The Skinny on Ski Cores

There is a tremendous mystique about the core in a ski.  And even more mystique about wood as a core material. Some of this is historical, some based on science, and some based on experience.

 

Skis started out as just wood so there is history and tradition associated with wood. When aerospace materials and sandwich construction came on the scene the new structural materials (primarily aluminum and then fiberglass) were just glued to the top and bottom of the wood already there. The wood went from being the structural layers of the ski to just being the core. The wood core became a support structure for the new structural materials. Dramatic performance and durability improvements followed.

 

Many ski companies still promote wood as the essence of their skis. Foams were developed for core materials but it took quite a while to get good performance foams that were durable and strong enough for the task. Early foams, and the injection ski technology still used today for very low-end skis, gave foam as a total family a very bad reputation. That technology challenge has been overcome with high performance foams and well executed designs.

 

The core is part of a whole structure. In its simplest form the core keeps the top and bottom structural layers in their correct position and transfers loads or stresses between them when the ski bends, twists and gets hit with impact loads. So when is the core material important and when is it less so? In a fiberglass ski the stiffness (or modulus) of wood is around 50% of fiberglass. So wood has quite an influence on a fiberglass/wood core ski. So when comparing fiberglass skis the use of wood core is a significant part of the overall feel of the ski.

 

In a carbon ski, the carbon is about 10 times the stiffness or modulus of wood, depending on the fiber lay-up orientation. The net effect is that the core material has a lesser effect on the overall performance of a high modulus ski made out of aluminum, steel, or carbon.

 

All foams, unless they reach the density or weight of wood have a fraction of the stiffness or modulus of wood. But since foam cores are so much softer than wood the difference in a fiberglass ski is really pronounced with wood being so close to fiberglass. So, fiberglass skis made with wood cores frequently perform better than foam core counterparts. But for the aluminum, steel, and carbon skis a switch to foam is not as significant because the foam and wood core properties are so much lower than the structural component. Hence the actual core material in aluminum, steel, and carbon skis does not dominate the ski performance as much as it does in a fiberglass ski.

 

The different mechanical properties of cores can be adjusted for in the overall design. Strength and durability becomes the big issue with foam. Foam is dramatically lower strength than wood. So foams are reinforced or made with very high density to get their strength and durability high enough. Or else they just don't last. Foam impact strength is also much lower than wood.

 

Some very light foams are so weak and soft that they need ribs to reinforce them, especially at the cure temperatures of a ski. A good series of vertical ribs can even negate the need for a foam or wood core at all. But it has to be well executed. Less than successful efforts have been made to create ribs structures in previous iterations of carbon skis. The ribs are folded with air pockets and lack adequate pressure to create a solid rib. The resulting core structure is compromised making the ski weaker and less durable than using a higher performance core material. Those vertical ribs also add weight, which could be traded off for a higher density and therefore stronger core, whether it be foam or wood.

 

Sidewall construction skis usually use wood as the core material. The wood core can withstand the high pressures of molding to create a very solid laminate. Using a foam core usually requires a cap ski design to prevent the molding pressures from crushing the core. Foam core in a sandwich ski, without a special mold design, cannot withstand the high temperature and pressure of aerospace carbon prepreg.

 

DPS uses a lightweight wood core to achieve the best combination of ski performance, excellent laminate properties, and field durability. Wood cores in our skis are field tested with finely tuned flex designs to provide the best in both performance and feel.