I know when one company has a bow that doesn't go up to 30 inches they say at what draw length they measured it at. I think the Mathews Mustang is an example of this. Don't know if they adjusted anything else for the test but on the back of the manual it will say something like @28 inches of draw or whatver the max is for that bow.

Then it could have been measured at 31in.

Like I said before the bow shoots fine and I am happy with the penatration and trajectory of the arrows with very little drop in my shooting distances. I just always wondered how they post speed measurements on bows outside of the 30in draw length. I never even thought about the shorter draw bows but the same applies to them as well I guess.

As was stated it is a standard used to measure a bows speed. IBO speed has nothing to do with a 31 inch draw.

IBO speed is always tested as follows. It is a standard used to rate a bows speed as horsepower is used to rate an engines power for an analogy. You never deviate away from the test specs for a bow just like you don't when testing an engines power.

70 pounds
30 inches
350 grains.

Except for childrens and ladies bows that do not draw 30 inches. They are then tested at 5 grains of arrow weight per pound of draw weight for their speed.

A longer draw is just on top of the IBO speed. Kinda like your car was rated at 200 HP then you add a new mainfold and now it has 210HP.

I hope that was an OK analogy. Probably not though.

I think I understand what your saying.

So it would be correct for me to say that my bow has a listed IBO speed of 290 if it were at a 30in draw even though the shortest I could make it is 31.....right? I just need to apply the 10-12fps gain per inch that it is set at and I have the actual IBO of the bow (not to be mistaken for actual speed of course.)

Right?

Maybe this will help.

A.M.O.(Archery Manufacturing Organization) and I.B.O. (International Bowhunters Association) both have a method for testing arrow speed from bows and although they are different both can be used to compare equipment for relative speed.
Both of the speed testing standards use a constant drawlength, arrow weight and bow weight to test bowspeed. These are the way the two differ.

A.M.O.

Under this standard the bow being tested will have a maximum pull weight of 60lbs. The arrow will have a grain weight of 540(9 grains of arrow weight per pound of bow weight). The draw length will be set at 30 inches. The chronograph used for measuring the speed will be placed at point blank range for testing.

I.B.O.

Under this standard the bow being tested will have a maximum pull weight of 70lbs. The arrow will have a grain weight of 350(5 grains of arrow weight per pound of bow weight). The draw length will be set at 30 inches. The chronograph used for measuring the speed will be placed at point blank range for testing.

What is important about these two speed ratings is that they are only to be used to compare bows speed tested under the same standard. They should NOT be used to tell you what you will personally shoot for speed. For example lets take an average archer……Bill Bowshooter……..Bill has a 29 inch draw length shoots his bow at 65 lbs and shoots a 455 grain aluminum arrow. In our example neither the A.M.O. or the I.B.O standards will accurately reflect the speed of Bill’s bow. Since Bill is shooting a shorter draw than both standards, shooting an arrow that weighs 7 grains of arrow weight per pound of bow weight, and is shooting 65 lb peak weight he will shoot at a speed very different to either standard.

If we try to make some generalizations about the two different ratings we could say that the I.B.O. speed rating is much faster than most archers could achieve, and conversely the A.M.O. speed rating reflects a speed that is less than what most shooters could achieve with the same bow. If a bows I.B.O. speed rating is 320fps and its A.M.O. speed rating is 245fps that would mean the average archer would shoot that bow somewhere in the middle of that range

Buckeye,

Thanks for the responces.

I understand how the IBO and AMO ratings work and that is the premise of my question. How did they come up with my particular IBO when my bow's shortest draw is 31in and therefore is not able to be adjusted down to the IBO standard of testing?

Perhaps an email or a call to your bows manufacturer would answer your question.

It should also be noted that some of the different bow companies fudge a little on the speed ratings. Most of them do not put a peep sight, a nock, or string silencers on their test bows as these items will also slow down your arrow speed.

I couldn't tell you what my particular bows IBO or AMO rating is, but I can tell you that at a 30.5" draw, set at 65#'s, shooting a 403 grain arrow, it is moving at a very quiet 271 feet per second.

It is more than likely that your particular bow model the draw lenght is changed by swapping out the cams.

I dont think there is a bow out there that wont adjust below 31"


Just for curosity what bow do you have?

I have several but the one in question is a Browning Boss Tracker: Contour XP limbs, Heat Cams, 65% let-off 8in Brace height, 42in axle to axle and the draw length is 31-33in. There are no cam replacements you just move an allen screw to change the draw length in 1/2in increments so this is the only range for this bow. The actual listed IBO is 287 (I think) I rounded up for ease of discussion. It is set up with a Cobra Venom sight, Wisker Bisquit rest, True Glow stabilizer, cat wiskers, two tied on nocks with a string loop, Surz-A-Peep (three strand kind) a Bohning Lynx 4 arrow quiver and it's about 3-4 years old.

I'm not a speed freak and this is actually my fastest bow LOL I was just wondering as to how they came up with an IBO number for my bow. (or for any bow that starts over 30in for that matter)

I am not sure but I believe Browning also made this bow in a shorter draw range. Could it be that they just tested the shorter range bow (30in) and used that IBO speed fot this one as well?

I don't know, this has been a question I have had for a while and I thought I would post the question.

The bow will be able to change cams. Your particular cams will adjust a specific ammount of length. They will have different cams for that same bow that will also adjust to different specific draw lengths.

Look at Hoyts cam 1/2. They have numbered cams. A #6 cam on a v-tec will adjust from 28.5 - 30 inches if I remember correctly. But they also have a #5 cam which is the same cam design but a tad bit smaller. This one will have a shorter adjustment range. They also have a # 4 cam etc... etc... All the same cam geometry but each made to adjust a specific ammount and outside that ammount you would go to a larger or smaller cam.

Just like anything outside a 30 inch draw, 70 pounds, and 350 grain arrow will give different speeds.