I've posted this a few other places, figured I'd throw it up here too.
Background: Recent owner of a 54# @ 29" Omega Longbow. It's going to be my deer hunting bow this year. Realized a day after getting it that I was going to need new arrows. Wanted woods at first, but basically realized carbons are going to be the best for my purpose. Bought 3 Rivers' Traditional Only bareshaft kit last week, been testing when I've gotten the chance.
The bowyer says his bows like a stiff arrow. His chart tells me a 340 @ 30" w/ ~150gr up front is about right. 3 Rivers' chart says about the same. I tested the 400, 340, and 300 shafts (all @ 30.5") with 175, 145, 125, and 100 grain field points.
First day of testing, all of the 400s showed weak. All of the 340s showed weak. It wasn't until I got to the 300s w/ 145 that they went basically where I pointed them, and then they showed a little stiff. That was quite a bit stiffer than I expected. That's what I learned the first day.
I did some additional testing a few days later to confirm/refine the initial results. That day I found that 340s w/ 100 and 125 flew pretty well, showing a bit weak, and the 300 w/ 145 and 125 flew pretty well. Today, my third session, has pretty much confirmed all of that, but also exposed a few idiosyncrasies.
I toy around with Stu Miller's Dynamic Spine Calculator quite often mostly to compare arrow setups. For my last recurve it was pretty dead on with the suggestion. With this bow it was WAY off, but I figured the calculated dynamic spine numbers would still be useful for comparisons. The main point of confusions is this: While the 300/145 combo flies pretty well, and is close in figure to the 340/100 (arguably the "best" as far as I can judge right now), the calculated spine is less and the flight shows stiff. Throwing a 175 up front should weaken it, right? The calculated figure is considerably lower, and intuition confirms this -- but the result is an arrow that flies much further to the left (right-handed shooter). I cannot explain this, but it has happened during each session. Some unintended effect of higher FOC?
All of that said and done, I'm still not sure what arrows I'm going to end up getting. I could air on the side of slightly weak, which seems to be the more normal approach, and get the 340s. I can cut them to 30", like most of my arrows, and maybe use a heavier head. I wanted to use a 125 broadhead MINIMUM if possible. Or I could get the 300s, make them longer, and use whatever weight head works. Of course practically no one suggests a 300 for a 54# bow -- but if it works it works.
The main thing that is throwing me off is the mystery of the heavier point showing stiffer. I want to go with the heavier setup, but don't want to get shafts that I can't end up tuning.
So that's that. Just a few other little things.
Someone elsewhere said that the weaker arrow flying left might be similar to when you have your nock point set too low and the arrow "jumps" off the shelf, giving a nock high reading while being low. This may be a side-to-side version of that. Either way, I still can't really explain it.
The reason for wanting to keep the point weight above 125 if possible has less to do with desired FOC and more to do with broadhead choice. I have a few "first choices" in mind, but when getting down into the 100 grain range I would have to settle for second and even third choice heads.
The advice that makes the most sense to me is to shoot what seems to be working best -- which in this instance would be the 340/100 combo -- and to leave the length alone. That's fine advice. The only thing keeping me from following it is the grain weight/broadhead choice issue, and the fact that the arrows I get are likely to be a different brand, so will possibly tune differently as well.
I've shot a 55 bow at my draw which is around 30". I've tried the Traditional Only in the 340. I left them full length and was having the best luck around 175 grains. I would spin test them though and found wobble on the tip of the arrow,even without a tip on them. I ended up going with Heritage Carbon Express 250's and I'm having better luck. I'm using a 175 grain tip on them also,and they spin very true.
A couple thoughts:
If you are getting contact with your riser it usually shows up as the back of the arrow bouncing up or out. The first would cause a bare shaft to impact low, the false high you mentioned. The other usually sends the bare shaft to the right for a right handed shooter...false weak. The thing is any contact is going to mess you up and I'm not sure that things are always going to act in the "usual" way.
I have a recurve that is 56#@29" and I draw slightly over 29". I use a 31" .340 with 175 grain points with that bow. I shoot the same arrow with 250 grain points out of my 50#@29" recurve. That doesn't mean those setups will work for you, just an example that the bowyers recommendation of a .340 sounds pretty reasonable.
I think you just haven't got the tuning down yet. It often takes a few sessions. Make sure you are shooting groups of bare and fletched shafts, two or three of each, and comparing the relative impact points of the bare and fletched shafts. This is a good link to the process... acsbows.com/bowtuning (http://www.acsbows.com/bowtuning.html)
Re-reading your post it looks like you only have one arrow of each spine. Sounds like you need to make a decision. In my opinion I'd go with the .340s, but don't cut them any shorter than you need to. Full length shafts to start is never a bad idea, you can always cut them back later. Make sure you have a selection of point weights when you start tuning. Buy at least the point weight you would like to use and some points heavier and lighter. There's no way to know what point weight is correct until you start tuning. Start close, work your way back to at least 20 yards, 25-30 is even better. If you end up with a good bare shaft tune but your tuning has you using a point that is lighter than the broadheads you want to use, that's when you cut your shafts back a bit. Make sure you do the tuning first, then buy broadheads of the same weight as the field points that tune the best.
I should add, and no offense intended, that if you are relatively new to stickbows, tuning might be a big frustration and not get you anywhere. If you don't have consistent form, go with what the bowyer recommended, enjoy your bow and practice...leave the serious tuning for later. As long as you are getting good arrow flight, no serious wobble on the way to the target, that's usually good enough for starters.
It sounds a lot more complicated than it is... ;)
Yeah, I only have one of each. So I know I'm not doing the bare shaft test 100% by the book, but my results so far have been repeatable. I will probably go ahead and give it another session or two before I decide to buy anything.
I'm new in the grand scheme of things, but I've been shooting for over two years now. I did my first fairly serious tuning last year with good results.
Try loading up that 400 with maybe 250 -275 up front and see what happens. I shot bows around 50-55@29" both D@R LB's and recurves. I would be way over spine with anything more than a 400 shaft, and most times they are a bit stiff unless I get a good bit of weight on the front.
Where are you in MI? Lots of members in MI that could possibly help you out.
My results are similar to xtrema312's. I have found that carbons behave differently when loaded heavy up front. I had very similar results to yours in the beginning. My arrows were "twitchy" and required perfect form too with anything up to 175 to make the signs of over / under-spine go away. I reluctantly jumped up to 225-250 up front and they shoot very well in that range. I am using a Howatt Hunter (55@28) drawn to 29.5" and I usually use 55/75 or .340 with 1.5" aluminum footing. In the heavy shafts the .400s worked too, but I am cheap and use the inexpensive shafts :D . I shoot these same shafts out of other D/R longbows and recurves too and they work good. Carbons really like high FOC due to their quick recovery time and seem to be very forgiving when set up that way. That has been my experience anyway.