Elasticity

[DC]

A couple of times people on here have mentioned that a wood has high or low elasticity. How would a person recognise this?

[PatM]

By bending it and noting how it reacts.   You can sometimes get a feel for how elastic a wood is while working it.   Many woods have well known elastic properties so you are then judging and noting how elastic it is relative to its typical levels.

 Then you have your hard elastic and soft elastic woods.

 Likewise there are soft and hard non-elastic woods.

[Badger]

DC, you often hear guys talking about letting the wood talk to you. It is usually in very vague language so it doesn't offer much help. What talking to the wood amounts to is simply monitoring very closely the condition of the wood as it progresses. Anytime it shows signs of stress simply back off on the area showing stress. If the entire bow is showing stress back off on the entire bow. We get to select the dimensions we start at but then it is up to us to make sure it can tolerate what we are asking of it and come out unscathed. I have made bows up to 100# that didn't break but really should have been closer to 60#. Much better to stay within the boundaries of elasticity.

[DC]

So it's an experience thing? I have to learn to gauge the thickness against the bendyness? Is there a difference(for bows) between a thin stiff piece of wood or a thicker elastic piece? Why do people chose elastic wood when they can use a thinner stiff piece?

[PatM]

   The difference is more between the woods in that scenario.   The wood itself may demand one option or the other.   Say Black Locust and Yew for example.  For some woods it's said that "thicker is quicker" so if it can be narrower and deeper it should be if you want to get the most out of it per mass.

People choose elastic wood because it allows a bit more leeway in a bending application.   Not to say dozens of bows aren't still made out of woods that are bit more touchy about max bend limits.

 If you have a highly elastic readily available wood then you're probably going to use it.  If not you'll likely choose something like Ipe.

[bushboy]

If you bend a popsicle stick to the point of breaking,release,it will show deformation.on the other hand bending a piece of Osage it will spring straight again or almost.a simple bend test will tell the tale.

[George Tsoukalas]

You can look it up.
If you go to TBB 1, I think in the bow design chapter, you'll see some of  the woods classified by elasticity.
It's nap time or I'd look for you.
I believe osage and yew are high in elasticity and BL is medium.
So you could make a bow shorter and not as wide from osage than you would from BL.
Jawge

[upstatenybowyer]

Just to through another wrench in things, I think there are woods that are elastic in tension, but not compression, and vise versa. 

[PatM]

Just to through another wrench in things, I think there are woods that are elastic in tension, but not compression, and vise versa. 

 Relative to their ability to actually stretch it doesn't  seem that any wood is really elastic in tension.  Most woods really do seem to stretch very little before failing.   Maybe they vary in elasticity in that tiny range of stretch they do have but that doesn't seem like it would figure prominently like compression elasticity does.

[Marc St Louis]

Just because a wood springs back without deformation after being bent doesn't necessarily mean it is elastic.  There are species, Black Cherry for instance, that will spring back quite well after being bent but take it too far and presto, chrysals all over the place.  You can also find low in elasticity wood in species known for their elasticity.  Wood high in elasticity can be bent pretty severely and they will take set but will not chrysal, you can see this in low density Yew.  Elastic failure in compression is essentially the wood cells crushing.

I am quite sure that elasticity also plays a part in tension as well, just not as much as in compression.   

[DC]

Apparently wood stretches 1 or 2%(correct me if I'm wrong) 1% of a 66" bow is .6" That's actually quite a bit from my way of looking at it.

[Badger]

Apparently wood stretches 1 or 2%(correct me if I'm wrong) 1% of a 66" bow is .6" That's actually quite a bit from my way of looking at it.

 D/C, not sure exactly but I think typicaly slightly less than 1% is all we have. Most of that is compressed on the belly maybe 3/4 and maybe 1/4 is stretched on the back. Very seldom is the entire bow bending so even less.

[Selfbowman]

We know sinue helps in this elasticity for the back of a bow. I wonder has anyone laid say 10# fishing line on to a back of a bow for backing and elasticity. I know it's not primitive material but it might be interesting to see the results. It might be lighter than sinue.   Arvin