com/s_nezhnyj/S...alculator.html (double click on numbers to type the value)
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