I’ve been lurking here for a couple weeks now (well, on and off for years to be honest), read your posts, looked at your awesome bows and useful advice with much enjoyment. I started making bows a couple of years ago, but gave it up when I had success and winter came; just lost the drive. I’m now very keen and getting back into making them. I’ve made my bows from Australian timber only, Silver Ash actually, and I think what really inspires me about bow-making is this feeling that goes back to the days of gentlemen scientists, discovering stuff in their labs as a pastime. Australian woods are so many, so amazing in their strengths, and so unexplored: it’s a great world of mystery to be explored and understood.
Now this is probably going to be an unusual first post, but I’d like to put in my 2c about bow wood properties. I've read much of the great work that has already been done here (building bows with maths, Dennis's wood tables etc) and I'm hoping to contribute without repeating what has already been said. I don’t have the practical experience many of you will have, having made few bows, but I have a keen desire to learn and explore. Don’t take this as the new guy charging in to tell you he’s right, just my way of pitching in to try to make sense of it all. Discuss, dispute, disagree, it’s all to the benefit of the craft.
On with my thoughts…
A bow is a spring, designed to store energy by deforming elastically, without deforming plastically or breaking. Elastic deformation is a change in shape due to a force which does is not permanent (i.e.: it disappears when the force is removed). Plastic deformation is a permanent change in the material that remains after the force is removed (e.g.: set). A bow needs to deform elastically, resisting a force to store energy, while avoiding plastic deformation.
Given that bows need to do so many different tasks at once, it’s unsurprising that many measures and ideal properties have been proposed. As has already been speculated to some depth here (in Dennis’s wood tables most of all) it is most likely that a combination of properties is desirable.
Actually identifying the combination of properties most desired is a difficult task. Many measures of strength overlap, for example modulus of rupture measures of the force required to impart plastic deformation, as does the concept of elastic limit (http://en.wikipedia.org/wiki/Elastic_limit). Different units of measurement (Americans using PSI and lbs doesn’t help) have also made the maths one step harder. Really we’re talking about ratios of ratios to ratios here (e.g.: density to MOE to MOR) and the extra conversions make life “fun.”
Looking at the excellent work and speculation that has already been done here I was inspired to have my own go at cracking the problem. The problem, as I see it, is this: in the US and Europe there are a long line of bowyers who have laid the grounds for modern bowyers to select woods and know their properties, which are the best etc, etc. In Australia we have little to no history of bows (I read somewhere that there were a very few in Cape York from PNG), yet we have a plethora of amazing and varied woods, with some of the toughest timbers in the world and only anecdotal evidence as to their usefulness for making bows. I have seen it argued that the lack of any bow-making tradition in Australia is evidence of the fact Aussie timbers are just no good for bows. I think we can safely invite any such person to look at the variety in Dennis’s short list of Aussie woods or perhaps to contemplate the sheer size of geography and variety of flora in Australia and they’ll rescind their argument.
So what’s my take on this… I’d like you to follow my thought path if you have the time, because I think it is important to understanding the end result.
I started from looking at Dennis’s list of timbers (to reiterate, that was some great work!). It just bugged me though, that the traditional king-pins of Osage, Hickory and Yew were rated so poorly! And why were they so low in bending strength (and to a lesser extent, rupture)? I wrote out the generally accepted (or why I considered generally accepted) hypotheses regarding bow wood qualities. In brief I reasoned that high MOR, MOE and low mass were desirable. I also considered that greater limb thickness (back to belly) was a secondary desire, when combined with mass and necessary width to make weight (bow weight, not draw weight).
I also wrote out an alternative hypothesis: lower MOE is better, OR ratio of MOE to MOR is better. I reasoned that a bow is a spring and so it must have some ability to bend without breaking: I was thinking of the design of buildings in earthquake zones: they bend with the quake, to avoid snapping. I was also inspired by the comparison of Iron and Steel: Steel is harder as more carbon is added, but it also becomes more brittle. The MOE of wood is far, far larger than its MOR: it breaks more readily than it bends! Therefore I arrived at my first theory, which was that the ratio of MOR and MOE was important. I also took a bit of a look at some of the desirable properties of springs using reputable sources (read :Wikipedia
), which seemed to back this up.Sorting Dennis’s data by MOR/MOE looked like a good step, top 3: Osage, Hickory, Yew. I considered the idea of mass being important also though, so I revised my thinking and divided mass by MOR/MOE. Top three were now Yew, Hickory Osage… even better, the classic bow wood was on top. The Euro/US timbers Dennis included in his data were near to the top of the list, and a few Australian candidates were there, yet I felt that something was missing. I toyed with the maths on and off for a couple of days, trying to figure out how to incorporate maximum limb thickness into this theory. A limb twice as thick, as is often stated, has eight times the draw weight. A limb twice as wide has… what is it, four? Two times?
I couldn’t crack it, sad to say. Fortunately, I had a look around and found someone else already had (using US timbers)! All I needed to do was to repeat the calculations for Dennis’s list and I could estimate maximum limb thicknesses for Australian timbers.
Here’s the link to the original thread: http://paleoplanet69529.yuku.com/topic/ ... -for-a-bow
So, I’ve done some calculations for Dennis’s list of Australian timber properties, and the top three are Osage Orange, Hickory and…. Saffronheart? Yew drops to number 9 (I’m fine with that, since the whole sapwood/heartwood thing is really unique, probably means it needs more maths), and we’ve got a nice mix if the Australian and EU/US timbers at the top.
I calculated these figures for a 140cm long flatbow, 18kg draw (approx 40lbs) with a bending radius of 55cm. The final figure is the weight in grams of a 10cm long section of fade-width/thickness wood. I don’t know about you, but I want to get some saffronheart and give it a go!
Note: this assumes the same thickness and a straight bend I think, so no actual tiller. This isn’t a guide to actual thickness, it’s a comparison of properties with all factors held constant.
Having trouble attaching... here's a link to the spreadsheet:
http://dl.dropbox.com/u/77412936/BowWoo ... 10.12.xlsx
