Designs for Selfbows made from Australian Timbers

[Glenn]

A lot of Australian timbers used for selfbows have very high mechanical properties which raises some problems to designing a selfbow from these timbers that wont self destruct. Timbers like osage-orange have a very high modulus of rupture on the belly side of the limb of around 177 but only has a modulus of elasticity on the back of the bow of 13, whereas grey ironbark for example has a modulus of rupture of 181 and an elasticity strength of 24. This high elasticity strength will give the bowyer problems but the bowyer has to design a limb that will counteract and at the same time utilise these qualities the timber possesses to add to the performance of the bow and not detract from it.
To counteract the strength of the back from collapsing the belly as the bow is being drawn the bowyer needs to take these mechanical properties into account when designing the limbs of a bow from different species of timber. One limb design does not suit all timbers.
I have found from past experience that when using timbers such as red ironbark and grey ironbark I prefer to use staves that still have sapwood intact as I have found that you can use almost half and even more than half of the limb thickness of sapwood, by doing this I have found that I can considerably reduce the amount of set the bow takes. On one red ironbark selfbow I hunt with I have made it with more sapwood than heartwood and infact I have made the bow with the sapwood coming into the belly of the limb and this bow still shoots well at my draw length of 26” after many years of service and has taken practically no string follow at all. If you don’t have access to staves with sapwood and you have to use floor boards or other stock timber which will be all heartwood you can will still make very good self bows with the right limb design.

http://img.photobucket.com/albums/v475/Glenn51/autofocus003.jpg
These are some selfbows made from Australian hardwoods by Alan Jones from Kilcoy. Note the very thin tips and the wide limbs from the handle section out. Timbers are yellow stringybark, red ironbark and tallowwood...

For an all heartwood selfbow from Australian hardwoods you would do well to make the belly wider than the back of the bow to resist the forces of the high mechanical properties the back of the bow will exert on the belly. The other problem with Australian hardwoods is their high density. Some of these hardwoods are over 1000 kilos per cubic metre which can mean bad hand jarr if careful attention is not payed to tillering and limb design.
I like to have most of the working part of the limb down near the handle and the last quarter to one third of the limb as it nears the bow tips to be drastically reduced which gives better arrow speed and less hand jarr. The lower two thirds of the limb have the most amount of timber where it is needed. A good hunting bow if it is to shoot heavy hunting arrows will need to bend more down near the handle where it is needed...Glenn...

[Warlocke]

Glen,

How would these timbers, especially the Ironbark, lend themselves to an ELB "D" limb configuration?

I have sourced a section of Grey Ironbark that I intend to make into a 70# longbow and any help in the process would be greatly appreciated.

After this project I would like to also make another around the 100# mark.

Would this timber be suitable?

[Glenn]

Grey Ironbark is my prefered timber. If you are going to make a bend through the handle ELB it wont have any problems with this design. What length are you thinking of making the D section bow?...Glenn...

[Warlocke]

Around the 68-70".

I am thinking more along the lines of the shorter length after handling my Olympic bow at 69".

[adam]

Off to the local timber supplier today to give one of these timbers ago.

[Glenn]

Warloke you will probably be able to make an ELB around the 66" mark without too many problems. If your draw is over 28" then I would build a bend through the handle bow and if your draw is under 28" then you will get away with a stiff handle bow...Glenn...

[Warlocke]

Thanks Glen.

Hopefully I can pick your brains for more information when the wood has cured.

[perry]

A hint with drying or seasoning bow timber . I never wait for a piece of wood to dry before I begin work on it , I rough out the bow stave until it is ready for floor tillering , sometimes even to the point of bracing the stave I just be carefull not to flex the stave while its still green to the point where it begins to develop string follow just enough that you get an idea of the developing bend of the limbs . Why ? Because thick wood dries much slower than thin wood , thick wood can check [ split ] , and warp in the seasoning process due to uneven escape of moisture , thin wood particularly wide thin wood will not develop drying checks or warp anything like the same degree . I cant imagine ancient people particulaly nomadic peoples carried around lumps of wood for months or years until they seasoned enough to craft into bows . They thined them down , sometimes hung them over a fire place in a shelter where the wood dryed and to a degree tempered and away they went . I have cut a tree worked it to floor tiller , put the bow stave in my car in the sun and finish tillered the bow into a very durable low string follow shooter in as little as 3 days . I knew the stave was dry as I took the stave from the car , weighed it on scales and after 3 days it had stopped losing physical weight [ water ] There is a lot of rubbish sprouted in oversea's books and in accepted dogma in traditional archery circles about drying wood believe me thin wood is a lot more reliable than thick wood and besides you can make more bows quicker . regards Perry .

[bowhunting4eva]

hi perry
i found with the drying of whole logs put fat or paint the ends of the wood so they do not split or check. even if you have big bits of wood cover them with fat so the moisture does not escape unevenly
when i shapped my staves i put fat all over it when not using it becuase it drys to quickly and you will get twists. more twists means more steaming. just my experience. oh and you are meant to have a cetain moisture content in the wood arent you. i thought like 15% or something( could be totally wrong)
but if your way works why change it.

[perry]

Drying whole logs in the manner you do works fine , have done it myself - its just so slow , why wait months actually years if its a big log when if the staves are thinned down to about an inch thick except for the handle section by 2 or 2 1/2 inches wide they will dry much faster should not warp as much as split staves [ie quartered ]by all means coat the ends immediatly after felling just come bowmaking time get it floor tillered while green , work all the staves from a log in this manner they will store fine if they are kept in a sensible enviroment and will be seasoned in shorter order as a bonus. Around 8 to 10 % moisture is the accepted level , without a moisture meter its hard to tell , this is why I mentioned the scale method , wood will not dry out naturally below these levels. regards Perry

[Glenn]

I do similar to what Perry does, I rough the stave down and then I store them to dry, preferably in a place out of direct sunlight but where the stave will get good air flow around them, wind is better. I seal the ends and the back of the bow and to allow stave to dry out through the belly. When I feel it is dry I will then start the tillering process but if I find the stave is still holding moisture I just store it again. The less timber the quicker the drying process will be. Air seasoned timber is stronger than kiln dried timber, I nevre force dry them but a hot box with a low wattage light bulb in the bottom can help bring the stave to the required moisture level...Glenn...

[Warlocke]

That's what I like about this site, all the usefull input from informed people.

Thanks fellas, this will speed up the process.

I was of the opinion that you just de-barked the timber, sealed the ends with wood glue, weighed it daily until the weight levelled out then start shaping and tillering.

Rough shaping of green timber is much easier than cured.

[Glenn]

Weighing your stave is another way to do it but if you are wanting to make the bow when there is a lot of humidity about you may to put it in the hot box to persuade it to loose some more weight-moisture...Glenn...

[perry]

Warlocke if we have trouble working seasoned hardwood with tool steel and power tools imagine the drama's and how blue the air would be if all you had was stone tools . There is nothing new in archery , just variations of a theme if you want answers look to the past . regards Perry

[Hefty]

Lol, I'm hooked on both of Glenn's threads!
What are the main tools you guys use for shaping and tillering? One or two of the articles I've read say don't cut the stave to its initial shape but rather split it then use a rasp/spoke shave/draw knife/cabinet scraper to tiller.
I figured with the spotted gum board bow I'm going to make, I could just cut the main shape on the bandsaw and then tiller from there. Would this be acceptable?

Jono.

[Glenn]

Hefty, cutting your stave to shape with a bandsaw will be no worries, after I have cut the profile to shape I always clean up the profile with a rasp and smoothing file. I never use cutting hand tools on a selfbow such as planes and spoke shaves as you run the risk of tearing out some timber if the grain is a bit irregular. I only use files and scrapers for the tillering stage. I do use an electric plane and a circular saw or band saw to get rid of the excess timber but you wont need to do to use a planer on a board bow as there isin't a lot of timber there to start with, a good rasp will do the job on the belly to start with...Glenn...