Stiffness of solid pultruded carbon fiber

[Alric]

On the off chance that anyone here knows, I'm wondering what thickness of solid pultruded carbon fiber I would need for a 7' lightweight spear core. A bit of flex is ok, but I want it to be stiff enough to hit what I'm aiming at (and pass weapon check, of course). Would .375" work? Or would I need .4" or .5"? I have no idea how stiff this stuff is compared to solid fiberglass.

[varadin]

Alric the only person i know who has had any luck with the stuff is Spaztic of the dragonhood. I know dudley can put you in touch with him.

[Tails]

What's wrong with bandshop pole?

[Alric]

Weight, I'm looking for something in the 8oz range ideally.

[Dabbanoth]

I've never had Pultruded Carbon fiber last for more than a few hits before - i cant remember the thickness that i bought, exactly, but it was the thickest walled kind they sold on the website.

[Oznog]

CF is substantially stiffer than FG.

But the diameter is a VERY large factor. Stiffness increases with the CUBE of the diameter!

That's so significant that, for a fixed outer diameter, thicker walls on the inside only have a marginal increase on stiffness because their location is at a smaller diameter on the inside. That is, say you have a 1" tube with a 0.10" wall of a certain thickness and you say "ok, I want it a little stiffer, but DON'T add more on the outside, add a little on the inside of the design, reducing the ID". Well, ok, then we're adding material on the inside where it's 0.9" dia, where it only adds 73% of the stiffness it would add if placed on the outside.

Stiffness of both the FG and CF fibers vary widely depending upon construction, and we didn't specify wall thickness, so it's impossible to compare them with any degree of accuracy. Say CF is a 3x stiffer material though (comparing CF arrows to FG, yeah, about right). You would still need a 0.67" OD tube of CF to equal the stiffness of 1" FG.

In short:
0.5" OD ain't gonna cut it.
CF is very expensive by weight. Ask for twice the material, expect twice the price.
Saving money by specifying a thin-walled large OD will meet your stiffness criteria, but will break easily.

Pultruded does maximize its stiffness- but it's more breakable in two ways. One, the wall's crush strength is very low, esp when you're doing thin-walled construction. Like an egg. You step on it, it breaks. For this reason most CF arrow shafts, despite the need for strong axial stiffness, go for larger diameter thin-walled wrapped construction.

Two, the minimum radius it can be bent into before splitting is substantially lower. This affects how much of a swing it can take or someone trying to lean on or push through it. In short- probably not much.

Most of the tubes you'll find made won't be 100% pultruded. They may have a wrapped core or may have an outer wrap to strengthen against splitting over a pultruded core.

[Solusar]

Alric the only person i know who has had any luck with the stuff is Spaztic of the dragonhood. I know dudley can put you in touch with him.

Those cores are pure gold. Expensive, but GOLD.