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What does turn radius actually mean

post #1 of 27
Thread Starter 
Is that the optimal place where you can turn the ski? Because I know you can turn before the turn radius of the ski. What makes a turn radius a turn radius?

Can someone expound on this mystery?
post #2 of 27
Yes, perfect efficient turn whereby the full edge is in contact with the snow with quasi equal pressure distribution and no scrubbing.
post #3 of 27
Typically people mean the sidecut radius when they say turn radius. The sidecut radius is the radius of a circle that would fit most closely to the side of the ski. The actual circle scribed in the snow will have a radius close to the sidecut radius times cosine of the tipping angle.

In deeper snow the flex of the ski determines what shape of turn it bends into. A turn radius based on flex can be different from one based on side cut.

A good ski has matching flex and radius for the speed the ski is designed to be used at.
post #4 of 27
Thread Starter 
What happens if you make a turn at 30m on a 23m turn radius ski?
post #5 of 27
Turn diameter would be more accurate. I guess radius can mean arc but the radius is the arc from one point of the diameter to the opposite side.
'It doesn't take into consideration the loaded radius but is calculated by the side cut differences.
Do RR track turns and you will get feel for the turn radius of different skis. They are on edge and with out any added rotation they will turn a predicted course. It would vary depending on the weight of the skier .
Flexing(decambering ) will shorten the radius.
post #6 of 27
Quote:
Originally Posted by Bohemian View Post
What happens if you make a turn at 30m on a 23m turn radius ski?
You are gonna gain some speed.
post #7 of 27
It won't be a pure carve.
post #8 of 27
Thread Starter 
Would the turn be as smooth? Is the point to hit the turn radius every time on a turn to have the cleanest, fastest turn?
post #9 of 27
Quote:
Originally Posted by Bohemian View Post
Would the turn be as smooth? Is the point to hit the turn radius every time on a turn to have the cleanest, fastest turn?
It would seem to me that is so NOT THE POINT. The point is to learn how to make the ski make the turn you want to make. Some ski's will make doing some things easier and other things will make doing different things easier--if one thing (say long fast turns on groomers) is what you like to do, you might as well get a ski that's good at that, but that doesn't mean you shouldn't point it into the trees if the snow looks sweet and you think you can manage to do so without hitting one with your head.
post #10 of 27
Quote:
Originally Posted by Bohemian View Post
Would the turn be as smooth? Is the point to hit the turn radius every time on a turn to have the cleanest, fastest turn?
No.

In a broad way, you want a longer sidecut radius when you intend on making longer radius turns, and vice versa. When you make a turn of long radius on a ski with a small sidecut radius the ski will not be as stable as a longer radius ski. The edge at the tip and tail will be sliding across the snow, and a combination of that with the dynamics of the ski and surface interaction will cause all sorts of undesirable things like "chatter" and rapid rolling torques you feel being fed through your ankles. The ski wont feel like it has grip on hard/rough snow, and trying to tip it further up on edge will magnify these behaviors until you get your CM far enough inside and the ski rolled up enough to engage cleanly, which might happen violently.

As skis get longer and the sidecut radii get longer, its less of an issue to make turns that are "too long" though understanding all the factors that influence that is beyond me.

The flip side is that at very high edge angles the turn isn't purely carved either. The tip and tail are deflected into a tighter arc on the mean than what you are actually skiing, and they'll flap about like all hell.

In reality, very little high performance skiing is really purely carved. (flamesuit on)
post #11 of 27
Quote:
Originally Posted by Bohemian View Post
Would the turn be as smooth? Is the point to hit the turn radius every time on a turn to have the cleanest, fastest turn?
If you make a turn larger than the turn radius it cannot be a pure carve.

If the turn radius is MUCH larger than the ski's radius, the ski will "hunt" as it will try to turn at it's "natural" turn radius. Note that this hunting can be extreme when you try to ride a ski with a deep sidecut (Short radius sidecut).

As Ghost says, if SR is sidecut radius and TR = turn radius, then:

TR = SR * Cos(edge angle).

The "natural" turn radius will depend on the angle of the ski.

As far as "the point" goes, that's a matter of intent. If you want to carve turns of a certain size at a certain speed, you'd do well to match the "natural" turn radius of the ski to the radius of the turns you want to make. Then choose stiffness to handle the forces you intend to apply.

If your eyes have not yet glazed over, then let's see if I can really put you to sleep, and see what affects the force on the ski.

Remember, force F=ma, (mass x acceleration) and a=(v**2)/r. Where v = velocity and r the radius of the turn.

So, at the same speed, you will have to deal with lower forces on longer radius turns than short.

Force goes up as the square of the speed. So, if you prefer slow medium size turns, you would likely prefer doing that on a softer ski than a really stiff WC race machine.

If you're heavy, you will likely prefer a stiffer ski too, although weight alone is not enough to launch you onto WC race stock -- you still need speed and radius to generate enough forces to really bend the WC ski.

It's a good idea to choose the right tool for the skier and their intent.
post #12 of 27
Quote:
Originally Posted by Garrett View Post

In reality, very little high performance skiing is really purely carved. (flamesuit on)
Agreed.
Consider the following:
The sidecut radius times the cosine of the tipping angle (theta), will be the radius of a carved turn on boiler-plate hardpack.

At each tipping angle, the acceleration toward the centre of the turn divided by gravitational acceleration (how many gs in the turn) in order to be balanced with force pushing directly into the base of the ski and not dislodge it from the platform can be obtained from geometry, a/g = 1 / (tangent (90-theta)

Using the formula for centripetal acceleration the a=v^2/R, we get the maximum speed you can carve a given turn for any sidecut (You can carve slower with angulation).

I made up a little spreadsheet to look at varius sidecut radii for a complete range of tipping angles. You can carve up to about 35 mph with 26 m skis. If you want to carve faster than that your cm has to turn less than your skis. 13 m skis about 25 mph. 70 m skis 58 mph.
post #13 of 27
Hi Bohemian--

You've gotten some good answers to your question. I'll try to explain how sidecut works, and how it relates to turn size, with a couple illustrations.

As Ghost suggests, when people speak of "the turn radius of a ski," they are usually referring to the radius of the sidecut of the ski--which may have little to do with the actual radius of your turns. If you extended the round (ish) arc of the ski's edge into a full circle, sidecut radius would be the distance from the center of that circle to its edge--the red line in this diagram:


It's common to assume that this measurement describes the size of the arc the ski "wants" to carve, but that is not at all correct. Many factors combine to determine the radius of the turn a ski carves, including softness of the snow, ski flex, edge angle on the snow, and amount and distribution of pressure on the ski--as well as sidecut radius.

What makes a ski carve a turn is not its sidecut, but the arc the ski itself bends into when you apply pressure to it. In soft snow--powder, crud, or very soft groomed conditions--sidecut actually has little to do with the ski's carving. If you place a thin wooden board with perfectly straight edges on soft snow and stand in the middle of it, it will bend into an arc.

Sidecut comes into play on firm conditions--the firmer the better. If you place a ski on a hard, smooth table top and tip it up on edge, it will only touch the table at its tip and tail, because of sidecut. If you then press on the middle of it, the ski will bend into an arc until its whole edge contacts the table, as in this animation:



The deeper the sidecut (in other words, the smaller the sidecut radius), the more the ski will bend before its edge hits the surface (assuming you press hard enough on it). In that way, a ski with a shorter sidecut radius "wants" to carve a shorter turn. But you can also cause any ski to carve a shorter turn by tipping it to a higher edge angle.

So sidecut radius is only partially related to a ski's "carving radius." The pure-carved arc a ski carves will always be shorter than its sidecut radius, roughly according to the formula Ghost described: turn radius = sidecut radius X cosine of the edge angle to the snow (thanks to Tom/PM "Physicsman" for the formula). That means that a ski tipped 60 degrees to the snow surface and pressured sufficiently on its "sweet spot" will bend into an arc one half its sidecut radius--which means a "12 meter" slalom ski tipped 60 degrees will "want" to carve a 6 meter radius turn.

Indeed, sidecut radius describes the longest turn a ski could theoretically possibly carve cleanly. And that's only theoretical, because the ski would need to be perfectly flat on the snow--so it could not actually carve at all! There is no theoretical limit on the other end--even a ski with a very long sidecut radius could carve a very short turn if you tip it far enough and apply enough pressure.

In practice, sidecut radius is only one of many factors that determine the shape of the turns you make--even when you are making "pure-carved" turns on hard snow. In soft snow, powder, and bumps, sidecut radius plays a very secondary role at best. And I agree with Garrett that "pure carved" turns are neither as common nor as important as many people think they are in many real skinig situations.

Best regards,
Bob Barnes
post #14 of 27
Quote:
Originally Posted by Ghost View Post
I made up a little spreadsheet to look at varius sidecut radii for a complete range of tipping angles. You can carve up to about 35 mph with 26 m skis. If you want to carve faster than that your cm has to turn less than your skis. 13 m skis about 25 mph. 70 m skis 58 mph.
Very interesting. This squares well with my intuitive feeling of the speed beyond which skis of those radii are not "rock solid" anymore.

This angle required for engagement without lateral motion across the snow, is there a technical term that has been agreed upon for it yet?
post #15 of 27
If I recall correctly, Bob Barnes posted some good graphics referring to it as a "critical" angle, but I don't think there is general agreement. Basically if the resultant force vector of you pushing on your skis pushes them up out of the groove, your tipping angle and the critical angel have passed each other and your about to discover why over-banking is not recommended.
post #16 of 27
I recall seeing those graphics from Bob, I've found them now that you've given me hints for the search terms. I think its quite reasonable to call that a critical angle, or a critical angle of edge engagement.

http://forums.epicski.com/showpost.p...&postcount=104
post #17 of 27
Hi BigE,

This explanation looks very sound on hard pack. What about soft snow? It looks more complex to me. Care to explore further?

Quote:
Originally Posted by BigE View Post
If you make a turn larger than the turn radius it cannot be a pure carve.

If the turn radius is MUCH larger than the ski's radius, the ski will "hunt" as it will try to turn at it's "natural" turn radius. Note that this hunting can be extreme when you try to ride a ski with a deep sidecut (Short radius sidecut).

As Ghost says, if SR is sidecut radius and TR = turn radius, then:

TR = SR * Cos(edge angle).

The "natural" turn radius will depend on the angle of the ski.

As far as "the point" goes, that's a matter of intent. If you want to carve turns of a certain size at a certain speed, you'd do well to match the "natural" turn radius of the ski to the radius of the turns you want to make. Then choose stiffness to handle the forces you intend to apply.

If your eyes have not yet glazed over, then let's see if I can really put you to sleep, and see what affects the force on the ski.

Remember, force F=ma, (mass x acceleration) and a=(v**2)/r. Where v = velocity and r the radius of the turn.

So, at the same speed, you will have to deal with lower forces on longer radius turns than short.

Force goes up as the square of the speed. So, if you prefer slow medium size turns, you would likely prefer doing that on a softer ski than a really stiff WC race machine.

If you're heavy, you will likely prefer a stiffer ski too, although weight alone is not enough to launch you onto WC race stock -- you still need speed and radius to generate enough forces to really bend the WC ski.

It's a good idea to choose the right tool for the skier and their intent.
post #18 of 27
Quote:
Originally Posted by Garrett View Post
I recall seeing those graphics from Bob, I've found them now that you've given me hints for the search terms. I think its quite reasonable to call that a critical angle, or a critical angle of edge engagement.

http://forums.epicski.com/showpost.p...&postcount=104
That's the one I was thinking of. The same thing happens when the ski stays at the same angle but the force vector aims more to the outside (rotates counterclockwise) as you try to do the same turn at a higher speed.
post #19 of 27
Quote:
Originally Posted by Garrett View Post
I recall seeing those graphics from Bob, I've found them now that you've given me hints for the search terms. I think its quite reasonable to call that a critical angle, or a critical angle of edge engagement.

http://forums.epicski.com/showpost.p...&postcount=104
That's the one I was thinking of. The same thing happens when the ski stays at the same angle but the force vector aims more to the outside (rotates counterclockwise) as you try to do the same turn at a higher speed.
post #20 of 27
Turning radius on ski is over blown...
post #21 of 27
Quote:
Originally Posted by rockdude View Post
Turning radius on ski is over blown...
Uh, ok.

Thought:

For skis where radius is small (<20 perhaps) the radius figure seems to be a dominant characteristic, and two skis different by a given ratio (say 15m, 10m) will perform markedly differently regardless of other factors like construction. OTOH, the differences between a 40m ski and a 60m ski will probably be dominated by layup, flex pattern, etc.
post #22 of 27
Garret,

Don't forget length.
post #23 of 27
Hmm, good point. Not sure where length fits in there...probably just plain dominates everything else in both of those cases.
post #24 of 27
have you seen physic's man sidecut radius calculator? You can play with tip/waist/tail and length and get a darn good idea of how a ski with those dimensions will work.
post #25 of 27
Oh I know what you mean now. I'm speaking of the radius as an independent variable, i.e. radius staying the same as length changes.

The (relatively few) skis that are built this way are very interesting to test in different lengths. Usually when we test skis of different lengths that are relatively short radius, the difference in radius is significant. For instance, women's vs. men's slalom skis.
post #26 of 27
I think the difference between a 40m and 60m ski will be dominated by length. That's all I'm saying.
post #27 of 27
I agree with that. But if you have a 40 and a 60m ski of the same length, I think the radius difference will be less important than other stuff.
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