IBO or AMO when it comes to arrow weight?

[jimg]

Everyone knows the ole rule o' thumb of 5 grains of arrow weight per pound of draw. Example 350 grs. for 70 # or 250 grs. for 50 #, etc. However there's an old AMO chart floating around the 'net and in some of my old Archery Digests that suggests otherwise. The chart claims that higher brace heights and shorter draw lengths change the equation. The powerstroke on high BH and short DL bows is much shorter than say a 30" DL and 6" BH speed bow. For example my current setup is as follows-

26" draw
70# (actually 69 # with limbs maxed)
8.25 brace height
single cam
typical 'stuff' on string- Bowjax, loop, rubber tube peep

The common wisdom dictates 350 grains BUT the AMO chart says that with the following specs-

Energy cam
8" BH
26" DL
65-70# DW
minimum arrow weight should be 291grs.

OR

Speed cam
7" BH
26" DL
66-69# DW
minimum arrow weight should be 323grs.

These are the two that are closest to my setup. Sooooooooo........the million dollar question is do I HAVE to shoot 350 grs? I'm not worried about the warranty as my local dealer does not care about minimum arrow weight as long as you don't get silly (like 3 grs. per pound). He says nothing bad will happen. My bow is still very quiet shooting 309gr. arrows compared to the 391gr. shafts I normally shoot. Trajectory is dramatically different (in a good way!). What say all of you?

[BT]

Your talking about a 10% change against minimum (65#) which would equate to about 5fps which wouldn't change impact, so we are looking at this strictly as a tuning issue.
It's very early in the morning but I will at least throw some stuff up here for consideration.

#1: When was this established?:

Many standards back in the day are not relevant today.
For instance, bows used to change draw length by cranking the limbs inward and outward, off the riser, while weight was added by adding tension to the cable system.
(Just the opposite now-a-days)

#2: Limb/Risers material/designs have changed:

While materials are far more advanced, they are used sparingly compared to yester year.
A 3/8" Limb made of fiberglass had alot more room for error than a 3/16" limb made of graphite.

#3: Cams are radically different today :

When the term energy cam was first coined, it wasn't even close to being in the same league as todays cams that share the same distinction.
A Barnsdale wheel is as fast or faster than the speed cams of the early to mid 80's....and thats really something to consider since it is the cam that loads and unloads those limbs.


Although these points may sound like they come from the dark ages of archery, the fact is that not even 20 years has past since such designs were on the shelves in mass.

All this said....I wouldn't sweat 10% either

[smj]

So, it sounds like you are trying to determine whether or not you can shoot an arrow of 309 grains... Rather than your usual arrow of 392 grains. So, we are talking a difference of 83 grains, which takes you from above the 350 grain mark, to below it. Sounds like you are shooting at 69 pounds, draw weight. So that puts you just a tad below 4.5 grains per pound, rather than 5 grains per pound - about a 10% reduction.

A rule of thumb is a rule of thumb... I can remember when this particular rule of thumb was more like 7 gpp! Then going to 5 gpp had many saying that you would break your bow if you when below 7 gpp... And, if you did so with the older bows out there, yeah, maybe you would!

So, what to do with your bow... I say, if you have a good pro shop, sounds like you might, and they are not concerned about the 4.5 gpp you want to shoot at, don't worry about it. If they say you are crossing the line bit - then you should probably reconsider! I don't know what bow you shoot, how old it is, or really anything about it other than it has a high brace, 26" draw, and pulls at 69 pounds with a single cam. Age, materials, and design all come in to play on this one, and the guys at your pro shop should know. That said, I think you are right about the noise. I have found, in general, that when the noise goes up, the arrow is to light. If the noise/hand shock seems to hold stable, you are probably just fine, 10% is not a huge change, but since you are crossing the 5 gpp line, certainly worth while to consider! I agree with BT, probably not an issue. But I'd still make sure to pass it by the guys at the pro shop if the manufacture calls for 5 gpp. Just call me cautious!

[jimg]

--------------------------------------------------------------------------------
"Your talking about a 10% change which would equate to about 3fps which wouldn't change impact,"




I don't understand. If I drop 82 grains in arrow weight I gain 19 fps over my F1 Chrony, not the 3 fps you say I should gain. Yes 3 fps is nothing but nearly 20 fps is a useable difference in my book. Maybe I missread your post?

[BT]

Feb 19, 2009 9:17:46 GMT -5 jimg said:

--------------------------------------------------------------------------------
"Your talking about a 10% change which would equate to about 3fps which wouldn't change impact,"

I don't understand. If I drop 82 grains in arrow weight I gain 19 fps over my F1 Chrony, not the 3 fps you say I should gain. Yes 3 fps is nothing but nearly 20 fps is a useable difference in my book. Maybe I missread your post?

I also said it was early ;D

ANYWAY...

I would have to see the article (original text) to know where they were coming from (other than the points I referred to before) but bottom line, I can agree on some points provided that conditions permit.
However, I dont see many bows that I would say permit that 4gpp with a degree of comfort....warranty aside.

As an example of this, look at the factory spokesmen that are putting the kibosh on the bow turbo which effects about the same changes as are noted in this old AMO statement.

The AMO standard has always been 7 and IBO 5, so I dont know where this artical was going with this.

[jimg]

Here's the chart (I hope!!). Let's see if I can get this inserting thing right.

home.att.net/%7Esajackson/amochart.html

[BT]

Feb 19, 2009 11:18:40 GMT -5 jimg said:

Here's the chart (I hope!!). Let's see if I can get this inserting thing right.


home.att.net/%7Esajackson/amochart.html

Excellent!

O.K.....this is making much more sense...although AMO started 7 and if it has changed..when?.
Whatever the case may be, it really isn't relevant today and probably should have been changed (if it wasn't) back in the mid/late 80's.

So....
As the table suggests, the stored energy change is dramatic (agreed) and therefore the AMO stated standard takes into account that the high average is 30" and therefore the chart compensates for that.
**(Spine deviation also varies across the range with shorter being allowed less spine rating)

Looking at the chart closely, it seems to me that this chart is pre 90's (based on brace heights listed) certainly could be early 80's even
It would really be a help to know the actual year that AMO was changed to further evaluate reasoning but I will just assume a mid 80's standard for bow construction and go from there.

IMO.....I would consider this chart to be safe at the old AMO standard of 7 when adjusting for the construction factors when viewed in the light of limb construction.
The Listed standard @ 6 would no doubt be borderline reasonable with quality constructed limbs such as the ever popular Barnsdale for example.

All that said....
Having spent more years hot rodding bows than most, I personally would never recommend going more than .5gpp below IBO.
I have shot 3gpp many times and the limbs generally collapsed/split at around 5K rounds....which is easy to get to if your a serious shooter.
I have seen some bows come apart @ 4....but to date, none @ 4.5

Could you do it?...sure.
How long?....who knows....it depends.

[weagle]

Skirting just below the edge of minimum arrow weight is kind of like shooting hot .357 Magnum loads in a Model 19 medium frame S&W revolver. Yea it can handle them, but for how long. Instead of practicing with .38 SPL loads and hunting with the full .357 loads. Eventually something will give, and maybe at the worst possible time.

[Buckshot06]

This chart brings up some interesting numbers.

I don't mean this to sound negative if it does sorry.

Are you looking for a chart to justify going bellow 5gpp

I have a 29" DL and a 71# DW
I shoot 28.5" arrows that weigh 376 grs.
So I shoot at roughly 5.3 gpp.

According to this chart here I need to do the opposite of you and increase my arrow weight
That is why I asked the question.

With this chart I should be shooting an arrow of 416 grs. @ 5.9 gpp or 454 grs. @ 6.4 gpp.
With this a loss of fps also and there is no added gain IMO.

It is your equipment and you can do what is best for you but I agree with BT here and wouldn't go below 4.5 gpp.

[jimg]

I'm just trying to flatten my trajectory without buying an PSE 'X' bow or some other speed contraption. I like my stable high BH bow but................