Double bow backing?

[markinengland]

I have been thinking of double backing a bow. A form of two-step Perry Reflex. I wonder if anyone has tried this?
My thinking has been along the following lines.
Backing a bow using Perry Reflex locks a certain amount of tension inside the bow around the glue line. A thick belly and thin backing glued into relfex means that the belly is put into a certain amount of tension and the backing into some compression. Some of this pulls out when taken out of the form but some of it stays within the bow. The bow is now a higher draw weight than in would have been if glued up straight. Work can be done so that the bow is pulled to straight so that the outer surfaces of belly iand backing are now under no forces, meaning that the force put in is the glue line. The bow performs better and is oddly under less stress at the outer critical surfaces.
My thinking is that if this is done once to good effect it may be possible to do it twice for even better effect.
My problem is trying to think out how this would work and how to best use it. This is part of what I love about bow making. So many things to get your mind around, and so much that doesn't actually fit any of the things you do work out!
Type one. What if the bow were glued into 2 inches of reflex, some pulls out but another backing is put on and glued into 2 inches of reflex once more. The first glue-up locks forces into the glue line even though some of the reflex pulls out. The second glue up locks forces into the second glue line and less will pull out. In this way the Perry Reflex is added up and should give better performance, or does the second set of Perry Reflex reverse or under the first?
Type two. My other thought is a little more strange and harder to get the grey cells around but may I think be the better one. What if a bow was first glued into 2 inches of deflex or reverse Perry Deflex. This would lock forces into the glue line (it must) and the force locked in should be somewhat the same as if into reflex. Now you have a backing under slightly more tension and a belly under slightly more compression, as if they were in actual fact each a higher performance material. Now the stave is glued into 2 inches of reflex. The bow looks like a 2 inch reflex bow, but is actually in 4 inches or reflex. The initial force locked in on the fiirst backing are still there but have come back the other way and are added to by the second. This could perhaps be quite a good combination. The only way to tell is to try it, but has anyone tried it or heard of this?
If the weather stays good I plan on doing a few glue ups for some bows I am making so may be able to report back. I could make identical straight backed, type one and type two bows and see how they come out. Trouble is, at the speed I can work at it may be summer by the time the bows are finished!
Mark in England

[Pat B]

Mark. In TBBIII, page 91, paragraph before ...Perry Reflex... is about just what you are talking about...I think.    Pat

[markinengland]

Pat,
Thanks. What TBB3 says is failry different to what I am suggesting. The section you mention talks about the reverse of my increasing reflex thought, basically iusing thin lams, ncreasing deflex from extreme reflex. Maybe the only way to see what happens is to do it and see what happens!
What started my thinking some years ago was reading about how turkish bows were made, being pulled into increasing reflex as each layer of sinew is added.
Mark

[tom sawyer]

I've thought about this too, building more tension into a tri-lam.  I sort of favor the Type 1 option for adding stress to the interior and having similar amounts of stress on the two glue lines.  Although depending on the bow's dimensions you might want to spare that interior wood as much as possible and that might be accomplished by the Type 2 method.  In either case, you have to be careful not to esceed the elastic properties of the wood around these glue lines.  You can only build so much stress into a bow before it fails.

Good luck, it is an interesting proposition.

[Rich Saffold]

I've glued the belly and middle lam into about 1/2" of reflex in an r/d profile and bumped it to just over an inch and a quarter with the backing.  I think for competition and high performance bows its a good method for achieving  high end speed, but the low limb mass slender limbs makes for interesting tillering.   I used ipe belly.yellow boo for center lam, and black bamboo for the outer lam.

I know it works, but there's a limit, and bows that get made for friends will get a dampened version of this style..

Rich,

[tom sawyer]

Another possibility, would be to glue the belly and middle lam into say 6" of reflex, then glue the backing on in just 4" of reflex.  That'd put the backing into compression when unstrung, and load up that belly-side glue line even more when strung and pulled.

While we're at it, you might use a R/D design and possibly focus your extra forced reflex only in certain areas, like near the handle.  Seems like that design is pretty stressed at mid-limb already, so you could enlist the inner and outer limb sections more with extra built-in stress.

Now should we talk about the permutations of four lams?  Yikes, of course the bow would be mostly glue at that point.

[bowmo]

You mean like a tri lam? I've glued in middle lams and then increased the reflex an inch or two more when I glued on the bamboo...

dan

[Justin Snyder]

Like Rich said, it has its limitations.  I have done it with the first glue up at 2" and the second at 4" and it wasn't all that good.  All the reflex pulled out by the end of tillering and it seemed to shoot like an average bow.  My center cores were to dense of wood I think.  I am going to try it again with a lighter weight core and dense belly. Justin

[markinengland]

Tom,
You could make a wood A, wood A bow backed by wood B. It would not be a tri-lam bow as much as a backed bow in which the belly was two parts of the same wood glued into relfex of deflex before adding the backing. I wonder if the basic reverse fnction of the "simple" Perry Reflex would work best in type 1 or 2? Maybe the reverse of what you think would work would work? (like in normal Perry reflex!).
Yes, there would be a limit. but I think we could maybe get a lot more that we currently do out of the woods we use. Woods tend to fail at the surface, yet Perry reflex moves the work to within the wood. Moving more in may increase the performance we can get out? Increased Perry Reflex effect may move the point of failure to within the bow! Maybe we can invent ther exploding bow grenade if we go over the basic limits? 
Richard,
What effect have you found when you have done this?
I can see that tillering can be interesting. I know that with Perry reflex you can remove wood and get increased reflex as you go. Obviously there would need to be a degree of pre-tillering as you go to avoid mind numbing effects!
Tom,
Would glueing in high then low reflex increase or decrease the compression of the backing? My thoughts would be that glueing the second backing into delfex would reduce the performance and undo the previous work done on the first backing. is there any real way to test this? I like your idea of using this to move and spread the stress away from mid limb. Of course, a man could go mad trying to work out how to do this!
Justin,
Do you think your second lam somehow undid the first? It sounds like it.
Mark in England

[tom sawyer]

As far as effects on the neutral plane, I tend to think there would not be too much.  No matter how you glue up the lams, you still have the decidedly "unstretchy" bamboo backing causing just about everything inside it to be in compression.  I agree that moving more stresses into the interior of the wood is what makes a Perry reflex perform better, and having two glue lines both under some shear stress would seem to work even better if nothing failed as a result.  I would worry about having a glue line too close to the belly surface.  After all, the belly is usually the weak link and putting it under shear stress in addition to compression stress might be its undoing.  That is where I was coming from with thinking Type 2 might be preferable, there would be even less shear in that belly-side glue line in that instance, than if you glued it up straight or reflexed.

Since we are ultimately pulling all three lams back in one direction, gluing them all into reflex intially (Type 1) would theoretically result in the most stress at full draw which I suppose is what we want.  However, since the neutral plane is the spot of no compression/tension stress, and this is usually near the bamboo backing glue line (from what I understand), then that is where you'd want to "bury" most of your additional stress (doen best with Type 2).

To summarize, heck if I know.  Makes a good case for a selfbow though.

[Jesse]

I made a r/d style bow with two layers of ipe and a boo backing. It performed better than a regular r/d boo backed ipe. I think 4 or 5 lams would perform even better yet. You end up with a light, fast bow but its more that can go wrong.
                                                                                                      Jesse

[Justin Snyder]

Mark, I think mass was my biggest enemy. My choice of woods was not very educated.  I should have used a super high compression wood on the belly not on the core lam. 

If we are trying to shift the high stress area away from the back where failure occurs it would make more sense to do the center lam and back in high reflex then reduce the reflex while adding a belly lam like was mentioned in TBB.   Justin

[Rich Saffold]

Mark, just what you said remove wood and watch the reflex increase. Plus our humidity is been very low and this only amplifies it.. I have found its better for me to start with less than I would think I need, and let the humidity, and how I tiller it determine the final shape.

My favorite lately is tri-lam ipe, two boos, elb style. It stays a little more tame than having a long rigid grip and short limb sections..but shoots very well since the limb mass is very low for this style..

Rich

[Marc St Louis]

Doesn't Chris Boyton make some of his lam bows like this?

[markinengland]

Marc,
Chris Boyton glues two thin hickory backings into extreme circular shape over a former. He then pulls these down onto the belly stave. This pulls the belly stave into reflex. I guess this is another way of doing pretty much the same thing.
Mark