spine vs arrow weight

[Riverrat43]

For instance, Beman Centershot 500's weigh 8.3 gpi. Gold Tip Velocity hunters are also 500's but they weigh 6.3 gpi. How can two arrows have such different weights per inch yet both be spined to a 500 deflection?

[CoilSpring]

Wall thickness is different and/or outside/inside diameters can be different from manufacturer to manufacturer.

[Friend]

Numerous factors such as material strength, density, design and configuration.

[Riverrat43]

Then wouldn't your bow shoot differently with an arrow that was say, 70 or so grains lighter but with the same 500 spine weight than what you normally shoot? Can you tell a big difference with 70 grains less weight in the arrow shaft?

[Diamond Paul]

I can't tell you the physics of it in terms of carbon, but it's in the construction process.  Easier to understand with aluminum.  You have a skinny shaft with a thick wall, or you have a fatter shaft with a thinner wall.  Both have about the same spine, and you pick weight or speed as your primary objective.  Spine wise, they should shoot about the same, only one will have a higher velocity and flatter trajectory.  The lighter shaft is often more fragile and subject to damage on hard hits or glance-offs, too.

[ChuckC]

weight and stiffness (spine) are two different things.  

Lots can affect it including shape, size of shaft diameter, shaft wall thickness, the materials in the shaft wall and more.  

Your bow may very well shoot differently.  For certain, that is less weight, by a somewhat substantial amount.
ChuckC

[Diamond Paul]

I forgot to mention this, but on the carbons, at least till you get below .500 spine, the outside diameters will usually be the same, so centershot doesn't come into play like it would with aluminums.  Also, .500, .400, etc., are kind of catch-all designations, and the actual spine of the arrows might be slightly more or less than that number; there will usually be some fine print that gives the actual spine of that arrow rather than the general spine group.  Carbons also tend to vary more than aluminums in my experience, so there is no promise that two .500 spine carbons of the same diameter will shoot exactly the same for you, but normally you would decide whether you wanted speed or more weight for penetration and pick accordingly.

[Riverrat43]

Thanks guys. I believe I'll go with weight and penetration.

[McDave]

As an example, I shoot Gold Tip ultralight 600's in some of my lighter weight bows.  A 29" finished arrow weighs about 300 grains.  There was a sale on some Gold Tip 1535 traditionals that are also rated at 600 spine.  I bought them and the finished arrows weigh about 380 grains.  80 grains makes quite a difference in trajectory when used for targets at distances of 30 yards and out.  I wouldn't have bought them if I had known they were that much heavier.  I assume the difference is because the GT Trads have the wood grain wrap, whereas the GT ultralight 600's are just the basic black carbon, which I assume adds 80 grains to the overall arrow weight.