spine

10 point shooter · 08-20-2006, 09:53 AM

Speaking of spine. How do you tell the spine of an arrow? Is the lower number stiffer or the higher number?

phil 5oh · 08-20-2006, 10:32 AM

The number re: spine depends on the manufacturer. With PSE and Carbon Express the higher numbers are stiffer. Easton and Gold Tipare the other way. Higher numbers are weaker spine. With aluminum it's different from carbon. You have to consider the shaft diameter (first 2 numbers) and wall thickness (last 2 numbers). A 2413 is 24/64" in diameter with a .013" wall.

10 point shooter · 08-20-2006, 11:30 AM

That pretty well answers my question. The pro shop yesterday recomended I switch from the gold tip 7595 to the easton epic 300s. They paper tuned better fixing to give them a try right now.

Paul L Mohr · 08-20-2006, 01:36 PM

With most easton carbon arrows the model number is the amount of spine deflection the arrow has, which is the actual spine of the arrow.

When talking arrow spine the lower the number the stiffer the arrow, but the model numbers don't always correspond, you need to look up the actual spine deflection for that arrow.

Spine is measured by putting an arrow on two contact points (28 inches apart I believe, but could be wrong) and suspending a specific weight in the middle ( can't remember what it is off the top of my head) and measuring the amount of flex or deflection that it has. A weaker arrow will obviously flex more than a stiffer one thus giving it a higher number. The measurement is in thousands of an inch. An easton 400 series arrow will have .400" deflection.

Paul

10 point shooter · 08-20-2006, 02:22 PM

That is very cool! 51 years old and I learn something new every day. I think I had read about that once many years ago. But I have been shooting the same bow, same arrows, same broadheads for so long, alot of the things I read and here now are like I am learning them for the first time.

Thanks

Jim B

10 point shooter · 08-20-2006, 02:28 PM

PS. The eastons are shooting great with field points. Next step broadheads.

Paul L Mohr · 08-20-2006, 03:36 PM

Since you seem to be on knowledge quest I will try and confuse you even more

. There are two types of spine related to arrows. I may get the terminology wrong, but you will get the point.

One is static spine, which is what I described in my above post. It is simply a specification given to a particular arrow. Like lets use a 2213 for example since that is what I shoot. It has a spine specification of .460. So if I use a spine calculating program and it tells me I need an arrow of X amount of inches with X amount of spine I can just look at arrows with that spine. It is independant of length, it is just a way to classify arrows and their relative stiffness.

Then there is dynamic spine. This is how the arrow reacts to your particular set up. How it flexes when it leaves the bow and on it's way to the target. Which is why we need the arrow charts and spine software. There is no real measurement for this. Which is why we do all sorts of testing and tuning. If you take a given arrow, such as the 2213 depending on how it is set up the dynamic spine will be different. Things that can effect it are length of the arrow, tip weight, fletch and nock weight, cam design and draw length. Even little things like what type of release you use, if you nock under the arrow or behind it (string loop), nock travel and how well your bow is tuned can all effect dynamic spine to a slight degree.

On a spine tester the length of the arrow and tip weight doesn't matter (as long as it's long enough). But with a bow a longer arrow with a heavier tip will always have a weaker dynamic spine than a short one. Which is easier to bend, a 3 foot rod or a 1 foot rod?

This is why two people with the same bow set at the same specs may require a slightly different arrow, or different adjustments to get the perfect spine.

Paul

BGfisher · 08-21-2006, 07:32 AM

Very good explaination Paul. Quite detailed without being too technical.

Oh, static spine is measured with a 29" shaft, suspended on two contact points 28" apart and pushing down (or hanging) a 1.97 lbs weight in the middle. Don't know where the 1.97 lbs came from, but that's what it's always been. Never did a check of the history of who came up with it, being as I don't have a spine tester anyway--too expensive and unnecessary at my age (yeh, what does age have to do with it?).

I'll bet the 1.97 lbs correlates to some number in grams. Probably originated in Europe about 10,000 years ago.

Paul L Mohr · 08-21-2006, 03:33 PM

I knew someone would know the exact measurements.

Paul