

ARROW SPINE & TIP WEIGHT



BACKBONE OF THE ARROW` If you've ever gone fishing, you probably already understand this concept. A fishing pole shouldn't be too limber or too stiff. You wouldn't take your heavyweight fishing-rod when you go Bluegill and Perch fishing? It's simply too stiff for the job and would perform poorly. A stiff rod doesn't cast light baits very well, and dragging in small pan fish on a heavyweight rod would be no fun anyway. On the other hand, you wouldn't dare take your ultra-lite fishing-rod for an afternoon of Florida Tarpon fishing. The ultra-lite rod wouldn't be stiff enough to fight such large fish, and it might even break if you hooked a good one. Right? For arrow selection, the concept is essentially the same. The arrow must have the appropriate strength and stiffness for the task - not too stiff - not too limber. Before we go on, please note that the official term is "spine" - as in backbone. Not "spline" - as in gears and sprockets. Arrow spine refers to the arrow's degree of stiffness - how much the arrow resists being bent. Some arrows are very stiff, others are very limber, and neither the arrow's diameter or physical weight necessarily correlate with the spine stiffness. So we have to figure this one out. If you ever intend to achieve serious accuracy with your compound bow, you'll need to choose an arrow that's just stiff enough, but not too stiff for your particular bow setup.



NOT A LASER BEAM AT ALL` Most people think an arrow flies just like it looks when at rest - perfectly straight. But nothing could be further from the truth. Once fired from a bow, an arrow immediately begins flexing and oscillating. That's not a defect. Each arrow bends and flexes in a particular cycle as it leaves the bow (archer's paradox). If the timing of the cycle is correct, the tail of the arrow clears the bow without making contact with the arrow rest, riser, or cables. If the timing of the cycle is not correct due to improper arrow spine, the over- or under-oscillation of the arrow results in serious fletching contact and/or paper-tune tears which cannot be corrected. So we have to get this one right, both for the purposes of performance and safety.



STATIC ARROW SPINE` There are just two main ingredients which determine an arrow shaft's static (at rest) spine characteristics: the stiffness of the actual shaft material and the length of the shaft. But it's not quite that simple. How stiff an arrow seems while being flexed by your hands is one thing. How that arrow behaves when its accelerating from 0-200 mph is another. When the arrow is at rest, we refer to it's stiffness characteristics as static spine. But when that same arrow is in motion, it's stiffness is a matter of dynamic spine - which adds more ingredients into our consideration pot. So pay attention. This gets a little tricky. 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. And those familiar arrow "spine sizes" like 340's, 400's, 500's reference the arrow's static characteristics only.



DYNAMIC ARROW SPINE`An arrow shaft's static spine remains constant. But the 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 flexes and behaves 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, and you take that same arrow and shoot it with 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 beyond just the shaft material and length.



TIP WEIGHT AFFECTS DYNAMIC SPINE` When an arrow is fired it bends because it is effectively being compressed. The arrow is momentarily trapped between the forward motion of the string and the static load of the arrow's tip. And the longer the shaft is, the more easily this compressive force can bend it. But it's not quite that simple. The static load of the arrow tip plays a role as well. The heavier the tip, the more it resists being put into motion. Remember those laws of motion from high-school? An object at rest tends to stay at rest unless acted upon by a force. It's like that. The arrow's tip is the "object at rest" and the forward movement of the string is the "force". The stationary mass on the end of the arrow resists the forward motion of the string, and since the heavy tip of the arrow is where most of the arrow's mass is concentrated, that's the area of the arrow that resists the most. So the forward motion of the string and the resistance of the tip create the opposing forces. The greater the tip weight, the greater the compression (and flexing) of the the arrow shaft when it's shot. The lighter the tip, the lesser the compression (and flexing) of the arrow shaft when it's shot. So a heavy tip DECREASES an arrow's dynamic spine (makes it act more limber). A lighter tip INCREASES an arrow's dynamic spine (makes it act more stiff). See? Who doesn't love Physics?



MACHO-MAN CHECKPOINT` Before we go on, this is a good time nip something in the bud. Some archers are hopelessly stricken by the Macho-Man Syndrome when it comes to choosing arrows and arrow tips. Some guys simply cannot dispense with the macho idea that bigger is better and more is meaner. We assure you, bigger is not necessarily better - at least not when it comes to selecting arrows and arrow components. Choosing an excessively stiff arrow shaft and/or an excessively heavy arrow tip will likely yield no benefits whatsoever for bowhunting in North America with a modern compound bow. In fact, MMS sufferers are often at a technical disadvantage to other bowhunters with proper setups. With today's hot new compound bows often pumping out 60, 70, even 80+ ft-lbs of kinetic energy, much of the "old school" thinking (largely from traditional archery conventions) about hefty arrow mass and heavy tip weights is no longer applicable. Some of the most popular broadheads are now only available in the common 100 grain variety. Of course, other common tip weights (notably 85 grain, 90 grain and 125 grain) still command a share of the modern archery market. Nevertheless, the useful application for the heavy 150+ grain head is limited. For modern archery anyway, the availability of heavyweight tips serves more of a psychological demand than a technical one. We respectfully suggest, if you absolutely must supersize some part of your bowhunting gear, get an extra big bow case. But get arrows that actually fit your bow.



BOW OUTPUT DRAMATICALLY AFFECTS DYNAMIC ARROW SPINE` The physical features of the arrow (the shaft's static spine, the shaft length, and the arrow's tip weight) all play a part in giving the arrow its spine characteristics. But as we mentioned earlier, the arrows final dynamic spine (how much it will actually flex when shot) will greatly depend on the output of the bow. Your draw weight, draw length, cam-type, let-off percentage and bow efficiency all contribute to the actual output of the bow. And bows with more powerful outputs will require stiffer arrows to achieve the proper dynamic spine when shot. Bows with less powerful output will require more limber shafts. But don't worry. You won't need to make a speadsheet to figure all this out. Arrow company engineers have already crunched the numbers for us on their spine selection charts. All we have to do is understand how to read the charts and interpret the spine sizes. Are you ready? Go on to the next chapter. If you've ever gone fishing, you probably already understand this concept. A fishing pole shouldn't be too limber or too stiff. You wouldn't take your heavyweight fishing-rod when you go Bluegill and Perch fishing? It's simply too stiff for the job and would perform poorly. A stiff rod doesn't cast light baits very well, and dragging in small pan fish on a heavyweight rod would be no fun anyway. On the other hand, you wouldn't dare take your ultra-lite fishing-rod for an afternoon of Florida Tarpon fishing. The ultra-lite rod wouldn't be stiff enough to fight such large fish, and it might even break if you hooked a good one. Right? For arrow selection, the concept is essentially the same. The arrow must have the appropriate strength and stiffness for the task - not too stiff - not too limber. Before we go on, please note that the official term is "spine" - as in backbone.- as in gears and sprockets. Arrow spine refers to the arrow's degree of stiffness - how much the arrow resists being bent. Some arrows are very stiff, others are very limber, and neither the arrow's diameter or physical weight necessarily correlate with the spine stiffness. So we have to figure this one out. If you ever intend to achieve serious accuracy with your compound bow, you'll need to choose an arrow that's just stiff enough, but not too stiff for your particular bow setup.Most people think an arrow flies just like it looks when at rest - perfectly straight. But nothing could be further from the truth. Once fired from a bow, an arrow immediately begins flexing and oscillating. That's not a defect. Each arrow bends and flexes in a particular cycle as it leaves the bow (archer's paradox). If the timing of the cycle is correct, the tail of the arrow clears the bow without making contact with the arrow rest, riser, or cables. If the timing of the cycle is not correct due to improper arrow spine, the over- or under-oscillation of the arrow results in serious fletching contact and/or paper-tune tears which cannot be corrected. So we have to get this one right, both for the purposes of performance and safety.There are just two main ingredients which determine an arrow shaft's static (at rest) spine characteristics: the stiffness of the actual shaft material and the length of the shaft. But it's not quite that simple. How stiff an arrow seems while being flexed by your hands is one thing. How that arrow behaves when its accelerating from 0-200 mph is another. When the arrow is at rest, we refer to it's stiffness characteristics as. But when that same arrow is in motion, it's stiffness is a matter of- which adds more ingredients into our consideration pot. So pay attention. This gets a little tricky. 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. And those familiar arrow "spine sizes" like 340's, 400's, 500's reference the arrow's static characteristics only.An arrow shaft's static spine remains constant. But the 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 flexes and behaves 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, and you take that same arrow and shoot it with 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 beyond just the shaft material and length.When an arrow is fired it bends because it is effectively being compressed. The arrow is momentarily trapped between the forward motion of the string and the static load of the arrow's tip. And the longer the shaft is, the more easily this compressive force can bend it. But it's not quite that simple. The static load of the arrow tip plays a role as well. The heavier the tip, the more it resists being put into motion. Remember those laws of motion from high-school?It's like that. The arrow's tip is the "object at rest" and the forward movement of the string is the "force". The stationary mass on the end of the arrow resists the forward motion of the string, and since the heavy tip of the arrow is where most of the arrow's mass is concentrated, that's the area of the arrow that resists the most. So the forward motion of the string and the resistance of the tip create the opposing forces. The greater the tip weight, the greater the compression (and flexing) of the the arrow shaft when it's shot. The lighter the tip, the lesser the compression (and flexing) of the arrow shaft when it's shot. So a heavy tip DECREASES an arrow's dynamic spine (makes it act more limber). A lighter tip INCREASES an arrow's dynamic spine (makes it act more stiff). See? Who doesn't love Physics?Before we go on, this is a good time nip something in the bud. Some archers are hopelessly stricken by the Macho-Man Syndrome when it comes to choosing arrows and arrow tips. Some guys simply cannot dispense with the macho idea that bigger is better and more is meaner. We assure you, bigger is not necessarily better - at least not when it comes to selecting arrows and arrow components. Choosing an excessively stiff arrow shaft and/or an excessively heavy arrow tip will likely yield no benefits whatsoever for bowhunting in North America with a modern compound bow. In fact, MMS sufferers are often at a technical disadvantage to other bowhunters with proper setups. With today's hot new compound bows often pumping out 60, 70, even 80+ ft-lbs of kinetic energy, much of the "old school" thinking (largely from traditional archery conventions) about hefty arrow mass and heavy tip weights is no longer applicable. Some of the most popular broadheads are now only available in the common 100 grain variety. Of course, other common tip weights (notably 85 grain, 90 grain and 125 grain) still command a share of the modern archery market. Nevertheless, the useful application for the heavy 150+ grain head is limited. For modern archery anyway, the availability of heavyweight tips serves more of a psychological demand than a technical one. We respectfully suggest, if you absolutely must supersize some part of your bowhunting gear, get an extra big bow case. But get arrows that actually fit your bow.The physical features of the arrow (the shaft's static spine, the shaft length, and the arrow's tip weight) all play a part in giving the arrow its spine characteristics. But as we mentioned earlier, the arrows final dynamic spine (how much it will actually flex when shot) will greatly depend on the output of the bow. Your draw weight, draw length, cam-type, let-off percentage and bow efficiency all contribute to the actual output of the bow. And bows with more powerful outputs will require stiffer arrows to achieve the proper dynamic spine when shot. Bows with less powerful output will require more limber shafts. But don't worry. You won't need to make a speadsheet to figure all this out. Arrow company engineers have already crunched the numbers for us on their spine selection charts. All we have to do is understand how to read the charts and interpret the spine sizes. Are you ready? Go on to the next chapter.