Drenalin performance question?

[canny]

I'm setting up a drenalin for a buddy. He had to get a new cam and limbs so the choice was his I'm just the assembly guy. Anyway he got 60-70 limbs with a 30" 65% let off cam. Today I got it tuned and I will say it's a beast.

However I'm not getting the performance I expected. At 69 lbs with a 420g arrow it's getting 273fps. I kinda figured it would be closer to 290 but I'm sure I could get it there with about a 360-370g arrow.

[JBS]

Original IBO for a Dren was 320 with a 350 gr arrow.

To verify the speeds obtained with a 332.5 grain arrow, here is the manual
computation using the standard 10 fps/in of DL, 2 fps/lb DW, 3 fps/grain
arrow weight and string weights.
IBO= 320
minus (70-67.3)x2 = -5.4 fps for DW
minus (30 - 28.5)x10 = -15 fps for DL
plus (350 - 332.5)/3 = +5.8 fps for Arrow weight
minus (14+5+2)/3 = -7 fps for peep, loop, nock point

320 - 21.6 = 298.4 fps, Chronograph results were 299, 298, 298, 299, 299,
299, 298 for a seven shot string, avg = 298.6 fps

I'd say that is close enough for government work ...


My Setup

IBO = 320
Minus ( 70 – 70) X 2 = -0 fps for DW
Minus ( 30 -27 ) X 10 =-30 fps for DL
Plus ( 350 – 351 ) / 3 = -.33 FPS for Arrow Weight
Minus ( 5.4 + 5 + 2 ) / 3 = -4.13

-0 + -30 + -.33 + -4.13 = -34.46
320 – 34.46 = 285.54

Easton Axis .400 27” w/125 @ FFP360 w/4” wrap

IBO Your Setup
70 70 <--- Your Draw Weight 0 2 0
30 27 <--- Your Draw Length 3 10 30
350 353 <--- Your Arrow Weight 3 3 1
Wt on String 9

IBO Fps Neg. Fps Est FPS
320 40 280


Mathews ATA SETUP

There has been much discussion about ATA and setting it properly. I have had many bows that needed tuning and one thing I have noticed is that rarely is there any consistency in how the idlers are aligned. This is due to the cable harness settling after the initial setup. Every single bow that I've tuned has exhibited excellent flight characteristics with the following method:

I max out the limbs to take my spec measurements. I press the bow and take the cable loop and twist until the ATA measurement on the shelf side corresponds to the following specs:
2 twists for every 1/16
ATA Shelf Side RH Bow ATA Sight Side RH Bow
29.75" for the DXT……………………………………………29 11/16” sight side
37" for the Drenalin LD……………………………………..36 15/16 sight side
33" for the Drenalin………………………………………….32 15/16 sight side
31" for the Switchback XT…………………………………30 15/16” sight side
33" for the Switchback……………………………………..32 15/16” sight side
31.5" for the Outback……………………………………….31 7/16” sight side

Once this is set with the limbs maxed out I look at the cam to see where it's orientated. I adjust accordingly until the timing holes run parallel to the bowstring putting equal twists in both loops of the string to maintain the balance of the factory twist. After this is done I set my nocking point so the arrow sits perpendicular to the string. I then set my centershot to 13/16" 11/16” dropaway at the berger button.

Once this is accomplished I run an arrow shaft flush with the shelf side of my idler wheel. I then twist the cable harness so that a gap of a light 1/8" exists between the shaft and the string at the nocking point. (see pic) At rest the idler is canted inward from the riser on the top and outward at the bottom but at full draw it's straight up and down.