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Turn radius

post #1 of 27
Thread Starter 
Is it possible to carve a turn on a ski in less than that skis turn radius? For instance, if I'm on a ski with a 20m turn radius (just for aruments sake), will I have to skid the ski to turn in less than 20m? I just want to make sure I understand the terminology.
post #2 of 27
I don't know what anyone else will say, but my own response would be a resounding "yes".

I think the design turn radius for a given ski assumes a certain amount of force bending the ski to a certain arc. If you apply significantly more force than that, the ski will bend into a deeper arc with a shorter turn radius.

This all assumes that the skier has the skill and strength to put the ski in that position in the first place and then stay in balance through the turn, but I think the World Cup racers prove that it's possible every day.

Just my $.02.

Bob
post #3 of 27
Yes.

The "radius" number that's associated with skis isn't the "turn" radius, it's the radius of the sidecut. Tip it on its side and bend the ski, and you create a curve with a smaller radius. If you were carving perfect turns on a skating rink (if you could) the radius could be considerable smaller than the sidecut. (Of course, in the real world, there are various other things at play, like how much the snow deforms, how deeply the ski penetrates the surface of the snow, etc. etc.)
post #4 of 27
Thread Starter 
Well, there you go. I was wrong again. Thanks for the comments.
post #5 of 27
Colossus178,

Might want to do a search for posts by PhysicsMan ...I think about a year or so back (maybe more) he posted a formula that would calc. the 'possible' radius depending on edge angle.
post #6 of 27
Just to add to what has been said, I would suggest that it would not be possible to carve a turn as large as or larger than the stated raduis of the ski. The measured radius is with the ski flat on it's base, and because of that lack of edge contact/pressure, you can't carve that turn. As soon as you put the ski on edge and decamber it, the radius shrinks quite a bit. The more you decamber the ski and the higher you tip it on edge, the smaller the radius gets.

Just as a general hint as to actual turn radius, go out on a freshly groomed blue run and carve a turn from 90 degrees to the fall line on one side, to 90 degrees on the other side. You now have a turn that shows its complete diameter. If you have a ski with a 15M radius, that means that the diameter of that turn would have to be 30 meters or about 100 feet. A somewhat talented skier who knows how to carve could probably do it in about half that distance.
post #7 of 27
Quote:
Originally Posted by cgeib
...Might want to do a search for posts by PhysicsMan ...I think about a year or so back (maybe more) he posted a formula that would calc. the 'possible' radius depending on edge angle...
I'll save him the trouble since it is such an easy relationship. For a pure carved turn on a surface which is essentially unyielding:

Turn_Radius = (Sidecut_Radius) x (cosine of the edging angle)

So, from flat up to 10 or 20 degrees of edging, the turn radius doesn't decrease by much. However, crank it further up on edge, and (flex permitting), you can really tighten up the turn.

Also, as JohnH correctly pointed out, you can see from the above formula that you can never get a pure carved turn larger in R than the sidecut radius.

Finally, if you are interested in what happens on softer surfaces and the ski can flex more deeply, a search should turn up a couple of posts where I addressed this issue.

HTH,

Tom / PM
post #8 of 27
Quote:
Originally Posted by Bob Peters
I don't know what anyone else will say, but my own response would be a resounding "yes".

I think the design turn radius for a given ski assumes a certain amount of force bending the ski to a certain arc. If you apply significantly more force than that, the ski will bend into a deeper arc with a shorter turn radius.

This all assumes that the skier has the skill and strength to put the ski in that position in the first place and then stay in balance through the turn, but I think the World Cup racers prove that it's possible every day.

Just my $.02.

Bob
Bob, the trouble is that you are working with, as Physics Man says, an "unyielding surface" (hardpack) that restricts the bend of the ski. If you cut an arc on a piece of paper (the ski) you see that, keeping full contact along the arc, you get the most bend in the paper as you approach 90 degrees.
post #9 of 27
Quote:
you see that, keeping full contact along the arc, you get the most bend in the paper at 90 degrees (cosign of 90 = 1). For lesser angles the arc is less. The maximum arc you can produce (at 90 degrees) is the turn radius.
Si--you'd better stick to your day job. (Oh--wait, you ARE a scientist!)

The cosine (not "cosign") of 90 is zero. Yes, you do get the maximum bend (minimum radius) as the ski approaches 90 degrees edge angle. But intuitively, with your "paper ski," you can see that at 90 degrees, you cannot make full contact with the "ski's" edge. You could fold the ski right double--radius of arc=zero--and you STILL couldn't make full contact with its edge!

Furthermore, the higher the edge angle, the greater the proportion of the skier's force against the ski goes into bending it, with less pressure forcing it "down" into the snow. At 90 degrees, there will be little-to-no "downward" pressure on the snow. (The only reason there would be any at all is if the ski is tipped more than 90 degrees to the "line of action"--in other words, the skier is angulated.)

You are right, though, that the ski will bend into a tighter arc (on a hard surface), the higher the edge angle, as most skiers have experienced, and as Physicsman's formula shows. As the edge angle decreases, the radius will approach the ski's built-in sidecut radius, as others have described. The LONGEST radius a ski could theoretically carve (pure carve) is its sidecut radius.

Best regards,
Bob Barnes
post #10 of 27
That's what I get for multi-tasking and not reading what I typed which was jibberish. I'll just never be a Bob Barnes with my posts here on Epic.

I obviously needed to have used "minimum" turn radius, properly stated that the cosine of 90 = 0, spelled cosine correctly, talked about tightest arc, referred to approaching 90, .... And this from someone who has taught calculus. That is possibly the most mistakes I have made in such little space. Maybe there are times I have had a higher density of errors but I just can't remember! I guess senility has set in sooner than anticipated.

The only valid point and the one I intended to make was that there was a limit to how far any ski could bend and the minimum radius of a turn in a full carve.

I have edited the original post but your quote will guarantee that my mistakes will still be visible to all.
post #11 of 27
Don't feel bad, Si - Wear a hair shirt for a few days and you will be absolved of your sins and granted a papal (whoops, I mean PSIA) indulgence. If it makes you feel any better, I knew what you meant.

Tom / PM
post #12 of 27
So that's what you have to do to get one of those "indulgences"
post #13 of 27
Here is the basic formula used by F.I.S. to determine the sidecut radius of a ski. As was already pointed out this is not the turning radius which is less.


L2
___________

2( s + h- 2w)

L = Length from contact point to contact point in mm's squared.
S = Width at forward contact point in mm's.
H = Width at rear contact point in mm's.
W = Width at narrowest point in mm's.

On many GS skis the sidecut will continue to widen beyond the forward contact point. This is a way around the rules because they measure at the contact point.

There is some more to the formula but this is the gist of it. The easiest way to decrease a skis sidecut radius is to decrease factor L. This can be accomplished by getting a shorter length with the same S, H, and W dimensions, or by tipping it on it's side and allowing it to bend. This also decreases factor L and leads to carving of turns.
Hope the math keeps you busy until it is time to "shut up and ski!".
jl
post #14 of 27
Or, if you don't feel like pushing numbers around, you can use a little spreadsheet I developed to automate this calculation:

http://forums.epicski.com/showthread.php?t=2681

Read the directions in that thread, click on the link at the top of the first page, fill in your numbers, and presto, out comes a sidecut R in meters.

HTH,

Tom / PM

PS - BTW, neither me nor my spreadsheet are FIS approved.
post #15 of 27
Quote:
Originally Posted by SLATZ
So that's what you have to do to get one of those "indulgences"
Hair shirt, heh ... that's nothing. You should see what they asked HH to do. As penance, they had him perform gliding wedges until he couldn't stand up any more.



Tom / PM
post #16 of 27
Quote:
Originally Posted by PhysicsMan
Hair shirt, heh ... that's nothing. You should see what they asked HH to do. As penance, they had him perform gliding wedges until he couldn't stand up any more.



Tom / PM
braking not gliding
post #17 of 27
Hello BOOTech, Inc--Welcome to EpicSki!

Hey--that doesn't look like the formula you showed as a couple years ago at Fall Training...the one that took up a whole flip chart page.



Everyone--with BOOTech here, we have among us another one of the world's top experts in boot technology, fitting, and alignment. Listen to what he says!

Jim (I assume that's who this is), thanks for joining us! I hope you can find lots of time to keep us informed (and entertained). Watch out--it can be addictive!

Best regards,
Bob Barnes

PS--to anyone with a serious interest in equipment setup--check out the BOOTech web site. And make your appointment well in advance, if you plan to be in Aspen. He's a busy man!
post #18 of 27
As PhysicsMan notes:

Quote:
Originally Posted by PhysicsMan
Also, as JohnH correctly pointed out, you can see from the above formula that you can never get a pure carved turn larger in R than the sidecut radius.
Which is one of the reasons why super-G and downhill skis have a less radical sidecut, built for speed and more gradual (but close to pure carved) turns.

The other reason is that longer, straighter skis glide faster--I don't know exactly why length matters that way, but as long as I've got this assembled physics brain trust on-line, why is it that longer skis generally glide faster than shorter skis given a skier with the same weight?
post #19 of 27
Can we get BOOTech and GMOLfoot an a ring and sell tickets????
post #20 of 27
Bob Barnes et. al.,

Thank you for the warm welcome to epicski.com! I stumbled onto this site a couple of weeks ago. I was impressed not only with the quality of discourse but also how many industry luminaries were regulars here. Bob Barnes, Gregg Hoffmann, Jeff Bergeron, P.J. Dewey even Weems (what the hell is a "weems" anyhow?) and others whose nom de plumes I have yet to identify.

Those of you looking forward to a pissing match between myself and the other bootgods assembled here will most likely be dissappointed. : Most of us have known each other for sometime and for my part at least I have found we agree far more than we disagree.:

I look forward to participating where my time and expertise allow and thank you for welcoming BOOTech, Inc. to the epicski.com community.

jl
post #21 of 27
I'm excited to say I'm making the trek to Aspen in the morning to see Jim at Bootech.

I'll post a full review!
post #22 of 27
Back from Aspen!

First of all it was snowing around the tunnel and as I drove over Independence Pass. I would say it amounted to two or three inches on the ground near the tunnel.

It was thirty degrees at ten at the top of Independence Pass with ice on all the ponds.

I can't say how impressed I was with the entire experience at BOOTech. I have talked to and/or visited a great many folks who claim to be boot technicians. Suffice it to say there are folks out there who do not have a clue.

Jim has clues and answers!

A variety of things impressed me. First of all he was on time. I can name a few folks who can't seem to run anywhere near ontime. His personality was markedly different than others that I have encountered. Simply stated, some of the boot gurus around here can be a tad on the cocky side, somewhat akin to tempermental hairdressers. Jim is very self deprecating, exceedingly modest, and armed with that personna the experience is great.

I had a couple of key questions that I wanted to cover in order to satisfy myself that he knew what he was doing. We got into a neat discussion about the ankle being tri-planer (sp?) and he passed the query with flying colors. He knows his stuff.

I walked out with two pairs of boots that feel great. Obviously the final test will be how they perform on snow.

Anyone remotely close to Aspen ought to make the drive. Call first! Unlike one perpetually empty alignment center that I passed on the way, Jim has a steady stream of clients this time of year and a two week backlog in mid season.
post #23 of 27
FOr us non-scientist/physisists (sp) what is a Cosine?
post #24 of 27
Though I'm not a physicist or a scientist, I did take trigonometry at some point in my life.

Picture a right triangle, and look at one of the angles (other than the right angle). The cosine is a function that, given the size of that angle, tells you the ratio between the side adjacent to the angle and the hypotenuse of the right triangle.

(That's the quick 'n' easy definition. It's actually a bit more complicated, because there is such a thing as the cosine of an angle greater than 90 degrees).
post #25 of 27
Yes, cosine is "adjacent over hypotenuse." In practical terms, that means that (on hard snow) if you tip your ski 30 degrees to the surface, its sidecut will allow it to carve a turn 2/3 the radius of its sidecut. If you tip it 60 degrees to the snow, it will carve a turn 1/3 of its sidecut radius. All this is theoretical, assuming that the ski does not sink into the snow and bend to a tighter arc, that there is sufficient pressure to fully bend the ski, and that the skier manages that pressure accurately over the ski's "sweet spot"--not levering forward or aft.

A little change in edge angle goes a long way with today's skis. Theoretically, on ice, a modern slalom ski with a 12 meter radius sidecut will carve an 8 meter radius turn when tipped 30 degrees to the snow, and a 4 meter riadius turns at 60 degrees! Edge angle is critical on these skis, and it's easy to get too much. This, combined with the increased inclination (leaning your body into the turn for balance) due to heightened G-forces, is one of the reasons we typically see less angulation ("sideways" bending in the hips, knees, ankles, and spine) in today's racers than in the past.

Best regards,
Bob Barnes
post #26 of 27
Another way to look at it is that cosine is almost the opposite of sign.

Sorry, couldn't resist.
post #27 of 27
Jim, is it Boo Tech or Boot ech? Do you sell high tech holloween costumes, or smelly ski boots?
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