Part 1
Basics & Measurements 		Part 2
Arrow Length & Mass 		Part 3
Spine & Tip Weight 		Part 4
Fletching & F.O.C 		Part 5
Speed & Kinetic Energy

Arrow Spine:  The Backbone of the Arrow

If you've ever gone fishing, you probably already understand this concept.  Would you take your heavyweight fishing-rod when you go Bluegill and Perch fishing?  Of course not.  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 shark fishing either.  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.

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 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.  You may have noticed that most arrows come in different sizes denoted by some kind of number system:  Gold Tip 5575's, Beman 400's, A/C/C 3-60's, Carbon Express 4560's, etc.  While each manufacturer's number system is often different, the important thing to note is that the number on the arrow specifies the spine (the stiffness) of that particular shaft, and doesn't necessarily denote the exact draw weight of the bow from which it should be shot.  For example, don't assume a Gold Tip 3555 will only work in bows ranging from 35# to 55# of draw weight.  That may or may not be the case.  The actual stiffness ( static spine ) of the shaft material is only one factor in determining the effective, or dynamic, spine of your finished arrow. 

And don't assume spine sizes transfer from one brand to the other.  In some brands, higher numbers represent the stiffest spine sizes.  In other brands, lower numbers represent the stiffest.  So don't assume if "300" is your spine size in one brand that it will be the same in the next brand.  Each manufacturer has their own system.  Be sure to check the individual manufacturer's charts (more on this in a moment) before you decide which spine size is right for you.
Arrow Spine:  Why Does it Matter?

A properly spined arrow is simply safer, and flies better than an improperly spined arrow.  To begin, if an arrow is too weak (underspined) for a particular bow, there is a risk that the arrow could break when shot.  The more grossly underspined the arrow, the higher the risk of breakage.  In a few rare instances, arrow failures have even resulted in serious injuries.  If your arrow breaks upon release, there is a remote possibility that the remaining half of the arrow could be driven through your hand or arm.  Ouch!!!  To avoid the embarrassment of ever needing to explain to an emergency room doctor how you managed to shoot yourself with your own bow, we strongly suggest you NEVER shoot an arrow that's underspined for your bow.

But aside from avoiding the freak accidents, choosing a proper arrow spine will give you the best possible arrow flight and result in dramatically improved accuracy.  Most people think an arrow flies just like it looks when at rest - perfectly straight.  Hence the phrase "straight as an arrow".  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.  Arrows are supposed to flex and bend some.  In fact, with respect to accuracy, an overspined (too stiff) arrow actually flies just as badly as an underspined (too limber) arrow.  So don't choose an overly stiff arrow either, tough guy.  Choosing an over- or underspined arrow yields absolutely no benefit - regardless of what you hunt or how you shoot.

Each arrow bends and flexes in a particular cycle as it leaves the bow (archer's paradox).  In slow-motion video it becomes obvious - especially for finger shooters.  An overspined arrow undercycles and leaves the bow with it's tail too close to the bow's riser usually resulting in serious fletching contact as it passes the arrow rest.  An underspined arrow overcycles and the arrow's tail leaves the bow too far away from the riser with similar results the opposite way.  Arrows fired with a mechanical release often cycle more vertically rather than horizontally, but the business of timing the cycle is still the same.  When the spine is just perfect, the fletchings zip through the rest unimpeded and the arrow flies.....uh, well.......straight as an arrow.  So we have to get the spine right.
What Factors Affect Arrow Spine?
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There are three main ingredients we'll need to stir-in when trying to brew up the perfect arrow spine:

   (1) the stiffness of the actual shaft material 
   (2) the length of the shaft 
   (3) the tip weight that will be used 

But it's not quite as simple as 1-2-3.  How stiff an arrow is when it's sitting still on the workbench, and when it's busy accelerating from 0-180 mph as it's fired from the bow, are totally different issues.  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 one will be on the quiz.

 

STATIC SPINE OF THE SHAFT MATERIAL
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Let's start with static 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.  The shaft's resistance to being bent this way is known as it'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 SPINE OF THE SHAFT
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Unless your arrow shaft breaks or is altered, it's static spine remains the same.  But your arrow's dynamic spine can change dramatically.  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.  When you shoot an arrow, the explosive force of the bow compresses the shaft and the shaft momentarily bends under the strain.  Unlike the unwavering characteristics of an arrow's static spine , the dynamic spine of two identical arrows, shot from two different bows of varying output, could be drastically different.  How's that possible?  If your arrow has a perfect amount of dynamic spine when shot from your modern 70# hard-cam bow - it's 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 shot your son's arrows in your 70# bow, it's likely the arrows would 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.  So hang in there.  We're almost through. 

SHAFT LENGTH AFFECTS ARROW SPINE
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An arrow shaft bends, not because it is being pulled down in the middle, but because it is being pushed inward from the ends.  It is being compressed when it's shot.  And the longer the shaft, the more easily a compressive force can bend it.  Imagine a brand new pencil.  If you put each end of the pencil between your palms and began compressing the pencil by squeezing your palms together, this would be similar to the forces that cause an arrow to bend when shot.  So, with your standard length pencil, could you push your hands together hard enough to make the pencil bend?  Probably not.  A short pencil is surprisingly stiff and resists bending this way.  But if that same pencil were 2 ft long, you could bend it easily by compressing it.  Under a compressive load, the longer pencil had less spine than the short one, even though the shaft material (the wooden pencil) remained the same - with the same static spine per given length.  Again, same concept applies for arrows.  Longer arrows have less spine (more limber), shorter arrows have more (more stiff).
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TIP WEIGHT AFFECTS ARROW SPINE
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Every arrow should have a tip.  The tip is the business end of the arrow.  It could be a simple steel practice point, a razor sharp hunting broadhead, a small-game judo tip, or a number of other tips designed for a variety of purposes.  Each of these arrow tips is also designed to a specific grain weight.  The most popular weights are 75gr, 85gr, 90gr, 100gr, and 125gr.  However, some specialty tips can be much lighter or heavier.  

OK.  Now remember how a bow compresses the arrow shaft?  It's not hard to figure out what's pushing in one direction - the forward movement of the bow's string.  That's an easy one.  But what force pushes back the other direction?  You can't get that kind of compression if you don't have two opposing forces - one pushing on each end of the shaft, right?
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Right!  So what pushes on the other end?  Oddly enough, it's the arrow's TIP.  Of course, the tip doesn't actively do anything.  It's just a weight - hanging out at the end of the shaft.  But surely you must remember learning about Newton's Laws of Motion in school, eh?  Remember the one that says "An object at rest tends to stay at rest unless acted upon by a force" ?  AH-HA!  Well think of it this way.  The arrow's tip is the "object at rest", and the forward movement of the string is the "force".  The stationary mass 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 resistance of the heavy stationary tip and the forward motion of the string create opposing forces and.....Viola!....compression.

So, the greater the tip weight, the greater the compression (and flexing) of the the arrow shaft when shot.  The lighter the tip, the lesser the compression (and flexing) of the arrow shaft when shot.  See where we're going?  You guessed it.  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). 

This concept is a bit more abstract, so consider a dramatic example to illustrate the concept.  Imagine if you screwed a bowling ball on the end of an arrow and tried to shoot it.  Upon firing the bow, the arrow shaft would bend dramatically as it grudgingly inched forward, trying to get the heavy stationary mass of the bowling ball into motion.  The arrow would be highly compressed between the forward movement of the string and slowly accelerating mass of the bowling ball.

MACHO "BIG-MAN" TIP WEIGHTS

In sections #4-5, we'll discuss the topic of tip weight selection and explore it's effects on arrow mass, front-of-center balance, loss of shot trajectory, and kinetic energy in greater detail.  But for now, it's worth noting that many archers choose a ridiculously heavy tip weight for their hunting arrows.  There's often no logic behind the selection, other than the macho idea that bigger is better, and the often distorted and misunderstood notion that heavier tips "hit harder".  So don't get too puffed-up bragging about how you " always do better when shootin' them big XXX grain super-ultra-magnum broadheads ".  With today's hot new compound bows often pumping out 50, 60, even 70+ ft-lbs of kinetic energy, much of the "old school" thinking about hefty arrow tip weight is no longer applicable.  

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BOW OUTPUT AFFECTS DYNAMIC ARROW SPINE
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If all this talk of spine is becoming a real pain in the neck, don't worry.  We have just a couple more details to cover, then we'll sum up the discussion on 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 it's spine characteristics.  But as we mentioned earlier, the arrows final dynamic spine (how much it will actually flex when shot) will also depend greatly 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 will less powerful output will require more limber shafts.

Fortunately, the engineers have already crunched the numbers for us.  In just a few moments we'll take a look at a sample spine-size selection chart.  You won't need to solve any equations or plot any graphs today.  But before we go to the charts, you should understand which attributes affect the output of a bow and the spine requirements of the arrow.  Most arrow manufacturers publish charts which take some, or all, of these bow output factors into account when recommending a particular arrow spine size.

Less Spine Required

More Spine Required

Lighter Draw Weight Heavier Draw Weight
Shorter Draw Length Longer Draw Length
Lighter Tip Weight Heavier Tip Weight
Less Aggressive Cam More Aggressive Cam
More Let-Off % Less Let-Off %
Less Efficient Bow More Efficient Bow


APPLYING SOME COMMON SENSE

As we said, some manufacturers have very complex charts that take many variables into account.  But other manufacturers offer a more simplified chart, like the one on the left, that just represents an average bow setup.  So you may have to apply a little common sense if your particular bow setup isn't exactly "average".  For example:  If you shoot a typical 300 fps compound bow, with normal 100gr tips, and 60-75% let-off, all you'll need to do is follow the chart.  If your bow is set for 60# and you use 29" arrows, you just follow the dots on the chart and choose the 200 spine.  Easy!

But what if you shoot a very aggressive low let-off speed-bow with an IBO speed over 330 fps.........say a Bowtech Black Knight or a Mathews Black Max.  In that case, your bow will have more output than an average 60# bow, so you would need to accommodate by choosing a little stiffer spine like the 300 shaft.  So be prepared to use your best judgment, should your bow setup have some special characteristic that needs extra consideration.  
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A FINAL THOUGHT ABOUT ARROW SPINE
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Remember that changing your bow's draw weight or draw length will change the energy output of your bow.  If you order arrows to match your 70# bow, then later decide to turn the bow down to 60#, your arrows will likely be too stiff.  Before you order your new custom arrows, make sure you're comfortable with your bow's current draw length and weight.  If you're a pathological tinkerer, a growing youth archer, or you're "working up" to a heavier draw weight, you may need multiple arrow sets with more than one spine size to ensure you get the best arrow flight throughout your "transitional periods".