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#### christophe

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Hello everyone! i have a 63#@28" longbow and i shoot GT 5575 arrows. My draw is 27". Shelf between +1/4 and +3/8
Arrow Dynamic Spine Required (pounds) would be 50#, according to Stuart calculator.
The 5575 are at full length, 32", with 100 gr. points with an extra 100 gr.

So far, everything looks fine except the FOC is almost 18% and the arrows have a curve trajectory at 30 yards. I would like a flat trajectory, which would increase my accuracy at different distances.

I started to play with Stu's calculator and found that the ideal arrow for my bow would be a GT1535 (!!!) with 100 gr. point, extra 70 gr. for the point and an extra 50 gr. for the nock. The dynamic spine would be the same as the 5575, but the FOC would be near ideal, 10.7%. The "perfect" arrow for this bow is extremely different from what the factory suggests....(http://www.goldtip.com/selection.htm)

My question is: how can it be that arrow builders tables show such useless values and why did we have to wait so many years to have some smart guy to understand that dynamic spine was everything ? i still cannot understand....

- chris

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#### christophe

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No, i have GT5575 at this time: fly well and straight, but curve trajectory at 30 yards, which was predicted by Stuart's calculator. I plan to order GT1535 (not easy since i have to import them: i live in South America) but would like an opinion from experienced archers first...

- chris

#### Viper1

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chris -

Couple of things: Most of my "better" bows are shooting around the 200 fps mark, and there's a noticeable trajectory at 20 yds. My idea of noticeable and yours may however be different.

Regarding arrow selection s/w. My take has always been, if they work for you, it falls into the dumb luck category. Basically with even minimal understanding of what's going on, they just aren't necessary for finger shooters.

Now, I don't claim to be an expert on carbon arrows, but a 1535 from a 63# bow does seem a little bizarre.

BTW - regarding FOC, I've never really worried about it, but that's a personal preference, the arrow nor the game animal will care one bit.

Viper1 out.

#### rraming

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Before you change your mind about the 18% FOC you should read Fred Asbell's article in TBM - you may just leave it alone:mg:

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#### christophe

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I forgot to say i do not hunt: just target shooting... anyway, thank you for your comments.

- chris

#### Flying Dutchman

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Chris, I just took a look at this calculator, but it doens't seems to calculate right. I know for sure that the dynamic spine for my bow is 53lbs, and this calculator gives me a 40 lbs!

I also shoot with GT's with a 100 grains tip and the calculator gives me a arrowspeed of 188 fsp, while I measure always a 173 max. I also don't beleive that all longbows witht he same drawweight offer the same speed. The only thing that seems to be right is the FOC calculation.... So I would recommend that you forget about this calculator...

Here I have a better calculator, it is German, but I recon you'll understand it:

http://www.bogenblog.de/index.php?/pages/spine-rechner.html

I would try the following: see what dynamic spine you get with this calculator.

Then take a GT which is the closest at this. Dont make the mistake of looking at the "spine values, such as 11-35 or 35-55, but use the calculated AMO spines (which you need):

15-35 = 0.600” becomes: 26/0,6 x 1,2115 = 52,5 lbs
35-55 = 0.500” becomes: 26/0,5 x 1,2115 = 63 lbs
55-75 = 0.400” becomes: 26/0,4 x 1,2115 = 78 lbs
75-95 = 0.300” becomes: 26/0,3 x 1,2115 = 105 lbs

I'll give you an example. At my drawlength of 29 inches my R/D longbow delivers a 38.5 pounds. When I use this German calculator, I get an dynamic spine of 53 lbs. When I make my own calculations I end up with pretty much the same. This is because this calculator is based upon the same parameters as I use. If you want to know the exact parameteres, let me know!

Which means I use a Goldtip 15-35! I shoot this arrow with the standard insert at the front, an 100 grains fieldtip and the standard GT nock.
I choose for the 100 grains because intensive bareshafting prooved in a convincing way that this was the right weight.
My FOC is a bit heigh (13 %) , but since I shoot selden over the 60 yrds, this is no problem.

So my advice to you would be to do the same. From what I know from your bow-parameters ( I don't have all info), I would say you need a GT 35-55 with a 110 or 125 grains point. The only way to be sure you have the right bow/arrow combination is to shoot with it! But start with bare-shafting to fine-tune your arrow! I am instructing people here for bowshooting in The Netherlands am writing an booklet about how to tune arrows and this method prooved itself!

Oh yeah and forget the charts of the manufacturers, they really s*ck!

#### Flying Dutchman

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Further, I am with Viper one all the way! Getting the right arrow configuration right away from s/w would be sheer luck indeed. However, you can get reasonable close; use it as a starting point. There are too many facors involved to calculate this. Even two people with exactly the same bow and exactly the same drawlength can end up different; due to their shooting-style. Paper- or bareshaft -tuning is a good way to get the right configuration. Where you have to keep in mind that you must be able to reproduce consistent groups when choosing bareshafting. When you are just starting with archery, I would recommend papertuning. Both techniques are explained on the GT site.

As far as FOC concerned: I did my best for my cedar arrows to end up (after bareshaft tuning) with a FOC of 9%, which is believed to be ideal for 3D shooting. I ended up with 9.1%. The best I can get with my Goldtips is 13%.
However, I hardly can notice any difference. I mean with this that both my cedars as my GT's are deadly accurate...

So Viper is right again here too....

Good thing I bought your book Anthony....

#### Str8 Shooter

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In reading the original post and the desired goal I don't think that you are going to be happy switching to a 1535 GT. First, that spine arrow is extremely limber for the draw weight of the bow. Even if you manage to get the arrow to tune it will be critical to shoot. I've tuned arrows that were too limber and made them work (as opposed to a spine that is close to start and fine tuning with length and tip weight) and they were always less forgiving of mistakes.

Second, the change in FOC and predicted speed isn't going to change your trajectory drastically, especially shooting 30 yds or less. I've shot trad bows at speeds from 140's up to 230+ fps. This past summer I shot my bow with two pairs of limbs and the exact same arrows. One shot at 225 fps, the other was 208-210. I shot both setups extensively and out to 50 yds I used the same holds. Past that distance I could observe a change in drop with the slower setup. Same thing with FOC. I've done a lot of testing with different arrows, weights, FOC, etc and realistically the change in FOC for the distance you mentioned won't be very noticeable. I tested two arrows setup at the same weight, same speed, both tuned to my bow, one with 10% and the other at 24%. Out to 40 yds had the same holds and point on. I expected to see a change in trajectory but it never really materialized. In fact, I found that an arrow with a little higher FOC 12+% was more stable in flight at longer distances.

Just my opinions. If you try the arrows out let us know how they work.

#### christophe

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Thank you for your detailed answer. I checked the http://www.archerytalk.com/vb/showthread.php?t=1079897 and found that if you increase the weight point in this form calculator, the dynamic spine increases accordingly, which does not look logical to me. Also, the string seems to greatly affect the dynamic arrow spine: i did not know this had any effect?

About the FOC: i understand that it has less effect on the arrow flight than what i suspected, which is good since it will make me save the cost of a dozen 1535

Anyway, i would be interested to get Stuart's opinion about the calculator showing the perfect match for a 63# longbow with a 1535. This would mean that playing with weights, you could almost use any shaft with any bow.

- chris

#### BLACK WOLF

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This would mean that playing with weights, you could almost use any shaft with any bow.
Yep...an archer knowing what they are doing can just about make any shaft work within reason :wink:

Ray

#### Viper1

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Chris -

This would mean that playing with weights, you could almost use any shaft with any bow.

- chris
That's the theory - but I wouldn't bet the farm on. There's latitude, but everything has limits. Even if you could make "anything" work, you might not like "how" it works. Best to get in the ballpark and tune from there.

Viper1 out.

#### Stu Miller

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Hello Chris,
Actually, I think your GT 5575 looks great! The issue I see with the GT1535 is getting the extra 50 grains to the nock end. Everything is possible in theory but to run a lot of front end AND nock end weight with a very flexible shaft in between seems to be unneccessary just to get a lower FOC.

As an alternative, you may want to consider giving a Carbon Express 90 a try. The attached screen grab that shows a setup that should be very close for your bow dynamically, has a low FOC%, and has a higher speed which will probably be the biggest factor affecting the trajectory curve you are seeing.

-Stu

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#### christophe

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Thank you Stuart! but i would like to insist on this dynamic spine, which could translate in "arrow bending at release". It sounds logical to me that it is related to static spine, weights, shaft length and string acceleration. The shelf location does not directly influence the dynamic spine, but higher values will require a lower dynamic spine to allow the shaft go around the riser. So far, so good: this is exactly what your calculator is about.

The amount of acceleration induced by the bow design is also an important factor; i like string follow longbows, even if i know my arrows will be 5fps slower than a backset longbow due to a smoother acceleration curve. I suppose we could deduce the type of acceleration the bow will produce measuring its poundage at 25", 26", 27", 28", 29" and 30". The harshest the acceleration, the higher the dynamic spine required for the arrow.
Anyway, the resulting GT1535 i get for my bow does not look that silly: it is an interesting result that deserves investigation

- chris

#### Viper1

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Chris -

Anyway, the resulting GT1535 i get for my bow does not look that silly: it is an interesting result that deserves investigation
While you're investigating, please wear body armor. In addition to functional spine (the amount the arrow needs to come out straight), there's also what might be called tensile strength spine, for lack of a better term. If an arrow flexes too much, as in the case of a VERY weak arrow tuned to shoot from a bow that's it shouldn't be in the same room with, you might end up with an arrow failure at the worst possible time.

I know some people here have played with very light and weak arrows for speed, but I've asked them in the past to warm me when they are going to be shooting.

OK, somewhat tongue in cheek, but while a lot of things are possible, some things just don't make good sense.

Viper1 out.

#### Sanford

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The shelf location does not directly influence the dynamic spine, but higher values will require a lower dynamic spine to allow the shaft go around the riser.
- chris
The shelf location does directly influence dynamic spine. The riser wall acts as a fulcrum against which the outboard swing of the string causes bend in the arrow. Yes, you need a more limber arrow to get fully around the riser and back to center on exit, but as others have said, there are limits. As Mr. Miller stated, you were able to get a formula driven model to spec an arrow but did so by adding 50g to the tail with 170g up front and all against weak spined shaft, at least weak by common knowledge for a 63# bow.

In modeling there is the acronym GIGO (garbage in = garbage out). IOW, the output of the model is only reliable if the inputs were good, reasonable, and applicable. With any formula driven model, one can force favorable results by using unfavorable inputs.

#### Flying Dutchman

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GT's are virtually indestructable is my experience after shooting them for a year. Even when you stand on them, they won't break. The only thing they don't like is a Robin Hood. I once tryed to destroy one, that was damaged by a Robin Hood and unusable. Whatever I tryed, I couldn't break it....

Remember that a GT 15-35 still puts a 53 lbs on the spinemeter. No need to be afraid for strange things when shooting it from a 63 lbs bow...espacially when your drawlength is 27"

#### rattus58

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Don't know how we got on weak spine, but I've in my lifetime had two arrows disintegrate at release, one wood and one a carbon that must have been damaged. The carbon turned itself into a hawaiian puili (split bamboo music instrument) and created a crack on the limb of my martin longbow in the process.

The other was a wood arrow that broke, sending and imbedding the nock-end into my garage... talk about a learning curve there. I was VERY fortunate in not having serious splinter damage in my arm.

Aloha.... :beer:

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