Sandwich (Sidewall) Construction
Sandwich construction is a type of construction where an ABS-plastic or P-tex material is placed on either side of the core, above the edges. This type of construction is a bit more expensive, makes the ski heavier, and can sometimes reduce the torsional stiffness of the ski. However, durability is increased and compressed edges are easier to repair. Edge pressure is better distributed and more tenacious through the plastic sidewalls of sandwich construction. Sandwich construction is more often seen in high-end alpine skis and very high-end touring skis?
Cap construction is where the composite wraps around the top of the core and down to the edge. While this method doesn't provide the most stable edge or the most durable finished product, the ski is usually fairly torsionally rigid. With fewer materials used, the price goes down and the weight is kept pretty light. Generally cap skis are more often seen in lighter weight touring skis and intermediate level alpine skis.
Ski cores are constructed as laminates; thin pieces of wood/composites adhered together to form unique flex characteristics. Some skis use only one type of wood and mill the wood to make it thicker in parts that need rigidity and strength. Conversely, manufacturers will use thinner pieces of wood in areas of the ski that require flex and reduced weight. Other skis are layered with different varieties of wood and composites to yield the same results, blending strength with reduced weight.
Typical Core Materials
Wood - By and large, most skis are made with a natural fiber core (wood or bamboo). Natural fibers provide the longest flex life before they begin to degrade. In addition to that, the material is very strong and durable. Popular woods used in ski construction are Ash, Maple, Aspen, and Poplar. Ash and Maple are among the stiffest, heaviest, and most durable woods available. Conversely, Poplar and Aspen are much lighter weight, more flexible, and less durable. Occasionally, you will find skis made out of Paulowina (Karuba). While it isn't as popular, it is worth mentioning that these skis are going to fit into the lighter weight and flexibility categories while maintaining better strength properties.
Composite - Some skis still utilize composite cores, sometimes in the honeycomb structure, which has a very desirable stiffness-to-weight ratio. The hexagonal shape allows manufacturers to minimize the amount of materials used, thus reducing weight and not sacrificing any strength Some ski will use various types of ISO-Core material that includes blended foam/epoxy and fiberglass, again offering a high stiffness-to-weight ratio. These composites offer fantastic flex without the weight penalty of wood, are often less expensive, but do tend to degrade a bit faster than their natural counterparts.
The reinforcement of the core is a crucial part of the ski. Different materials greatly affect how a ski performs on the snow. Also, the use of certain materials are telltale signs of what the ski makers intentions are for the product's utility. There are three common materials utilized in the reinforcement of the ski core; fiberglass, metal, and carbon.
Typical Reinforcement Materials
Fiberglass - By far the most prevalent composite. Inexpensive, this material comes in a variety of weights and weaves. Fiberglass makes for a very "damp" ride. The heavier weight tends to absorb more vibration, so fast speeds and uneven terrain don't transmit the energy and vibrations into the legs quite as much. Using fiberglass, the manufacturer sacrifices a degree of durability. Fiberglass degrades over time regardless of use, it generally begins to lose it's pop after 5-10 seasons of regular use. Fiberglass is always used in conjunction with metal, and often carbon as well.
Metal - The use of metal reinforcement makes for the "dampest" ride. Due to the strength and durability of metal, this adds a ton of rigidity to the longitudinal and torsional flex of the ski. Using metal as a reinforcement comes a price; added weight. Because of this, metal is less than ideal for touring skis.
Carbon - This is the lightest and strongest reinforcement material used. In addition to that, carbon does not suffer from the natural degradation of fiberglass. Therefore, the natural spring and pop of the skis lasts much longer. On the other hand, it lacks the damping effects of fiberglass and metal, so you'll often see carbon reinforcement combined with one of the other options.