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Crossbows - Everything about Building, Modding, and Using your Crossbow Gear

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    Optimal Bolt

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    Post by cnunley on Wed Dec 05, 2012 11:25 pm

    Hey guys, I'm building a crossbow around the Alcem 1316, 120 lb prod and I want to make my own bolts out of 3/8ths dia shafts. Is there an optimum weight/length I should be shooting for? And as I want them all to be the same length, what is the best way to fine tune the weight for a matched set of bolts? Lead shot in the bodkin? lastly what is the +/- range for the weight in a matched set of bolts?

    Thanks,

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    Post by panne on Sun Dec 09, 2012 7:22 am

    i've also been searching around for a basic formula to calculate the minimum and maximum weight and length bolt for a crossbow. i'm sure there is a basic formula that takes into account draw weight and draw length, but i have yet to figure it out or find an example.

    in the 120# and up draw weights you can get a general idea of where to start by looking at the weights of the bolts supplied with recurve crossbow package deals. most will come with a bolt that is heavy enough to prevent damage to the prod.

    adding lead shot to the bodkin tip of a homemade bolt to balance the weight between bolts should work if they are not too far apart and you have room to add them. you can also remove a little bit of weight from the heavier points and shafts.
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    Post by Todd the archer on Sun Dec 09, 2012 9:03 am

    Well as a starting point modern crossbows are around 2 grains per pound of draw weight. So 120 pound would use 240 grain min. 200 pounds a 400 grain. However this is a minimum and heavy (physical weight) steel prods are ineffiecient with light arrows so you are better served to use heavier arrows without much loss in speed.

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    Post by hullutiedemies on Mon Dec 10, 2012 12:01 pm

    panne wrote:i've also been searching around for a basic formula to calculate the minimum and maximum weight and length bolt for a crossbow. i'm sure there is a basic formula that takes into account draw weight and draw length,
    You do not need to know those.
    You need the "virtual mass" of the bow.
    This is the inertia that the bow slows itself with when accelerating.
    For traditionally shaped bow this is normally about 1/25 of the limb mass.
    So optimally bolts should weight ca 1/12 of the physical mass of the prod. This should give about 70% mechanical efficiency and optimal combination of speed and hitting power .
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    Post by Zardoz on Mon Dec 10, 2012 9:06 pm

    Nerd, where did you find that formula for determining the weight for the bolt? I would like to look into that in more detail.
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    Post by hullutiedemies on Sun Dec 16, 2012 10:52 am

    Concept of "virtual mass" or "ghost arrow" is well known to bow bulders. Especially in flight shoting community.
    Normally if you test a bow with different weight bolts, you will notice that the efficiency improves as bolts get heavier and bow get slower. Here you will commonly notice that the bow behaves like there was a second invisible arrow of fixed mass riding along the bolt.

    This invisible arrow represents the inertia of the bow and string themselves.This virtual mass also determines the dry-fire speed of the bow.


    So using a bolt about twice the virtual mass will make the mass of the bolt ca 70% of total inertia the bow has to accelerate.

    Wight ligter bolts efficiency of the bow diminishes quite rapidly. With heavier ones the speed of the bolt goes down respectively.


    Assuming virtual mass 1/25 of bow mass is based on observation. It can also be calculated from limb geometry.
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    Post by Zardoz on Sun Dec 16, 2012 2:46 pm

    Thank you, Nerd. I am going to use that info.
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    Post by cnunley on Mon Dec 17, 2012 11:30 am

    Ok, what is the target over/under for a set of "matched" bolts? Or is "close" good enough?

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    Post by mac on Tue Dec 18, 2012 7:00 am

    cnunley wrote:Ok, what is the target over/under for a set of "matched" bolts? Or is "close" good enough?

    CEN

    It depends on how well you shoot, and how fussy you want to be.

    If I recall correctly, the last time I made a matched set, I got them all to within two tenths of a gram. This was probably a better set than I needed.

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    Post by Joulesbee on Tue Dec 18, 2012 7:50 am

    May I ask, does the relative mass of a prod pretty much match its draw weight regardless of material used in its construction.

    Steel or Alloy weighs approx. same as a composite/wood for the same draw weight ?
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    Post by mac on Tue Dec 18, 2012 9:06 am

    Joulesbee wrote:May I ask, does the relative mass of a prod pretty much match its draw weight regardless of material used in its construction.

    Steel or Alloy weighs approx. same as a composite/wood for the same draw weight ?

    A steel prod is significantly heavier than a composite or wood prod of the same draw weight.

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