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 won’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.
- 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.
- 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.