Radius Sidecuts and Turning Radii

[Papasteeze]

Bumped to the top since my mission is accomplished This information should be a lot more useful should the topic ever arise. Keep in mind that this only addresses True Radius not "shaped" skis..

 

Side Cut Radius:

 

There are web sites and formulas that can be used along with the measurements of the ski dimensions to figure it yourself, including the length of the ski (only the part that meets the snow- not the total length with the tip and tail that does not meet the snow). The issue is the ski length in the brochures is overall length so you have to physically measure the boards (which requires having access to the model and length you want). Some manufacturers will give a table or chart with the length and radius for every length like this brochure on Atomic web site

http://www.atomicsnow.com/english/overview_alpine.pdf

 

A good reference book on this (explains it well and has the calculation formula too if I remember) The NEW Skiing Mechanics by John Howe (there is a Skiing Mechanics too):

http://www.clawskis.com/new_skiing_mechanics.htm

 

And the web site calculator:

http://www.math.chalmers.se/~olahe/Fri/skiradius.html

 

 

Turning Radius:

 

When a ski is turned on edge and flexed until the entire edge touches a flat plane.

 

Theoretically, the side cut radius and the "pure" carve radius should be the same. It may differ a bit, dependent upon the taper in the ski.

 

The USSA uses the width at tip, mid-section, and tail to calculate the radius:

 

1) Call "W" the narrowest measurement, typically near the mid section.

2) Call "L1" the length from tip to where W was measured.

3) Call "L2" the length from the tail to where W was measured.

4) Call "S" the width of the tip at the contact point. Estimate the tip contact point at (.2*L1 ) back from the tip, measure the width of the tip there.

5) Call "W" the width of the tail at the contact point. Estimate the tail contact point at (.1*L2) forward of the tail, measure the width of the tail there.

6) Call L the contact length estimated as (.8*L1) + (.9*L2)

 

Then the turning radius can be calculated as:

 

R = (L^2)/(2000*(S + H - 2*W))

 

So the carve radius is a static number which is based on the geometry of the ski. (Again, simplifying assumptions are made.) In a pure carve, the ski would follow the arc inscribed in the side cut.

 

A great way to learn this is to make a paper ski using a compass. Take an 8.5x11 sheet of paper and fold it in half vertically. Now, measure a distance down from the top, say 2", and put a dot on the crease. Measure the same distance up from the bottom and put a dot on the crease.

 

Take a compass and pick a radius, say 1.5". Use the compass to draw two arcs in the center of the page. (Make sure you pick a radius that is small enough that the two lines don't touch or cross.) Now, draw in your tip and tails by hand. (Make sure that you don't make the ski wider with any lines you draw free hand.) The end result will look something like a fish. Cut out your paper ski.

 

Homework:

 

--Tip your ski a little and make the edge touch. Put a another sheet of paper on the table. Bend the ski on a table top so the whole edge touches the sheet of paper. Have someone trace where the ski meets the paper. What is the radius? How does that compare to the compass setting?

--What do you need to do to your ski to make it turn a tighter radius?

-- Measure your ski using the USSA procedure above and calculate the turning radius. How does that compare with what is marked on the ski?

 

Extra for experts (this might be tough for our younger racers)

--Tape newspaper together to make a sheet of paper big enough to fit under your real skis. Work with a parent to trace your real skis and cut out the resulting shape. Play with that paper ski by tipping and bending it.

Does the side cut form a perfect circle? If not, what shape is it? If your ski doesn't have a circular side cut, why would the mfg do that?

[adrian]

thats all rather intimidating. i rmember i had a program for all this on my computer, but from a quick google search, this might help:

 

http://www.natew.com/pages/software/snow/html.Reverse

 

the sidecut radius and the pure carve radius and the actual radius all depend on how far you push the chamber. so many things are involved, it can make your head spin.

 

lest we forget, shpaed skis are a borrowed development from the you know whos.

Edited November 20, 2005 by skitzo

[skimom]

Looks like Thanksgiving homework.

[Papasteeze]

It was good summer homework thanks for bringing it back up. That whole radius sidecut thing was an very enlightening thing for me in relation to EBay and educating a somewhat clueless 10k + seller on careless misrepresentations. HA!!

[adrian]

do you really need to calculate the sidecut radius yourself? im surprised they dont tell you this in the product specs. i dont recall any snowboard manufactures that neglect this.

[Papasteeze]

do you really need to calculate the sidecut radius yourself? im surprised they dont tell you this in the product specs. i dont recall any snowboard manufactures that neglect this.

 

UGGGHHhh - the wound hasn't healed yet. Kinda a sore subject for me. But I guess I have an obligation. I was surprised to learn some things about marketing and manufacturing and the representations of sidecuts this summer. They use the "greater than" and "lesser than sign" frequently on product ratings and then there are those who don't do that. I noticed some discrepancies and ...... Ughhhhh I don't have much nice to say.............

 

Want to be sure of the (true) radius sidecut on your skis or board? THEN you Better do the calculating yourself, is all I can say. When a particular Ski Line is represented as Radius = 14 meter at 140 cm don't assume that applies to the 140 cm ski you just bought. That MAY only apply to the widest points of the tip and tail and the ski might acutally be 155 cm tip to tail.. Or it might only apply to that particular ski, the radius will change (obviously) if the tips and tail widths are the same at different lengths hence the < or> sign.

 

Measure your skis at the points specified above, use the calculator and then compare the number to what is prinited on your ski or literature. You might be surprised!!

 

And the web site calculator:

http://www.math.chalmers.se/~olahe/Fri/skiradius.html

 

 

Turning Radius is can be something entirely different than a sidecut radius: When a ski is turned on edge and flexed until the entire edge touches a flat plane.

 

Theoretically, the side cut radius and the "pure" carve radius should be the same. It may differ a bit, dependent upon the taper in the ski.

 

The USSA uses the width at tip, mid-section, and tail to calculate the radius:

 

1) Call "W" the narrowest measurement, typically near the mid section.

2) Call "L1" the length from tip to where W was measured.

3) Call "L2" the length from the tail to where W was measured.

4) Call "S" the width of the tip at the contact point. Estimate the tip contact point at (.2*L1 ) back from the tip, measure the width of the tip there.

5) Call "W" the width of the tail at the contact point. Estimate the tail contact point at (.1*L2) forward of the tail, measure the width of the tail there.

6) Call L the contact length estimated as (.8*L1) + (.9*L2)

 

Then the turning radius can be calculated as:

 

R = (L^2)/(2000*(S + H - 2*W))

Edited November 23, 2005 by Papasteeze

[adrian]

thats pretty ridiculous. the greater and lesser than symbols dont refer to a quadratic sidecut do they?

 

thats very, very odd. they cad-up the designs for every model. why keep the consumer in the dark?

[Timeless]

thats pretty ridiculous. the greater and lesser than symbols dont refer to a quadratic sidecut do they?

 

thats very, very odd. they cad-up the designs for every model. why keep the consumer in the dark?

 

Do ski's use quadratic, progressive, degressive side-cuts? this would probably cause papa's head to explode....

[adrian]

they do indeed. he ought to put a helmet on before he tries to wrap his mind around that.