Self bows

[DylanK]

Gday All

When making a self bow, to get some curve to the limbs, could you not string the bow backwards for a night or two until the limbs set, then string it the right way around? Wouldnt it save steaming the limbs and the like?

Just a thought,

Dylan

[Dennis La Varenne]

Dylan,

The short answer generally is no. This is a complicated question which requires a somewhat detailed answer to which I hope other selfbowyers on Ozbow will probably want to contribute.

Intuitively, yours might appear a reasonable supposition, but wood is highly elastic, meaning that it has the property of wanting to return nearly to its original shape. It is also plastic, meaning that its shape can be permanently changed, but only under specific conditions, principal of which is applied heat. This softens the 'glue' which holds the wood fibres together. Once softened, it can often be bent into another shape. However, not all wood is amenable to this treatment. Victorian Red Ironbark, for instance, is one such. It simply will not permanently bend from heat - at least the kind of heat which we bowyers normally have at our disposal - and is listed as having a very low index number (almost at the bottom) on any list of heat-bending properties.

One way to induce bending in the manner in which you propose and which is ancient and well tried is that of carefully and gently bending a stave into a reflex while it is green and lashing it into a desired shape until dry. However, I haven't seen any kind of wood retain its induced shape change permanently after tillering and shooting. It will maintain a good deal of it if done well, but wood is not perfectly elastic.

Most sources of the mechanical properties of wood will list the modulus of elasticity (MoE) of the listed species which is a number in Gigapascals indicating how much force must be applied to a standard sized piece of that wood where it begins not to return to its original shape. Obviously, the higher the number, the more elastic (springy) the wood.

Complementarily with the MoE is the modulus of rupture or MoR which is a number in Megapascals indicating the force applied to a standard piece of wood where the fibres of that piece of wood begin to rupture or break. Obviously, when that starts to happen, the wood cannot return to its original shape and will break under further bending stress.

MoR is measurable in both tension and compression. So, on the back of a bow it is used to indicate the amount of resistance to tensile (stretching) forces and on the belly is is used to measure the resistance to compressive (crushing) forces.

Almost all wood is stronger in tension than compression by somewhere in the range of 10-20%. That is why most selfbows take some degree of 'set' or 'string follow'. The back is more tension resistant than the belly is compression resistant in the ratio of something like 5:4 or thereabouts. That is the nature of wood. Even selfbows which have been built with reflex into them still take a set, that is, they almost always lose some of their original reflex. Allowing for this is part of the art of selfbowyery.

On Ozbow a while back, I did some experimentation on removing the set of an English pattern longbow which I had by doing just what you write about. I simply bent the bow over a former very gently and slowly from something like 60mm of set (as I recollect) into 300mm of reflex using gentle heating (the hot summer sun).

Surprisingly, this particular bow, probably/possibly because of its tropical hardwood belly, retained about 10mm of reflex after a long rest without shooting, but finally took some slight amount of deflex ( I forget how much) after shooting, but nothing like its original set.

I performed this experiment just to test an hypothesis like yours - that could a bow be simply bent into a new shape by force-reflexing it. This particular bow was partially successful, but I would not like to generalise to any other bow because of the highly variable mechanical properties pertaining to any specie of wood and within a specie depending upon its original growing conditions.

Importantly with my experiment, I performed a minute inspection of the bow's belly with a strong magnifying glass to see if there were any signs of minute frets which would indicate that the belly fibres had begun to rupture. Obviously, once rupturing or breaks in the individual wood fibres had begun, they could not be pulled into strong tension and stretched. All that would happen would be that the cracks would open more and result in catastrophic failure. In the case of this individual bow, there were no discernible frets. Only because of that initial investigation did I decide on progressing with my experiment, and that may be a clue in any further serious investigation of your proposal as a technique.

Certainly, I have the courage now to use the technique again to remove some set, but only after careful scrutiny of the belly and back for signs of rupture, and only if there is no evidence of rupture anywhere. Bear in mind also that a bow will take a set without rupture because of shortening of the wood fibres under compression. It appears that wood will do this in a self relieving manner up to a point which is its MoR and then begin to rupture.

Conversely, rupture under tension is generally abrupt and spectacular with little or no warning. All that can be checked for in such an examination is that there are no apparent breaks in the integrity of the back. But, that still does not preclude that a back can simply let go without warning as it has done several times in my bowyery career despite having an ostensibly perfect back. But, any obvious damage to the back of a wood bow usually spells doom unless remedied by re-backing.

Anyway, some others may wish to say something on the issue. It is an interesting topic and thank-you for raising it.

Dennis La Varenne

[looseplucker]

DylanK

What Dennis said.

Also - I think it is counter intuitive because I also believe that the tillering and shooting in of a bow 'teaches' the wood a way of behaving - and that is within its elastic limits as well, which is about 66% or so of the MoR. Bear in mind as well that when you bend a bow the back experiences tension, the belly compression. Reverse bending to impart set would, in my view, compromise the tension/compression qualities of the timber causing breakage or even more set.

There is a chapter in the TBB series that deals, however, with 'reverse tillering' - where you do tiller the bow by bending it in reverse. I have not tried this and from memory it is something done with green wood (I could be wrong).

If you want to impart the bend to seasoned timber, use the methods described by Denniss - also, if you can get it, the Jim Hamm book Bows and Arrows of the American Indians contains some fantastic pics and descriptions of applying heat to impart reflex in self-bows - and relatively short ones at that.

If you google 'Pyramid bows' there is a buildalong on another site which shows how a guy heat treated a self bow to impart reflex (I think it was hickory) and then reinforced the reflexed tips with strips of another hardwood on either or both the back and the belly. Looked great and seemed to work.

At the end of the day I still regard myself as a novice bowyer (although these days I don't seem to break bows nearly as good as I used to - after going through a fairly consistent period of doing it) and the best thing to do would be to experiment with the tried and true methods and then see what you come up with. In general though, at least in my experience, set will occur selfbows - but we can control that by how we tiller it and whether we put in reflex etc.

I recently made a semi recurve - it was a laminate and it was straight limbed until working recurve tips. Following shooting in, however, a small amount of set appeared from mid limb to the beginnings of the inner limb. The tips are still recurved and the bow has a very nice deflex - reflex profile.

[DylanK]

Thanks for the detailed info. I understand now.

Dylan