What is the spine of the arrow i need?
I have a Fred Bear Sparrowhawk II.
40-50 lbs.-Draw Weight.
What size do I need?...I am new to the bow world
- fisher1221usLv 71 decade agoFavorite Answer
Spine is one of those things that everyone talks about, but few completely understand. I'll try to provide some basic information and hopefully won't make too many mistakes.
Spine is basically a measure of the stiffness of an arrow shaft. More accurately, it is a measure of the deflection a shaft exhibits when a two pound weight is suspended from the middle of the shaft when the shaft is supported at two points 26 inches apart and the shaft is rotated so that the grain of the wood is vertical. This measurement is generally made with a device called, surprise, a spine tester. It is important that the grain of the shaft be oriented properly since a grained material exhibits different stiffness with and against the grain. Arrow shafts are measured to determine the greatest stiffness and that measurement is across the grain. Note that this is important because it tells you how to orient the fletching on the shaft.
There are tables that convert shaft deflection into pounds, a more usual description of the stiffness. This conversion can also be done using the simple formula Spine (in pounds) equals 26 divided by the deflection in inches. This says that if a shaft under test bends half an inch then is it has a 52 pound spine measurement. I suspect that the formula is simple because of the selection of the measurement method (the two pound weight seems to be the item most easily used to tune the conversion). In any case, the above information should be enough to allow a handy archer to jury rig a spine tester if one was needed.
The spine of an arrow is important because of a phenomenon called Archer's Paradox. Archer's Paradox is essentially the process by which an arrow shaft bends around the bow and shoots straight. Note the word bend. The force of the bow string on the arrow causes the arrow to bend during the process of accelerating the arrow off the bow. The amount of bend affects the flight of the arrow and the accuracy and consistency of the shot. This is particularly important where the arrow rest is significantly offset from the path of the string such as with longbows shot off the hand. It is less important in center shot bows where the motion of the arrow is essentially along the path of the bow string upon release.
For the bow and arrow combination to work well, the arrow must bend just enough to get around the riser of the bow. If it doesn't bend enough, the back end of the arrow will impact the riser and the arrow will deflect. If it bends too much, the back end of the arrow will fly off to the side and the arrow will deflect. Just right and the arrow goes straight and everyone is happy.
Selecting arrow shafts for your bow is a little more complicated than simply buying shafts with the same spine rating as your bow weight. There are a couple more things that affect the effective spine of a completed arrow. The first is the weight (mass) of the arrow head or point. The heavier the point of an arrow, the lower the effective spine of the shaft. This is because of the increased inertia provided by the greater mass. With a heavier point, acceleration of the arrow will be slower, and more energy will accumulate in the arrow shaft reflected in greater bending and a lower effective spine. All other things being equal, an arrow with a 30 grain target point on it will act like a more heavily spined arrow than the same arrow with a 125 grain field point.
The second thing that affects the effective spine of an arrow is the efficiency of the bow. All bows of the same draw weight are not created equal. My 35# 70" Wing recurve will put significantly more of its stored energy into an arrow shot from it than someone's 35# "D" section hickory selfbow. Once again, more energy in means more bending during acceleration and a lower effective spine.
One more thing to remember is that spine is measured over 26 inches of the arrow shaft. If your draw length, and consequently your arrow length, is significantly different than 28 inches, the effective spine of your arrow shafts will be different. The rule of thumb here is that you require about three to five more pounds of spine for every inch increase in arrow length over 28 inches. You require two to three less pounds of spine for every inch decrease in arrow length under 28 inches. Stiffness increases faster as a shaft is shortened.
All this said, however, the best way to select shafts with the correct spine for your bow and shooting style is through bare bow testing. Here you shoot unfletched arrows of differing spines while observing the flight of the arrows. The ones that fly straight and true even without fletching are correctly spined for your shooting. If you must err, err on the side of stiffer arrows. It's safer and allows easier archer adjustments.
- targetbuttLv 61 decade ago
go to www.eastonarchery.com, they have a tool for spine selection. You'll have to know the length of arrow you want to make and the weight of the bow also the cam type. I'm not familiar with the Sparrowhawk, but from the way you describe the draw length and the huge variance in weight, I assume it's a compound bow.
If you're shooting with a release, spine is no longer such a big issue. The easton website tool usually give you a spine that is a bit stiffer than what you really want. When you get your arrows leave about an inch of room, so if you really want a 25" arrow, cut it to 26". Where spine matters most is with recurves finger shooting. With fingers the arrows deflect side to side instead of the vertical up and down deflection that compound release get.
When you're over at the easton website also make sure you download their tuning guide.
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- ole max.Lv 51 decade ago
take your bow to your local archery shop and have them choose the proper arrow for the size of your bow, you also have to be measured for drawlength so they will know where to cut off the arrows for a proper fit. you might consider a carbon shaft for arrows because they are almost indestructable, they do cost a little more than alluminum shafts but are worth the money. good luck and practice before you go to the woods.
- Anonymous1 decade ago
Well I was once like you I would have to say go to your local outdoor store like *****, Gander mountain, etc. and ask them this question would kind be hard for a civilian to know you have to go to the experts