Elasticity versus poundage

[gfugal]

So that makes sense that the core stiffness wouldn't matter as much since the outer fibers are the ones that do most of the work. But surely the core is stressed still. If it wasn't then it wouldn't be so important to have a blemish free prime wood core, or we could use a knotty piece of pine. So that tells me it is stressed at least to some degree, and if that's true its stiffness will have some factor. Maybe not as large as I was thinking but still present right? or is there something else I'm missing.

[BowEd]

gfugal...Yes of course.With any of these type stressful designs a good flawless core is required though.As are the other components.It does'nt make any sense to make a bow with materials with flaws in them in these type designs.
A less strained one possibly so.To walk that fine line is up to the bow maker.

[willie]

greg makes a good point, about the work done in the core. just because work is concentrated at the outer surfaces, a fair percentage of the work is done as you get closer to the center. much of the finessing done getting a bow to it's best potential means that all parts of the limb would be well served to be optimized.

Understanding theory may not be necessary to inherit knowledge, but understanding whats going on sure helps lowering the experimentation curve. I am all for learning both. Sometimes there are factors that we overlook in our theorizing.  Experience or history can often show, that the best is yet to be re-found.

BTW, is the flight record for the Turkish bow that has stood since sometime in antiquity, considered to be fact or myth?

[BowEd]

I don't know about the Turkish distance record.I've read some shots were not recorded.So who knows.Understanding their language describing things takes a little time.I'd say Adam has got a handle on that info.In fact he's testing bows right now on the atarn web site.
Core is important for sure.A dense elastic one with good glueing qualities would be my choice.Others may say different.Maple has been a go to core material for quite some time.I would say if you can't make a bow out of the core or it take the bend needed from a flat profile then your taking a chance using it is another way to look at it too.

[willie]

one aspect considering stress in the core, is the shear forces that come into play. typically with a self bow, tension and compression limitations at the surfaces demand very good wood selection, and govern the designs and materiel selections. As a point of interest, there was recently a bow posted that had a hidden flaw in the limb that resulted in a unexpected failure. A cursory examination of the accompanying photo, looked to me,  to be non-typical for a tension or compression failure. It would not surprise me if that bow failed in shear, at the central flaw, resulting in a weak spot that ultimately induced the total limb failure.
Shear failures are also often seen when a glued handle "pops off".

what reading I have done on composites, often mentions the need for careful core selection, leading me to believe that shear limitations often dictate in composite designs. this makes sense, as the sinew and horn is selected for their exceptional tension and compression qualities, asking the core to handle more shear stress that attends more radical bend radii.

I have often wondered if the preference for maple is on account of its diffuse porus qualities. It does seem reasonable that a shear force could be easily localized on the spring growth portion of a ring porus wood, especially if it were flat sawn, making it a poor choice.

[BowEd]

Yes the sheer forces going on need to be accounted for although if constructed properly not a problem,but proper construction won't override poor quality.
Yes your right about maples' diffuse porus qualities making it a good choice for glueing.Using ring porus with large early growth would not be my choice especially flat sawn.
I've been getting away with using hickory on mine.Very fine ringed.Early wood like dots.Flat sawn.Time will tell if I get away with it.So far it has.

[mikekeswick]

Willie - you are dead right it is a sheering force that the core feels, as it is on the neutral plane (give or take). If the sinew is too thin then the core starts to feel tension and will fail everytime....don't ask me how I know this!
Maple is indeed chosen for it diffuse porous properties, ring porous woods are likely to fail along the earlywood layers, or at the very least each earlywood layer is a 'built in' weakness. Maple will of course also handle being steam bent into a pretty extreme shape and glues very well. A good core wood will have all these properties.
People have used ash (and other ring porous woods) for cores on less stressed bow designs, however if you are going to use a ring porous wood then it is a very good idea to use it 1/4 sawn therefore 'homogenizing' it compared to flatsawn. The only issue with doing this is that wood is stiffer laterally with a flat sawn piece compared to 1/4 sawn. Flat sawn will make the bow quite a bit easier to stabilize - this is a major factor and becomes apparent when you first brace a bow.

[willie]

Making another composite bow here.Going with less thickness of a core by 1/16" and more composites.

just curious why you are changing the proportions on this bow.?  Are the working limbs bending to a tighter radius than the earlier bow, or are you shoooting for a lesser stringfollow? or better performance?.......

[Redhand]

Those are some excellent example Beadman.  I really like your bows. I need to attempt one of these.

[BowEd]

I did'nt give strict heed to a few of the rules making a bow like this with materials and design,but it has worked.Like with any bow time and usage is the ultimate truth extractor....Ha Ha Ha.
I think your bows are a work of art to be truthful.I need to sometime try an all horn or an all antler bow too.All these type bows seem to be the meaning of patience in primitive archery.

[BowEd]

Talk earlier on this thread was about how much work a core is doing.Think it was gfugal.I look at it this way.When 10% thickness of the limb on the outside[a backing] does 50% of the work yes the core is doing 50% of the work[one reason too for a decent ring on self bows] and 25% thickness of the limb on the outside[a backing] is doing over 80% of the work yes the core is dong around 20%  of the work too along with the sheer forces discussed.Quite a load off it already.With 35% thickness of the limb thickness[a backing] on the outside it is doing close to 100% of the work but the sheer forces strain will still be the same the way I understand it.Engineering minds come up with these findings of which I don't argue their point.The asians/indians/turks and other cultures figured this out long before PHD diplomas were handed out through trial and error I imagine.
That's why the 1/3,1/3,1/3 ratio of components they've shown are  ideal for horn/wood/sinew bow composites.This is old hat to those who have been making these but for someone new to it not and it's an explaination why it works so well.It does not mean though that is written in stone like Mike said but a good starting point.It can be 25%/50%/25% ratio too with very good results.

[willie]

there seem to be many different composit designs out there, some with shorter working limbs than others. Wouldn't a shorter working limb have to bend into a tighter radius?, and consequently need to have a higher proportion of sinew/horn to wood?

seems like longer and easier bends could use more wood, although the trick might be to properly balance the sinew to the horn strengthwise, so that the wood remains centered about the neutral axis of the entire limb?

[joachimM]

Willie, yes you see this actually, when examining cross-sections of different designs across the length of the limb.

But also bear in mind that moisture content management also makes a huge difference. Both horn and sinew strongly increase their stiffness from 12% MC (which is way too high for sinew-horn bows) to 4%. In my climate (temperate humid) I can hardly get to below 12% MC without a hotbox or similar.

[willie]

waterbuffalo horn, from a buildalong at       

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http://paleoplanet69529.yuku.com/topic/57441/Turkish-Bow-4-Year-Journey
the horn appears to be twice as thick as the sinew. I do not have adams book, so I cannot look it up for myself, but does he mention the relative strengths and working strain limits of the various horns and sinews?

[BowEd]

joachimM is right I feel to a degree.Depending on what a person can live with.Most times actual turkish horn bows are sealed with layers of finish and paint and guilding also slowing the infiltration of moisture.Adam talks of using them in rain storms with no ill effect.Just part of the landscape owning one you have to put up with using natural materials.Mike lives in England and owns many.Very humid there.Although it can take twice as long to lose moisture as acquiring it in them.Horn is affected only in half the degree with moisture as sinew and glue will be so it's resiliency is better.
Putting the bow back into a say 7% to 8% MC warm setting over a period of a week from a few days of shooting[braced throughout] it would return itself to original profile it took after tillering.
willie...As far as different strengths of horn goes I believe he mostly was satisfied with water buffalo horns' strength and use.Some very early bows made from ibex horn.He talked of cow horn being too thin,twisted and having delamination problems of which I've seen for myself.Gemsbok horn is ok too if you can find some thick and wide enough.I have myself here.Sheep or possibly goat horn done by a few fellas on here make great horn sinew bows from it.Works of art.Chuck,Goat,and Redhand has one going too.I feel it could function in a horn/wood/sinew construction.Bahleen I would like to try also.I'm sure it's a lot denser then wood and could be used.
Sinew of most types is good as far as strength goes he feels.He preferred moose leg sinew.Did'nt like any backstrap as it being too course and hard.I feel Pat M's method of wrapping and heating helps eliviate this.Actually sinew will shorten when it swells.To dry and shrink to that state.I've used beef backstrap,buffalo backstrap,elk leg the most of,moose leg,and deer leg and backstrap.All are strong enough to use.As long as the grains of weight add up to being what you need.I have not used ostrich.Getting rid of the grease is the thing to watch out for using any.
I would say that the bow you saw with thicker horn depending on proflle of course would still have to have enough sinew on it to a thickness to stop any chance of splintering yet.
I can say in truth I shot the previous bow I made close to every day for close to 4 months straight for sure 6 to 8 hours a day bracing times in misty rainy days too[then unbraced overnight] with maybe 1/4" of set showing to the next day.With one full day of rest it was right back to original profile when first tillered.That's being stored inside the house then too inbetween shooting.Very little stress shown on these for the amount of reflex it held.Resilient.