Arrows, FOC, and Tuning for Penetration

[mrpenguin]

OK, so I have been playing around with arrows b/c I just got a new (used) Robertson Stykbow TD.  It is a 48# @ 28" (though pulls 50# on the scale at the shop).  It has a fast string on it (BCY material... can't remember the name).  The bow has qivuit silencers, seal skin arrow rest, and a detachable bow quiver (which I took off today).  I was really excited to try the Alaskan Bowhunting Grizzlystik Sitka, so I got one from a buddy and could not get it to tune.  It always shot weak, until we removed the tip and shot a 28" arrow with just the 45 gr brass insert.  Then it shot like a dart... Similarly, I found that using brass inserts with Beman arrows produced poor flight.  Everything I have EVER read says "the more FOC, the better the flight" but I cannot seem to get any extreme FOC arrow to shoot for me.  Right now I use Beman MFX Classic 400's cut to 29.5" with a standard insert and 125 gr field points.  It shoots nicely with 100 or 125 gr broadheads... but my FOC is around 10% (as I recall).  I would like to use a heavy broadhead (like the Muzzy Phantom SS at 220gr) but am afraid they will not tune... SO, I am curious if someone can help me understand why I cannot seem to get a good EFOC arrow to work for me??

[overbo]

are you getting any ware on your shelf?I would say a .400 deflection w/ a 125gr point w/ your setup would be way too stiff.I'm shooting the same arro w/ 250grs up front w/ a 60lbs @ 27''.
Are you hitting high ?
How much above center is your string nock set?

[IronCreekArcher]

You should probably try a few differnt types of shafts.  My bow ended up liking the Easton Axis N-Fused with a 100 grain brass insert and a 217 grain broadhead out of my Black Widow recurve.  I am pulling 49 lbs at my 28" draw and my EFoC is 24.38%.  I am getting bullet holes through paper from 2 feet to twenty yards bare-shaft...

[GMMAT]

When you add tip weight, you weaken the dynamic spine.  If you're gonna try for an EFOC setup, you'd better start with some shafts that'll handle it.

[Jesse Peltan]

Can you cut the shaft shorter? If so cut from the thinner(nock) end. This will stiffen spine weight. You could then add point weight. Really weak might also be a false stiff. You should try adding lots of point weight then tune. Also if you are shooting at a target the arrow can kick so make sure you paper test.

[James Wrenn]

You will have to go to a 340 spine or cut your arrows shorter if they are tuned with a 125gn point now.Like you I can not tune heavy points like a lot do without going up in spine.I can't use brass inserts.I had rather have any extra weight out front in the form of heavier broadheads anyway.

[mrpenguin]

Thanks for the advice everyone!  I have tried 500 cut to 28, 500 cut 30, 400 cut to 30.5, all with 125 gr field tips.  The MFX 400 cut to 29.5 with 125 gr seems to fly best.  That said, Overbo, you mentioned wear on the shelf... I did notice that today which is why I switched to the seal skin arrow rest... The deer skin wore out.  My string nock has been set using bareshaft testing and is slightly over 1/8 up.  It puts the arrow into the target slightly angled up or basically straight... I figured that the feathers would take care of shooter form errors.  My arrows usually hit on the mark or a bit high.  They do not hit left nor indicated strong spine when bareshaft tested.  After doing some reading, I think my string is a bit faster "than the average bear" if you will... and perhaps that's putting the bow up to 55+ lbs, which may be why the 400's work and 500's don't.  What I think I'm going to do is get some heavy field point tips and a set of Phantom SS bh's.  I can try those out and see how they all fly.  I'd like to see an EFOC of 20-24% for the Fall.

[Running Buck]

Erik,
I have a morrison dakota that deaws 49lbs at my 28" draw length. When I put a 8 strand string of 8125 on I had to go to 55/75 goldtips with 250 grain points and they fly like darts. I get bullet holes out to 20 yards. Its amazing what a high tech string can do for performence.

[mrpenguin]

Thanks for that confirmation Running Back!  That's what I anticipated... I think it may be time to try some different points and see if I can use a heavy broadhead.

[overbo]

Erik,
My opion is .If you where getting wear on your shelf  quickly and hitting high,then you need to weaken the spine of that .400.1/8'' above center w/ too strong of spine is probally the culprit to the wear.
I would work the other direction.Start w/ weak and too long w/ a heavy point and work the shaft down.The Robertson I had shot well at 1/2'' above center w/ string nock.Also,make sure the bow isn't moving at the shot.I squeeze my grip to set the hand and relax slightly before release.
I've had the same problem in the near past and got thru it this way.

[Old York]

"the more FOC, the better the flight"

I believe the idea is that with high FOC, the better the  penetration . Without perfect arrow flight, it's all a wash anyways.

[mrpenguin]

Overbo, thank you.  I actually have some lighter spine arrows to fool around with and will try that.  I did find my nock moved a bit, so I moved it back up just above 1/8 on my bow square.  The arrows did shoot better after that.  I am also going to try using a heavy broadhead/tip and see if I can get the bow to tune better.  I think the 400's will work, but I need more weight in the front to break the spine down a bit more, as you suggested.  

Old York, thank you for that clarification.  I have heard both arguments... not sure if I believe the first, but the later is for certain.

[George D. Stout]

"the more FOC, the better the flight."

Ahhhh No!   That would be wrong.  There is a line of dimishing returns on both ends of the spectrum.  You would be much better off to tune for perfect arrow flight and then see what for FOC you have.  That has worked for me for five decades 8^).

[Bill Carlsen]

Given the choice between a properly tuned well tuned arrow with lower FOC than a poorly tuned arrow with high FOC I would go with the arrow that shoots the best. Perfect flight trumps all other variables, IMO, in regard to penetration. However, I have found that heavier FOC does give me better penetration but true flight of the arrow is the primary goal so until you find the right combo that gives you great flight and heavy FOC stick with the great flight.

[GMMAT]

Give me an arrow flying true with a razor sharp BH every time.  There IS a point of diminishing returns when adding weight to your arrow.  I'm absolutely tickled with my first hutning arrow I've built from the tip back.  I couldn't honestly care less what the FOC is.

[metsastaja]

have you tried Stu Millers dynamic spine calculator.  I have found it a great tool for getting in the ball park real fast. You need microsoft excel to run it.

you can download it at.
 http://www.heilakka/stumiller/

the 6-12 is latest version and is at bottom of page.