EpicSki › The Barking Bear Forums › On the Snow (Skiing Forums) › Ski Gear Discussion › Ski stability versus skier weight?
New Posts  All Forums:Forum Nav:

Ski stability versus skier weight? - Page 2

post #31 of 44
Thread Starter 


 

Quote:
Originally Posted by mogulmuncher View Post

Well, a current model GS ski at 183 cm is more stable than a 208 cm GS ski from about 20 years ago, so there must be a little bit more to it than just length.

 

Specific to your question though, how about a 17-year old J2 racer who is skiing on a K2-targeted GS ski, e.g. 165 cm & 18.5 m?  A decent J racer will out-ski this equipment fairly easily.

 

However, a 14-year old K2 racer, maybe 35 lbs. lighter, will manage very well with that ski, and might even beat the J2 racer in the same course, even when the J racer gets on a more appropriate 23m FIS ski.

 

How can this work?  A given ski has the capacity to manage a certain amount of energy, and if you exceed the limit (or can't reach the target range), results are compromised.  F=ma again, and there comes a point where more is not always better.

There was more to it than length even 20 years ago.  A 208 Kästle SG was more stable than any >208 GS ski I was able to demo. I think it has to do with the skis ability to dampen oscilations, a trait in which total mass has a part to play, both in how much mass needs to be stopped from oscilating and in how much mass is available to stop other parts of the ski from oscilating.   The 215 Kästle SG was more stable than the 208 SG, in part because it had more mass.  I think the amount of rubber and other energy absorbing materials and the strength of the ski in transmitting that energy to other parts of the ski plays a bigger role.

 

If a ski has the sufficient structural integrity and energy absorbing material, then the longer ski will be more stable at high speeds and have a higher ultimate speed limit.  If the ski doesn't, then the longer ski will be less stable.   While a well-designed SG ski is more stable at a greater length, take a softer gentler ski like an old Rossi Bandit and crank it up to 60 mph, longer than 176 will be worse, because you will just have more ski flapping around to wrangle.

 

As to the J2 racers above.  The too-light for the ski skier will have a worse time on the longer ski because he isn't producing the forces to make the ski work, and the heavier skier will have a worse time on the shorter ski because he is overpowering it.  True enough, but I don't think either skier would reach either ski's speed limit on a GS course unless he skipped the gates and straight-lined the run.  You can easily overpower a ski at speeds well below the ski's stability speed limit.

 

You could be onto something with the energy input though.  Let's explore skier mass as a factor in putting energy into the ski straight-lining or nearly straight-lining it down an icy run at 60 mph.


 

post #32 of 44

Skiers mass will have a significant effect on the energy of the system. At the top of a run the skier will have a potential energy of m*g*h joules. m=mass, g=gravity or 9.8m/s^2, h=height. As a skier moves down hill, that potential energy will transform into kinetic energy given by 1/2mv^2. v=velocity. Obviously, kinetic energy is directly proportional to the mass of the system. Hence, traveling at a constant velocity of 60mph, a higher mass will lead to a higher kinetic energy. Basically the system has more energy just because there is a higher mass.

 

I can only guess that a skier with more mass will be able to transfer more energy to the ski from his or her kinetic energy. That and the fact that gravity is already imposing a larger force on the ski due to a larger mass. Like I said before, I think that tension (or torsion?) in the middle of the ski creates a loss of contact with the snow and a counteracting force acting on the tips and tail of the ski......which may cause it to be unstable. Lengthen the ski, and you create a larger area for gravity to act upon which will counteract the larger force applied to the ski by the skiers mass.

 

skiing.png

Intuitively, if you increase the length of the ski you increase the cumulative effect of gravity on the ski which will attempt to counteract the force of the mass at the middle of the ski. Decreasing that bend, one will keep the ski more firmly on the snow and decrease the effect of other forces like wind.

 

Pardon the drawings and my possibly completely wrong explanation. I'm only 18 - at least I tried.

post #33 of 44
Thread Starter 

I don't think the skier's kinetic energy will change much as he is skiing down at terminal velocity for the slope (constant speed), but combining your ideas with Mudfoot's...

 

The heavier skier keeps the pressed down harder.  The ski hits a bump, let's say it looks like a curb going across the path of the skier

..............___________

_______|...................

 

The ski tip hits it first and the ski bends as the middle is kept pressed down longer with the heavy skier.

The ski tip hits it first and the ski bends, but not as much with the lighter skier as he is partially lifted, the ski acting like a bridge.  Result, heavier skier is deforming the ski a bit more for every bump he hits, while the lighter skier is feeling the bumps more.  Because the the ski is being forced to greater deflections for every major impact, it has more energy to dissipate.

 

Myth resurrected!

post #34 of 44
Quote:
Originally Posted by Huckle View Post

Pardon the drawings and my possibly completely wrong explanation. I'm only 18 - at least I tried.


Your real name isn't Sheldon Cooper is it?

post #35 of 44
Quote:
Originally Posted by mogulmuncher View Post

...

How can this work?  A given ski has the capacity to manage a certain amount of energy, and if you exceed the limit (or can't reach the target range), results are compromised.  F=ma again, and there comes a point where more is not always better.


This is it. 

 

For the overall experience of stability, sidecut, camber, dampening, etc. etc. can matter at least as much as the overall length and stiffness of the ski.  E.g., a short section of 2x4 would be pretty unstable.  (MTB fans will know you can affect stability in lots of little ways -- the bike's a system.)   But, fortunately ski manufacturers do a pretty good job, and if you take their input on who and what a ski is meant for, you'll come off pretty well. 

post #36 of 44
Quote:
Originally Posted by L&AirC View Post




Your real name isn't Sheldon Cooper is it?

Nope.......Jacob is my first name....and I really am 18.
 


Edited by Huckle - 6/20/10 at 12:57pm
post #37 of 44
Quote:
Originally Posted by Ghost View Post

I don't think the skier's kinetic energy will change much as he is skiing down at terminal velocity for the slope (constant speed)


It is changed by mass more than you would think. Let me illustrate with another fine piece of drawing.

 

Skiing 2.png

At the top of the slope velocity is equal to 0. Therefore initial K = 0 because .5m(0)^2 will always equal zero. Therefore a skier only possesses potential energy initially given by mgh. As the skier moves down the slope height decreases and so does potential energy....in order for delta k and delta u to remain equal, kinetic energy will have to increase. Kinetic energy is given by the equation for kinetic energy. It's just basic mechanics. 1/2mv^2. If you have more mass you have more kinetic energy which can be transferred to the ski. With conservation of energy we have at the end of slope, u=0 and k = its highest value.

post #38 of 44
Thread Starter 

I'm not saying the heavier skier doesn't have more kinetic energy than the lighter skier.  I'm just saying that there is not donation of that kinetic energy from the skier to the ski; the skiers energy remains constant at 1/2 m V^2.  The skis' energy is 1/2 m(of ski) V^2 + whatever vibrational energy the ski has due to the tips bouncing up and down and the ski vibrating out of control.  For the ski to be less stable for the heavier skier the ski's vibrational energy has to be higher when being skied by the heavier skier. 

 

You could argue that the heavier skier is skiing faster, but then we have a ski is unstable at a higher speed, not at the same speed for a heavier skier.

post #39 of 44

Because they are moving at the same speed the equation can be simplified 1/2(Mski + Mman)v^2. I've never really heard about vibration energy and I'm sure it's negligible both because of opposite vectors (a ski tip will bounce both up and down almost equally effectively negating any energy of a bouncing ski tip) and because if it does exist it doesn't create that many joules. If a person has a high kinetic energy they are able to do increased amounts of work on a ski - FScos(theta) or F Dot S. This work is in fact a transferring of energy from skier to ski. Or from skier + ski to ground.

post #40 of 44
Thread Starter 

An oil tanker has more Kinetic energy than a Seadoo, but it isn't any less stable.

The kinetic energy of the skier is not going into the ski.  It is fixed at 1/2 MV^2 (period).  The system having this amount of kinetic energy does not make the ski unstable.  The vibrations of the ski are the instability.

 

We need to get into how more mass on board leads to greater ski vibrations.  How is work being done on the ski?  How and from where is energy being put into the vibrations?  The linear velocity and associated kinetic energy of the skier is not changing.


Edited by Ghost - 6/20/10 at 5:13pm
post #41 of 44

Quote:

Originally Posted by Ghost View Post

An oil tanker has more Kinetic energy than a Seadoo, but it isn't any less stable.

The kinetic energy of the skier is not going into the ski.  It is fixed at 1/2 MV^2 (period).  The system having this amount of kinetic energy does not make the ski unstable.  The vibrations of the ski are the instability.

 


Alrighty. We know that work = FScos(θ). That is F dot S or force times distance times the cosine of the angle between them. From that we can see that a force on a ski ridden over a distance will have work done upon it. In the case of downhill skiing.........much likely that there is less work than the product of force and distance. If on level ground, there is no work done because cosine of 90 degrees is equal to zero. This work is basically saying there is energy put into a ski over a distance.

 

What I am saying is not that there is a transfer of kinetic energy to ski from skier, but that the increased mass of a skier, relative to a low mass skier, will greater affect the kinetic energy which will in turn increase the amount of work done on the ski which increases the energy put into a ski. (After all substituting for F in the work equation we get work = macos(θ)) A lower mass person will do less work on a ski......a higher mass person will do more. Therefore a ski that was meant to handle a certain amount of energy may very well be overpowered by a skier that exerts a greater force and in turn does more work upon the ski. This overpowering leads to, as you said, vibrations. That and the wind resistance caught under tips going 60 mph is quite strong, leading to more vibrations.

 

 

skiing.pngAs I put here......when that ski bends there is a greater area that wind resistance can do work upon. This is a possible source of vibrations. Increase the length, and gravity helps to lessen the effect of the skier's mass and the bending of the ski.

post #42 of 44
Thread Starter 

Let's agree that with a bigger mass, the ski will be bent more as it goes over bumps, potentially contributing to a greater vibrational energy in the ski.  The ski will also be pressed down into the snow more, which will act as a stabilizing influence.  Hence the heavier skier may find a ski more unstable over refrozen cut-up crud, but more stable on regular snow.

post #43 of 44
Quote:
Originally Posted by Ghost View Post

Let's agree that with a bigger mass, the ski will be bent more as it goes over bumps, potentially contributing to a greater vibrational energy in the ski.  The ski will also be pressed down into the snow more, which will act as a stabilizing influence.  Hence the heavier skier may find a ski more unstable over refrozen cut-up crud, but more stable on regular snow.

I think the stability on the different types of snow lends itself more to friction than anything else. But I think it still acts as a stabilizer as you said. Greater force down, greater normal up.

post #44 of 44

 

Originally Posted by Ghost View Post

We need to get into how more mass on board leads to greater ski vibrations.  How is work being done on the ski?  How and from where is energy being put into the vibrations?  The linear velocity and associated kinetic energy of the skier is not changing.


Back to F=ma, more mass give more force, and too much force goes beyond what the ski can handle.

 

In my earlier example I was describing performance in a race course, where the course set is to some extent establishing a terminal velocity which is roughly the same for the J2 and K2 racer.  If you're thinking about a straight-lining example, the limits might work out a bit differently.
 

New Posts  All Forums:Forum Nav:
  Return Home
  Back to Forum: Ski Gear Discussion
EpicSki › The Barking Bear Forums › On the Snow (Skiing Forums) › Ski Gear Discussion › Ski stability versus skier weight?