Please explain spine to me,

[FunyFarm]

Hello world, first post / question.  I have just picked up the sport of archery, I have no interest in compound bows or the like. I want to make my own self bow from local historic material ( here it would be Juniper) With that said.
 I can not understand. SPINE. It appears to me that if you are talking about carbon it is one number that means ? Then there is another set of numbers for wood/ cane etc. like 55. 45 ,?  I do understand is that spine is the flexibility of the arrow shaft. I also know the spine needs to be matched to the bow it will be shot from. And it is extremely important to accuracy.  So here is the query: what number am I looking for. I am using a Samic Sage 52" 35# at 28" my draw is slightly longer 29 1/2 inch. So any help in understanding this would be greatly appreciated

FunyFarm

   Alluminati not all conspiracies are theory

[Dakota Kid]

Spine is really important to non-center shot bows. If you have a center shot, you can get away with much stiffer arrows. The farther you get from center the weaker the spine you'll need. Matching the arrow spine to the draw weight of a selfbow is typically a good starting point when matching arrows.

Archer' paradox is why selfbows need perfectly matched arrows. Read the wiki article on archers paradox and it will make more sense. It's been explained by better instructors than myself many times.
Basically the arrow has to bend around the handle using it's own inertia and the initial force supplied by release. After which the arrow straightens out, with help from the fletchings. Adding weight to the point weakens spine, by adding inertia to the resting arrow. Sanding the center of the arrow shaft will weaken the spine if you're close but still stiff.

[agd68]

Ok. they system of determining a bows draw weight is standardized at 28 inches of draw length, so is an arrows spine. If you increase the length of a shaft you will weaken the spine by about 5lb per inch. If you shorten the shaft you will increase the spine by about 5lb per inch.It can get confusing. Say you want to make an arrow, since you draw 29.5", you will want a shaft that spines at about 65lbs at 28 ". This will allow for the extra 2.5 to 3 inches your arrow needs to be.

[Pat B]

Spine is the deflection of the arrow shaft as energy from the bow pushes it. With too weak spine the arrow bends too much as it goes around the bow and the arrow will go left for a right handed archer. If the spine is too stiff the arrow will not get around the bow cleanly and will typically go right of where you are aiming. There are ways of determining spine of any shafting by placing the arrow on uprights that are 26" apart, hanging a 2# weight on the middle and noting the amount of deflection.

[aaron]

There are indeed several different standards for measuring spine. One way is to quote the "Deflection", or the ammount of bend under a standard pressure. Under this system, an arrow is said to be .400, .500, etc (Also known as 400, 500). To further confuse this system, there are two different standard pressures used . One system, called amo, calls for a 2 lb weight to be hung from the arrow, which is supported at two points 28 inches apart, the other system, called astm, uses a slightly lighter weight and slightly closer spacing of the supports.

Another way to express the stiffness is with numbers that represent draw weight like 50 lbs, 40 lbs. Or with GT brand carbons there may be a weight range like 15-35, 55-75.

It  gets even more complex when you take into account the fact that arrows act stiffer or weaker depending on point weight and other factors .

Finding the right spine for your bow involves making a educated guess followed by testing. For me, the easiest way to arrive at the educated guess is to use a program like "Stu millers spine calculator".

Luckily for you, you have a very forgiving bow that is almost centershot.

Do a google search for "carbon arrow state university" for a good article that also applies to wood arrows.

[aaron]

Here are some possible starting places, all based on a 30 inch arrow:
.500 spine carbons with a 125 to 175 point
Gt 1535's with a 125 to 150  point
wood 40 or 50 lb arrows with a 125 point
aluminum "1916's" with a 125 point

[Danzn Bar]

Shoot cane .....and don't worry about spine.....that's what I've heard some say......
DBar

[Pat B]

Aaron, the AMO standard is measured between 2 points spaced 26" apart not 28" and it is for a 28" finished arrow with a 125gr point.

[aaron]

Ok, thanks, pat.

[iowabow]

I agree with all of what has been said. I bare shaft tune my arrows and try to hit the target 3 inches to the right before adding feathers. I adjust the spine by sanding shaft as it comes to the right and try to balance with tip wt. This wt will also affect spine. More wt the more it moves right. The heavier arrow falls. This is a long hard process. Making good arrows can be as hard as making good bows.

[Outbackbob48]

John, not sure of your reason to be 3 "right with bare shafts , feathers do not weigh enough to affect impact point I believe bare shaft and fletched should impact same spot. Feathers are only needed to stabilize broadheads or bad releases or arrows that are not properly tuned. your other instructions are also for right handed shooters vs. left handers. Personally I like cane because of natural taper is much more forgiving of spine. Funny Farm sorry for getting away from your OP, Like some others have stated spine is all about deflection measured in thousands with a given weight and distances between 2 points, Some arrows are marked in thousands like carbons 500,400 and ect. others like wood are in spine groups of 5# such as 46-50 Aluminums come a little different with 2 numbers 1916  the first numbers is dia. in 64ths as in 19/64 ths. second number is thickness of wall or 16
 thousanths. I know clear as mud.     Hoping everyone had a Happy New Year.  Bob
 

[iowabow]

My understanding was not about wt but drag.

[iowabow]

Bob this is kinda what I do ...maybe not right but works for me.

[JackCrafty]

Just try a bunch of different arrows until you find one that shoots great.  Then make or buy arrows that match that one.  Simple. You might have to try shooting 100 different arrows but that's all part of paying your dues.

Or you can take the hard route and get a PhD in Arrowcraftology.  You'll need to be in school for 6 years, minimum.  Then you'll need to do what I suggested above before it makes any sense.
 

[Outbackbob48]

John, very interesting Bottom line is if they hit where your lookin good enough.  As many deer as I have missed who am I to be giving instructions   Bob