Core glass...the elephant in the room

     For many years, I've heard and read many statements from archers wondering why I put a strip of core fiberglass in my bow limbs.  I've heard all sorts of theories and been amused that not one single time, has anyone ever asked me why I did it.  Why do you suppose that is?  Why the secrecy?    I don't know but I think it's time to say why I do it.  There are probably a lot of reasons why some other bowyers do it and my reasons will not be theirs I'm sure, but these are mine.

    Bowyers I know about that use a core glass, or did in the past, are Don Brown, Rocky Miller, Black Widow, Styrian,  Fairbow, and John Schulz.  Wait...What?  John did?  Below is a photo of John and his brother Dan's first bow brochure.  Notice the bottom right corner.  The deluxe bow option. Core glass.

    Many are the comments I've heard that "Schulz never did it.  Why does Sunset Hill do it?"  Well, Schulz did do it.  

        So there's that.  John's son Dave is doing the core glass these days and also Dave Miller. Why?  Well, in today's world of lesser quality bamboo, it helps in an all-bamboo non-glass belly bow to keep the bamboo from collapsing, folding and developing a hinge.  But that's not why I do it.

       When I started building specifically Hill style longbows, I wanted to match the feel of an all bamboo bow, but with the durability of glass backing.  Bowyers that make backed selfbows know that adding a backing changes the durability and shooting feel of the bow.  John Schulz's "Naturals" had a bamboo backing to the stave whether the stave was osage, yew, hickory, or bamboo.  What happens when you add a backing material to a self bow?  You make the bow inherently similar to the physical properties of a self yew bow wherein you have sapwood and core wood working together.  Sapwood stretches / tension, and core wood compresses.  Where those two forces meet, in the limb, there are shear factors involved.  Yew happens to be able to handle the forces.  Some woods cannot and need to be backed.  The backing and the added glue line, become the area that the internal shear forces are concentrated.  

      The placement of the backing is specific to the overall thickness and shape of the limb and to how it performs in durability, speed and stability / lack of hand shock.  The backing placement dictates where the shear forces are concentrated.  By adding a core glass to my limb, I am specifically placing the shear forces between tension/stretching and compression exactly where I want them because I'm introducing something greater than just a glue line.  I'm introducing a material that can do both and make the shear transition more constant.  I am telling the bow exactly where I want the transition to happen in the limb and therefore I can replicate as close as possible the feel of a selfbow or backed selfbow...in a glass backed bow. This reduces stress at the shear point and allows the bow limb to be flexed more without undue strain. It helps in the area of the fadeouts of a short riser bow because of this.  A Hill style longbow that has limbs bending all the way into a short handle adds stress in the fadeout area. A common remedy for this with some bowyers is riser "shims" but the core glass is a key helping factor.

     The placement of a core glass in my bows is very specific, not just "in the limb".  Its main purpose is what I just mentioned, but it has two other positive side effects.  One... it gives some torsional support to the limb at the tips, helping make the limb more stable without having to add tip insert wedges.  Two... it gives strength through the handle of a short-handled Hill style longbow.  

      John asked me why I added the strip of glass.  He'd made comments at a 3d shoot in Mississippi to some in attendance, that he didn't understand or know why I did the core glass.  Those comments didn't mean he wasn't in favor of it, as he had done it before as well.  When he studied my bows and knew the reasoning why I did it, this information lead to his comments that "I'd moved to another level of bowmaking". I had his approval.

      So there it is.  The secret's out.  Some people will embrace it and some will deem it not necessary.  Either way, I like it, will contine to use it in my very specific way and I wasn't the first to use it.  I don't know if other bowyers did it before the Schulz brothers, and John never told me why he stopped doing it.  But he embraced it at the end and recognized it's place in today's world of acceptable longbow building materials, while yet keeping to the basic Hill style of longbow making.

     Shoot straight...