Newbie Spine sanity check

[HenrikBP]

First time poster - some time lurker here; I want thank everyone for the tremendous amount of great archery related information available on here on Tradgang. I recently dusted off my old Martin Hatfield TD after a 20+ year hiatus, and lurking and reading here has been a great help in getting things sorted.

I  have read all I could find here on Tradgang and elsewhere, but still have a question:

I ordered a 17" Morrison ILF riser with Morrison "Long" D recurve limbs. 62" 48# at my 26 3/4" draw length, 10 strand D10 string. I believe the shelf is cut 1/8" past center, but I could be wrong. I shoot 3 fingers under.

I am trying to decide on carbon arrows for the new bow and have been recommended to go with .500 spine shafts - which matches some posts here. I am looking to get about 10 GPP and would like 15 - 20% FOC so I would have to go with a lighter weight (6 - 7 GPI) arrow with a heavier point.

However, when I run the numbers in Stu Millers spine calculator .500 spine arrows come out much too weak.

Thoughts?

Thanks,

Henrik

[bigbuckmalik]

All the carbons that tune well to my bows come out 15 to 20lbs weaker than the calculator tell me to shoot. Right now i shoot a Bob Lee recurve 50@ 28.  It tells me to shoot a 300 spine to get close.  I'm shooting a 500 with 175gr tip.

[HenrikBP]

Thank you - yes that's pretty much how it adds up. About 20# low depending on length and point weight.

Henrik

[metsastaja]

the morrison Ilf is cut -3/16. You just need to enter the bow in the calculator and then add in strike plate thickness.

Remember BOP is not arrow shaft length. it is a combination of shaft length plus valley of nock to end plus any insert length.

[HenrikBP]

Thanks Les for the correction and BOP info - I'd been wondering :-)

I'm using the only Morrison ILF bow listed in the spine calculator: Morrison ILF Mini 14", but don't know how closely that choice matches my actual bow.  I was surprised that any change in the center cut number in the calculator makes no difference in the dynamic spine number. Any change in strike plate thickness however does.

Henrik

[xtrema312]

I don't think you will have a problem doing or getting real close to what you want with a 500 spine arrow.  You will get that shaft to tune for you with some combo of length and point weight.  It should be a good set-up.  I don’t think you will need as stiff an arrow as the calculator indicates so I think you will get real close with the GPP and FOC you want. Even if you don’t get there you will have a very good arrow for that bow.

I can't shoot 500 because they get too light weight for me by time I get them close to tuned, but I have over 29" draw and shoot about 54-55# at that length.  I also shoot 15-20# weak, but that is with 400 spine. I find the 500 to act a little closer to the calculator.  

I believe that carbons act very different based on length.  Cutting makes a bigger difference than point weight for me.   I shoot the 400 about 1" longer than needed to get it tuned with the point weight I want.  I also have a 400 spine arrow with 1” shorter shaft and heavier point.  The shorter one should act weaker based on the calculator and it shoots stiffer.  I think  the 500 shaft acts closer to the calculator because I shot it at minimum length.

Best thing to do is go get a 500 and 400 shaft.  Start shooting them at least 2" longer than your draw.  Get brass inserts and a wide range of point weights.  Then start bare shafting for general flight.  When you get close you can get a couple fletched shafts to fine tune.  You can get something like Beman Hunter or Bow Huner shafts at any archery shop.  Strip them if you can't get a bare shaft.

[metsastaja]

here is the location for instructions on how to use calculator. It is worth the read.
Bow center cut is calculated internally for database bows and only changing the strike plate thickness will change the spine.

 http://www.heilakka.com/stumiller/calculatorinstructions.pdf

This is the download link for bowcenter cuts created by TG members,Stu Miller and bowyers
 http://www.heilakka.com/bowcentercuts.xlsx.pdf

[HenrikBP]

Thanks Extrema; that's good to know. I was thinking about getting a shaft test kit, but it seemed silly when only 2 of the 4 shafts might work.  I do have point test kits on order, and know from Tradgang threads to leave the shafts full length until testing with several point weights indicate I should shorten the shafts a hair.

Thanks again Les: I know - RTFM  (hangs head ;-)). I did download the manual and started, but never made it to the center cut section.

Henrik