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.
Now I know this is an old post but the entire topic of spine is both dubious and bothersome to me to some extend. I currently shoot a 105@31" yew bow with tapered shafts and never gave much attention to spine as the store I bought them from doesn't spine their 1/2" shafts. I forge my tips myself and keep them at a relatively even weight, maybe 30 grain or so difference between each of them but from what I read a heavier tip reduces the dynamic spine. As such I mostly paid attention to the arrows weighing the same.
ReplyDeleteI wanted to build a spine tester soon but I have not even got any idea on what kind of spine I'd want for me bow nor if reducing the spine by thinning the arrow would be ideal as then I have varying arrow weights.
I'd appreciate your input or suggestions on how to procede or any other sources where I can learn about the topic in regards to warbows.
Cheers, Simon
Hi, With heavy bows at a long draw, spine doesn't really make much difference. Obviously too weak with a heavy head could cause excessive flex, but tbh virtually any 3/8 or 1/2 shaft will be stiff enough. If you want a number, anything form 60 to 90 will be fine. If you start to look at flight arrows from heavy bows, it gets a bit more complicated (I've shot 40 spine flight arrows from 100# plus bows !) For an easy to build spine tester, have a look at this post.
ReplyDeletehttps://bowyersdiary.blogspot.com/2017/10/spine-measurement.html
Thanks a lot, this makes it simpler. I have read that for wooden selfbows one should go for the spine equal to the power of the bow in pounds plus 10 for some reason but I suppose that is a load of tosh then.
ReplyDeleteI will check it, thank you very much!
Simon
The general rule is bow poundage MINUS 10! But it's only a general rule for average bows. Not all 40# bows shoot at the same speed, and a 70# bow doesn't shoot at twice the speed of a 35#
DeleteBTW. The reason it is minus 10 is that an ELB doesn't have a cut away arrow pass, so the arrow has to flex more to get round the grip and thus needs to be a bit weaker.
DeleteOh that is my bad then, must have remembered that incorrectly. That makes sense at any rate. I will likely just build a spinetester, see which specific arrow shoots well annd with as little contact as possible out of my bow and then try to match my other arrows to that.
ReplyDelete