This answers it better than i can..hope it helps.
Static Arrow Spine
If you support an arrow shaft at two points a given distance apart, then hang a weight in the middle of the arrow - the weight will cause the arrow shaft to sag. How much the shaft resists this type of bending would be a function of the arrow's static spine. The actual static spine of the arrow shaft is determined by the elasticity of the materials in the shaft and the geometry of the shaft. In multi-layered arrows (carbon/aluminum, etc.) the bonding materials also contribute to the static spine. The inside diameter, the cross-section shape, and the thickness of the material all contribute to the static spine of the shaft material. However, arrows don't perform under static conditions - like a floor joist or a curtain-rod. Arrows perform under dynamic conditions, with motion. A hanging weight doesn't really represent how forces are applied to arrows when they're actually shot, so static spine is really used as only a benchmark for predicting dynamic spine.
Dynamic Arrow Spine
Unless your arrow shaft breaks or is altered, its static spine remains the same throughout its life. But your arrow's dynamic spine can change dramatically depending on how it's used. The real mean-n-potatoes of arrow performance relies on the arrow's dynamic spine. The dynamic spine is how the arrow actually bends when shot - and there are many factors which affect the dynamic spine. The static spine of the shaft is only part of the equation. As you fire the arrow, the explosive force of the bow compresses the shaft and it momentarily bends under the strain. The more powerful the bow, the more the arrow bends. So the dynamic spine of two identical arrows, shot from two different bows of varying output, could be drastically different. If your arrow has the proper amount of dynamic spine when shot from your modern 70# hard-cam bow - its stiffness is just right - not too limber - not too stiff. But, if you take that same arrow and shoot it out of your son's 40# youth bow, it will be dramatically too stiff. The arrow will have too much dynamic spine. Likewise, if you shoot your son's arrows in your 70# bow, it's likely the arrows will be dramatically too limber - not enough dynamic spine. Determining a proper dynamic spine is a bit more complex and requires examination of several contributing factors.
Topic: static/dynamic (Read 241 times)
