Fiberglass bowyers, a question

[StanM]

I've seen fiberglass reflex/deflex bows that are trapped from wide back to thin belly (RER longbows I believe), and other bows that are trapped wide belly to thinner back.

My question is which method is better and why?  Thanks,

Stan

[O.L. Adcock]

Stan, I think you asked a question that can't be answered. The internal forces going on in a bow limb are extremely complex with hundreds of variables. You'll hear a lot of theories and guesses supporting both sides with no proof one way or the other. The only thing I know for sure in any structure if you weaken one part of it, you put more load/stress on the other. We know materials are stronger in tension then compression. That means you can probably get away with less material in tension is one logic...The other is why remove material that's doing you the most good?? I'm not going to find the strongest element in my house and saw half of it away!    The fact they can be built either way with no measureable differences indicates the structures are over built with lots of room for error. I'm sure one way is better then the other but you don't know where the line is till you go too far and then it may not be obvious....O.L.

[StanM]

Thanks O.L.,

Trapping the back is what has always made sense to me for the tension vs. compression reason that you mentioned.

My main reason for asking is that I'm about to start another archery class in my middle school this year and I've got a great formula for straight limbed bows, but I'm going to try to build a reflex/deflex form for the students to try out this year.  My first attempt at getting the weight right for them may be off a little and if it's too much weight I thought we might trap the limbs to take some weight off and didn't know which way to go.

Always appreciate your insight.

Stan

[O.L. Adcock]

Stan, In a flat cross section limb "trapping" can be used to help hit draw weight and or adjust where the limb bends and how much. Heavily trapping the tips and transition to no trap in the middle to the fades will make a bow stack sooner. (soffening the tips) Trapping from the fades to the middle and transition to rectangle at the tips will do the opposite. (stiffening the tips) Trapping the entire limb changes nothing except draw weight. It's just a tool.....O.L.

[StanM]

Thanks again, O.L.  Good info for helping with the tiller.

My plan for the outline of the bow is deflex the limbs about 1 inch and then reflex the tips 2 inches, putting them approximately 1 inch behind the handle.  I'm building the form to make bows out to 68", but plan on 64" - 66" as the norm.

Any thoughts on ball park glass and core laminations to hit weights of 30-40 lbs.?  I know there is some trial and error, but I'm just looking for a jumping off place.  All help is greatly appreciated.

Stan

[O.L. Adcock]

Stan, You are asking the wrong person. It's been so long since I've built flat limbed bows I wouldn't have a guess other then it'll be about 50% thicker then I'm used to!    Maybe someone else builds something similar that'll get you going in the ballpark....O.L.