I was waiting for my turn at the range the other night and was looking at the bows. Not counting the compounds every bow there(except mine) was a FG recurve.( I can be a little slow sometimes 
) Ignoring the FG part all the designs were deflexed recurves. Not one FG longbow and I know they made them when I was a kid. This got me thinking that somewhere in the past bow making companies must have done a lot of testing of various designs and all decided to make basically the same design. There must be a lot of data squirrelled away on the effect of various tests. Does anyone have access to any of this stuff? I realise that FG data won't be directly transferable to wood but a good design is a good design.
I go back and forth between natural material bows and glass bows. I don’t know if there is much data squirreled away for glass bows. Most glass bow builders tend to copy prior designs as a starting point and this gets them something that works well and reliably most of the time. I feel the primitive bow builders have a much better understanding of materials and design because they have to. The wood bow builder has to design their bows much closer to the limits of the materials in order to achieve similar levels of performance.
So why do most glass bows look very similar to one another? It is because they are very economical to build this way. All you need is a fixed thickness of glass on the back and belly, a decent core with a constant taper, and a tried and true recurve profile, and you end up with a decent performing bow that should last almost forever. Performance with 10ggp arrows and 28” draw length doesn’t vary much from the best to worst. The deflexed limbs help make the bows easier to string. There are also various standards in place (for example AMO) which standardizes some features of the bow limbs. For example, it standardizes the string length relative to the limb length, and this limits the design possibilities for those manufacturers who conform to these standards. There are other formal and informal standards for takedown bows that help insure interchangeability of various manufacturers limbs and risers.
Some bow builders try to get a bit of an edge over others by experimenting with materials and taper rates in their bow limbs. Carbon fiber doesn’t guarantee a boost in performance. Most designs that report an increase in performance are pushing carbon much closer to its durability limits than their original glass designs. For example, there was a trend for awhile to make high performance compound bow limbs using carbon, but the limbs were a disaster when it came to durability. Now, the pendulum has swung the other way and short glass limbs are used to store the energy. This combined with big cams takes full advantage of the capability of glass to store tremendous energy in bending. The idea is similar to using the material of horn and sinew to store energy and a large stiff and light wooden ear or throwing arm to perform a similar role as a cam.
Glass is able to store much more energy in bending for its weight than unidirectional carbon. But unidirectional carbon is a much more efficient structural material (better stiffness to weight) than glass. A bow requires the materials to store energy in bending and provide structure to hold the limb shape and control bending, so the two materials have their pluses and minuses. A carbon limb requires more limb length to store energy in bending. This allows room for more energy robbing vibration modes which counteract some of the advantages carbon has.
There are a new breed of modern bows showing up called “Super Recurves” that use huge recurves to give force-draw curves that rival a compound. Some even show some letoff at the end of the draw. The designs are not really new. Hickman proposed a similar design for a wood bow in the 1930’s. The difference is that lighter carbon fiber designs are using interesting combinations of weaves and fiber orientation to allow the material to behave a little more like unidirectional glass (much lower stiffness, but better energy storage capability in bending). The lighter materials reduce the limb mass to the point that these can outperform most conventional designs with heavy arrows. They also do not rely on reflex to improve the force-draw curve so the limb is under very low stress when the bow is braced, which favors keeping the bending stresses under control for a carbon composite limb. Unfortunately, the efficiency drops off so bad with lighter arrows that a conventional glass bow will run blow the Super Recurve away in a flight shoot with very light arrows. The old saying “the fastest bow with heavy arrows will be the fastest with lighter arrows” does not apply to a modern Super Recurve.
Alan
