Sunday, 13 May 2012

Spine Tester & Matching Arrow to Bow


Before rushing in and reducing the arrow thickness I thought I should measure the spine (flexibility) of the arrow before I started. There are various slightly different definitions for spine measurement, but basically you support the shaft at it's two ends, hang a weight in the middle and measure how much it bends.
I've seen stuff saying the supports should be 28" 23" or 26" apart. I went for 26" as it is convenient for my arrow length. Two nails in my shelving and a small steel angle bracket (adjustable for different thickness arrow shafts) to act as a reference point does the job. A 2lb weight from the kitchen scales put in a plastic bag and hung onto the arrow with a piece of bent coat hanger and the job's a good 'un.
If you divide the distance between the supports (in inches) by the deflection (in inches) it gives a figure for the spine (or thereabouts). The reading with the unmodified arrow gave a spine of 31, I scraped along the central 20" of shaft with my scraper and then spun it up in the electric drill whilst rubbing sandpaper along it. The drill grips onto the point and the nock end was supported in a block of  wood with a hole in which was clamped in the vice.
I measured the spine again and it was down to 26.
The test shot, ok, it's just a single arrow but I had a good feel for the bow and I also know how to hit the spot at 10 paces. I took care to aim dead straight, no allowance for the kick... thwack the arrow thumped home about an inch left of centre.
A very obvious improvement, as yesterday I was having to aim way off right on every shot.
It's nice when the experiment does actually tie in with all the theory, normally my bows aren't to sensitive to spine, but the combination of low draw weight and very wide grip made the Hazel hypersensitive.

Measuring the arrow, it is 8.05mm diameter where I haven't reduced it and 7.45mm in the middle where it's been thinned. Not a vast amount, but enough to make a difference, the arrow is under a huge acceleration as it is loosed so I suppose it doesn't need much to have an effect.

It didn't take long to do two more arrows to be similar, (I didn't go mad looking for perfection). The results speak forthemslves, maybe a tad left still, but a vast improvement. I might take 'em down a bit more, but for now it's gardening weather.

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