Saturday, 31 October 2015

"TakeDown" Explodes!

I'm going to a shoot tomorrow at Avalon (weather permitting) and I'd made half a dozen new arrows.
I thought I'd test shoot them through TakeDown which is the bow I was going to take tomorrow. It's 40# which is a bit lighter than Twister.
First shot I hear a slight cracking/creaking, was it the string in nocks? I felt over the bow, nothing untoward. Second shot, there it was again... still couldn't see anything. Third shot BANG and I got a nice whack across my left tit from the upper limb.
The point where it broke is fairly predictable. It looks like the bamboo back gave way where it's thinned to go into the socket. Easy to be wise after the event (Any fool could see it would break there!) and it didn't help that the bow had an arrow pass inlaid on either side.
I'll know next time to leave the backing going right down into the socket next time and to barely (if at all) cut into it. It could have done with more thick grip section above the join*, but it was never originally built to be a takedown. All good learning experience. The fibres in the 'boo show up nicely in the lower pic'

The upside is that the lower limb is fine and I can made a delightfully mismatched upper limb to investigate the phenomenon of limb timing with odd limb lengths, weights, materials etc. It will make a novelty bow... If I take it to a shoot I can say it's half longbow half primitive!
Dunno what I'm going to shoot tomorrow now, but fortunately I have a few to choose from, although I might have to change bow category.
Shame as it was a V fast smooth bow to shoot and I don't have a takedown to show.

The distance from nock to the break is 31", and from the break to the top of the socket 3".
If we consider the limb as a lever and the point where the limb thickens into the grip (the fade) as the fulcrum. Then near enough 10:1 leverage, so with 40 pounds on the end, it was exerting 400 pounds! (Or about 1/5 of a ton if you think in those terms).
Looking closer, it's maybe nearer 32": 2" which is 16:1 and gives 640 pounds, so in reality it's prob' somewhere between.
No wonder it tore the bamboo where it is stepped down.
I'm sure this analysis isn't probably technically correct in it's detail, but the whole point of physics and maths is to make stuff understandable. In the context of natural materials I think it's probably reasonable. If anyone feels like drawing it up into into a CAD system, I'd be V interested to see one of those pictures that shows the stress in different colours.

I was looking for a bow to shoot tomorrow, and picked up a scruffy character Yew stick bow (here:- )
I thought I'd check out my arrows with it... first arrow, thwack, dead centre in the scrap of paper pinned to my target boss! I immediately decide that the runty little stick bow had booked it's place on the shoot.
I checked it on the tiller, just about 40# at 28" I tidied it up a bit round the nocks and arrow pass and gave it a good wipe of bees wax polish.

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