How to Measure Arrows

Carbon Arrow Selection and Research Guide / Chapter 1                                          Read Arrow Safety Notification


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BASICS AND MEASUREMENTS

carbon arrows from a big box storePROPER ARROWS ARE ESSENTIAL` If you're one of the many bowhunters who select arrows each season by just grabbing a handful from the miscellaneous arrow bucket at the local super-mart, you may be surprised to learn that you've been cheating yourself. Shooting the proper arrows will greatly improve your accuracy and success in the field - and for less money than you might think. If you want reliable and accurate performance from your compound bow, your arrow must be specifically matched to YOUR bow setup. There is no such thing as a "one size fits all" arrow. An improperly sized and/or poorly constructed arrow will not only fly erratically, profoundly degrading your accuracy, but it may present a safety hazard for you and your expensive compound bow. If you are serious about bowhunting, you owe it to yourself, and to the game you pursue, to shoot the right ammunition. Modern archery is a semi-technical sport. So there are a number of technical considerations to juggle when selecting arrows: proper spine, FOC balance, weight, straightness, fletching material, fletching angle, arrow length, etc. And if you're feeling a bit lost, don't worry. This isn't exactly rocket-science - this online guide will provide you with all the information you'll need to choose the right arrows for your bow. These next sections will take you step-by-step through the process of selecting and ordering custom carbon arrows to fit YOUR bow, purpose and budget. We hope you find this help section useful.

PARTS OF AN ARROW` The parts of a modern hunting arrow are pretty straight forward, but these parts will be referred to throughout this help guide. So before we really get going here, let's take a moment to bone-up on our arrow jargon. The foundation of every arrow is the SHAFT, a long hollow tube usually made of aluminum or carbon/graphite composite materials. The rear of the arrow is fitted with a small piece of molded plastic called a NOCK, which allows the arrow to physically attach to the bow's string. At the front of the arrow is a small aluminum (sometimes plastic) sleeve called an INSERT. The insert gets glued into the end of the shaft and provides a threaded hole in which to screw in the arrow's TIP. The tip is the business end of the arrow, and it doesn't necessarily have to be a practice point (as pictured here). A standard 8-32 insert allows you to screw-in and use of a variety of tips in the same arrow (broadheads, judo-points, blunt-tips, field points, fishing tips, etc.). The last component is the arrow's FLETCHING - the flight wings. The arrow's fletching is usually done with colorful parabolic shaped pieces of soft plastic (vanes) or feathers. In most cases, the three fletches are glued onto the shaft in an equally spaced circular pattern, with two fletches one color (the hen-fletches) and the the third fletch a different color (the cock-fletch).

parts of an arrow diagram

ARROW LENGTH


STANDARD OF MEASUREMENT` The standard AMO Method of measuring an arrow is the distance between the bottom of the groove of the nock (where the string rests in the nock) to the end of the arrow, not including the tip or insert. We measure and trim all arrows to length using this standard AMO (now the ATA) method. Be advised that some archery retailers may be unaware of industry AMO standards and may confuse the AMO length of the arrow with the arrow's shaft length or the arrow's tip-to-tip physical length, which will both be different than the AMO measurement. So don't trust anyone else's declared measurement of your arrows. If you are buying replacements for your existing arrows, be sure to MEASURE FOR YOURSELF before ordering custom carbon arrows. Once an arrow is cut, the process can't be undone. So as in carpentry, the measure twice and cut once philosophy must be observed. If you already have existing arrows which fit your bow correctly, simply measure one by this method and order the same size. If you are unsure about what arrow length is appropriate for your bow setup, the next section may help.

REQUIRED ARROW LENGTH` The proper length for your arrow will depend upon several factors: the draw length of the bow, the type of bow you have, and the position of your arrow rest. Before we dive into this issue, we should briefly discuss how the draw length of a bow is measured. Officially, a bow's draw length setting can be found by measuring the distance between the groove of the nock to a position 1.75" beyond the grip pivot point when the bow is at full draw. Confused? Not to worry. There's a simplified method too. Conveniently enough, for most bows, 1.75" beyond the grip pivot point is roughly at the outer edge of the bow's riser. So without splitting too many hairs, we can say that a bow's draw length is approximately from the nock point to the front of the riser - when the bow is drawn back. So if you drew back a 29" arrow, and the insert of the arrow lined-up with the outside edge of the bow's riser, the bow is set for approximately 29" draw length. Whew! Glad that's covered! Many people think the bow's draw length and arrow length have to match. This is not necessarily true!. On modern centershot cutaway compound bows, the arrow rest typically sits well inboard of the outer riser edge. So on most setups, it's perfectly acceptable to use an arrow that is slightly shorter than the bow's adjusted draw length. As long as the arrow sits comfortably beyond the arrow rest (we like to see 1" minimum overhang), then the arrow length is sufficient.

arrow too short obstructed path shotSAFETY ALERT` Arrows which are too short for your bow setup are a serious hazard. Even an arrow that is just long enough is too short. The best safety practice is to make sure your arrows sit at least 1" beyond your arrow rest when the bow is at full draw. A little bit of extra arrow length gives the arrows an important margin of safety. A little too long is okay. A little too short is not. An arrow that is too short can lodge behind the arrow rest at full draw. If this happens and you don't notice it before you fire the bow, the arrow could buckle and snap upon release, possibly sending shards of carbon into your bow hand or arm. This kind of obstructed path shot can be a very very bad thing. See our Arrow Safety Warning page for the gruesome details. Unfortunately, some shooters (and shops) deliberately cut arrows too close to the arrow rest, usually to minimize arrow mass and get the fastest possible arrow speeds. But this practice regrettably comes at the expense of safety. The extra 1-3 fps you gain by cutting arrows just long enough isn't worth risking an arrow shaft stuck in the forearm. So never shoot arrows which are too short.

KNOW FOR SURE` Be particularly cautious if you make draw length changes on your cams. For example, if you change your draw module setting from 28" draw length to 29" draw length, and your original arrows had a 3/4" overhang, at the new setting the arrows will be 1/4" too short. Also, we recommend you not automatically trust the factory sticker on your bow that indicates draw length. Measure for yourself. In many cases, the manufacturer's sticker and the ACTUAL draw length of the bow do not match, particularly on bows that have been around the block a few times. And since changing your draw length may necessitate changing arrows too, we can avoid some trouble here by thinking ahead. If your bow does not already fit you comfortably, you should have the draw length adjusted before ordering your custom arrows. Arrows which may be perfect for a bow at 29" draw length, may be totally inappropriate for the same bow set at 27" draw length. So, to purchase the correct arrow, you must know the draw length of your bow. And of course, the draw length of your bow should correspond to your body's draw length requirement (which is an entirely separate discussion). If you aren't sure of your (personal) draw length, please read our Bow Fitment Guide before moving on.

MORE ISN'T MORE` On the other hand, shooting an excessively long arrow isn't so smart either. If your arrow length is excessive, your arrow will have additional (and unnecessary) mass and the additional length will increase the arrow's spine requirements (more on this in a moment). Basically, extra long arrows significantly decrease your arrow speeds and limit the performance of your bow. So we shouldn't assume that more is more either. Choosing a safe yet optimally performing arrow length is the goal. For most of us, it's really not so complicated. If you have a modern center-shot cutaway riser bow which is already setup to fit you, finding your optimal arrow length is easy. Simply draw an arrow back to full draw and hold, while another person (safely standing to the side of course) takes a Sharpie marker and makes a mark on the arrow approximately 1" forward of the arrow rest. Then measure the arrow from the groove of the nock to the mark on the arrow, and you've got it. Obviously, this doesn't apply to older bows or traditional bows without centershot cutaway risers. But for virtually any compound bow made in the last 30 years, this method works like a charm.

carbon arrow discussion highlight length affects arrow spineARROW LENGTH AFFECTS ARROW SPINE` Before you make up your mind about your arrow length, there's one more detail we'll need to consider. The length of your arrow is a factor in determining the proper stiffness, or spine, for your perfect arrow. The longer your arrow is, the more limber it will act when shot. The shorter your arrow is, the more stiff it will act when shot. We'll cover this issue in more detail in the next section, but you should be aware that shooting an extra long arrow often results in a double-whammy regarding arrow weight. If you shoot an excessively long arrow, not only will the excess shaft weight result in a heavier and slower flying arrow, but the added length may necessitate changing to an even heavier/stiffer arrow spine. For those of you looking to bulk-up your carbon arrows to gain a little KE, a little more arrow length may be a good thing. But most shooters want to get as much zip as possible out of their high performance compound bows, so keeping an eye on excess arrow weight is a consideration.

NO! NOT THE HACKSAW! When you purchase your new set of arrows, you have two choices regarding arrow length. Most raw shafts come in stock-lengths of 30-33", so that they can be trimmed to make a proper AMO length arrow to suit virtually any bow. You may choose to receive your arrows UNCUT (full-length) OR you may receive your arrows already trimmed to length and inserted. There is no added charge for trimming and inserting your arrows at our pro-shop, but here are a few things to consider before you decide. Carbon arrows should only be cut with a high-speed abrasive-wheel saw. Attempting to trim your new carbon arrows with your hacksaw or your plumber's tubing cutter will result in splintered fibers and a weakened arrow shaft. And if you can't make a clean 90º cut, your insert flanges will not fit in perfectly straight, so your arrow tips will all be pointing in slightly different directions. If you're the "handy" type, be sure you know the challenge you're accepting by ordering full length shafts. If you would rather avoid the handyman hassle and you're already sure of your arrow length, we would be happy to professionally trim and insert your new arrows free of charge. We even include practice tips installed in every arrow. But it's up to you.


ARROW SPINE & TIP WEIGHT

carbon arrow discussion highlight length what is arrow spineBACKBONE 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.

question iconSTATIC 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.

carbon arrow tip weight affect spine support photoTIP 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.

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