"Today's modern bow is made up of laminations of wood and Fiberglas, the
wood serving as a neutral core or spacer between two laminations of Fiberglas.
Actually it is the Fiberglas that does the work of the bow, carrying 88 percent
of the load while the wood core carries only 12 percent. As you increase the
spacing between the two Fiberglas laminations by using a thicker core, you
automatically increase the strength of the bow by the square. Thus, if you
double the thickness of the core, you increase the weight of the bow four times.
Since the thickness of the laminations is measured in thousandths of an jnch,
it is easy to see how just a few thousandths of an inch more thickness in the
core can make a bow too heavy for your use."
This came from some older instructions on building a recurve bow..
They can be found here along with others.
http://www.vintageprojects.com/archery-plans.html