Know your Bow!!
Content Provided By: Brian Stephens, StickemArchery.com Pro Staff
One question that comes up often is how to estimate a bow’s speed under different setups. By knowing the IBO/ATA specification of a bow, it is possible to get a decent estimation of how fast a bow will shoot with different arrows, draw lengths, draw weights, etc.. We did another article on this on our resources portal that looks at all of these factors including a page on calculating your arrow speed and kinetic energy.
Understanding Bow Speed
Let’s first talk about where a bow’s speed rating comes from and what it means. Most compound bows use the IBO (International Bowhunting Organization) specification. This calls for shooting a bow with an arrow that weighs 5 grains per pound of draw weight and is generally done with a bow set at 70 lbs draw weight and 30″ draw length with only a one nock set on the string. There is some wiggle room in the specs as well; the draw length is allowed to be +-3/4″ and the draw weight +-2 lbs. This means that the bow can actually be set at 30 3/4″ and 72 lbs shooting a 350 grain arrow.
Another specification that is gaining some popularity is the ATA (Archery Trade Association) spec. It is very similar, yet much more restrictive with very little leeway in the way the bow has to be setup. For ATA, the bow can be set to 50, 60 or 70 lbs of draw weight with only +-0.1″ variance and a draw length of 30″, +- 1/4″, also with only one nock set on the string. The arrow must be exactly 5 grains per pound of draw weight. Because it is a more restrictive spec, it gives an overall better view of what speeds a bow can really reach.
Estimating Bow Speeds From Specifications
The majority of bowhunters are not shooting at the 70 lbs, 30″ and 350 grain arrow that most bows are rated at. Therefore it’s useful to know how to estimate how a bow will perform under different setups. By knowing the IBO or ATA rating of a bow and also knowing what you will have the bow set to and the arrow weight, the actual bow speed can be estimated. This is not an exact science, but with a little math it’s possible to get close. It is easy to be lead to believe that when you pull your bow out of the box it will be shooting the blazing speeds advertised on TV. Again, most shooters will be fit the specs referenced on the TV ads. That does not mean your bow will not be fast. It just means that if you truly want to know a MORE ACCURATE SPEED for YOUR Bow you can follow these steps.
For every inch of draw length under 30″, subtract 10 fps, every inch over 30″ add 10 fps.
For every 3 grains of total arrow weight above 5 grains per pound of draw weight, subtract 1 fps.
For every 3 grains of weight on the bow string, subtract 1 fps.
Generally speaking, bow ratings are done at 70 lbs but quite often they are shot at 60 lbs. This will usually result in the bow performing at 3-4 fps slower, assuming that the arrow remains at 5 grains per pound draw weight.
The rule for weight on the string applies to nock sets, d-loops, kisser buttons, peep sights and anything else near the center of the string. The big exception is for nock sets and speed buttons precisely placed near the cams that can actually speed up the velocity. Do not count these items when calculating speed changes.
If you would like to calculate your Arrow Speed, Kinetic Energy, Arrow Weight click here to go to our Archery Calculators Page….
Originally posted 2012-06-30 11:26:54. Republished by Blog Post Promoter
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