by Stein, Eric's Son
Ski Construction
Most skis are made of synthetic materials these days,
so this article will discuss how these types of skis are
constructed. Basically, there are three types of construction:
sandwich, box and cap.
Sandwich construction resembles how wood skis used to
be put together. Usually two load-bearing layers of a
strong material are laminated over a lighter core to produce
a ski built something like an I-beam. The load-bearing
materials are usually man-made materials called composites.
These composites are made of fibers, fillers and resins.
The type of fibers and their orientation in the material's
matrix give the materials their different properties.
A plastic topsheet and base are then laminated to the
sandwich to complete the ski. A major variation on the
sandwich construction model is the injected ski. Skis
of this type are made by placing the top and bottom sheets
in a mold, which is then filled with a plastic foam that
expands, which when it cures forms a core.
Box construction (or as it is sometimes known torsion-box
construction) consists of a load-bearing box-like structure
in cross-section that is built up around a lighter core.
The box structures are produced in many ways. One of the
most common techniques for making the box is called the
wet-wrap method. In this method a synthetic-fiber cloth
is pressed or wrapped around a core and cast with an epoxy
resin in a mold.
A more recent innovation in ski construction is 'cap'
technology. Cap construction is a variation on the box
construction theme. A braided fiberglass 'sock' with fibers
running in different directions surrounds a foam core.
The design of the sock enables control of the ski's camber,
torsional stiffness and sideflex. Additional layers of
fiberglass or in some cases carbon fibers run lengthwise
down the ski to give it additional strength and stiffness.
This combination is covered with a one piece "cap"
or shell of acrylic plastic. The cap serves as a "monocoque"
frame for the ski that is both lightweight and strong.
The relative difficulty of construction and the price
of raw materials influences ski price. Sandwich skis tend
to be the least expensive and are usually best suited
to in-track skiing. They can be made to fine tolerances,
and stiffness and flex can be precisely controlled. Less
expensive models may also be more prone to de-lamination.
Box construction skis are heavier, but stand up well to
off-track use and are torsionally rigid enough to make
good turns. It's harder in the manufacturing process to
control the stiffness and strength of box skis, and pairs
of skis are typically made up by matching individual skis
with similar characteristics from available supplies.
Cap skis are high-tech and usually expensive. This sophisticated
construction technique is versatile enough that it is
suitable for use in skis designed for just about any type
of skiing; from telemarking to skating.
