Wood Core Lamination
Overview The core of a ski is made by
laminating pieces of wood together and profiling the core to get the desired
There are several reasons why a core should be laminated, as
opposed to using a single piece of wood. First, a laminated core minimizes
irregularities such as knots and
grain patterns that run in various directions. These irregularities are
often found in a single piece of wood, and they can affect the strength and
durability of a ski. Therefore, a laminated wood core is more uniform in
density and grain pattern, and more lively in terms of performance. Second, the
lamination process allows the core's characteristics to be custom designed by
mixing different types of woods.
There are several types of
laminated cores: vertical, horizontal, and symmetrical; they are discussed below
Vertical Lamination A vertically laminated core is
one in which the adhesive interface (shown below in red) is vertical:
In this arrangement, the wood
grain boundaries run lengthwise
along the core. This configuration provides strength and rigidity
(stiffness) in the longitudinal direction, but very little in the transverse
direction. As a result, a vertically laminated core relies heavily on composites
such as specially braided fiberglass for torsional support.
Step 1: Rip Wood Into Strips Begin by selecting your wood
of choice and rip it
lengthwise along the grain boundaries into thin strips (1/4" to 1" wide).
Use a table saw to rip the wood and be sure to use appropriate safety
precautions when operating the table saw. Note that thinner strips reduce the size of the irregularities
(e.g., knots) but it also increases the amount of adhesive during the lamination
process, potentially making the core weigh more. Epoxy is almost twice as
dense as wood, and the increase in density adds
more weight for the same volume. Thicker wood strips weigh less but are
not as torsionally stiff. The strips we use for our skis vary in width from 1/2 to 3/4
of an inch.
Rip enough strips so that when placed side by side they collectively measure
wider than the ski's width by a few inches.
Step 2: Laminate Strips Apply a generous amount of high
quality epoxy or wood adhesive to the strips and clamp them together as shown
below. Some claim that epoxy is the better choice for core lamination
because it is more durable and forms a stronger bond. On the other
hand, some believe wood adhesive is better because it is specifically designed
for wood applications.
Make sure that a generous amount
of adhesive is used, such that it overflows from the seams of the core.
Let the adhesive dry for a period of 24 hours or until completely cured.
To prevent your work space from becoming a mess, use painter's plastic to cover tables and work benches during the lamination
Horizontal Lamination A horizontally laminated core, or plywood, can be identified by the
horizontal direction of the adhesive interface (shown below in red) between the wood
Although not as common in ski construction, horizontally laminated cores can be
found in some snowboards and is very popular in skateboard
construction. The direction of the wood grains are varied with
every layer. This configuration provides rigidity in several directions (not just the
longitudinal direction as in vertical lamination). It's debatable whether this provides
for a better core.
Symmetrical Lamination This core is a version of the
vertically laminated type but symmetric around the longitudinal axis.
The goal of symmetrical lamination is to create cores with consistent flex and density.
Step 1: Laminate Planks Symmetrical cores are made by
first laminating several planks of wood together. The
height of lamination should equal at least half the width of a ski.
Step 2: Cut Planks After the adhesive has cured, the
laminated piece is run through a table
saw to cut out planks, e.g.,
Step 3: Create Symmetrical Core Two of the planks are
then laminated side by side to
produce a symmetric core.
As you can see the core is
mirrored about the longitudinal axis (highlighted in red). This process ensures that wood
from the same batch is used at the same distance from the ski's center,
resulting in a symmetric core.
NOTE: A laminated core doesn't have to use the same wood throughout.
It is possible to combine the properties of a hardwood (stiffness, strength) with those of
a softer wood (less weight, more shock absorption). In fact, this
configuration is very common. Many
manufacturers create cores that have alternating strips of two different types
of wood to meet certain specifications, such as weight, stiffness and cost.
Often, strips of harder woods are
placed underneath the binding areas to maximize energy transfer.
Another configuration involves
placing stiffer woods near the edges to increase edge stability, e.g.,