Medieval Style Crossbow, Rubber Operated

[JoergS]

I have made many slingshot crossbows, but all have been long and narrow - more like a modern rifle.

But this time I wanted to make a slingshot crossbow that resembles a medieval style model! Of course with rubber instead of a steel prod, and shooting balls instead of bolts. But regarding the look and lock/trigger mechanics, I wanted to follow the ancient design.

It came out very good!

Here is the video:

http://www.youtube.com/watch?v=1fllqaJax88

It shoots great, and as you can see in the slomos, the bands never touch anything - they will last a long time.

The weapon is really accurate.

Jörg

 

[Tennessee Ned]

It appears accurate and powerful. Awesome as usual.

 

[M-Cameron]

that looks fantastic......the only thing it needs is a few coats of wood stain and then it would look REALLY nice.

 

[goon]

Can you show some close up pics of the trigger mechanism and how it mates up with the pouch? This is something I haven't seen before, but I have to admit it's looking cool!

And FWIW, there were some medieval crossbows made to fire stones, but I think they were meant for hunting and not for battle. Search here:
http://www.myarmoury.com/albums/

 

[glistam]

I have a mechanical question. With rubber as the powerplant on devices like this, do you have a greater mechanical advantage if the "prod" or "fork" is wide, or close together? I'm trying to figure out if a rifle configuration like this is more efficient with wide-set points like the prod of a bow, or close together like a speargun.

 

[M-Cameron]

I have a mechanical question. With rubber as the powerplant on devices like this, do you have a greater mechanical advantage if the "prod" or "fork" is wide, or close together? I'm trying to figure out if a rifle configuration like this is more efficient with wide-set points like the prod of a bow, or close together like a speargun.

mechanically speaking...the closer the forks are....the more efficient.....as more of the energy can be transmitted along the length as opposed to outward towards the forks

 

[glistam]

Interesting. Thanks.

 

[JoergS]

It is true, wider forks are less efficient. However, if you want to keep the weapon short, a wider fork reduces (even eliminates) the dead play in the bands, which increases efficiency.

My design even pre-stretches the rubber a bit. To get the same effect with a narrow fork, you would have to make the weapon 70 cm longer. It currently is exactly 1 meter long, which is reasonable. 1 meter 70 is very long, and it would be hard to cock the weapon with the stirrup method.

 

[MachineCrossbow]

Hi, Joerg!

 

[MachineCrossbow]

mechanically speaking...the closer the forks are....the more efficient.....as more of the energy can be transmitted along the length as opposed to outward towards the forks

Are you sure?
Conservation of energy says "no".
Horizontal forces don't even transfer work on the projectile.
They don't have displacement for the projectile in the direction of the force: no displacement for
a force, no work done: E=0=Fs=F0

Einstein said "A problem well-stated is a problem half-solved".
The answer to the original question is: The wider the fork,
the greater the pouch speed (lower mechanical (force) advantage,
higher "speed" advantage). The stretch of the rubber is not
a "waste of energy", again. It's being transferred to
the projectile by a certain levering action.(Assuming now the rubber itself has no weight.)
This is the very mechanism the bows work by: they magnify the
speed of the limbs by the special geometric arrangement
of the drawn string.
. See force parallelogram.(Conservation of energy-> smaller force-> bigger displacement->bigger speed)

 

[M-Cameron]

Are you sure?
Conservation of energy says "no".
Horizontal forces don't even transfer work on the projectile.
They don't have displacement for the projectile in the direction of the force: no displacement for
a force, no work done: E=0=Fs=F0

pretty sure.....


the 2 Fx forces will cancel each other out, leaving only the 2 Fy forces.......which equal F Sin (theta)......so the closer the angle is to 90* the more of their force they transfer to the projectile........

so the efficiency is calculated by the total force of the bands compared to the total force imparted to the projectile.


unless i screwed up in my math ( which is very possible, it is friday)

 

[MachineCrossbow]

HI.
Since then I made edits to my post, please re-check.

 

[M-Cameron]

the problem with the Bow analogy, is that a bows power is a direct relationship with the length of its limbs, so the longer the limbs, the more powerful the bow, and inevitably, the larger the angle........but with a slingshot, all the power is stored in the bands, so any angle off the direction of travel of the bands is effecting the effiency

i think Joergs next project needs to be a slingshot with adjustable width forks......that way we can quit the theoretical math and find out for sure.

 

[MachineCrossbow]

I know that the y net component acting on the pouch is smaller,
but this is not directly related to efficiency, nor energy, or work.

OK, in other words (density of rubber = zero for simplicity, we can
assume it if projectile weight is big comp. to that):

Where could the elastic potential energy of the rubber go else than
the projectile? In any configuration? According to the conservation
of energy, all the energy is being transferred to the projectile.
The apparent sub-100% efficiency of the bow comes from that
the bow accelerates itself too, being heavy, this requires waste
energy.

 

[M-Cameron]

Where could the elastic potential energy of the rubber go else than
the projectile? In any configuration? According to the conservation
of energy, all the energy is being transferred to the projectile.
The apparent sub-100% efficiency of the bow comes from that
the bow accelerates itself too, being heavy, this requires waste
energy.

it is transferred back into the slingshot, and into your arm.

 

[MachineCrossbow]

I don't think he'll do one, he doesn't like this kind of experiments. That's
OK, by the way.

 

[MachineCrossbow]

You're kidding
This idea wasn't even in your pic
The mount is supposed to be perfectly rigid and
to have infinite inertia. A good approximation.
(Don't worry, I did the same thing to my university teacher,
I got on his nerves.)

 

[MachineCrossbow]

the problem with the Bow analogy, is that a bows power is a direct relationship with the length of its limbs, so the longer the limbs, the more powerful the bow, and inevitably, the larger the angle........but with a slingshot, all the power is stored in the bands, so any angle off the direction of travel of the bands is effecting the effiency

Holy Buddha

You know...you're right. I apologize.
You were right in everything. There ARE
flying saucers. The alien invasion is
inevitable. Zombies! Zombies!!

("Why so serious?" Joker, The Dark Knight)

 

[M-Cameron]

^^^^^..........umm....i dont even...???


What?

 

[JoergS]

Folks, the great Richard Middleton has already tested this. Read it up in his groundbreaking book "Man powered bullets".

http://www.amazon.com/Practical-Manowered-Bullets-Richard-Middleton/dp/0811701565

Narrow forks ARE more efficient. This is my observation also.

However, that can be compensated with stronger rubber.

The wide forked crossbow has three advantages that help, too.

1. The wide fork allows me to prestretch the rubber. The weapon is a lot shorter this way.
2. The shorter length allow me to use the stirrup cocking method, which makes loading sooo much easier. I can use much stronger rubber this way.
3. The prestretched rubber bands can be slipped off and replaced with a new/different one in seconds, no tying necessary.

I did work on the concept some more.

Ordered some speargun rubber.

Got 16, 18 and 20 mm thickness, a meter each.

The stuff is beastly! I decided to test the "weakest" one first. 6,5mm walls, compared to 3mm walls on the already tough Thera Tube silver. Here you see TB black, silver and the 16mm speargun rubber in comparison.

Did a bandset for the medieval crossbow. The center hole is too small and the rubber is too tough for the knot in hole method, so I attached the rubber to the outside. Works fine.

Draw weight of this exceeds the 40 kg my scales can measure. I estimate it at 45 kg (about 100 lbs).

I reinforced the nut and tickler with steel because I did not trust the wood on wood method for this off the scale power. It holds fine.

It is surprisingly easy to cock the crossbow, the stirrup method rocks. Ancient weapon makers clearly knew their stuff.

It shoots absolutely frighteningly fast. My most powerful sling-x-bow for sure.

But there is room for improvement - see the 18 and 20 mm beasts in comparison... stay tuned.

Jörg

 

[MachineCrossbow]

OK, I won't argue with you, Joerg.
But with Cameron...

 

[MachineCrossbow]

^^^^^..........umm....i dont even...???


What?

I r o n y.

I guess all the engineers around the world should be
fired, they're useless, because they calculate before build.
Cars, airplanes, guns are just grown on farms,
unless built from LEGO by trial-and-error.

 

[MachineCrossbow]

M-Cameron:

Here's the proof.



l= the total loose lenght of the rubber band
K=spring constant of the rubber band
F (not F(s))= maximum rubber load (one band)

No link, no book,
no "my experience".
I think the alleged change in efficiency in
practice is due to that many things
are changed simultaneously
with the fork width -> no clear experiment,
loss of causality.

 

[JoergS]

Well, I am not a physicist (though my brother is, maybe I should ask him).

Mr. Middleton and myself have both tested the influence of such a wide fork (that eliminates all dead play in the bands). We both got the same data - about 25% less speed with the same bands, same drawlength. I am not trying to explain why, and Mr. Middleton says there is "obviously some other effect" involved.

But as I said, that can be compensated for with stronger bands.

With the stirrup and the short enough draw (70 cm = 27"), it is easy to manage bands that only the "Hulk" and maybe the "Thing" can draw on a normal slingshot.

Clocked the thing with the 16 mm bands, 150 fps with the .87" lead balls weighing about 1050 gr.

Will fire it some more and then maybe check out the 18 and 20 mm tube.

 

[Mp7]

....

Now that one is good enough to actually sell!!!

some wood stain ... and maybe a rear sight ... and a fluorescent line on the tiller to line it up with target ... i want one!!

Nice Balester, Sir!