Help me understand spine numbering!

[LimbLover]

Pardon my newbness, I always thought I understand this but now I don't think I quite get it like I should.

A lighter arrow - like a GT 3555 has a .500 spine at around 8.4 G.P.I.

A slightly heavier arrow - like a Beman ICS 400 has a .400 spine with a greater G.P.I.

I understand the lighter versus heavier concept and had always thought that .500 was a weaker spine than a .400 as heavier arrows usually have a smaller spine numbering.

Is this backwards?

[vermonster13]

The spine number represents the deflection(how much the shaft bends) when a weight(approx 2#s) is placed in the center of a 28" length(used to be 26"). So the larger the number the more bend in the shaft and thus a weaker shaft.

[LimbLover]

So help me figure out why I have been told by multiple friends that a 2117 would should better out of my bow (55# at 30") than my 2018s with a 31" arrow?

Specs for 2117:

12.0 GPI, Spine .407

Specs for 2018:

12.3 GPI, Spine .464

The 2018 is heavier with a smaller diameter BUT thicker wall, the 2117 has the stronger spine.

Because the 2018 has the thicker wall and is heavier, shouldn't it have a stronger spine?

[LimbLover]

Well I think I've got my answer from another forum - now I just have to figure out which shoot better.

[Onehair]

It must still be 26" as the easton axis 400 spines .400 @ 26 with a 2lb. weight

[Orion]

Nope.  You're confusing physical weight with spine weight.  A thicker walled shaft would be a heavier spine than a thinner walled shaft if they were both the same diameter.  However, diameter of the shaft also affects spine.  That's why a larger diameter, but thinner walled shaft can be a heavier spine than a smaller diameter, thicker walled shaft.