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Ultra high powered slingshot crossbow


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Old 10-19-2016, 02:36 AM   #1
Setarip
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Default Ultra high powered slingshot crossbow

Hello everyone,

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!



Attached Images
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Last edited by Setarip; 10-19-2016 at 02:49 AM.
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Old 10-19-2016, 02:39 AM   #2
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Default Ultra high powered slingshot crossbow

"Frictionless system" that sounds amazing dude

How about a video?


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Old 10-19-2016, 02:41 AM   #3
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Just posted

It's also not frictionless! THat would be impossible. Speed ratio simply represents what the mechanical advantage without friction would be.
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Old 10-19-2016, 08:03 AM   #4
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Nice construction. Always, when i see videos like this, i want to finish my own
slingshotcrossbow and have fun with it.
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Old 10-19-2016, 11:08 AM   #5
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When I see this video an inner voice whispers into my brain "Of course we can always add more rubber...."


Great Work!
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Old 10-19-2016, 12:56 PM   #6
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Quote:
Originally Posted by kiozen View Post
When I see this video an inner voice whispers into my brain "Of course we can always add more rubber...."


Great Work!
Oh yeah I plan on increasing power as much as I can. I do want to sort out some of the inefficiencies such as the shackles first and see what I gain there. I did notice that there is a speed limit due to friction when the string lightly rubbing against the stock when firing. It was kind of setup like a traditional crossbow with the string on the surface of the stock.

I was clocking around 280-290 fps, then added more weight and saw a drop to 230-250 fps! I figured it was string height so I added about 1.5mm of height and suddenly was well above 300 fps.
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Old 10-19-2016, 06:21 PM   #7
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Small update: I achieved 323.3 fps @463 grains for 107.49 ft-lbs or 145.7 joules of energy! I purchased lighter shackles, used a lighter string, and added two more theratube silver bands to the total.

In the video below you will see that the bands came off of the mount. Version 2.0 will fix this issue. This has only happened at this power level. I was not hurt at all because most of the energy went into the bolt! Still slightly terrifying when this happened!



Last edited by Setarip; 10-20-2016 at 01:08 PM.
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Old 10-19-2016, 09:23 PM   #8
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Quote:
Originally Posted by Setarip View Post
Small update: I achieved 323.3 fps @463 grains for 107.49 ft-lbs or 145.7 joules of energy! I purchased lighter shackles, used a lighter string, and added two more theratube silver bands to the total.

In the video below you will see that the bands came off of the mount. Version 2.0 will fix this issue. This has only happened at this power level. I was not hurt at all because most of the energy went into the bolt! Still slightly terrifying when it happened.

https://youtu.be/ABMtZbewEv0
Now you're only 5 Joules away from the legal mininum power limit from a class 4 hunting rifle in Sweden. At that power level you are legally allowed to hunt hare, capercailzie and white grouse. Impressive that you are able to reach the power achieved by a .22 LR rifle!
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Old 10-19-2016, 11:49 PM   #9
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Hehe! That's interesting JonS! I am trying to get to 22lr energy (40gr @1200fps 127fpe). Only thing is with an arrow it's beyond enough to take even elephant with! Which brings a big advantage rubber powered crossbows have over regular crossbows...weight! This is a remarkably light weapon of this type with this much power.
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Old 10-20-2016, 06:51 AM   #10
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Well, you are already getting the momentum of a .45 ACP round, at 2,9 Ns. Energy isn't everything! Great work.

The only problem with rubber based crossbows is the hysteresis effect... after four hours in the tree stand with the crossbow fully cocked, the performance will be much, much lower. Plus also the poor performance in cold temps.


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