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How to Measure a True 30 inch draw
Last Updated 02/03/2003
 


 

Determining an exact 30 inch draw length on your bow.


 

The figure to the left shows a picture of the scenario described herein. In order to determine the correct speed of a bow at 30", you need to measure your bow as follows. First determine the point at which your bow is at a full 28¼" draw. The 28¼" is used to compensate for a standard riser width of 1 ¾" (for an overall 30" draw). However, since some risers are not 1¾" wide, and draw length is measured to the front of the riser, the following means of measurement will work exactly the same. To find 28¼" on a particular bow, you first need to measure the perpendicular distance from the lowest point on the inside of the grip to
thestring, also known as the brace height

Next make a mark on the sight window, using the distance you just measured, by measuring from the string back to the riser. This mark should be marked on the riser, in the sight window and align perfectly over the lowest point in the grip. Again this method is used since not all bow risers are the same width from front to back. Now take a 540 grain arrow with a nock and measure from the inside of the nock throat (see figure on the right) down the shaft 28 ¼". Mark the shaft at that length as if you were marking it to cut. Since a fingers shooter will use more string than a person shooting a rope release, the optimal performance is to measure the speed using a system that is in between. This is achieved by using a mechanical release such as the Gator Jaw. The Gator Jaw grabs the string above and below the nock, similar to fingers but takes up more string than a rope release. Finally, draw the bow until your mark on your sight window and your mark on your arrow are perfectly aligned and shoot it through a chronograph. When perfectly aligned, you will be at a "true" 30" draw. You may be surprised at the result of the speed achieved unless you’ve played this game before. These draw length specifications are guidelines used by the AMO when testing bow speed. When you use these guidelines, you will be able to check bow speeds on an apples to apples basis with no bias. This method will not however, compensate for a bow speed differences with different let offs. A 65% let off bow will shoot faster than an 80% let off bow. This method will, however, make sure that both bows have the exact same amount of forward power stroke when trying to make a fair comparison.
 

IBO Testing


 

There is no standard for IBO testing other than the 5 grains per pound ruling. Therefore don’t be fooled by advertised test results when draw lengths are not indicated. When testing for IBO speed, set the bow for 30" draw lengths, (as described earlier) 70 lb. draw weights and shoot arrows that weigh 350 grains. The difference between a 60 lb. bow, shooting a 5 grain per pound arrow, and a 70 lb. bow also shooting a 5 grain per pound arrow can amount to several feet per second if the draw lengths are exactly equal. As a bow breaks in from continuous shooting, it may gain several feet per second. When comparing speeds you need to be aware of these facts to guard yourself against false advertising claims.


 

Why measure the draw length?


 

Some bows available today do not seem to conform to any sort of uniform standards when testing IBO and AMO speeds. Manufacturers and dealers should use the same uniform means to determine draw length when measuring for a speed rating as they do when measuring a bow for a customer. If they do not, then their speed claims should be highly scrutinized. If you are testing a bow that doesn’t come close to the speed advertised, then begin asking the manufacturer why? The difference probably lies in the draw length used. What they may interpret as a 30" draw may actually be a 31¾" draw. Even a difference as little as 1 inch in draw length may change the speed of a bow by 8 to 10 feet per second. Some manufacturers are lucky Ralph Nader is not an archer, otherwise the book "Unsafe At Any Speed" would not be about a car but rather about a bow and might be titled "Unreliable At Any Speed".