Face or back

[Longcruise]

Here's a question that I've been trying to muddle through on my own.   I'm not going to throw my opinion out there because I'm more interested in the opinions of our esteemed conclave of Bowyers.

This is in regard to glass/wood lam bows.

Which side of the bow should be optimized for to obtain highest performance?  IOW is it better to have more tension than compression or vice versa.   Or, should the two be in perfect balance?

What do you think and why?

[EvilDogBeast]

I would say you'd want as much tension as possible since that's where the majority of your cast power is coming from.  Since the belly glass is mostly there to keep the stack from collapsing on itself, reducing the amount of material there to the bleeding edge of failure at full draw would theoretically lighten weight and improve speed.

These are all just speculations on my part.  I'm not an engineer by any means, but perhaps one will weigh in.

[williwaw]

you could investigate the energy storage capability of fiberglass in both conditions, and the efficiency of how the energy is transferred  to the arrow. I suspect Chris's speculation is in the right direction.

Question for all if Michael doesn't mind,   
When testing new designs by varying back/belly glass ratios, what is the most common type of limb failure?

[Mad Max]

I would think thicker belly glass than the back would help

[Crooked Stic]

Back years ago when there was a thing called Walk the Talk on another forum several guys there were building hot rod bows. After optimizing the design they found thinner belly glass give gains in speed. But not durable in the long run.
They checked the bows at 28 in. with 10 GPP of draw weight. Then at 9 GPP with the same brace on each bow. 

[Shredd]

  Oh chit...  I can hear some of the guys shuffling around for some paper, sharpening their pencils and scrambling for their calculater...   

Here's my speculation...  First place I believe what you are really talking about in essence is shifting the neutral plane...  Me and Flem never settled that one...   

  Quotes from EDB and the Mad Man...  "I would say you'd want as much tension as possible"... 
                                                         "I would think thicker belly glass than the back would help"...

   I think both of these statements have some air of truth about them...  Tension and compression work together You really can't have one without the other in a bending limb...  Since tension trumps compression in this case I believe the neutral plane is a hair more toward the tension side...  Meaning the tension side is compressing a bit more of the limb than the compression side is stretching more of the limb on the tension side...  I also believe the trick is to increase the strength of the compression side as to put more stress on the tension side without increasing the thickness of the limb...  If you increase the thickness of the limb you will gain more tension and you will also increase arrow speed but you will also increase draw weight with no added performance of draw weight / arrow weight...

  I like to call dabbling in this direction the Alchemy of Bow making...  Kinda like turning Lead into Gold...  How much can you cheat physics and stack more performance into something??

   I just read Stics post and that proves my theory all wrong... Something to think about...  But who wants a bow that is gonna self destruct...

[Shredd]

   I just thought about what Stic said...  And if this is true that they decreased the belly glass for more performance there is the possibility of decreasing the draw weight while keeping the same stack thickness which in turn could possibly give you more performance... 

   You don't really know until you spend month's of experimenting... 

 Alchemy I tell you...      

[williwaw]

they found thinner belly glass give gains in speed. But not durable in the long run.
 

Stic,

"not durable"....   can you recall how the bow(s) failed?

[EvilDogBeast]

I am pretty sure we are overbuilding with glass as it is right now, and the glass used could actually handle quite a bit more stress.  Personally I suspect we are at the limits of wood cores.  To increase arrow speed we need to increase the rate of return of the limbs.  The use of composite core material could produce something that returns to neutral faster than wood, while allowing for a narrower profile.  Thinner glass reduces weight, composite cores increase return speed, narrow profile reduces air drag... bang bang faster bow.

On the topic of failure and glass thickness/overbuilding:  Has anyone ever seen the glass itself fail?  I don't recall ever seeing a bow that blew the glass.  It always seems to be a disbond or a failure of the core.

[Shredd]

  Willi...   I am pretty sure that they probably buckled...

[Mad Max]

This one is pushing the limits, No core problems
50-1/2" NTN 7-1/2" reflex






Here is a link to Pete Ward bow review
"All readings  were taken at one time, with a static release, Cavaler Elite Tab, Fresh Batery, and draw lenght's witnessed and coached by my wife Pat."
http://peteward.com/2007%20new%20pages/test.sheepeater.html

[Bowjunkie]

I've seen glass fail on some recurves. They all failed on the back in the area of the fadeouts, or fadeout lams, or wedges in the case of takedowns. They did not blow or delaminate at all because they owners stopped using them before complete failure. Any experienced bowyer could look at these bows and deduce exactly why they failed. But that's not the point. The point is, not a single one failed in these compromised areas on the belly. It was always the glass on the back that fatigued and then failed. This leads me to believe glass is stronger in compression resistance than tension resistance, but I don't know that for a fact.

These bows were brought to me to refinish because in each instance the owners believed the stress marks were only in the finish. I refinished two of them, and they looked almost like new when I was done. Their glass was brown, and the faintest smokey shadow was barely visible afterwards. Even if I pointed it out to you, it was almost impossible to see. But after a few weeks or months of additional use, it came back, and then with continued use, the glass fibers eventually began to raise to the point you could feel them. This happened in an area about 2-3" long and the whole way across their limbs.

[williwaw]

  Willi...   I am pretty sure that they probably buckled...

If I understand the principles of materiel strength correctly, then it is reasonable to assume an excessively  strained glass lam in compression will buckle on account of an underlying core failure?

I've seen glass fail on some recurves. They all failed on the back in the area of the fadeouts........... and then with continued use, the glass fibers eventually began to raise to the point you could feel them.

And an excessively strained glass lam in tension, seems to fail when the epoxy in the lam separates from the glass fiber?

Which side of the bow should be optimized for to obtain highest performance?  IOW is it better to have more tension than compression or vice versa.   Or, should the two be in perfect balance?

What do you think and why?

Since the strain levels at failure in tension and compression are failures of different natures, the "balance" for highest performance could be a design that maximizes the acceptable working strains on each side of the bow. They are most likely not equal. 

The point is, not a single one failed in these compromised areas on the belly. It was always the glass on the back that fatigued and then failed. This leads me to believe glass is stronger in compression resistance than tension resistance, but I don't know that for a fact.

Jeff,
did your appraisal of the failed bows take into consideration the thicknesses and widths of the glass lams both back and belly?  Your observations about the failures occurring in the fade areas complicates matters if the fades were not symmetrical front and back.

It always seems to be a disbond or a failure of the core.

Chris,
Did the failures you examined have pieces of the core still epoxied to the glass, ruling out a glue failure?

[williwaw]

This one is pushing the limits, No core problems
50-1/2" NTN 7-1/2" reflex

that's short, how wide are the limbs?

[Mad Max]

Start with 1-1/2" glass

[EvilDogBeast]

Hard to say for sure will.  The broken glass bows I have seen have all been pictures.  Add to that the relatively little effort it takes to peel the lams apart once an edge comes up and it's hard to say.  However, in all the ones I've seen the core is sheared and the glass is mostly, if not completely,  intact.  I have probably reached the end of my usefulness in this conversation, I am merely an aircraft structural mechanic lol

[mmattockx]

Which side of the bow should be optimized for to obtain highest performance?  IOW is it better to have more tension than compression or vice versa.   Or, should the two be in perfect balance?

Perfect balance will be the most efficient way, but in this case 'perfect balance' would mean that both the belly and back are strained to the same percentage of their failure strain. The belly side should fail first because compression is an unstable condition and tension is stable, but that assumes perfection in the construction which may or may not be the case.

I would think thicker belly glass than the back would help

This goes along with the compression side failing first and makes sense to me.

I am pretty sure we are overbuilding with glass as it is right now, and the glass used could actually handle quite a bit more stress.  Personally I suspect we are at the limits of wood cores. 

We never use more than about 50-60% of what fibreglass has to offer. You are correct in thinking that the cores are often the weak link. Care must be taken to not overstress the cores as the wood can take ~1/3 the strain of fibreglass.

I've seen glass fail on some recurves. They all failed on the back in the area of the fadeouts, or fadeout lams, or wedges in the case of takedowns.

That's interesting and indicates there were other issues causing the failure beyond simple tension/compression loads. It also means the glue is doing a great job of a holding the belly lams onto the core.


Mark

[Bowjunkie]

"And an excessively strained glass lam in tension, seems to fail when the epoxy in the lam separates from the glass fiber?"

Perhaps that's partly what I saw. But the glass fibers themselves were failing too. If you fold your hands by interlocking your fingers and then straighten your fingers so they stand up a little, that's what the glass fibers were doing. Did they 'come unglued' and then break, or vice versa? I don't know.

These bows had seen decades of regular use before failing.

No glue joint issues in those bows. They stayed together. Yes, there were other root causes concerning how the fadeout was executed, it acted like a hinge of sorts, creating much strain of both types in a very specific, isolated spot, and it seems to me at least that if glass was weaker in compression resistance, that it would have failed there in that spot in compression and not tension.

Has anyone ever seen glass fail only in compression? As in crushing, fretting etc... without wood or glue joint failure?

[Roy from Pa]

Most glass bows break just outside of the fade.

That area is a high stress area.

With thousands of shots out of a bow, sooner or later a weak spot is created there.

My old bear 1972 takedown, with prolly 10,000 shots out of it, broke just outside of the fade.

Wasn't a glue failure, just finally got weak enough and broke while at full draw.

That was exciting as hell:)

[Walt Francis]

This one is pushing the limits, No core problems
50-1/2" NTN 7-1/2" reflex

that's short, how wide are the limbs?

.

If I remember correctly, Brent narrowed the limbs on The Sheepeater to 1.40”.  I narrowed the ones made on his forms to 1.40 and 1.35.  I bought one at an auction that was really beat up and narrowed it to 1.30” to clean up the edges and it still shoots like the other sheepeaters.