Bamboo back failure
Posted: 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.
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.