Author Topic: sisal backed shortish hazel pyramid in progress (Read 6508 times)

joachimM · « **on:** June 16, 2015, 06:55:05 pm »

Hi folks,

I'm working on a new sisal-backed hazel stave.
Some people commented in the past that sisal is low quality "mexican twine", but that doesn't stop it from being about as strong in tension as bulletwood, while still being able to stretch a good bit further. And since I like to make low budget bows from scraps, here I am. The sisal was recovered from hay bale twine, cut into strands of 40 cm, washed and combed a bit.

I cut a hazel stave from a hedge in the backyard, it was about 5.5 cm diameter. I cut it to 55", decrowned it completely (the back of the stave actually follows the pith, as a result of which it is a backward bow) and reduced it to 53". It's a pyramid design, being just under 4.5 cm wide at the fades, tapering down to 1 cm at the nocks. The back is slightly trapped, so as to have a stronger belly.
I had to steam out a few unwanted bends and center the handle, and offset the lower limb a bit to even out things. Didn't work out completely though.
The wood is very light, I guess it's only about 0.45 SG. (I'll oven-dry a sample of the stave to check this; but it's scaringly light...). EDIT: it's only SG=0.36
the stave's back was scraped to increase adhesion of the backing. Stave is about 9 mm thick, backing (slightly crowned adds 3 mm or so).
Backing was glued in a single session, took me about one hour. Afterwards the stave was completely wrapped tightly with bicycle tire to press the sisal, heated it with a heat gun to reliquify the glue and pressed out as much as I could.

The pre-tillered hazel core initially weighed 250 g. After backing and curing it weighed 280 g (so I added about 30 g of sisal and hide glue).
As tillering is progressing, it weighs again 250 g. I'm aiming for a 40# draw at 25-26". I get an increase of 3# per inch approx.
Currently, it is at 33# at 22", so if it holds I should get there, but I'm getting a bit of (visible) set right now (juster under 1" after unbracing, returing to zero set within minutes).
If I don't get to my target draw weight, I'll just add some more backing material and keep it at 23-24".

The strain on the back is now just over 1%, which is approximately the strain at which I get tension failures in wood backs. The sisal should take at least 1.5%, but the belly can't take that much. Hence the trapped back and belly toasting.

I'm not sure the tiller is OK right now. It's pretty symmetrical, but that could mean I'm making two mistakes at once. It seems still too stiff right out of the fades for a pyramid. I'm keeping the tips a little stiff.

any comments are appreciated

Joachim

alwayslookin · « **Reply #1 on:** June 16, 2015, 07:38:42 pm »

Is that on a long string in the drawn picture?

sleek · « **Reply #2 on:** June 16, 2015, 08:37:27 pm »

Yeah, if thats long string time to stop and put it on the real string. Your tiller looks perfect btw.

joachimM · « **Reply #3 on:** June 17, 2015, 01:44:10 am »

This is a shortish string braced at 4"
I'm pretty comfortable with low braces, typically 5"

alwayslookin · « **Reply #4 on:** June 17, 2015, 02:19:11 am »

Looks pretty good to me.

Del the cat · « **Reply #5 on:** June 17, 2015, 03:17:31 am »

V interesting... lookin' good.
Del

Pappy · « **Reply #6 on:** June 18, 2015, 06:43:39 am »

Looks good to me, I would for sure go to full brace and see how it looks, bet it will look fine.

Pappy

joachimM · « **Reply #7 on:** June 18, 2015, 07:53:35 am »

Thanks for all your comments.

took it to full brace (5" in my case), and further drew it to 26" where it goes to 37#.
Burnished the belly to make it a bit denser in the hope that it won't take too much set.
Haven't shot it so far.

All in, it wheighs 240 g, which is just a few g under the expected mass for a backed bow of these proportions, so at least I'm not clearly at risk of making the bow too heavily strained for its mass.
It's density is only 0.36 g/cc DW

Still, I'm going to reverse brace it and add another thin layer of backing, to gain a few pounds and relieve a little bit of stress from the belly surface. Hopefully I don't mess it up that way, so I can start shooting it in before finishing it.

Pappy · « **Reply #8 on:** June 18, 2015, 10:32:25 am »

I would think more sinew on the back might over power the belly if it is weak in compression anyway. $:-\$ Don't know about that wood but just saying might be something to think about.I rarely do back bows of any kind but do know you can over power weak in compression wood with the backing causing them to fret.

Pappy

joachimM · « **Reply #9 on:** June 18, 2015, 01:30:42 pm »

Hi Pappy,
thanks for your thoughts. I do have the same doubts, but that's why I want to put the back first under compression before adding a thin strip of backing. That way the belly is first put under a bit of tension before being asked to take compression. Thereby I hope to be able to add 3 pounds without causing extra strain on the belly. Perry reflex follows the same principle, albeit at two surfaces instead of just one.
I'll let you know how it turns out. I've done a similar hazel-backed bow before 40#@ 26", which was a bit narrower but was also denser, but it had approx the same final mass. However, that one was entirely backed with the back under compression (3" of reflex).

Mind you, it isn't sinew, although it seems to be the most sinew-like of plant fibers I've used so far.

Joachim

ty_in_ND · « **Reply #10 on:** June 18, 2015, 05:42:25 pm »

All I can say is good luck with what you decide to do! Whichever way you go, if you do get a shootable bow, it'll be great to see how it looks when finished. It's always good to read people trying new things out, especially when they do it in a thoughtful way as you are. Did you lay the sisal on the back like you would with sinew? I'm guessing you did by the way you have it arraigned in your 3rd picture, but it would still be good to know. Thanks!

mikekeswick · « **Reply #11 on:** June 19, 2015, 03:28:31 am »

Occasionally I find one of these 'floaty light' hazel staves. So light that when you cut it from the tree they sort of hover for a while before floating gently to the ground!
Mostly hazel is around 0.50s.g. here.

simson · « **Reply #12 on:** June 19, 2015, 04:24:24 am »

Interesting project, tiller looks pretty good.

I personally would never back a hazel, too much effort. Just make a HLD with high crown shorter staves or look for long enough and try kinda ELB style. Just my 2 cents.

joachimM · « **Reply #13 on:** June 19, 2015, 08:49:08 am »

Actually, this isn't that much effort. I doesn't take me more time to make such a backed bow than to make any decent wood selfbow, branch bows aside.
The backing is stripped from cordage in a few minutes time, so no hours of sinew pounding and shredding. After glueing (1 h), curing takes about a day (in a pretty dry and warm place), I let the wood come to equilibrium MC for another few hours to a day (and it doesn't take much more than that).
Moreover, hazel is very easy to work (low hardness), so you quickly reduce the stave to basic proportions while still green with a hatchet, dry it in a few days, start the pre-tiller with a farrier's rasp ... For a pyramid design you just get an even thickness all along the limb, so not too much pre-tillering work either.

Another advantage of such backed bows, is that you can correct tillering mistakes by adding backing and retillering the stave. (which makes this forgiving for rookies like me).
Compression fractures, however, cannot be cured this way (did my fair deal of trying in the past).

I wouldn't back such light SG wood with hemp or flax, as these are much stronger (higher MOE) than sisal. but sisal can stretch a bit further. I did back a red oak RD bow with flax the other day (as it lifted a splinter on the back), we'll see how that one comes out one of these.

The below pics are after I added a strip of backing (one hay bale cord per limb, 6 g

) under forced reflex, will retiller the bow to 40# at 26".

joachimM · « **Reply #14 on:** June 19, 2015, 06:07:00 pm »

retillered it today starting back on the long string with the now setback bow (due to glued-in reflex), and over to short string and finally full draw.

Had to scrape the upper limb only once mid-limb, but otherwise no extra tillering was required.
Did take a bit of visible set during tillering: 1.5 cm set right after unstringing, now a few minutes later back to 1.5 cm setback. And yes, a few chrysals in one lower limb

. No wonder, Just 40#@26" for a 53" bow from wood I should have chucked in the stove.

Overall, it feels like a victory to get wood this light at a stage where it could be used as a hunting bow

, thanks to sisal.
Still, its mass (261 g) is exactly as predicted by the mass principle for a backed bow of those dimensions. To me it shows again how extremely useful this rule of thumb is (thanks Mr Gardner!).

Next thing: shoot some arrow through it. That'll be for tomorrow. And then some finishing, and a decent string.

Joachim

Primitive Archer