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Main Boards => PowWow => Topic started by: Morning Star on February 14, 2009, 01:18:00 PM

Title: Speed / draw length
Post by: Morning Star on February 14, 2009, 01:18:00 PM
Just curious.

Given the "same weight" at each length, what kind of gains do you get out of longer draw lengths?

Compare....say

50lbs @ 27"

50lbs @ 28"

50lbs @ 29" and so on.
Title: Re: Speed / draw length
Post by: vermonster13 on February 14, 2009, 01:19:00 PM
About 4% per inch
Title: Re: Speed / draw length
Post by: KSdan on February 14, 2009, 05:05:00 PM
4% of what??
Title: Re: Speed / draw length
Post by: WESTBROOK on February 14, 2009, 05:25:00 PM
Dan, say 50@27" goes 170 fps. 50@28" will go 176.8 fps, 4% faster.

Eric
Title: Re: Speed / draw length
Post by: Dartwick on February 14, 2009, 05:30:00 PM
Wouldnt be more sensible to have a % of energy?
I know this an off the cuff estimation, but it just seems odd to use velocity.
Title: Re: Speed / draw length
Post by: O.L. Adcock on February 14, 2009, 05:59:00 PM
Yep, Bows put out KE...Looking at fps is like looking at rpm of an engine..It's not tied to horsepower...Any given bow will gain of lose about 4% every inch up or down. At 9-10gpp that works out to about 4fps.....O.L.
Title: Re: Speed / draw length
Post by: Morning Star on February 14, 2009, 09:22:00 PM
Thanks guys.
Title: Re: Speed / draw length
Post by: KSdan on February 14, 2009, 09:45:00 PM
Help me understand power stroke then. . ???  I thought the longer draw meant longer power stroke which meant more "horsepower"?  No??  
Appreciate an explanation.  

I am shooting 55# at 31.5".  How is that better than 55# at 28"?  

Thanks for educating me guys.
Title: Re: Speed / draw length
Post by: vermonster13 on February 14, 2009, 09:47:00 PM
The arrow is on the string longer and thus absorbs (for of a better term) more of the energy from the bow resulting in more KE/Momentum.
Title: Re: Speed / draw length
Post by: John Havard on February 14, 2009, 09:55:00 PM
Stored energy goes up with each inch of draw.  Calculate the area under a triagle.  One has a 50# peak at 27" and another has a peak at 28".  One-half of the base times the height equals the area of the triagle.  A longer draw with the same draw weight has more stored energy than a shorter draw.  Simple.  

Therefore, with dynamic efficiency staying about the same at most normal draw lengths (see our website's description of "energy storage") then the longer draw length (at the same draw weight) will shoot the same arrow faster because it stores more energy than the shorter draw length.  

As much as we'd love to, none of us can fool Mother Nature or the laws of physics.