Flat belly, Round belly

[Badger]

SCP, I use specific gravity as in the mass principle only because it works on averaging things out. Once we get to know specific woods we go a bit one way or the other.

[Jim Davis]

    ... Strong in compression has nothing to do with good in compression. Many of the strongest compression woods are terrible about chrysaling. ...

But strength in compression compared to the same wood's strength in tension does make a difference in how a bow should be designed.

A wood that is "terrible about chrysaling" is a wood that has been overpowered by a back that is much stronger in tension. We can relieve the belly of some of the work by reducing the amount of tension wood in the back.

Black locust is infamous for chrysaling. But I have not had one chrysal since I started narrowing the backs. Before that, I made a BL bow that had 5" of reflex when unstrung. It was a pyramid design (as all of mine are) and well tillered. It shot very nicely, but it was a maze of chrysals from tips to handle. It performed fine with the chrysals, because they were everywhere--no hinges.

Badger, I think you are probably  correct about the neutral plane being farther from center because of the difference in compression and tension strengths. But that ends up meaning the belly does all the work.

jm $.02

Jim

[JackCrafty]

I actually spent some time in an engineering lab testing different materials until they failed.  Different materials fail in different ways.  Some do not show much sign of damage before failing and some get all ugly and scary before they fail.  Point is, just because something might take a lot of abuse before failing doesn't mean it looks pretty doing it.  Like Badger said, we are concerned with the elastic limit.  That's the point where the material will still go back to it's original shape (or close to it) and before it starts to get ugly.

[George Tsoukalas]

If you tiller a bow with hand tools you will automatically have a slightly rounded belly more than likely. Hand tools function best with rounded bellies so the tool you use will determine the design. The bellies of my bows are slightly radiused or rounded because I tiller with hand tools. Jawge

[Marc St Louis]

There seems to be quite a bit of confusion about what is desirable in bow wood.  Compression strength has nothing to do with how good a wood will be for making bows.  Some wood species have very high compression strength yet will chrysal readily.  Bubinga for instance has very high compression strength but it will chrysal fairly easily if over stressed, Black Cherry is even worse for chrysalling.  The most important property that a wood can have is elasticity.  That will determine how well it will tolerate a round belly in a bow.  A wood's compression/tension strength will determine how physically small a bow can be made for a given draw weight, generally speaking its mass will still be the same though.

[crooketarrow]

  Badgers TOTALLY RIGHT also like someone said you can't build wood bows from spec's. Its not lams of wood and glass. DESIGNS not specs either. I 've went throuhgt the stage where I built all types of bows and designs. I have built many wooden bows both ways flat and rounded bellys with all woods both ways and keeping the limbs a flat ovel seams to be in between the scale.
   I can't see a difference. If your build like D bow designs thats designs to the exstream. Then woods like OSAGE, YEW  are far better with that design than say HICKORY or some other white woods which prefers a flater belly. But it dos'nt have to be straight flat with a starp edge.
 I've found that the flat ovel limbs are in the middle of that scale. And don't have any affect on set or string follow as long as you don't make your limbs to rounded.. And the rounded limbs (FLAT OVAL)  helps with chrystalizing on all woods.
  Thought the years I come to build bows my way, my own design and center shot, flat ovel limbs. Produce a releiable, fast good cast, spine tolerant, little set little string follow hunting bow. I just start with a straight 2  inchs of reflex added stave. Any swists or slight curves you can take out in tillering. If the stave so out of line I have to use heat I wou'nt ues it. Matter fact I would'nt have cut and seasoned it in the first place. I know you'll say well we have to use what we can get. Thats fine makes you a better a building bows.
  But you went through the trouble of getting that suckie stave why not just get or cut only good ones.
   I'm not trying to get anyone to build your bows my way. Just stating the way I've came to build all my bows and what works for me.

[scp]

The perennial question is whether it is wood, design, or both. I'm still in the camp of design, within reason, of course. I would not say that a pine bow can shoot as well as a yew bow, no matter how well it is designed to fit its wood. But I do think a well designed oak bow would outperform a mediocre yew bow.

As for the compressive strength, its number alone does not mean much in bowery. It needs to be used in conjunction with the wood's elastic strength. Hence, I'm interested in the ratio of elastic/crushing strengths. The index might be useful in determining the width/thickness ratio of each wood. At least in the beginning stage of bow making. It also says that it would be dangerous to make round bellied longbow out of red oak (266%) without backing it. How far we can go with this simplistic heuristics is a matter to be tested empirically.

IMHO it would be silly to round the belly of wood with its index higher than 200%. That's about it, for now.

[crooketarrow]

  I understand where your coming from.With a plain wooden bows not lams with the index higher than 200 % how wide ,leanth and poudage you want.People never figger in poundage but say where a HICKORY BOW flat back a 55#bow would'nt chystalize the straight edges a 75 # will.  Most of all your tilling exsperance can take over design some what. With wooden bows straight or square edges chystalize far sooner than rounded edges no matter the design or wood type. Again tillering comes into play here again.
  I say this ,you can take a bow with the right design for the wood and if your tilling sucks your bow sucks. But I can take a bad design bow for that wood and with good tillering make that bow exseptable. It may not be as good as a bow as it could be with the right design but you will make a exceptable bow.
  I'D SAY IT'S BOTH THATS WHY A FLAT OVEL LIMB DESIGNED  WELL TILLERED  FALLS WELL IN BETWEEN DESIGN AND TYPES OF WOOD.

[rossfactor]

It might be helpful to post some definitions of different properties of wood: here are mine wrt to bow building, please correct me if I'm wrong.

Compression Strength: The resistance of a material to breaking under compression, or the amount of force it requires to crush the belly cells of a bent bow.

Tension Strength: The maximum stress that a material (back of a bow) can withstand while being stretched before it fails.

Elasticity:  The tendancy of wood to become deformed when a force is applied to it.

Gabe

[Badger]

Gabe, tension and compression strengths are how much force it takes to bend, or how hard it resists bending.
Elasticity is how far you can bend it before deformation starts to take place. Plasticity is the area between elasticity and failure. We don't even want our bows going into the plastic range. Woods that tend to chrsal have a very short range between elastic and failure, the plastic range is very small.

[rossfactor]

Thanks Steve, that's what I was trying to say, although you were more succinct. 

So... a large plasticity range isn't necessarily a good thing then.  Even if the bow doesn't fail, excessive deformation (set) damages performance.

So, like Marc said, elasticity is probably the most important feature in a desired bow wood. Because  tension and compression strengths can mostly be addressed with limb design.

As an aside (in another thread) I think it would be really interesting to evaluate the relationship between the properties of wood (e.g. elasticity), limb design and hysteresis.  There might be some useful info hidden in there. Maybe this has been done though.

Gabe

[Badger]

  Gabe, the study you mention would be a good one. The backyard laboratory work I have done kind of indicates that hysterisis goes up dramaticaly once we get into the plastic range. Woods like black cherry and locust for instance have very small plastic ranges and likewise have very low hysterisis compared to other woods. Some woods will allow us to work deep within the plastic ranges of the wood often still mainating a decent profile, invariably these bows will shoot like dogs inspite of impressive looking profiles. Osage can be a big offender in this one as well as some white woods like hickory and elm. Moisture aggaravates this greatly.

[rossfactor]

Ok, so what would be an ideal design to reduce hysteresis in a wood like black cherry, with small plastic range and high compression to tension strength ratio.

Theoretically it would be very different from the ideal design to reduce hysteresis in a wood like Hickory with a large plastic range and a low compression to tension strength ratio.

This could be fun.  Damn, not enough time. Steve, you'r retired, you should get on this .

Gabe

[soy]

Black cherry keeps popping up so o will ask a question using that as the example ...aside from mc, damage (set) is caused primarily on the belly side as is chrystaling so to reduce both trapping the back on said wood with a semi ovald belly with a pyramid design would be a way to make a good performing bow???

[rossfactor]

Black Cherry is  stronger in compression than tension. Trapping the back would focus the tension forces to a smaller area, which would be a bad thing for cherry.  Especially if cherry has a lower plasticity range. You'd rather trap the back on a wood that is strong in tension but weaker in compression.

Gabe