Flatbows - ratio of thickness to strength

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N.Vodden

New member
Ironman
Good Day all,

I've started to get back into bow building now that the weathers improved somewhat and there is a little bit of light to work with at decent hours!!

I've resumed the white ash/hickory longbow thats been sat roughed out in the corner for 6 months and done some work on it. Its tillering down nicely and is starting to take shape.

One thing that i do want to make though is a flatbow. 1.5-1.75" wide limbs, 70" odd, tapering towards the tips from the fades with a build up handle to support a slight cutaway for an arrow pass/shelf.


The thing i want to put to the floor is, what is the ratio of a flatbow's thickness to its draw weight? I imagine this like asking how long is a piece of string ( answer : twice as long as from one end to the middle ) as a lot depends on profile, thickness, edging etc. I get the impression if it is too thin it will be 'flappy' like a low poundage jellybow, but i have also heard that thickness is deceptive and even thin bows can kick a fair cast.

I have a lovely piece of maple backing that was going to be the front and it is .175" thick. Laminations for this purpose are generally .100". How many laminations of say bubinga/paduk, ballpark figure would i need to get a bow of approx 35lb? could i get away with the backing and one lamination? the handle would be rigid so it would be the limbs doing all the work.

Thanks in advance!

V
 

myownslave

New member
there are sites that quote dimensions for laminate bows length width and thickness. i cant remember which site, its a bow sales site that has fiberglass laminate kits.
 

ChakaZulu

New member
Yes it ultimately comes down to the actual design, but there are three features to bear in mind:
specific gravity (density)
thickness
width

Higher density means that the same design will be heavier (mass and draw weight). Lower density needs to be thicker/wider.

The other two are related. The formula suggested by TBB (I think it's Vol.1) is that if you half the width of the limb then you half the draw weight. If you half the thickness of the limb then you reduce draw weight by a factor of 8.

Therefore to reduce weight slightly you either scrape the edges or be very gentle with the thickness. To reduce the thickness of limb tips while keeping them stiff you just reduce the width a lot whilst keeping them thickish.

The other thing about thickness of course is the relative tension/compression strength of the particular wood. Top 10% of the limb does the huge majority of the tension work. After that it's compression.
 

Del the Cat

Well-known member
Yeah, stiffness of a simple beam is proportional to the cube of the thickness. hence the twice as thick 8 times as stiff rule.
Also as a bow gets thinner a half a millimter becomes more critical.
EG Half a millimeter off a 15mm limb won't make as much difference as half a millimeter off a 5mm limb...this is why we find it so easy to end up with a low draw weight...early on in the tillering we are removing wood quite quickly and it seems to do nothing....then all of a sudden as the limb gets thinner.... whoops.
Of course most bows are tapered in both width and thickness just to confuse the matter.
'Make haste slowly' as Confucious used to say when he was tillering bows ;)
Del
 

N.Vodden

New member
Ironman
Thanks for the advice everybody :) seems this is something i just need to dive right into and get cracking!

Ordered some laminates from Flybow last night and another backing strip to replace the one i'll be using so i have one on standby :)

Decided that im going to go with Paduk as the face of the bow because its only .1" thick and a beautiful colour, and im going to use the Maple backing board i have as the core wood that will be worked down and tillered because that is just shy of 1/4" thick. I'll make the handle out of white ash as its not far off the same colour of the maple, and i have a lot of it :) got enough ash in the attic with dead straight grain to make about another 10 longbows, and a board of red oak i could get 6 or so bows out of.

Come up with a rough design pencilled on the board, going to give it a try. Worstcase scenario ( barring a breakage of course! ) is that it comes out in a low draw weight. If that happens i'll just donate it to the club for the equipment cupboard so if anybody or a beginner fancies something a little different , its there!
 
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