Ahoy all,
Another Aussie bowyer here. In fact, I wrote that post Nezwin linked to above.
It's true that we're essentially still in the pioneering stage of exploring the potential of Australian timbers for making bows here. The list grows constantly, but from a relatively small base.
Let me just say early on that I'm not an engineer, and I nearly flunked maths in high school.
Let me also say that of course I recognise that there are thousands of bowyers who do not conduct bend tests or make bows by numbers. And they make very good bows. But there are also some of us who have gone down the engineering route, I think for the benefit of the bowmaking community.
Many years ago, in an effort to better understand bow design and to maximise my use of Australian timbers, I forced myself to learn the engineering of cantilevers (because a bow is just two cantilevers joined at their origins), and also learned how to conduct bend tests.
I learned quite a lot about wood, as it related to bowmaking. I learned that the published MoR of any timber is only indicative of its merits for making a bow in the same way the speedometer in a car is indicative of how quickly it will accelerate.
For bowmaking purposes the MoR is not terribly helpful, because although the MoR is a measure of bending stress; it is the bending stress at which the wood fails. The stresses in a working, effective bow are far below the rupture stress. In addition to this, how wood will respond to bending stresses leading up to that rupture point differ wildly by species and growing conditions, among other things.
Published data for timbers are rarely helpful for definitive (as opposed to indicative) guidance on making a bow. This is because two samples of the same species can have wildly different properties (like up to 20% in extreme cases), and the features we as bowmakers want is not generally tested, never mind published.
In my mind the best way to use the numbers to make a bow is to conduct a bend test of a sample of wood from the stave that will be used to make the bow. This will most reliably indicate the properties of the very wood inside the stave that you'll make a bow from.
What qualities are important? The strain the wood can withstand before it takes an acceptable amount of set; and, the stiffness of the timber when it reaches that amount of set.
When a bow is drawn 28 inches, its tips will move about 14. If about one inch of set is acceptable, then a set equal to 8% of deflection is about right. So when I'm bend testing, I look for that deflection that causes the sample to take 8% of the deflection in set. Then I also take the corresponding stiffness.
Strain, for those that might not know, is the percentage of lengthening or shortening due to tension or compression under load. Happily, multiplying the stiffness by the strain will give you the bending stress at that same load.
A wood that can tolerate a large amount of strain at the 8% set can make a thicker bow than a timber that cannot tolerate as much strain.
A wood that is very stiff can make a narrower bow than a wood that is not as stiff.
Yew does not make an excellent bow wood because its MoE is low. Far from it. Willow has a similar MoE. Yew makes an exceptional bow wood because it can withstand a monumental amount of strain before taking the 8% set. This means it can be much thicker than other woods. Being thicker, it does not have to be as wide as other woods of the same stiffness.
Incidentally, the wood database gives an MoE for Yew of 9.31 GPa. Some of our Australian woods are about 24 GPa, and they make exceptional bows too. Not because their MoE is high, but because their allowable strain is favourable.
I could go on and on about how the mechanical properties of wood interact and influence the suitability of wood for bows, but I have prattled on a bit too long already, methinks, so I should leave it there for now.
Cheers,
Dave
Topic: Bow Index (Read 1208 times)
