Experimental crossbow - What should a crossbow look like?

[SeekHer]

So that when shooting at bullseye-sized targets the arrow/crossbow accuracy is critical.

Accuracy is critical in anything--you don't want wounded game animals suffering so a shot to the "spot" is advised but if you hit a couple of inches either way they'll still be down.

What wins competitions is closest to the X with the tightest group -- nothing else...The only way that is done, is by consistency--in arrows, hold, breathing, stance etc.

 

[MachineCrossbow]

Accuracy is critical in anything

I used "critical" as the most important factor, which has the
most pronounced effect on the overall hit accuracy ,
as a bottleneck.
(For example: at 1000 yds, , arrow accuracy is not critical,
it doesn't make difference, because the drop is high.)
BTW this is largely academic, and I understand what you mean.

 

[MachineCrossbow]

In the meanwhile, I've measured material properties.
Sorry for the slow progress.
I found interesting results:

The elastic energy content per unit weight of rubber is 8 times more
than that of spring steel.
(No, I'm not stating that rubber is stronger.)

NERD WARNING

Formula: (Spec. Energy)=0.5*(sigma^2)/Y/Rho

Y: Young's modulus, "rigidity"
sigma: Elasticity limit (almost like "breaking stress")
Rho: density



So, rubber is weak (low sigma), but much more elastic (very low
Y), and lighter (low Rho) that's the reason. My rubber has
~5 kJ/kg=1707 ft*lbs/lbs.

(This is dedicated to Joerg, the rubber-slingshot-enthusiast.)

(Rubber is not linearly elastic, however, the value is an estimation.
The energy difference between steel and rubber is bigger than
the error coming from that rubber is not exactly behaves like
a spring.)

I have glass fiber mat at home, and glass is very strong (fiber!),
very elastic (as compared to steel), and comparatively light, but simply too
sensitive to surface scratches. So effectively it's weak when
exposed to any abrasion.

Strength of sheet metal turned out to be around 880 Mpa(=116 000 psi),
that's seems erroneously high for mild steel. I'm going to
repeat the measurement. BTW I expect around 400 Mpa,
and after considering fatique (cyclic loading) I could use
200 Mpa(26 370 psi), further weakened by safety factors and
stress concentrations at certain places. The
weight still can be low.
I collected many other material properties.

 

[Skyshot]

Interesting topic, After shooting several crossbows myself, I keep going back to the excalibur. For these reasons, 1. It's simple, I can do any repair in the field, with the exception of broken limb, which could be done if I carried limbs. 2. It's light weight. Most componds are a couple of pounds heavier, when hiking into backcountry mountains that's a lot. 3. It's durable, I've dropped it out of tree stands and fell on it while dragging a deer, no problem. My con's are, 1. Trigger pull could be better. 2. It's a little wide. As for arrow speed as long as it can send a bolt at 300 fps. is good enough for me. I believe the broadhead is the key to anchoring the animal. What I would like to see is the crossbow be more modular, as in the ability to change out components for particular types of game hunted or for personal fit and feel.

 

[MachineCrossbow]

Picatinny all over the cover?

"Groovy, baby!" (Austin Powers)

OK, I googled "excalibur trigger pull".
The trigger is a little stiff (3-3.5lbs), and there's a big pull
before the let-off.

 

[MachineCrossbow]

Most rails I've seen on crossbows looked Weaver rail to me.
Is that right? If it is, why?

 

[MachineCrossbow]

Sheet metal yield strength is 59 000 psi. Quite strong.
Good for a testbed.

 

[SeekHer]

Picatinny all over the cover?

"Groovy, baby!" (Austin Powers)

OK, I googled "excalibur trigger pull".
The trigger is a little stiff (3-3.5lbs), and there's a big pull
before the let-off.

You can not equate the release weight of the triggers between the two platforms because once engaged a completely different method of activation enfolds...The only commonality is that the CB is a shoulder fired weapon with a somewhat same thingee sticking down that makes it work--that's it, no more!

Let off/trigger pull is nice to have at 3# to 3.5# as long as the draw (creep)before the sear/whatever releases the firing mechanism is short...My BR rifles have 2 to 3 ounce triggers and with nearly zero creep or take up but my hunting rifles have from 1# to 1½# for my varmint rifles to 4# for my heavy bush guns and for the most part they're at 3# to 3½#...I went and checked the trigger pull on the other CBs and they went from 2.875# to 4.35# and everyone like their settings--probably just used to them by now.

 

[SeekHer]

Most rails I've seen on crossbows looked Weaver rail to me.
Is that right? If it is, why?

Commonality since the W.R. Weaver Company set the dimensions (5/8" and 1" et al) for optics and optical accessories back in the 1930s & 40s--mounts, base, etc. and most companies followed suit and built to those specs...This way a guy on Safari in Mozambique can install an Egyptian made flashlight to his rail with just a multi-tool and know that it will fit...Also, it allows the mounting on any position

 

[MachineCrossbow]

Thanks, SeekHer!

And what about Picatinny rails? Why Weaver instead of
Picatinny? Is the Picatinny rather militaristic?

 

[MachineCrossbow]

Do cables break at the cams frequently?

 

[SeekHer]

Do cables break at the cams frequently?

We have never broken a cable so I can't tell you and those that I've seen have been mostly between the cam, on the limb side as opposed to the nock side.

 

[MachineCrossbow]

I'm still working on a good combination of trigger and nock-string interface.
Very difficult.(Many unusual requirements at one place).

 

[MachineCrossbow]

Stuck with that. (trigger)
I don't give up!
Basically, it's not so hard to make one, but I want such one which stops
the "limbs", not the string. The reason is to launch light projectiles
at higher speeds. In that case, the string load would only consist of the inertia of
the accelerating projectile and string, so I could use weaker, lighter
string. But only if I don't draw and hold the crossbow by the string.

...

...

 

[MachineCrossbow]

Thank you for the feedback (poll).

 

[alsask]

I have been shooting crossbows for close to 30 years. Forget the compound cross bows, what you need is at least a 50% increase in limb length and a 50% increase in draw length from what you see offered in the stores right now.

The longer the distance that the limbs are pulling the arrow the higher the velocity. To increase the distance [draw length]you need longer limbs.

The best commercial cross bows made are in Canada at the moment, but even the best are no better than a 40 yard weapon and that is with a lot of practice.

 

[MachineCrossbow]

I have been shooting crossbows for close to 30 years. Forget the compound cross bows

Too late...I've unstuck!

For weeks I thought the same, it didn't seem to work.
I made ideas one after another, for parts of the
crossbow, but they didn't fit together (too big, too long,
too heavy, one part makes the operation of the other
one impossible...I have details, but they are technical
and boring.) I wasted at least one notebook for drawings,
and terrible amount of time making a spreadsheet
model. I don't want to end up with a crossbow with
surprisingly low performance, just because I don't
know exactly, how it works.
Finally, I'm more or less certain about how
it should be done. The hardest part was to
find out the part what will be the trigger
and a smooth ratchet in one piece. (There were more.)
It's absolutely needed to be done in such way.
Now I'm happy, really!!!!

So after a starting over, I'm going to make
this anyway!

For you alsask, you may be right,
but I love this kind of science safari,
so don't ask me not to do that!

By the way, I think this thread has fulfilled
its purpose, there's nothing much more
to discuss about the original question.
After ready, videos about launching various
objects will be posted in a new thread.

Thank you!

 

[MachineCrossbow]

Happy fourth!

 

[R.Ganister]

There are a couple of reverse draw compound crossbows made by Horton and Scorpyd respectively, you might be interested in examining, if you are not familiar with them.