Why the Glass?

[UnderControl16]

So simple question, I'm planning on making a longbow reflex/deflex laminated bow this spring.

Why do you use glass and do you need to use it?

I'm thinking of doing .12 Hard Maple on the back, .15 Action as the core, and .1 Jatoba as the belly. I'm shooting for 45-55#, 12" handle, 66" ntn, 1.75 wide with a straight taper to 3/8" at nock.   Thoughts????

[WILL CAROTHERS]

The point of glass is usually just for extra support for the lams, i don't know how hard maple would do as a backing so with that formula you may need glass, but if you put well grained hickory, it wouldnt really be needed.

Im no epert, so lets wait for the others to chime in

[Pat B]

With good grain configuration hard maple will work for a bow backing. There are better options like hickory or boo. I'm not familiar with the compression strength of jatoba but that should also be considered when you consider a backing. FG is never necessary as a bow backing unless you want a FG backed bow.

[Mike Most]

Another way to answer your question is this. Being able to use a variety of woods and utilize the glass for support and strength where things like compression and tension are questionable or unknown, (it gives you a working bow the first time out)  

[UnderControl16]

So then if you have a glass back and belly does that mean you can consider the other layers cores almost and not worry "as much" about the tension or compression strengths?

[Pat B]

In that case the tension and compression are handled by the glass.

[UnderControl16]

so there is no concern at all about what makes up the core then?

just trying to make sure that my first one comes out right.

[jsweka]

There's still some thought given to the core materials, but it is less critical.  For example, I wouldn't use some exotics like bubinga or bacote for all my laminations, but they work when you have laminations in the center of your stack that are made out of the tried and true woods like hard maple, bamboo,osage, or red elm.

[Living_waters]

A few thoughts on core material. From a performance point of view, when your glass is taking the majority of the tension and compression, core material has little effect. A core of maple or foam shoots relatively close to the same. The big difference is in the feel of the bow. I just built a maple cored short recurve with only two lams and it feels like a 60 lb bow at draw when it is only 50lb. A foam or softer core would have felt softer at draw.
Using thinner glass lames therefore decreasing the ratio of glass to core, makes the core more important in how the bow preforms.
Also I have noticed how the number of lams makes the feel of the bow change. few lams seems to draw harder then multiple lams, dont know why it may be the number of glue joints, the glue joints are points of stored energy. The more joints the more it spreads the work.

[Pat B]

Being how the glass doesn't stretch or compress I believe the core does little more than hold the glass on. Different cores might make a little difference because of their physical weight but I think that is about all.
  A while back a few commercial bowyers began making glass backed wood bows but I think the glass was too much and that practice shortly wained. I think Mike at Driad was one of the bowyers.
  If you want to build a glass lam bow, build a proper glass lam bow. If you want to build a selfbow or all wood lam bow then build it properly and your success rate will be better.
  Thinking that a wood bow is less durable, strong or whatever is rediculuous! I have selfbows and hickory backed bows that are over 10 years old and still shoot the way they did when built. With proper care a selfbow, wood backed bow or an all wood lam bow will out last you.

[rmorris]

Here are a few comparison numbers for you, the fiberglass most of us use has a tension strength of 150 KSI or 150000 PSI and a compression strength of 110 KSI or 110000 PSI. Hard maple on the other hand has a compression strength of about 7.3 KSI or 7300 PSI. There is over a factor of 10 on strengths of the material so when you use glass in a bow you get much more of the strength from the glass not the wood.

[okie64]

Your design sounds like it will work fine without glass but I would consider using 1/8" hickory instead of maple for the backing. Jatoba is plenty strong in compression, probably too strong for a maple backing. Like Pat said, I think on glass bows the core material is just something to take up space between the glass and doesnt make a whole lot of difference in performance.

[UnderControl16]

So how would you finish an all wood laminate bow then? Would you just polyurethane?
Would a .12 Hickory, .15 Action, and .1 Jatoba at the above dimensions give me the poundage I want?

So it is uncommon to only have glass on one side?

So Living Waters are you saying that it would be better to have three cores then one or two if you are putting on a glass back and belly?

[okie64]

There are a lot of different finishes you can use. Tru oil, tung oil, shellac, polyurethane, animal fat, paste wax just to name a few. As for your stack thickness, I would make the belly lam thicker to allow yourself room for tillering. 1/8" thick for back and core should work but your belly lam needs to be thicker. When I build trilams I use a powerlam also and let it extend a couple of inches out past where the handle stops. You dont have to use a powerlam but if you dont you will have to make your belly lam even thicker that way you can blend the fades into the limbs to keep your handle from popping off. If it were me I would go with 1/8" back and core, powerlam 1/4" thick in the middle tapered to feathered edges and a 5/16" belly lam.

[okie64]

[UnderControl16]

Can you or someone explain more what a powerlam and or post a picture of one? Thanks everyone for all this advice.

[Pat B]

A power lam is a thin(usually) piece of wood, double tapered, that goes between the back and belly(or core) at the handle area to prevent the handle area from bending and the handle riser popping off. If you are familiar with this particular problem and leave the handle area stiff enough a power lam is not necessary. I have made many backed bows and a few multilamed bow but have never used a power lam.

[okie64]

Like Pat said you dont have to use a powerlam but it allows you to be able to start out with a thinner belly lam since you dont have to work the handle into the limbs so much to keep it from popping off. I usually let mine extend out 2 inches past the handle and then thin the belly lam into the limbs a little bit. You could probably let it extend out a little farther and not have to blend the belly lam in quite as much as I do, thats just how I do it.

 
 

[UnderControl16]

The riser design that I am thinking about has long tapers and also has the belly flair onto the riser. It is basically the handles that binghams uses for their reflex/deflex longbows. Is the handle popping off something that I need to worry about?

[UnderControl16]

When selection a core is there anything that you should be looking for? If you chose a heavier wood would it absord more hand shock? However would a lighter wood be more flexable and not pop off as much?