AMO Measurements

[Nick559]

Can anyone tell me an accurate way to measure the AMO on a bow that is not marked with it?

Thanks

[Paul Mattson]

What are you trying to mesure? Lenght? Bow Wt?

[Orion]

The length is measured from nock groove to nock grove following the contours on the back of the bow. A flexible tape should be used to follow the contours. A tape stretched in a straight line from nock to nock gives an inaccurate/wrong number.

Draw weight is measured at 26 1/4 inches from the string to the deepest part of the throat on the riser. Usually, but not always, that equates to about 28 inches to the back of the bow.

[wingnut]

Actually AMO length is determined by the length of the string that braces the bow at the recommended brace height and then add 3".  That is the AMO length of the bow.

That's why most don't mess with AMO length.

Mike

[KHALVERSON]

Originally posted by wingnut:
Actually AMO length is determined by the length of the string that braces the bow at the recommended brace height and then add 3".  That is the AMO length of the bow.

Mike

isnt the 3 inches for bows made after 2000
and 4 inches prior to 2000 date of manufacture
kevin

[Nick559]

What I need to measure is the length of a bow.  The method that Orion stated, is it with the bow strung or unstrung?

I'll double check it with Wingnut's method as soon as I find the correct brace height.

[Swamp Yankee]

Orion is correct and it's measured with the bow unstrung.

[Nick559]

Using Orion's method, is the tape stretched straight over the riser section on a three piece recurve or do you follow the contours exactly including the grip?

[ishoot4thrills]

Originally posted by wingnut:
Actually AMO length is determined by the length of the string that braces the bow at the recommended brace height and then add 3".  That is the AMO length of the bow.

That's why most don't mess with AMO length.

Mike

That would work great except for a bow you may have purchased that doesn't have a string with it.  

[Jeremy]

AMO standards were adopted in February of 1968.  Mike has it right.  The AMO length of the bow is determined by the string length needed to properly brace the bow +3".  Doesn't matter if it is a longbow or recurve, if it's marked AMO that's what it is.  Also of note that the string is measured under tension.

The whole reasoning behind the standards is that you always will know what you're getting and what you should get.

I should be able to order multiple strings, made from multiple materials with multiple strand counts at the same length (determined by stated AMO length) and have them all brace the bow properly.  Out of the package they'll all measure differently due to different stretch rates, but measured properly they should all be the same.

A HUGE pet peeve of mine is bowyers who don't follow the AMO standards or, worse yet, don't follow the AMO standards and mark the bows "AMO".  Coming in slightly second to that is stringmakers who don't measure their strings correctly!

[vermonster13]

Jeremy is correct. Orion is incorrect. Arvid Danielson was one of the bowyers who wrote the AMO standards. Some what was made into the standards wasn't agreed upon such as AMO draw length, some wanted it measured from the back of the shelf but Norb Mullaney who was the chairman of the board decided the way it is measured from the pivot point would be best so it was adopted.

[George Vernon]

Nick559,
You raise a good question.  Especially for those who like to find hidden treasures of old bows at sales and estate auctions.  Seems like many bows show up at these sales with no string, or the wrong string.  I use the approach outlined by Mike and Jeremy, but it begs the question what is the recommended brace height?  The approach I use to get in the ballpark is to string the bow and adjust the brace height so the tips of the five inch fletching on my arrows are about a half inch short of the rear most edge of the sight window.  I then measure the string while the bow is still strung giving me a length I can use to order or make a new string.  This half inch clearance often represents the lowest brace height at which the bow can be shot.  Less than a half inch, and the feathers will usually hit the sight window before the arrow has left the string and one will hear the impact and/or get noticeable side to side movements in the nock end of the arrow.  This is a crude way to get some basic dimensions, but works pretty well.

[Paul Mattson]

Here is a link to the AMO Standards.  Print them out for future reference.

  http://www.texasarchery.org/Documents/AMO/AMOStandards.pdf

[Nick559]

George hit it right on the head when he said how we run into bows at yard sales and through estates.  I fell into three older bows from an estate that are beautiful customs with very little specifications on them from fading with age or never having the information written on them.  Hence, the question, what is the AMO string length.  
Paul's link is especially helpful too.  I used Orion's method first which was very close to right on.  Of course, wingnut's method is exact as to AMO standards.
Thanks for all the help guys.

[Orion]

Vermonster, I'll grant that I'm half wrong.  On measuring bow length. That measure of bow length was derived well before the revised AMO specifications in 2000.  However, in most cases,if a bow is measured that way, and a string is purchased that is three inches shorter, it will fit.

Regarding measuring draw weight, my definition jibes with the AMO definition. The deepest part of the grip is the pivot point.  And AMO draw weight is measured from the string 26 1/4 inches to the deepest part of the grip/pivot point.

[Swamp Yankee]

The new AMO standard for string length is of no help to anyone with a bow that does not have a correctly marked AMO length; as was the case with the original question.  In that case, the old measurement is the only option that will get you pretty close to a workable string.  Personally, I'd rig up an adjustable string, find exactly what length string the bow shoots best at and order a new string 1/4" - 1/2" longer than that (so you can still fine tune).

[Nick559]

Orion - I measured the bow and came up with a figure based on your system of measurement.  Then I strung up the bow with a string that was pretty close and adjusted the brace height the way George suggested with a five inch fletched arrow to one half of an inch from the rear most edge of the sight window.  I took my measurements (with a precision measuring device from my younger machinist days) from the string grove to string grove and added three inches.  I was only a quarter of an inch off from the measurement obtained the way you suggested.  Granted, the string had not stretched yet (I was not using a fast flight string) but I came up pretty close to the proper scientific method Wingnut originally stated and with the AMO standard that Paul added for our use.  In other words, pretty close as a starting point.

[Orion]

Glad it worked for you Nick.

[LBR]

Bow String Master that braces bow at proper String or Brace Height. Bow String
Master will carry only the bow length designation. Example: A Bow String
Master designated as AMO 66” (bow length) will have an actual length under
tension of 63”.

AMO DRAW LENGTH STANDARD
For Manufacturers
Draw length is a specified distance, or the distance at the archer’s full draw, from
the nocking point on the string to the pivot point of the bow grip (or the theoretical
vertical projection of a tangency line to the pivot point parallel to the string)
plus 1 3/4”. Draw length from pivot point shall be designed at DLPP and shall be
called TRUE DRAW LENGTH.
EXAMPLE: 26 1/4” DLPP plus 1 3/4” is the equivalent of 28” draw.  

FWIW, it's a rare occassion for a bowyer to use "true draw length".

[Nick559]

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