Spine tester

KidCurry

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Never bothered before, and now shooting barebow spine matching is probably the least of my problems, but lockdown needs something to fill the void. Works better than I expected. I did remove the return spring from the dial gauge to improve return accuracy and reduce force on arrow.
spine tester1.jpg
 

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dvd8n

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Very good, I'm impressed, but shouldn't the bearings be set up to allow movement along the length of the arrows, rather than at 90 degrees?

To allow it to sag frictionlessly rather than to allow it to roll around it's own axis?

I like the idea of using a dial gauge; I had one with a huge pointer and massive arc with spines written on it. The stupid thing was impossible to store.
 
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KidCurry

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Very good, I'm impressed, but shouldn't the bearings be set up to allow movement along the length of the arrows, rather than at 90 degrees?
To allow it to sag frictionlessly rather than to allow it to roll around it's own axis?
It's the spine change as it rolls is what I'm after, although if you tap the arrow it bounces back to the same reading every time so I suspect the bearings are not really needed at all. However it also works very well as a arrow straightness gauge unloaded, picking up bends down to 0.5thou". I'm wondering if any pre existing bend will have an effect on spine. In theory it should but which one is more important, spine or straightness?
 

Kernowlad

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So is spine simply the force needed to deflect the centre of the arrow from its normal position?
Does it have SI units?
Spine charts seem to be almost random and carbon or carbon/Alu ones seem to have a value but not pure aluminium ones. What about wood?
‘Tis a darrk arrrt.
 

KidCurry

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So is spine simply the force needed to deflect the centre of the arrow from its normal position?
Does it have SI units?
Spine charts seem to be almost random and carbon or carbon/Alu ones seem to have a value but not pure aluminium ones. What about wood?
‘Tis a darrk arrrt.
Spine is the deflection distance (fraction of an inch) an arrow bends when a specific perpendicular force is applied to it. Usually a known weight between two supports about 0.9kg or about 8.82N of force. No idea why aluminium arrows still use the old xxnn(1916) style. Should be just as easy to print the actual spine on them.
 
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dvd8n

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So is spine simply the force needed to deflect the centre of the arrow from its normal position?
Does it have SI units?
Spine charts seem to be almost random and carbon or carbon/Alu ones seem to have a value but not pure aluminium ones. What about wood?
‘Tis a darrk arrrt.
A carbon arrow of say, spine 500, will deflect 500/1000" when supported at centres of 28" with a weight of 1.94 lb suspended from the middle. It used to be 26" centres and 2 lb, but that obviously had to be changed because it was almost sensible.

Ally have a magic number made up of diameter (in 64ths ") and wall thickness (in thousands ") which has a theoretical connection to the spine but not in any way of any use to any normal human being.

For woodies the deflection is converted, by means of a wild-assed guess, into the poundage of a notional bow that may or may not have existed in the 50s that might have shot the arrow correctly, but which corresponds to no modern bow, which means that all modern bows need some sort of poundage fiddle factor to convert to that imaginary bow.

It's simple really - I can't see why you'd be confused.

I'm convinced that the AMO committee spends all it's time drunk.
 
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KidCurry

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Ahh... re ally, if you know the dia and wall thickness you know what size point will fit, I think. T'was a long time ago :(
 

dvd8n

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Spine is the deflection distance (fraction of an inch) an arrow bends when a specific perpendicular force is applied to it. Usually a known weight between two supports about 0.9kg or about 8.82N of force. No idea why aluminium arrows still use the old xxnn(1916) style. Should be just as easy to print the actual spine on them.
I must admit that when I quickly want an idea of the spine of an ally, I look at the tables and look to see what the spine of the carbons are that are in the same group.
 

Kernowlad

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A carbon arrow of say, spine 500, will deflect 500/1000" when supported at centres of 28" with a weight of 1.94 lb suspended from the middle. It used to be 26" centres and 2 lb, but that obviously had to be changed because it was almost sensible.

Ally have a magic number made up of diameter (in 64ths ") and wall thickness (in thousands ") which has a theoretical connection to the spine but not in any way of any use to any normal human being.

For woodies the deflection is converted, by means of a wild-assed guess, into the poundage of a notional bow that may or may not have existed in the 50s that might have shot the arrow correctly, but which corresponds to no modern bow, which means that all modern bows need some sort of poundage fiddle factor to convert to that imaginary bow.

It's simple really - I can't see why you'd be confused.

I'm convinced that the ATA committee spends all it's time drunk.
Perfect; thank you! :ROFLMAO:
 

4d4m

Active member
Perfect; thank you! :ROFLMAO:
As I understand things the woodies’ spine is converted to the poundage of the notional bow that spine will suit by dividing the deflection into 26. So a 1” deflection indicates a 26lb bow. 26/1=26
0.50” indicates a 52 lb (26/0.5=52) and so on.

For a basic AFB with a shelf it’s not a million miles off, so it’s actually quite a cleverly chosen standard. So obviously it needed changing.
 

dvd8n

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As I understand things the woodies’ spine is converted to the poundage of the notional bow that spine will suit by dividing the deflection into 26. So a 1” deflection indicates a 26lb bow. 26/1=26
0.50” indicates a 52 lb (26/0.5=52) and so on.

For a basic AFB with a shelf it’s not a million miles off, so it’s actually quite a cleverly chosen standard. So obviously it needed changing.
I had no idea that there was that much thought behind it. Thanks!
 

dvd8n

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It's the spine change as it rolls is what I'm after, although if you tap the arrow it bounces back to the same reading every time so I suspect the bearings are not really needed at all.
I guess if that if that's what you are testing for then it's a sensible choice. And if you are rolling the arrow then the position will settle.

I had heard that carbon arrows don't have an even spine due to the lay-up of the carbon and where the edges of the mat fall but I wasn't convinced that it is a real effect. Can you see it on your tester?

However it also works very well as a arrow straightness gauge unloaded, picking up bends down to 0.5thou". I'm wondering if any pre existing bend will have an effect on spine. In theory it should but which one is more important, spine or straightness?
Your project made me wonder if I could go the other way and convert my straightness gauge/straightener into a spine tester like yours, seeing as it's basically a dial gauge and mount with a lever attached - the work's 90% done!

Did you make the weight yourself or is it something repurposed?

Did you go to the effort of making it to the ATA standard (28" / 1.94lb) or are you just using what's convenient to spot the variations that you're interested in?
 

dvd8n

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Your project made me wonder if I could go the other way and convert my straightness gauge/straightener into a spine tester like yours, seeing as it's basically a dial gauge and mount with a lever attached - the work's 90% done!
I've just had a look at the dial gauge on my tester and it's range is 0-0.5" so the maximum spine would be 500. Doh! 🤦‍♂️
 

Kernowlad

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I guess if that if that's what you are testing for then it's a sensible choice. And if you are rolling the arrow then the position will settle.

I had heard that carbon arrows don't have an even spine due to the lay-up of the carbon and where the edges of the mat fall but I wasn't convinced that it is a real effect. Can you see it on your tester?



Your project made me wonder if I could go the other way and convert my straightness gauge/straightener into a spine tester like yours, seeing as it's basically a dial gauge and mount with a lever attached - the work's 90% done!

Did you make the weight yourself or is it something repurposed?

Did you go to the effort of making it to the ATA standard (28" / 1.94lb) or are you just using what's convenient to spot the variations that you're interested in?
As a holder of a Materials Engineering degree (Swansea 1997), I say, errrrmmm....
But carbon does vary a huge amount depending on how it’s laid, the binding agent, etc that’s it’s a tricky one.
But one thing that should NOT be made of carbon is flashy bike seat rails. They snap. And it hurts.
 

dvd8n

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As a holder of a Materials Engineering degree (Swansea 1997), I say, errrrmmm....
But carbon does vary a huge amount depending on how it’s laid, the binding agent, etc that’s it’s a tricky one.
Well, I did say that I wasn't convinced but on the other hand it doesn't seem unreasonable conjecture that if (say) a layup started at 12 o'clock and finished at 11 o'clock, the carbon would be thicker in some directions than others, resulting in spine variation.
 

oldnut

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A carbon arrow of say, spine 500, will deflect 500/1000" when supported at centres of 28" with a weight of 1.94 lb suspended from the middle. It used to be 26" centres and 2 lb, but that obviously had to be changed because it was almost sensible.

Ally have a magic number made up of diameter (in 64ths ") and wall thickness (in thousands ") which has a theoretical connection to the spine but not in any way of any use to any normal human being.

For woodies the deflection is converted, by means of a wild-assed guess, into the poundage of a notional bow that may or may not have existed in the 50s that might have shot the arrow correctly, but which corresponds to no modern bow, which means that all modern bows need some sort of poundage fiddle factor to convert to that imaginary bow.

It's simple really - I can't see why you'd be confused.

I'm convinced that the ATA committee spends all it's time drunk.
I just spine all mine to the same, there will be less aim off to the side for 1 of my bows, one way or another. not that it makes any difference as after the first end I will have broken one and just grab any old arrow to make up the numbers!
 

Kernowlad

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Well, I did say that I wasn't convinced but on the other hand it doesn't seem unreasonable conjecture that if (say) a layup started at 12 o'clock and finished at 11 o'clock, the carbon would be thicker in some directions than others, resulting in spine variation.
Yep it can be very inconsistent. Or if done well, pretty consistent.
It can be no better than poorly laid fibreglass or superior to titanium in the right application and manufactured very carefully. It’s a tricky one but it’s definitely a slightly oversold material in many cases.
 

Geophys2

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I went all round making various spine testers, never really happy with any, (but then I don't have KidCurry's engineering skills) so I ended up buying one of the Bearpaw arrow analysers. This plugs into a computer and weighs/spines and measures straightness, and keeps a record of the values for each arrow.

I make a lot of arrows, especially wood shaft, both for myself and about a dozen other archers, so measuring spine and weight, so I can batch select shafts, is important. I don't want to make arrows with shafts that are supposed to be say 30-35 spine as bought, but usually spine from 28-38, I want to get them into batches that are within 1 spine and 5 grains weight. I usually buy 12 dozen shafts at a time so I can batch them into say 10 sets of twelve with a couple of dozen canes for the garden left over.

I shoot both Trad (Hunting Tackle) field and target longbow to a fairly high standard (MB at both) and yes being this fussy does make a difference.


This is probably about half of my arrows!


Arrow analyser, bend the arrow until it touches the stop.


Readout
 

Rik

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Ahh... re ally, if you know the dia and wall thickness you know what size point will fit, I think. T'was a long time ago :(
Except they changed those figures from being actual measurements, to being "shaft codes"... (hence the whole 9.3mm debacle)
 
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