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Hysteresis effects on rubber powered xbows

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Old 11-30-2014, 06:19 AM   #1
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Default Hysteresis effects on rubber powered xbows

Did a little experiment today I have been keen to do for a while. We all know how powerful and efficient rubber powered weapons can be. Their Achilles heal is the rapid cooling down and subsequent loss of power and velocity. But is it really that bad and does it preclude rubber powered crossbows being practical weapons or hunting arms ? I tidied up and used my mongeral rail gun as the control. Dankung 5080 tubes were used for power. Just over 40 pounds of draw at 15 inches being stretched to just under a factor of 4.
I did a number of shots with a leisurely 30 seconds from cocking to shooting a 250 grain bolt as a control. This mild setup gave a consistent speed of mid 140 fps. Then I shot at delays of 2, 5, and 10 minutes for speeds of 141, 138 and 135 fps. I was surprised by the minimal loss of speed at 2 minutes expecting it to be greater. The loss of only 10 fps after 10 minutes was for me perfectly acceptable.
This augers well for the use of low stressed setups shooting modest weight projectiles at reasonable speeds. My holy grail would be a weapon capable of 180 -200 fps shooting a 500 grain edged projectile after being cocked for 5 minutes plus.
What is the ceiling with draw ratios and performance able to be extracted before a launcher becomes impracticable ? What do you guys think ?

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Old 11-30-2014, 08:21 AM   #2
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That is very interesting indeed! I would have expected a much greater loss of power. It has been that factor that put me off ever making a crossbow (well, that and my lack of mechanical aptitude!!) but I shall have to rethink this now!

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Old 11-30-2014, 10:04 AM   #3
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For minimal heat loss, those big tubes are the best you'll get. But for speed, why don't you try to find a good combo of smaller tubes? Test a whole range of sizes, and find the one that also has an acceptable heat loss while still being faster!
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Old 12-03-2014, 11:40 AM   #4
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Interesting results indeed! Though it would be good to know what was the outside temperature when you let the bands cool down? It'd be logical if the power loss was be far greater in a chilly winter day compared to a hot summer day. I wonder if you have got any sort of an equipment for measuring the temperature of those bands when drawn out?
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