Arrow Spine And Tip Weight
Tip Weight Affects Dynamic Arrow Spine
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?
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. Remember the one that says "an object at rest tends to stay at rest unless acted upon by a force?" 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 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 at rest create the opposing forces.
So, the greater the tip weight, the greater the compression (and flexing) of 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).
This idea is a bit more abstract, so consider a final 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 compress between the forward motion of the string and the bowling ball. The arrow would bend dramatically as the bowling ball started to grudgingly inch forward. This would be an extreme example of how a heavy tip would reduce the arrow's dynamic spine. So remember, lighter tips make an arrow act more stiff. Heavier tips make an arrow act more limber.
In sections #4-5, we'll discuss the topic of tip weight selection and explore its 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 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. 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 benefit whatsoever to most bow hunters. In fact, MMS sufferers are often at a technical disadvantage to other bow hunters 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 about hefty arrow mass and heavy tip weights is no longer applicable. If you absolutely must supersize some part of your bow hunting gear, get an extra big bow case. But get arrows that actually fit your bow.
Bow Output 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 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 with less powerful output will require more limber shafts.
Fortunately, the engineers have already crunched the numbers for us on their spine selection charts. 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.
Basic Arrow Spine Application Charts
Some arrow manufacturers have very complex charts that take many variables into account. But other arrow manufacturers offer a more simplified chart with an arbitrary number system, like the sample chart on the right which just references draw weight and arrow length. If you go by the simple chart method, then you'll need to apply a little common sense if your particular bow setup isn't exactly "average." For example, if you shoot a typical 310 fps compound bow, with normal 100 gr tips, and 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 2000 spine size. Easy!
But what if you shoot a very aggressive speed-bow with an IBO speed over 340 fps ... say a PSE X-Force or a Bowtech Destroyer 350? 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 3000 shaft. So be prepared to use your best judgment, should your bow setup have some special characteristic that needs extra consideration.
If you would rather not use the simplified method, then we suggest you get to know the concept of actual spine deflection.
Spine Deflection Measurements
Sometimes an arrow's spine stiffness is expressed as a more technical measurement, called a spine deflection. According to the modern standards (ASTM F2031-05) an arrow's official spine deflection is measured by hanging a 1.94 lb. weight in the center of a 28" suspended section of the arrow shaft (not to be confused with the old AMO standard of 2 lb. and 26"). The actual distance the 1.94 lb. weight causes the shaft to sag down is the arrow's actual spine deflection. For example, if a 1.94 lb. weight causes the center of a 28" arrow to sag down 1/2 inch (.500"). Then the arrow's spine deflection would be .500". Stiffer arrows will, of course, sag less. More limber arrows will sag more. So the stiffer the arrow is, the LOWER its spine deflection measurement will be. The more limber an arrow is, the HIGHER its spine deflection measurement will be.
Remember how we mentioned the cut-throat squabble about arrow straightness? Well, the same is true for spine consistency. If you test for spine deflection while slowly rotating the arrow, the spine deflection should remain constant. No matter which direction you bend the shaft, it should be equally resistant at all points, right? Nope! The wound layers of a carbon arrow will often have a seam somewhere inside the circle. This creates an imbalance in the spine consistency of the arrow, such that the arrow is a bit more stiff at one point around the circle. It's unfortunate, but the process by which carbon arrows are manufactured inevitably results in some imperfections. And if you want to get really technical, most arrow shafts aren't even perfectly round for that matter.
But the important question is, does it really make a difference? From a pure physics standpoint, yes. But again, few shooters have enough skill to notice. Small amounts of spine variance are realistically inconsequential to the weekend bow hunter and backyard enthusiast. But that doesn't stop the arrow companies from bickering about who has the best spine consistency and accusing each other of spine crimes. After all, every arrow company wants YOU to believe that their arrows will give you a technical advantage ... even if that's a little distortion of the truth. Again, we'll avoid the internal melee here by not pointing fingers, but if you wish to know more about spine variance, check the forums. There are a few forum regulars out there who own spine testing devices and Hooter Shooters. And they'll be happy to disagree with us on this issue.
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