In addition to Frank's discussion about aluminum arrows... Wall thickness has some effect on stiffness, but it's major effect is weight. Shaft diameter has some effect on weight, but it has more effect on stiffness. The thicker the walls, the heavier the shaft. The larger the diameter, the stiffer the shaft. Two shafts the same diameter, the thicker walls are stiffer. Anyway, that's why a 2512 is a much stiffer arrow than a 2018, but the 2018 is a good bit heavier. And a 2219 is a stiffer, heavier arrow than a 2213.

Thin walls are easily dinged and cracked, and they can fold up like an accordian. Smaller diameters are easy to bend. Thicker walls are heavier but more durable. Larger diameters are more resistant to bending. Go up to a 2419 and those bad boys are darn near indestructable!

I generally shoot 2315's out of my bows because they're a good compromise between weight and durability. I won't go less than .015" wall thickness on an arrow when I want decent durability.

My main rule of thumb for carbon... If the chart shows me within 5 pounds of the upper end of the scale for my setup, I ALWAYS go to the next stiffest spine. If the arrow has a reputation for being softer spined than they're marked, I bump up if I'm within 10 pounds. It works for me.