Having received my ballista back with a new chronied speed of 175fps with a 400 grain 20″ arrow (up 24fps from 154fps) after a minor tweak, I am ready to build the new one.
The goal here is to approach 300fps using the same pull of 45-50lbs, use the same Scorpyd 125 frame but drop the arrow weight down to something more reasonable for the pull weight say 300-325 grains from 400, while at the same time giving it a little wider axle to axle measurement, and using 3/8 diameter metal rod levers instead of the heavier 5/8″ diameter designed for pull weight in excess of 150lbs.
Before the minor tweak it was getting 154fps with 400 grains and 175fps with a 350 grain. A normal drop in velocity of 1fps per five grains – so the arrow is too heavy causing twice the normal amount of vel. loss. It seems logical to assume then taking the arrow weight down another 50 grains would result in another 25fps gain.
So bottom line is my current set up should be right at 225fps with a 300 grain arrow. The weight savings from going to a smaller diameter metal rod should outweigh by far the weight difference in arrows (about 15 grains per gram). I am hoping to lose a full one ounce per lever and think this should be enough to get me up to 250 fps with just 50lbs of pull.
If accomplished, I think that it would not be unreasonable to expect a 50% return on an additional amount of power, or about 375 at 100lbs pull. Since that is actually higher than the Scorpyd even a 40% increase in velocity would take me to the legal speed limitations of 350fps of a lot of the States.
I will do a weight savings test using one inch lengths of each diameter to calculate total weight savings, but figure it will be enough that a heavier arrow than 300 might still accomplish the speed goal for a 50lbs draw weight. The 50lbs pull is just right for backyard target practice or in my case basement or living room LOL.
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