I posted this over on Bowhunting.com...no responses. Let's make this forum less of a sleeper for Jeff

So, I had my 101st Airborne in the vise today, and decided to take a few measurements to see where I'm at now after shooting heavily for the past 6 months. I found my brace is still dead on, my cams are synched (finally got my modular stops hitting the cables at the same time after a lot of tweaking), but I'm at 36 5/8" ATA. How much can this difference (from 36 1/4" effect my my poundage? I don't have a scale, but I'm shooting 60lb limbs backed out to ~58lbs from what I can guess. This "guess" is probably off even more due to my longer than factory ATA. I've heard there are tolerances built in by the manufacturers for deviation above and/or below factory specs, but how much? Could this effect tuning at all? The reason I asked is when I broadhead tuned this summer, I had to back my poundage off a bit to bring my broadheads in to my field points. I would never had guessed ACC 3-49s, at 28", and 60# would be too weak of spine, but that's what my BH tuning showed (although I was able to get them pretty close by dropping a few pounds. So...thoughts?

Thought #1: broadheads always cause spine to react more weak so if the arrow spine is on the edge of too weak they will need the bow to be backed down in poundage. I would estimate that 28" 3-49's are on the edge of being weak with a 125 grain head for your rig (radical hard cam/high energy).

Thought #2: Shorter than spec. ATA usually means higher cable load which is higher poundage (and longer power stroke/draw length) so i'm betting that your poundage was more than you think for your spine estimate causing you to be even more over spined.

Thought #3: If you're going to get serious about doing your own tuning you've got to have a press and a scale so that you can keep cable tension, ATA and poundage in specified tolerance and matched to your arrow spine.

KA covered it well.

One thing I would add- Minor deviations from "spec" are normal (e.g your 1/8" deviation in A2A length- limbs always have a little variance) and frankly don't concern me. What concerns me is if the brace height is correct AND the cam/cams are in the proper rotational position- I dont mean just synched together but also timed correctly (single cams still need to be timed, but have nothing to be synched with)

As for particulars on allowable manufacturers tolerance, I'd defer to my former colleagues (and the new additions, congrats guys!) on the BT Corp. Staff, of which there are a bunch on these forums. I am not familar AT ALL with the Binary cams or the BowTech line from 2005 and later.

FWIW- this is one reason I really came to prefer single cam systems. Just get the cam timed properly, and it's easy to keep track of, and you rarely need more than a twist or two put in the cable if you have a decently made string/harness. Easy peasy minimal maintenance.

Thanks. Broadheads were 100gr, not 125gr which makes it a bit more puzzling. I can't imagine I would be too weak, but if I am, I wonder if going down 3/4 to 1" in shaft length would bring back into the green. I really need to invest in OT2 so I can run these things[&:]

I know shorter cable length equates to a higher poundage, but how much? Is there a correlation between distance from ATA spec and he effect on DW?

#3-Excellent point. These questions wouldn't even be asked right now if I had a scale. Already started on the rest of the tools (I already have the press), but I'm in a slowdown right now. I'm not buying too many gadgets as I'm closing on my first house with my fiance at the end of February, and we want to be able to put 20% down. Priorities you know

I would invest in Archers Advantage(instead of OT2) if you are going to shoot speed bows with fixed blade heads.

I ran a 28" arrow at 28" and 29" draw lengths and both show you are weak with the 3-49.


What is your draw length and arrow length(raw shaft only)

Not Jeff B, or anyone with staff knowledge. I have some idea because I have played with a few Frankenstein bows, and a few others where limbs needed to be replaced with non-factory limbs (Champions that have no factory limbs available). I can tell you absolutely, for certain, that it depends.

Most of the bows that I have messed with, 1" shorter ATA meant about 10# more peak weight. A few that I've tried, it meant a little less, maybe like 7-8. I think it is all due to limb deflection values.

Draw length is 28.5". Actually I went and measured my shafts, and I must have had some variation in the cuts (the arrows all consist of two sets I bought about a year apart). Anyways from shaft lengths )excluding inserts is 27.25" and 27 5/8".

Why do you suggest AA over OT2?

Also, I believe Rob shoots a similar setup to me, except his arrows may be 27" even. I know he shoots the same arrow, cam style, and has the same DL I do, but shoots higher poundage. Would the .5" arrow length difference make the difference?

First off,yes,1/2" makes a big difference for you because you are on the edge of weak,if you were on the edge of stiff,it wouldn't make a difference.


The reason I recommend AA over OT2 is that AA uses actual ibo speed AND actuall performance of a bow to determine spine.OT2 uses cam style and I have found that when you get into the extreme cases,such as a bow that has an ibo of 330 or more,that hard cam just doesn't cover it anymore.Not much more than an arrow chart imo at this point. IMO,how can a bow that shoots 315 and one that shoots 350 both have their spine determined by a hard cam selection?


You are close enough that you probably get a good tune with field points but broadheads are telling the story. So,if it were a target setup,you would be fine without lowering poundage.

Well it's my hunting setup as well as my target rig (I'm not full blown into the target scene, but I will be using it). I don't want to buy new arrows, but I suppose I could have them cut behind the front insert. Roughly how long is a standard insert? I don't have any on hand to measure.