[SpotShy]

I followed Ken's tuning technique to a “T” on my Dad's new Specialist. Let me pause and say Thank you to Ken because when I was finished we had it shooting a perfect hole through paper with a bare shaft and the bare shaft would hit on a line with the fletched arrows at 10 yds no problem. I could not be happier with the tune and arrow flight. However I am concerned about the speed we are getting. Here's a little background info:
A to A= just a 1/6 under 37.5"
Brace height = 7.5"
The cams are sync'd and hitting the stops together perfect.
The cam dots are aligned so that the cable intersects dead center on the bottom cam and just a touch closer to the string side dot from center on the top cam (a frog’s hair from dead center).
Bow is set at 63 lbs (maxed out)
Draw length is 26.5" / draw post in #9 holes 75% let-off
Arrow : GT Velocity 400 @ 26" weighing 325 grains
specialty peep
Nock loop
factory string with factory speed nocks and string silencers
Chrony speed: 280 FPS
Given that the bow speed is rated at 330 FPS @ 29" I have calculated the following
330 fps
- 25 fps for 2.5 inch reduction in draw length 29” to 26.5”
- 5 fps for 12-13 grain specialty peep
- 2. 5 fps for 6 grain nock loop
= 297.5
I assume that the silencers and the speed nocks are possibly robbing some speed if not positioned properly, but the birth certificate indicated that the bow actually achieved 330 @ 29 before it left the factory. So I am guessing that the Octane string was on it and all the speed nocks and silencers were in tack. What am I missing here? Are we expecting too much? I did try to twist and untwist the cable to find an optimum position for the cam rotation but any more that one full twist either direction resulted in less performance. I would appreciate any advice anyone could offer. I had really hoped this bow would get 290 with this set-up.

Thanks in advance.

 

[Ryanswoman]

The 330 fps rating is actually taken at 30 inches of draw length and 70 pounds with a 350 grain arrow. Also should take in to consideration that for every 10#s you take off of that 70# rating you will lose 8 fps at 5 grains per pound. So that being said at 63# and a 325 grain arrow and a 35 fps reduction in speed, with a peep and loop, you should be anywhere between 279 and 283.

 

[2-b One]

Ryanswoman is spot on. I had a bowtech 82nd Airborne rated for 345 fps. everything said and done, I ended up at about 294. Your speed may be a little higher or lower depending on temperature also. Great speed for your draw. Be happy with it !

 

[fletched]

IBO is at 30" and at 70 pounds.

So if you deduct the 35 fps due to the shorter draw length, you get 295 fps. If you deduct the weight of the peep and loop, that will be 6 fps. So you are down to 289. Your arrow is 10 grain too heavy so there is 3 fps. So you are down to 286. Since it's a 60 pound bow, you can expect to loose a little speed compared to a 70 pound max. You can take 5 fps due to it being a 60 pounder. That gets you down to 281 fps. Sounds like the bow is hitting it's ibo.

 

[baldyhunter]

IBO is at 30" and at 70 pounds.

So if you deduct the 35 fps due to the shorter draw length, you get 295 fps. If you deduct the weight of the peep and loop, that will be 6 fps. So you are down to 289. Your arrow is 10 grain too heavy so there is 3 fps. So you are down to 286. Since it's a 60 pound bow, you can expect to loose a little speed compared to a 70 pound max. You can take 5 fps due to it being a 60 pounder. That gets you down to 281 fps. Sounds like the bow is hitting it's ibo.

I couldn't agree more. The bow is shooting right where it should!

 

[SpotShy]

Thanks guys, I was for some reason under the impression that this bow was rated at 29" and not 30. I all makes sense now. Anyway kudos to baldyhunter for providing excellent information on tuning these overdrive binary bows.

 

[ohio moose]

Baldy is the man with the overdrive binary bows.

 

[Justgot2hunt]

Baldy is the man with the overdrive binary bows.

X2 on that !!

 

[td051]

That is moving pretty good for those specs. The birth certificate is based off 70 lbs and 30" DL. Also note they are very sensitive to string weight!