Stiffness distributions

When it comes time to change your old skis for a new pair, it's quite a challenge to choose the right model. Sure, you can go to a demo day to try out different models, but not everyone has the time and not all models will be there. To make our choice, we often have to rely on the information provided by the various manufacturers. However, everyone has their own way of presenting their skis, so it's not easy to find your way around.

Well this ski has 100mm waist, 20m radius and 180cm long. It seems to suit my needs, but is it stiff or soft?
"This model has a flex of 8/10." But what does that mean?

At Ferreol, we have made available the stiffness distributions of our skis. A quantified and objective way to inform you about this mysterious property of skis, but how important: rigidity.

More transparency, more information, but also more questions. What is the stiffness distribution of a ski?

In this blog, you will understand what the stiffness distribution is, how it is measured and what it is used for. You will see that this objective measurement can say much more about our skis than a subjective measurement like the famous scale from 1 to 10 that can be seen on the sites of other ski manufacturers. By giving an objective measure of stiffness, it is possible to compare any ski on this point!

What is the stiffness distribution of a ski?

Stiffness distribution is the graph of bending or torsional stiffness at any point on a ski. Here are some definitions to see more clearly:

Bending rigidity
How difficult it is to flex a ski
Symbol: EI, measured in Nm²

Bending of a ski (Source: Sooth Ski)

Torsional stiffness
How hard it is to twist a ski
Symbol: GJ, measured in Nm²

Twisting a Ski (Source: Sooth Ski)

The shape of a ski when a load is applied (force or torque). It's kind of a measure of how well the ski makes the banana.

Variation of Y (in this case, stiffness) according to position X (distance from the center of the ski)

Why are we interested in the stiffness distribution?

The stiffness of a ski will greatly affect its behavior on snow. Indeed, the stiffer a ski is in bending, the more stable it will be at high speed. Torsional rigidity is also important. The more torsionally stiff a ski is, the better the edge hold will be. However, a softer ski will be easier to skid and easier to turn. It is for this reason that the rigidity of the skis will vary according to the use that we will make of them.

In addition, the behavior of the ski can also vary depending on the local stiffness of the ski. Let's imagine 2 skis with average stiffness in equal flexion. If we were only to the average rigidity, one would think that the skis would have the same behavior on snow in terms of flexural rigidity. However, one ski could be softer at the back and the other at the front. They will therefore not necessarily have the same behavior. It is for this reason that the stiffness distribution is important and it is not necessarily enough to look at the average stiffness.

How is the stiffness distribution measured?

To measure the flexural stiffness distribution of a ski, a known force is applied to the ski and the deformation of the ski is measured at any point To measure the torsional stiffness distribution of a ski, a known torque is applied to the ski and the torsional deformation angle of the ski is measured at all points. A research group from the University of Sherbrooke supervised by Alexis Lussier-Desbiens has developed a machine that can measure these 2 parameters as well as the geometry and camber of the ski in less than 2 minutes. Now exploited by Sooth Ski, this technology is what Ferreol uses to measure the mechanical properties of its skis.

Case Study: Explo 96

Our design process helps reduce the number of prototypes needed to arrive at the final concept. After drawing the geometry of a new prototype and manufacturing a first scale model, we choose the different materials and their configuration to obtain the desired bending and torsion rigidity. Finally, we manufacture a first prototype in our Beaupré premises to test it on the slopes of Mont-Sainte-Anne. Snow testing is the last step in a design cycle. It's at this stage where we have to leave engineering aside and rely on the experience and evaluation of our testers. If the ski performs as expected, the design is complete. Otherwise, adjustments will be necessary.

In the case of the Explo 96, our testers reported that the ski was nimble and easy to control, but would benefit from being more spirited and stable in bumps or variable terrain. Our designers have therefore decided to increase the front stiffness (tip) of the ski to overcome this problem.

The figure below shows the stiffness distributions of an early prototype of the Explo 96 (orange) and the final version (blue).

Increasing the front stiffness (tip) allows the skier to have a more aggressive stance (more weight forward) and increases the stability of the ski in bumps and variable terrain. However, by keeping the stiffness lower at the tail (tail), allows the skier to exit the turn at any time and initiate a skilful skid to quickly control speed and direction.

The result? A fiery and agile ski that will perform as well in tight glades as on firm snow.

To sum up, if our designers had only been interested in the average stiffness, it would have been impossible to perform this analysis and optimize the behavior of the Explo 96. This is why the measurement of the distributions of stiffness is an important tool in our design process.