Arrow spine (broadhead vs field point) using Ontarget2 from Pinwheel Software

[jechicdr]

I'm shooting a Mathews Z7, 70# draw, 29 inch draw length, Beman MFX 300's cut to 28 3/4 inches with 125 grain heads.  If I switch setting on search filter in Spine Match on Ontarget2 from hunting to target (I'm guessing broadheads to field tips) then the dynamic spine of the target arrow is stiffer with target.  I've tuned the bow to get field tips and broadheads as close to each other as possible, but broadheads group 6 inches to right at 50 yard.  The difference in dynamic spine (weaker spined broadheads hitting to right of field points) would explain this pattern.  If I increase the length of my target arrows (or increase the weight of the field tip), I can effectively match the dynamic spines.  Does anyone have any experience with using different length arrows (or different weight tips) for target and broadheads to have them hit the same spot?  Will changing the weight alter elevation differences between the two arrow types.

[Todd_ID]

Never really thought about different lengths for each head, but it may not be too practical.  The change in FOC would affect the trajectory enough to notice; use the ballistics tables to see the change.  Same with different point weights.  Best I can tell is that the program is built toward both parts of archery: target and hunting, and that check box tells the program which side of the aisle you desire to stand on.  Broadheads don't recover from under-spined issues nearly as easily as field points, so the program factors in an additional spine increase using some unknown formula to account for this, and that's why you get different spine recommendations for broadheads versus field points.  I can say that the recommendations are accurate.  I had a thread on here where everyone put down their specs, and I input it to see what the numbers were.  Many were underspined according to the program, and I received lots of feedback that changing to the program's recommendations solved their flight problems.

[jechicdr]

Well, I bought the program.  The simulations are pretty cool too.  Can learn how high to hold for say 85 yard target using 80 yard pin, drift on arrow with 5 mph wind.  Well the current arrow setup I have would be close for my broadhead setup, but for field tips it is over spined.  With a weaker spined arrow for field tip, I can imagine a larger bend of the field tipped arrow with the shot would catch a hair more air and would place it closer to the impact point of the broadheads.  Lengthening the  arrow seems to be the best way to do this, the difference in velocity between the two was pretty small (velocity should determine most of the flight path) and the weight difference would be relatively small so wind resistance should not slow the arrows significantly unless extremely long distances.  If I wanted to speed up my target arrows, I could take off some of the offset on the vanes to match the trajectories a little more, but I don't think I'll need to.  The extra length on the field tipped arrows is enough that I could cut off the extra and convert them to broadhead arrows with new inserts.  I suspect with lower draw weights and less aggressive cams, the difference in spine between broadhead arrows and field tipped arrows might be small enough that having two different sized arrows might not be necessary.

I'm tired of having my bow set up for hunting with my broadheads and going to the range with my field tips and having my group "miss" the bullseye to the left every time...when other people are looking ;P

[Button Nubbs]

Just curious, have you broadhead tuned before? Do you reach a point where the field tip arrow and broadhead arrow just won't come any closer together while bh tuning?

[jechicdr]

Yup, 6 inches at 50 yards.  Move the rest any further left or right, then they start spreading out again.  6 inches may not sound like much, but I'm a little bit of a perfectionist.  The program says the dynamic spines are different enough that with a centershot properly tuned bow, the arrows would behave the way they are, so I suspect I can't "tune" the bow any better, but I can modify the arrows.

[Button Nubbs]

6" at 50 is way too much for me also. I was just curious.

[jechicdr]

Once I get the new shafts, I'll post update on how the different lengths fly.

[BULLBLASTER]

Do you have access to a bow press? Could you add a half twist to the left split yoke. To lean the idler top left a touch? Have you bareshaft tuned at all? I just tuned mine this week and my field points and broadheads group together to 50 yards. To the point I was ruining arrows. I have necver had a problem getting broadheads to hit with field tips unless substantially underspined.

[jechicdr]

You shooting a Z7?  If so, what is your arrow set up?  I doubt I am underspined.  I'm shooting Beman MFX 300's with 125 grain broadheads/field tips.  I may cave in and go to a shop and see what they think.  I don't have access to a bow press.

[CRITTERSTICKER]

what kind of broadhead are you shooting? sometimes its the broadhead and not necessarily the arrow. With the speed of your bow i would try different heads. maybe go from a 3 blade to a 4 blade or vice versa. just a thought

[jechicdr]

Last year shot thunderhead 100 grains.  Killed a deer last year.  Lost an elk in the rain and dark of night and didn't want to go with a trochar but a cut on contact and a heavier setup.  If I had had the arrow sticking out the other side, I probably would have found the animal.  Had the same issue with spine with the thunderheads.  This year shooting a 125 grain phantom.  100 and 125 grain phantoms both hit the same spot using the MFX 300's and hit the same spot using MFX 340's.  I have my MFX 340's and 300's and 400's (from when I shot my hoyt reflex at 60 lbs).  I can get any broadhead to hit just about where any broadhead no matter what spine in that range to slice fletchings off.  It's almost like I can't get the field tips to move.  I tried lengthening the arrows on my field tips (do make a weaker spine...which in theory should have put the arrows closer)  and got a little tighter, but still would have to adjust my sight when I go to practicing with my broadheads prior to the season.

I am shooting a drop away and wonder if the arrow is not sitting on the rest long enough to "move" the broadheads toward the field tip impact site before it drops away.

[BULLBLASTER]

I don't shoot a z7 but I have spent a large amount of time in an archery shop tuning b_as over the last few years. I would almost guarantee that you aren't underspined.   It sounds like your bow just needs some tuning. Easy stuff for any shop to do. I would try that before new arrows. Keep us posted. Pm me if you want some help as I.am a member at evergreen too.

[Matt]

BH or FP have no influence on spine.  It is the weight of the arrow.  When you select target or Hunting arrow in the programs it is just a selection of different arrows.  With BH's you want stiffer.  Lengthening the arrow or increasing weight weakens the spine.  If you have a 6 inch difference in impact then you need to take a look at your set up.  It could be several different things spine being one of them. 

Do a walk back tune with your FP's.  Sight in at 20 yards, left right perfect.  Now using your 20 yard pin shoot at 40 yards (remember that your arrow will drop quite a bit).  Shoot 3 arrows.  Now take a plum and run it in the middle of the dot you aimed at.  If your arrows are in a / pattern from the 20's then move your rest slightly to the right, if they are in a \ pattern then move your rest left.  Do this until the drop stright l .  This sets your center shot.  Now go back to 20 and adjust your sight if you are off a bit.  Then screw on your BH's and see how they fly.  If you are still hitting right then you need to look at the arrows.  If spine is good (remember that stiffer is better) try re fletching.  Good luck.

[BULLBLASTER]

Good advice matt. Another thing I do is strip the vanes off an arrow. Tape the back to.add the fletching weight back and shoot at 20 yards. Move the rest the direction that the bare shaft needs to go to be grouped with Fletched. If bare is lower than fletched. Raise rest. Once bare shaft and fletched group together broadheads should be there too.

[jechicdr]

I've done all those things.  I can tune "a" set of arrows better than most.  How's this for "walkback" tuning.  With field tips, everything from 6 yards out to 80 yards is "dead" center.  I can tune my broadheads to do the same, with the same setup for the rest and nocking position...but I have to move the sight.  The flight path of each arrow is straight from line of sight to the pin to the target...it's just the broadheads straight line is slightly to the right.  At 20 yards they would be about 2 inches, 30 yards-3 inches, 40 yards-4 inches...etc.  There is no wobble in the arrow as it tries to find it's flight path for either FT or BH.  If I try to move the rest to "bring the BH impact to the FT impact" I can definitely see a wobble.  I have bare shaft tuned, paper tuned and modified French tuned and this gets the line of sight from the string to the pins directly in line with the "laminated" center line on the bow grip leading me to suspect that the bow is tuned.

[Matt]

Well if they are flying true then move your sight.  If you are hitting what you are aiming at at 6 yards then there is no problem.  Good luck.