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Some Ski Stiffness Data.....WTF? - Page 2

post #31 of 57

How old are these skis?  Do they have the same number of hours on them, or have some been subjected to more fatigue than others?  It's fun to play around and measure stuff, but unless you have full control over most variables the results will always be questionable.

post #32 of 57
Thread Starter 

All my skis are well used but never abused or overstressed.

I'd love to try and generate some data that actually shows ski degredation with age and use.

Maybe you want to send me a new pair of RX 12's to test?

I don't really believe that modern layups "fatigue" unless severely overstressed by a strong skier in tough terrain.

Boeing certainly doesn't expect the Dreamliner to fatigue but that remains to be seen too.

The Volkls have exactly the same camber as when I got them.

post #33 of 57
Quote:
Originally Posted by dakine View Post

I don't really believe that modern layups "fatigue" unless severely overstressed by a strong skier in tough terrain.

Boeing certainly doesn't expect the Dreamliner to fatigue but that remains to be seen too.

The Volkls have exactly the same camber as when I got them.

You are one of those folks that cranks the DIN down on their bindings before storage right?th_dunno-1[1].gif

post #34 of 57
Thread Starter 

Actually, I found out I could ski at DIN 3.5 last year because I forgot to crank them back op.

It is true that Boeing expects the Dreamliner to be much cheaper to maintain because of composite construction.

It is true that really old glass sailboats like early Cal sailboats are going on 50 years without fatigue problems.

It is also true that I have seen some really bagged out skis, but they were driven by beasts or fools.

So, as always, I'd be interested in data.

 

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post #35 of 57
Quote:
Originally Posted by dakine View Post

Actually, I found out I could ski at DIN 3.5 last year because I forgot to crank them back op.

It is true that Boeing expects the Dreamliner to be much cheaper to maintain because of composite construction.

It is true that really old glass sailboats like early Cal sailboats are going on 50 years without fatigue problems.

It is also true that I have seen some really bagged out skis, but they were driven by beasts or fools.

 

You can feel a big difference in skis over time. My first pair of skis felt dead after the first season. Then, after about 30 days, my Salomon x-wing 10s (foam core) felt dead. The next season I lent those skis to a friend, and they snapped at the toepiece while in a bump field (so clearly structural wear happens). Recently, in a coach training session, the coach noted that one of our candidates' skis was worn out, which accounted for his excess effort to try and impulse his ski. I've also skied on a friend's old race skis - they're clearly worn out as they just bend into an arc and barely rebound. 

 

While I'm neither a beast nor a fool, I have felt the "spring" wear out in a couple of pairs of skis over time. Whether this happens at 30 days or 300 days is an altogether different matter! But you don't see many full-time permanent instructors skiing on gear from more than a couple of seasons back. The guy skiing without booting out at DIN 3.5 exerts far less pressure/torque to skis than the guy booting out at DIN 7. (My race coach suggested I move my DIN to 9, and I'm just consolidating my racing skills on blue terrain--hardly WC or FIS level; more like beer league.) You may be lucky in that you probably don't wear out your skis at the same rate.

post #36 of 57
Thread Starter 

Plastic foam core skis were a disaster.

Wood, which is mother nature's own perfect oriented foam doesn't have the breakdown problems that plastic foam did.

I would expect to see a loss of camber or a change in the stiffness of the ski if it has internal fatigue damage.

Don't think I'm saying skis don't change their mechanical properties with use.

Some do but I also think that many people just get tired of their old equipment.

Instructors get deep discounts and are expected to ski on new stuff.

It would be interesting to cut up a ski that is supposedly used up and see why.

Skis stored at high temperatures can creep into distorted shapes.

I'm a why guy and I keep trying to understand how a ski can lose its dynamic properties without changing shape or stiffness.

Carbon windsurf masts will creep if left tensioned in the sun but generally they last a very long time or they snap.

 

I was very surprised I stayed in at 3.5 while carving groomers, but, I do a lot of XC skiing which teaches you to stay centered.

In a race course I'd have left them behind at the start.

post #37 of 57

I measured my old Race Tiger SL 165cm skis. 1/8 inch camber per ski. The slightly newer Race Tiger SL 165 that my son uses had 3/16 inch camber per ski. I had no way to test the flex. They feel soft FWIW. That is quite different than what you measured. The skis have been used hard with lots of bashing bumps. They feel much less lively than when new. 

 

Maybe I should mold some more flex and stiffness into these skis?

 

Eric

post #38 of 57

Interesting. A crude measurement of my Race Tigers showed the flex was similar to @dakine 's flex numbers. I am looking at 165 SL skis which are different but I tried to recreate the tests faithfully. Crappy scale, calipers and batteries for the last scuba weight made my accuracy poor but I got a spring constant of 23 - right in the ballpark for dakine's data. But my camber is nowhere close. I can mold camber back into the ski but I'm worried that the graphite I add to do this will stiffen the skis too much.

 

Are slalom skis supposed to be stiffer than GS skis?

 

FWIW, the Frankenskis were over 4 times stiffer with the same weight causing barely 1/4 the displacement. The Frankenskis suck - but is it the stiffness or the camber (3cm per ski)? These skis will get the camber reduced if I get around to remold them but that will also probably soften them too. Too much work to reduce the variables - especially for a silly project.

 

Eric

post #39 of 57

The fact that the skis we are talking about are different lengths has a huge impact on the "spring constant", which is technically E*I/(L^3).  The important thing to note is the L-cubed.  How you choose to normalize your data for different ski lengths makes all the difference.  A 180cm ski that flexes 1" under a given weight is obviously stiffer than a 150cm ski that also deflects 1" under the same force.  Simply keeping your end supports close together on the longer skis inflates your spring constant by cutting out the most flexible sections of the ski's contact length.  We have compiled data on ski flexes over the last few years in order to verify our FEA program and found the Racetiger SL to vary greatly from year to year.

 

-Just noticed the thread is a year old (doh!)...wish I had seen this last year.

post #40 of 57

Nice @SandwichTech Are the flex data tables published online somewhere?

post #41 of 57
Thread Starter 
I avoided the problem of differing ski lengths by using all 175 cm skis.
Except the old P40's were 182 so there is a bit of confusion in the raw data

Unfortunatly, you guys didn't send me some FX 94's for testing so I had to buy some.
I really want to generate some data on them for comparison.
I also want to throw some old straight skis into the mix.

anybody want to try to guess where the FX's and some straight skis will fall in comparison?
post #42 of 57

Late 60's Fischer ALU ST  has some interesting info right on the skis. and early DYNAMIC VR17's had tip and tail stiffness numbers hand stamped into the tips.  Very cool thread.  thanks for doing this research.

 

Royal

 

IMAG0509.jpg

post #43 of 57

I would be willing to guess that the FX94s are torsionally stiffer than the others due to the width.  Bending stiffness is less obvious...old straights should be stiffer in the tips and more compliant underfoot than more current skis.

 

@Tog no, our data isn't published online.  However we are putting together a video that shows our torsion testing setup with a couple samples (Racetigers and Root 78s).

post #44 of 57

@royal Quite interesting!  Do you happen to know the testing standards that produced the 7 to 8 degree twist?  Normally (depending on the preferred method) units would be N-m^2/deg.

post #45 of 57
Quote:
Originally Posted by SandwichTech View Post
 

@royal Quite interesting!  Do you happen to know the testing standards that produced the 7 to 8 degree twist?  Normally (depending on the preferred method) units would be N-m^2/deg.


Sorry, all I know is what is printed on the skis.  I just thought it was interesting that they would put this info on a ski as a selling point.  I'm not aware of any other skis that have this info.  there might be info on how these tests were done in the older ski mags.  they used to publish some pretty cool and detailed stuff about the equipment back then. 

post #46 of 57

Having some fun.

I sanded off the plastic graphics sheet and epoxied a couple strips of graphite unitdirectional on the top in place of the graphics. I blocked the ski to add camber before adding the unitdirectional. When the epoxy kicked off, I ended up with skis that were a bit stiffer and quite a bit more cambered. So how did they ski?

 

Well behaved. Better on ice than before but not great. Good on soft snow - as good as before the change. They felt livelier. A subtle change for the better.

 

Eric

post #47 of 57
Thread Starter 

Some new data.

I did my test routine to my Kastle FX 94's before I put them away.

All the data I have taken is in this chart for your perusal.

 

Now I think I know why the Racetigers are well named.

post #48 of 57
Thread Starter 

I'm thinking that the location of the centerline of the ski sidecut radius must really affect performance.

Mathematically, if you know the sidecut dimensions of a ski and the radius, then you can calculate the lengths between the tip and mid sidecut measurement points.

Likewise, you can calculate the distance from the mid to tail sidecut measurement points.

Adding these two lengths gives and estimate of running length.

The location of the radius centerline as a fraction of this calculated running length explains some of ski performance.

I calculated this table for the skis I have been measuring.


No wonder that the Volkls with long stiff tails are springy but will punish bad technique.

No wonder the FX 94's seem to ski shorter than my RX's.

I do wonder how boot mounting centerlines are aligned with this?

post #49 of 57

Flex changes over the length of the ski. Wouldn't where a ski changes from stiff to soft relative to the bindings and sidecut be relevant? How could you measure the flex distribution along the ski?

 

Eric 

post #50 of 57

The flex will be a function of the cross sectional area of the ski at any point along its length. Isn't affected by length per se. Assuming the same materials in roughly the same proportion along that length, you can just calculate a few areas, run a regression, plot a line or fit a curve. Could even do it for tip cutouts, or stepped layers, just more of a PITA to measure area.

 

OTOH, likely that the proportion of different materials changes near tip and tail, and if oddly shaped metal or carbon is used, even other places (Blizzie IQ Max carvers, Nordies, or Lines come to mind), so unless you know the changes or the elasticity differences of the materials are small, maybe back to paint cans, gauges, and dots on graph paper. 

post #51 of 57

Some stiffness, bending research:

 

Quote:

Originally Posted by L&AirC View Post

 

So I was playing in the laboratory (i.e. basement) on trying to determine the perfect way to find a binding mounting position and in one of the experiments I wrapped a strap around the ski and tightened the strap causing the ski to bend.  The result is this:

Ski Energy 009.JPG

 

This is an Elan GSX 182cm race stock ski.

 

Once I saw how much pressure the strap had on it, I realized I should be careful taking the strap off.  So I had my daughter record it.  I don't recommend doing this because there is the potential to get hurt and/or break your ski.  It is also worth noting the different arc (tail and tip) radius.

 

Anyway.  Here is the release of the energy:

 

http://www.youtube.com/watch?v=BZZu2ikz11Q

 

 

 

Not sure how much energy that really is but I believe it is a bit more than needed to launch even a non anorexic mouse across the trail.

 

Ken

 


 

post #52 of 57

I'm surprised that there was no delamination failure on that release.

post #53 of 57
Quote:
Originally Posted by dakine View Post
 

I'm thinking that the location of the centerline of the ski sidecut radius must really affect performance.

Mathematically, if you know the sidecut dimensions of a ski and the radius, then you can calculate the lengths between the tip and mid sidecut measurement points.

Likewise, you can calculate the distance from the mid to tail sidecut measurement points.

Adding these two lengths gives and estimate of running length.

The location of the radius centerline as a fraction of this calculated running length explains some of ski performance.

I calculated this table for the skis I have been measuring.


No wonder that the Volkls with long stiff tails are springy but will punish bad technique.

No wonder the FX 94's seem to ski shorter than my RX's.

I do wonder how boot mounting centerlines are aligned with this?

 
Very interesting bending data!  I am always surprised that the racetigers dont have a higher torsional stiffness.
 
I am going to go out on a limb and guess that your tip and tail widths got flipped in your program which in turn made your apex location end up at 45% instead of 55%.  Have you compared measured running lengths to your calculated ones.  The racetiger running length seems about 5cm too long for a 173 (unless you include the tail completely).  I would guess that relying on the stated sidecut radius will skew your results.  Designers tend to tweak sidecuts to produce their desired taper, apex, sidecut extensions, etc...but they arent about to list it on the skis as: "40% 13m/10% 16m/50% 14m".  This might not always be the case, but measuring the ski with a caliper is the only foolproof way I have found.
 
I have noticed that racetigers I have tested have an unusually forward flex pattern (mark the section of the ski that is within .010" of thickest).  Interestingly, they also tend to have a higher than average setback of 6.5 to 7% (that is BC at 57% of RL).
post #54 of 57
Thread Starter 

Ya, mistake in the centerline location for the Racetigers.

I flipped the tip and tail measurements.

The centerline is at 65% not 45%.

I want to look at a newer pair than these Gen 1 orange ones, I'm betting they moved the centerline forward as they further developed the ski.

These skis have a really strong tip and ski very different from the Kastles or Atomics.

 

I did the calculation of centerline location from manufacture's data to see how it compares to what I can measure.

If the stated radius is wrong or not a radius the calculation will reflect that.

The FX 94 data with the short calculated running length is suspicious, I need to actually measure that ski to see why it calculates short,

It does ski short though so maybe there is something to it.

 

 

My overall comment about all this is that I thought I knew something about the design of these skis just from skiing them.

What I imagined and what I measured were a lot different.

The next time someone tells me that "ski X is really stiff" I will laugh.

I'm getting suspicious that the number of guys that really understand ski design is mighty small.

Many ski designs have evolved rather than being designed.

 

All of this reminds me of a visit I once made to a high tech semiconductor fab line.

After asking a bunch of questions it became obvious that nobody really knew how the thing worked.

If old Fred put some duct tape on a vacuum pump nobody would take it off because it might be critical to the process.

A lot of industry runs processes that are very difficult to improve because everybody there has forgotten what they did in the first place and why.


Edited by dakine - 4/25/14 at 11:17am
post #55 of 57
Thread Starter 

Here is another observation.

When you snap a boot into a ski that does not have a binding plate, the ski picks up about 1/4" of camber.

post #56 of 57

Does the DIN setting affect the change in camber? Or the stiffness numbers?

 

It kind of makes sense that the camber would increase a bit. The boot effectively thickens the ski and adds pressure to extend the top of the ski. But I wonder how significant this effect is when compared to the loads applied by the full weight of the skier plus any dynamic loads. 

 

Interesting stuff...

 

Eric

post #57 of 57

I suspect the forward pressure setting is what causes the extra chamber.  The look PX binding have an option which is called the MFX plate which isolates this from the ski (basically it locks the distance between toe and heel).

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