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    rules of thumb on the optimal bolt weight

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    rules of thumb on the optimal bolt weight  - Page 2 Empty rules of thumb on the optimal bolt weight

    Post by Hotspur Tue Jun 04, 2013 8:49 pm

    First topic message reminder :

    So I have done some reading on the threads here about bolt making and I want to get a little feedback from people on my take.

    There is a couple rules of thumb on the optimal bolt weight for a given bow...

    I want to make bolts for my steel bow:
    Draw weight: 250lbs
    Physical mass of prod: 806 grams

    One rule of thumb is 2 grains per pound of draw weight as a minimum starting point or in this case 500 grains - 32.4 grams. With a suggestion that steel prods like heavier bolts.

    The other rule of thumb suggests 1/12th the total mass of the prod as optimal efficiency or 806/12= 67 grams or 1037 grains.

    So here is a spread of 500 to 1000 grains.

    Does anyone have thoughts on these matters?

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    Post by African Archer Mon Jun 17, 2013 11:51 pm

    I think you might be ok with the carbon shaft, but what you might fnd is that if the dowls are not glued in tight , the inersha of the shot might force the insert out eventually, so maybe some epoxy is needed. but hn might be wrong.
    Just  thought.
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    Post by Geezer Tue Jun 18, 2013 10:36 am

    Solid fiberglass rod will make damned near indestructible bolts, but the are heavy as can be.  Some medieval clubs, like SCA use them for their combat-game bolts.  I've seen them stepped on, even stamped on deliberately, with no damage.  Geezer
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    Post by Hotspur Sat Jul 06, 2013 5:53 pm

    The following chart shows seven bolts of increasing weight from 515 grains to 1000 grains, released from a 250 lb prod with a 9’ power stroke.  The FPS is the average speed over 50’, not ‘muzzle velocity’.

    This is an off the shelf carbon bolt 22 inches with a 170 gn field point (total 515 gn).  The hollow bolt interior just accommodated a .25” split lead shot (25 grains each)  held in place with .25” dowel.   I used the same bolt and just dropped in more lead for each step.

    rules of thumb on the optimal bolt weight  - Page 2 Boltkinetics_zps67b6274d


    Interesting how the ft. lbf starts to plateau at 900 to 1000 gn, perhaps confirming the Nerds efficiency hypothesis:

    903.8 gn               166 fps                 55.32 ft lbf
    1000 gn                158 fps                 55.45 ft lbf

    However, I think I will settle on a 600 to 700 grain bolt based on this experiment for about 180+fps at 50 ft.lbf and 15% FOC.  Seems like a good compromise of speed and KE for deer hunting and the increased FOC gives the bolts a nice stable flight.


    Last edited by Hotspur on Sat Jul 06, 2013 5:56 pm; edited 1 time in total (Reason for editing : sp error)
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    rules of thumb on the optimal bolt weight  - Page 2 Empty Re: rules of thumb on the optimal bolt weight

    Post by tnetcenter Mon Jul 15, 2013 6:02 pm

    Pardon my ignorance, but what does FOC refer to??

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    Post by Stonedog Mon Jul 15, 2013 6:48 pm

    Forward of Center
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    Post by Hotspur Mon Jul 15, 2013 11:29 pm

    I shortened the commercial bolt to about 14" and 650 - 700 gn and am awaiting UPS for a fetching jig.  The FOC should be about 18% with the shorter bolt.  I am curious to see how a shorter bolt compares.  I will also go from the 3 fletch to a two fletch.  In an initial test with rough fletching the shorter bolt was slower for the comparable weight, but the rough fletching may have made for poor flight, especially with only two feathers.

    Jeff, this is a good video explaining FOC:
    https://www.youtube.com/watch?v=a0FmMr-QXtU
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    Post by Hotspur Mon Jul 15, 2013 11:42 pm

    Further on FOC, Jeff, the more forward the weight on a bolt, the more forgiving it is to aerodynamic vagaries and the more dart-like in performance.  The cost being trajectory and distance.
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    Post by tnetcenter Mon Jul 22, 2013 5:16 pm

    Hotspur wrote:Further on FOC, Jeff, the more forward the weight on a bolt, the more forgiving it is to aerodynamic vagaries and the more dart-like in performance.  The cost being trajectory and distance.

     Thanks for the video link - that helped immensely.  I'm also into high power rocketry.  In HPR, we use 2 measurements that relate to the stability of a rocket in flight (they may have some bearing here as well).  The first and most well known of those would be Cg (center of gravity) which is used here to calculate the FOC.  Then there's the Cp (center of Pressure)  which is based on some calculations related to the length, distribution of mass, diameter, and profile.  In HPR, the stability is directly related to the difference between the Cg and Cp and how far apart they are, measured in calibers (diameter).  I suspect that FOC may in fact be a crude estimation of the Cp.  The dynamics are amazingly similar.
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    Post by Hotspur Sun Jul 28, 2013 6:12 pm

    I got my Grayling Fletching jig and started on a matched set of 15" bolts. 

    Just to prove that this collection of wood and steel could make something go 200 plus feet per second I made a 437 grain bolt and clocked it at an average speed of 203 fps. I considered shooting something lighter but didn't want to dry fire my limbs. 

    I did a two fletch set of three bolts and will tweak them to just over 600 gn.  That will give me about 188 fps and about 48 foot pounds of force. These will be fixed with 170 gn broad heads.

    Will also make a couple blunts for upland game birds. 

    Less then a month to archery hunting season here in BC!  Fall is in the air Smile

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