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Discussion Starter · #1 · (Edited)
some data of my actual "device" -> Let me show you its features ! ;-)
  • rubber powered (10 parallel theraband silver on each side)
  • transmission ratio to bow string 1/4
  • push & pull energy storage
  • frame material : Aluminium
  • size: 80cm*20cm*20cm
  • stroke length 55cm (pusch&pull)
  • bolts: DIY 27.5g 10mm quadratic aluminium + steeltip
  • magazine (up to 10bolts , so far only used 5 DIY)
  • bolt velocity 70m/s
  • bolt energy 70J
  • efficieny about 50% (depending on temperature,...)
  • overall weight 2,8Kg
20210101_133315.jpg pic front.jpg pic full.jpg pic rear.jpg pic top.jpg 20201226_123822.jpg pic side2.jpg 20201226_132330.jpg 20201226_121859.jpg

Backdraws of design (to be improved in next step):
  • rubber tubes have bad efficiency at cold temperatures
  • heavy rollers -> bad efficiency
  • magazine not exchangeable
  • trigger (aluminium) wears after several shots
  • bolt holder broke at dry fire
  • too much power :) -> how the hell do i get the bolts out of the 27mm doka plate :) ??
Questions:
Did I post this in the correct category or would be "Slingshot Crossbow" the better channel ?
Anybody working on similar targets ?
 

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Hi Marcus

That is mean man! Personally I can't see you needing any more power. What range do you intend to use the weapon at? If it's a self defence weapon you don't need more than 10 to 15 meters in my opinion. At that range it's lethal. Is it heavy to draw?

You say the trigger wears quickly. I am interested to see where as my trigger is plastic and has shown no wear at all. I have nowhere the same power you do though.

Don't worry about the mag not being exchangeable. If you can defend yourself with 10 shots you're screwed anyway. Again just my opinion.

Excellent build, congratulations!
 

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Discussion Starter · #3 ·
Hi Algstone
Thanks’

No its not a weapon -> Its a fun toy ;-) ....but easily covers my property.

Drawforce:
As it is push & pull operation the actual max force is only 160N=16Kg. -> very easy for normal man (no bodybuilders needed)
But you can adjust drawforce within seconds/minutes by simply removing some of the rubber tubes on each side. Possible combinations: 2,4,6,8,10 tubes on each side.
Actually the elongation unloaded is 200% and armed 300%.
-> by reduction of string this schould be enlarged to 300% & 400% which would make the draw even more linear.
Up to now I did not see any comparable system that uses same energy storage for pull operation. -> this is the main advantage of this device -> please tell me if there is something similar out there.


Bolts:
The Bolts are DIY without fins.
By simple aerodynamic its not needed to have fins as long as center of mass is infront of center of aerodynamic forces. -> then its self stabilizing
Keep in mind that damaged or misaligned fins are bad for flight precision.
Actually I struggle to get the bolts out of the wood again.
I will try to hang my whole bodyweight (in line) on the bolt.
Hammering on the tip on the other side will damage it.
That’s maybe the reason for archery matts I guess ;-)


Trigger:
I think I have to put a detailed picture to present how the two moving trigger parts work. Anyway, actually there are pusching two 5mm thick aluminum parts against each other. -> material pairing bad (micropitting damages it quickly)-> bearing or different material pairing has to be used to avoid pitting (next version)


Rubber Tubes (Theraband):
I did not expect that massive temperature influence.
Efficiency drops extremely in cold temperatures.


Efficiency:
I’ve seen many crossbows with reasonable power output and speed.
(Often brute force -> no way to operate by hand)
Some products also have good efficiency with heavy projectiles.
But hand operated (1Hz shooting frequency) devices with serious power output are rare. (only Legolas compound bow,…and so on which are bulky)
And If you look for compact designs its getting even harder to find anything usable.

As you can see from Jörgs videos, the efficiency decreases rapidly with lower weight arrows -> thus speed increase with handoperated devices is quite tricky.
Unfortunetly efficiency is not often discussed in forums or videos.
Most people speak about “max drawweight” and “speed” as they are in direct relation to each other, - which is a little bit oversimplified,…

Cooperation:
It would be interesting to compare different systems with focus on compactness & efficiency with low weight projectiles. -> that’s what I am looking for to make very tiny but serious “toys”.
If there is anyone (ore any company) out there who want to cooperate for production of usable small but serious “toys” -> let me know.
I also tried to contact Jörg on his gogun Mail to make corporate video of this device (I don’t have youtube skills) but I guess I landed in the spam-folder.
If someone is interested -> I also have videos, but don’t know if I can put them here.

Best regards
 
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