Bamboo back failure

How to make a Bow, a String or a Set of Arrows. Making equipment & tools for use in Traditional Archery and Bowhunting.

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DunnoNuthin
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Joined: Wed Apr 30, 2014 10:13 pm
Location: Hobart

Bamboo back failure

#1 Post by DunnoNuthin » Sat Apr 11, 2015 3:44 pm

Hi to all the contributors to this site. I have learned heaps here, having lurked in the background for about a year, and searching through old posts. So thanks to all that take the time share their knowledge.
I am currently working on my third serious attempt at making a bow. All have been backed from the same piece of pole bamboo. None of them lived to launch an arrow, the backing has failed each time. The first failed because I broke the rules and flattened both sides of the bamboo (Sometimes you just have to find out the hard way!).9 The second I can't recall exactly (it's been six months) but I know I didn't taper it correctly as per Greybeards instructions, it was too thick towards the tips. I'm pretty sure the bamboo failed in tension on the tiller stick about mid limb.
The current bow was kinda starting to get me a little excited just might make it. I had it bending reasonably well, just past the point I could have put a short string on at a brace height of 7 inches. I took a little more off the bottom limb to get both tips around to the same point, and was exercising the limbs before putting it back on the tiller. That's when I heard 'tic'. Here we go I thought, gave it another flex and 'crack'. I couldn't find the break till I put it on the tiller and flexed the limb to open the fracture. It's about halfway down the bottom limb, halfway in between nodes, and extends across the centre third of the bamboo backing. I would post a picture, but I haven't worked that out yet. The bow is now well out of balance, with way more curve on the bottom limb. Not that it really matters any more.

I was exercising the limbs on a scale, so I know I wasn't pulling it past the design weight of 45lbs. The first two bows were on Merbau decking. The last one on Spotted gum.
The bamboo was from Bunnings, and the best of a sad bunch. It had numerous splits I had to work around and has grey streaks/stains through the "wood" (looks like water/fungal damage????)

The design was one posted on here by Greybeard, from the article in Popular Mechanics for an American Flat bow.

Lessons have been learned so far,
One,(as mentioned repeatedly here) not all bamboo is equal,
Two, The dimensions given for a Lemonwood self bow are way too thick to then add a bamboo back
(obvious at the outset but I figured I would have plenty of meat to tiller with). I now have a much better idea of how thin to set out the next bow.
Three, the way I interpreted the particular plan I followed has the lower limb under a lot more stress, as it's effective bendable length is much shorter due to the stiff handle section. The bow is set out from the centre with both limbs the same length. On comparing it to my other two bows (commercial made), the bottom limbs are longer by several inches.
Four, I'm currently put off using bamboo of unknown quality.

I figure the bow is salvageable. Get the 'boo off it, and re-back it. Question is, what with.

little arrows
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Joined: Sun Feb 29, 2004 2:14 pm
Location: Sunshine Coast QLD

Re: Bamboo back failure

#2 Post by little arrows » Sat Apr 11, 2015 6:58 pm

g'day,

Can't help you at all with the bow backing, sorry, I just shoot them. I love your forum name, although if you keep learning from all your workings you'll have to change it to - "Startin to learn Sumfin'".... :lol:

I'm sure a more knowledgeable bowyer will be able to steer you in the correct direction.

cheers
sue

DunnoNuthin
Posts: 13
Joined: Wed Apr 30, 2014 10:13 pm
Location: Hobart

Re: Bamboo back failure

#3 Post by DunnoNuthin » Sat Apr 11, 2015 7:21 pm

Thanks Sue.
The forum name goes back to a bloke making a play on one of my nicknames, which stemmed from my surname. If all that makes sense. Still kinda appropriate and just seemed a good fit for this application.

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greybeard
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Re: Bamboo back failure

#4 Post by greybeard » Mon Apr 13, 2015 12:53 pm

DunnoNuthin wrote:......The design was one posted on here by Greybeard, from the article in Popular Mechanics for an American Flat bow.
.......Two, The dimensions given for a Lemonwood self bow are way too thick to then add a bamboo back (obvious at the outset but I figured I would have plenty of meat to tiller with).......
As long as you are learning something from these set backs you are moving forward.

When using hardwoods in bow making you have opened a new ball game. Substituting timbers invarilaby leads to making design changes.

Substandard materials may be part of the problem and perhaps to a degree your bow building technique.
DunnoNuthin wrote:......That's when I heard 'tic'. Here we go I thought, gave it another flex and 'crack'. I couldn't find the break till I put it on the tiller and flexed the limb to open the fracture. It's about halfway down the bottom limb, halfway in between nodes, and extends across the centre third of the bamboo backing........
Was the fracture in the bamboo or the timber or a de lamination between the two? if it was a de lamination perhaps the join was starved for glue, too thin an application, over clamping or contamination on the gluing surfaces?

What type / brand of glue are you using and what is your clamping method?

Daryl.
"And you must not stick for a groat or twelvepence more than another man would give, if it be a good bow.
For a good bow twice paid for, is better than an ill bow once broken.
[Ascham]

“If a cluttered desk is a sign of a cluttered mind, of what, then, is an empty desk a sign?” [Einstein]

I am old enough to make my own decisions....Just not young enough to remember what I decided!....

DunnoNuthin
Posts: 13
Joined: Wed Apr 30, 2014 10:13 pm
Location: Hobart

Re: Bamboo back failure

#5 Post by DunnoNuthin » Mon Apr 13, 2015 10:25 pm

Daryl, the crack opened up in the bamboo, it did not penetrate through to the belly.

The bow was glued up using marine epoxy from a fiberglass specialist, clamped using inner tube strips and a couple of quick grip clamps in the problem spots. The epoxy brand is unknown to me, it appears to have been decanted from larger containers into the small pots I bought, with the bare minimum of info on the label, apart from "Part A" etc.

The epoxy seems to have done its job on the limbs well enough, can't say the same about handles that keep popping off. That's probably more due to my design flaws, but the glue pulled apart here without tearing the wood fibres, so it seems to not really get into the fibres.

I feel this particular piece of bamboo is my problem in this case, I've used it all anyway, so I need to source something else for the other half of the spotted gum board I have left.

This bow is now dead, sticking out of the fire pot awaiting the next bbq. Did learn how not to patch a limb along the way though. Kept tillering the other limb for practice sake. It did surprise me how thin the limb needed to be to get to what would full draw, until one stroke with the spoke shake put a dirty hinge in it. Use a scraper at this stage next time. All learning as they say. Would be great to get a shooter soon though.

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