I want to show you a rubber powered crossbow I am working on. I am developing it off of RalphG's demonstrator from a couple years back. It's a continuation of his concept.
The basic layout consists of 4 pulleys creating a greater than 2:1 mechanical advantage. I say that's it's greater than 2:1 and not 3:1 because I measure the Speed Ratio as precisely 2.46:1(that is the rubber stretches 8.625 inches and the string is drawn 21.25"). The speed ratio is unaffected by friction and demonstrates what the MA would be in a frictionless system.
There are a couple of things I want to explain first. The version you see here is more like version 1.5. V.1.0's fork was made with 3/4" multiplex. During testing last week with six theratube silver fully drawn, the bow dry fired and the multiplex "fork" holding all of the weight ripped apart in tension and injured me lightly. This can be extremely dangerous and I only suffered minor cuts and scrapes. I always wear protective eyewear and so should you when working with large amounts of draw weight.
This new versions "fork" is made from 3/8" 6061 aluminum and is significantly stronger. The pulleys or sheaves are 1.25" diameter steel patio door hardware found at the local hardware store. They come equipped with decent 626zz bearings. All of the crossbows hardware is stainless steel.
The main body is 1" thick multiplex that is sandwiched with two 3/8" sheets of ply wood and one 1/4" sheet, allowing the perfectly aligned central channel for the bolts. The trigger is a simple "rolling nut" made from 1/4" thick 6061 aluminum. The d shackles are stainless steel and are each rated at 530lbs.
The bow string is 3mm 1050lb spectra. Overall this crossbow weighs 3 lbs, 7 ounces. The bolts used are carbon express crossbow bolts with 125 grain field tips. Overall bolts mass is 463 grains. The crossbow is powered by 8 Thera tube silver tubes, and 14 Thera tube red.
Today I achieved 307.8 feet per second, which equates to 97.43 ft-lbs of KE, or 132 joules! I was able to hit 350 fps with a 290 grain maple dowel, but it was good for one shot and then snapped when loosed. Yesterday with less rubber and a 162 grain dowel I achieved 375 fps.
There are a couple of things I plan on doing to increase the overall efficiency. First is to obtain lighter d shackles. These each weigh 20 grams! At full power they each absorb 10.5 ft-lbs of energy. 21 ft-lbs that's robbed from the bolt. I am also not using a pulley wheel on the shackles, therefore the friction is very high because the cables are rubbing against the bare steel.
Finally, RalphG's demonstrator had a 70cm power stroke, mine is only 54cm. I plan on increasing it on v.2.0.
Pound for pound this crossbow produces over 28 ft-lbs of energy per pound of mass that the bow weighs! That's significantly more power per pound than most commercial crossbows on the market. The Scorpyd VTEC Extreme produces only 20.6 ft-lbs per pound of mass!
Let me know what you all think!
Edit: Here is the video of how I draw the bow and the shot that hit 307.8 fps!