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# '99 Volkl P40 F1 -- Turn Radius??

I am curious if anyone has an idea of the turn radius on these skis. I have them in 183cm. I am in the midst of trying to determine it mathematically. So if anyone has a rough estimate, i would appreciate it.

Also, if you have an idea for the P40 SL (carver type) in 163cm, please let me know.
If I remember correctly its about a 23m
Actually, it should be about 21 in the 183 length. Give me the tip tail and waist widths and I'll tell you exactly.
David2002, great site. thanks.

UP Racer, the dimensions are 102,65,88 i believe. I am at school and my skis are at home, so i can't confirm. I estimated my SL's to be about 15~16 and my F1's to be about 22~23 using a math program i threw together.
If you contact Physicsman, he has a program in Excel that's pretty good.
For those dimensions, in a 183, the radius is 22.1. I have the same program.
I'm not at my home computer, so I can't give you an exact number, but I owned a pr of 188 P40's, and as I recall, the sidecut R was something like 23.1 or 23.3 m by the spreadsheet calculator I put together.

Tom / PM

PS - KeeTov, you've got a good memory!
HEY!!!! WHY WON'T YOU PEOPLE LISTEN TO ME?????!!!!!
IT IS 22.1 BASED ON THE DIMENSIONS GIVEN!!!

Ok, i have to appolgize, the true dimensions are 100,63,88. They changed slightly from the '00 model to the '01 model. Earlier, I quoted the '01 (black and green) model. I am curious to see how your spreedsheet works Tom. Would you mind sending it to me? (jluecht@nd.edu) I have been using MATLAB to run my estimates. With the correct measurements, I seem to get what the manufacturers claim.

Jason
oops,typo... tail=86
Actually, I just came up with 22.1 again. Evidently they kept the same radius on the green and black ones, just widened them a bit all the way down.
May I add myself to jluecht request to have PM pgm, please?
Hey guys - I just got back home and spotted this thread again. Give me a few of days to get some pressing things done, then I'll have a chance to tidy up my little program and send it to whomever wants it.

In fact, maybe AC or DC would be interested in hosting it as an interactive calculator for users of Epic. That would be cool.

BTW, I should caution that in all of these sorts of programs, an assumption always has to be made about how to turn the published length of the ski (usually the chord length) into the parameter which is really needed, the distance between the widest parts of the tip and tail. With varying ammts of tip and tail turn-up, there can be errors of order 5% in this conversion, and hence in the overall calculation of sidecut R.

Bottom line: don't get too carried away thinking you have three significant figure accuracy in the radius.

In my latest version of the program, I display the calculated contact length, so if you have access to the ski and can actually measure this number, you can tweak the overall length to get the correct contact length (and hence a more accurate answer).

Tom / PM
Quote:
 Originally posted by PhysicsMan: In my latest version of the program, I display the calculated contact length, so if you have access to the ski and can actually measure this number, you can tweak the overall length to get the correct contact length (and hence a more accurate answer).
That's the same first error I had when I attempted to do equivalent float with consideration as to midpoint moment.

PM

I'm interested in the parameters for your calculation. I had always understood the advertised turn radius was an approximation bearing in mind the numerous variables at work and has never been simply the radius giving that particular sidecut arc as some suppose. The ski's flex and torsion as well as the skier's mass, speed and technical ability, as well as snow conditions all contrive along with the side cut to produce a turning characteristic which is probably more usefully used to compare different skis, than to measure an absolute figure.
Comprex - Fortunately, the sidecut radius calculation is pretty insensitive to the fore-aft position of the waist. OTOH, if you want to separately compute the fore and aft sidecut radii, then you definitely need to know the position of the waist, and even small errors in that play havoc with the results (as you apparently saw in your calculation).

daSlider - For exactly the reasons you mention, my program only calculates the geometric sidecut radius (ie, the number printed on some skis), not the turn radius. No one *ever* attempts to calculate the turn radius, they just get on the skis to find out .

Tom / PM

[ April 21, 2004, 12:42 AM: Message edited by: PhysicsMan ]
Try this website:

I ran my skis through it and it seems pretty accurate. I have a 163 P40 sl I measured and I can't remember what it came out to be, but it was tighter than I thought just looking at the ski, 15 or 16 or so. No wonder I still like those skis!

If you want me to measure them again email me.

newfydog
My program requires the waist location to be measured or approximated; therefore there is alot of estimation when I am judging from photos on websites. When i measured my P40SL's my output was very close to what was posted on the voelklfanclub site as well as that last math site posted.
PM

if 'No one *ever* attempts to calculate the turn radius' as you say, why then do people go on about the techniques of getting skis beyond their specified radius as if this was an official limit to their carving possibilities?
Quote:
 Originally posted by daslider:PM if 'No one *ever* attempts to calculate the turn radius' as you say, why then do people go on about the techniques of getting skis beyond their specified radius as if this was an official limit to their carving possibilities?
With suitable snow conditions, and with suitable technique, one can carve turns with radii that fall in a range of values. The minimum value is equal to some fraction of the sidecut radius (say 20% at hypercarving edge angles approaching 80 degrees, ie, cos(80) = 0.18). The maximum radius is almost equal to the sidecut radius itself (ie, when doing RR tracks with the absolute minimal edge angle you can get away with and still leave sharp tracks). Outside of this range, your turns must have some skidding component to them.

I think that the "official limit" that you perceive is when a skier needs to make a turn of radius less than the 20% (or so) number and simply can't do it by carving. At that point, to do what they want to do, they can't just add more edge to tighten up their turn. They have to intentionally use movements that introduce some skidding into the turn.

IMHO, "all the talk" that you refer to has to do with how exactly is this done, what you call it (ie, "Active rotary"), what is the most efficient way to do it, etc. It's not that the 20% or so number is a hard limit, its just that you have to do other, non-carvey (sp?) things when your turns get this tight and you still want as high performance as possible.

Finally, I would add that the above is a bit of an intentionally odd way of looking at turns. Most people obviously don't start out carving and introduce skidding when they run out of carving possibilities. Rather, they start out skidding and introduce carving as they get better. I put my argument the way I did to try to answer your question (paraphrased) of "why does there seem to be a big-deal limit on carved radii".

HTH,

Tom / PM

PS - Another reason that people don't attempt to compute the actual turn radius is that all of the parameters that would be needed to make the computation even moderately accurate are never available even to world-class ski team members, and often will vary from moment to moment (eg, snow compaction, edge angle, rotary torques applied by the skier to the skis, etc.). The simple geometric sidecut radius of a flat ski at least allows skiers to have a ball-park idea of one aspect of how the ski will perform.

Tom / PM
PM

you wrote 'The simple geometric sidecut radius of a flat ski at least allows skiers to have a ball-park idea of one aspect of how the ski will perform. '

but, there must be more to the published figure than that? If 2 identically shaped skis have different flexes, they will turn differently, surely?
Quote:
 Originally posted by daslider:PM - you wrote 'The simple geometric sidecut radius of a flat ski at least allows skiers to have a ball-park idea of one aspect of how the ski will perform. ' but, there must be more to the published figure than that? If 2 identically shaped skis have different flexes, they will turn differently, surely?
The usual numbers (ie, the three widths and the sidecut radius) have to be taken at face value - they simply describe the shape of the ski (looking down from above), nothing else is implied.

As you point out, two identically shaped skis with different flexes will indeed turn differently (on a deformable surface) and the radius of those turns is called (no surprise), "the turn radius", not "the sidecut radius".

Unfortunately, as we talked about earlier, the turn radius is influenced (and influenced dramatically) by just about everything under the sun including the sidecut radius.

Some of these things are properties of the ski (eg, longitudinal and torsional flexibility, swing weight, etc.), but many do not (eg, the skier's edging, rotary and pressure input, the compressibility of the snow, speed, etc.). Because of the importance of these other inputs, no single "turn radius" can be stamped on a ski.

I suspect that you were hoping that one might at least be able to wrap up all of the ski-related turning properties in one number and give it its own name. Unfortunately, even this lesser goal isn't possible. The reason is that the different physical properties of the ski rise and fall in importance depending on how the ski is being used and the snow underfoot. For example, the swing weight (ie, the polar moment of inertia) is all important in pivoted turns, but almost negligible in importance in carved turns. Torsional flex is important on hard snow and of negligible importance in more than a few inches of soft snow. Longitudinal flex is of supreme importance in determining turn radius in powder, but of lesser importance to the radius on a hard surface.

I hope these concrete examples give you some idea of the impossibility of ever coming up with a "turn radius" number for a ski.

Cheers,

Tom / PM
Tom

I am not so deluded as to be seeking a magic number, I am just bemused by constant references on Epic to people tightening turns beyond such a magic figure and therefore being allowed a rotational/skidded component, which on the face of it is ok. But I really do think that many may actually believe that figure to be absolute (and probably published if not emblazoned in the graphics), rather than it being a different figure for every skier and probably every turn.

So really the 'sidecut radius' figure doesnt actually tell you what the tightest carved turn might be, although I have heard it said that the 'magic' turn radius figure is produced by manufacturers, assuming the vaiables, as a comparative rating figure to allow ski performances to be set against eachother. Hence my question to you, as this didn't strike me as something calculable with only the edge geometry.
Quote:
 Originally posted by daslider:...But I really do think that many may actually believe that figure to be absolute ... rather than it being a different figure for every skier and probably every turn...
Unfortunately, you are correct in thinking that many recreational skiers probably think that the sidecut radius is some sort of magic, absolute quantity with 4 significant figure accuracy. :

Quote:
 ...I have heard it said that the 'magic' turn radius figure is produced by manufacturers, assuming the vaiables, as a comparative rating figure to allow ski performances to be set against eachother. Hence my question to you, as this didn't strike me as something calculable with only the edge geometry.
Ah, now I think I understand where you are coming from in your questions. I think that ski mfgrs certainly do try to optimize each model's performance for the target population of skier (eg, intermediate men of average wt on hardpack who, because of their average athleticism, can probably only muster 30 deg of angulation). When the mfgrs set up their design process this way, there will indeed be a turn radius that is easy to lock in to, hard to perturb, within the constraints of the the target skier's speed and max-comfortable-G's, etc. I think the mfgrs would likely call this their "magic number turn radius". This number is no limit, just a value such that if you go away from it, the target skier's performance on these boards will tend to degrade. As you point out, you can't predict this "magic number turn radius" from just the sidecut geometry, but I'd bet that if this is to be a ski for recreational skiers and consequently assume that most of these skiers don't edge more than 30 degrees, you'd be doing pretty well. So, if you then calculate cos(30)*Geometric_sidecut_radius", you would probably be well within a meter or two of the "magic" number for that ski.

Tom / PM
PM

so what is the point of a 'sidecut radius' if it doesn't really tell you what the ski can do?
I suppose the point of the 'sidecut radius' is to rate the shape of the ski. Think of it as a parameter determined form the static dimensions of the ski.

It seems the 'sidecut radius' came into the lime light as skis got more shaped (hence the reason i posted the question about my older P40s where the radius wasn't advertised). Now, it seems that skis are categorized by how radical the shape is... e.g. r<14m for SL racing, r>21 for GS racing (FIS has placed restrictions on these dimensions). Practically all skis now are advertised with their 'sidecut radius'.

As Tom wrote, the geometric radius is not actually the radius of the arc that you can create on the mountain. However, it still tells you a lot about the shape of the ski and therefore, gives you an idea of what kind of turns it is capable of making/carving. It certainly gives you an idea as to what the ski can do. So again, It's merely a quick reference parameter... not an exact performance value.

Jason
jluecht - I couldn't have said it better.

Thanks,

Tom / PM
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