One question that comes up often is how to estimate bow/arrow 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.
This article describes how you go about estimating bow/arrow speed. ArcheryCalculator.com has an automated calculator that will allow you to enter your numbers and will do the calculation for you.
Understanding Bow Speed Ratings
First it is important to understand where a bow’s speed rating comes from and what it means. Most 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 lb. 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
There are few archers that shoot 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.
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.
There is often controversy on how accurate manufacturer’s IBO/ATA speeds actually are. Because the IBO specs are fairly loose, manufacturers can (and will) push the specs to the limit in order to get higher rating numbers. Unfortunately, when the consuming archer sets up their bow at their specs they can be disappointed that their bow does not seems as fast as they think it will be.
As long as the specs remain so loose, this will continue to happen. I would personally like to see all manufacturers switch to the much more restrictive ATA specs to give a more realistic speed rating that archers will more likely achieve.
If you have your numbers and are ready to go, head over to ArcheryCalculator.com and punch them in!