Experimental crossbow - What should a crossbow look like?

[MachineCrossbow]

Hello,

I'm about to build a platform, which is intended to try different
arrow types and to be used as a crossbow, too. It may not have
any limbs, however. I have many ideas, and I think I can make
a crossbow comparable to the commercial ones, but I don't
know exactly, what are the general user's preferences for a crossbow,
what's important for them. It's not a commercial survey,
I won't get any money from it, it's just a study. I'd like to
build a practical one.
I most probably can't make an acceptable repeating or self-loading,
self-cocking crossbow, so I left these possibilities out. Also,
my aim is not to make some rifle substitute gun, but
to make a good crossbow, if possible.
Preliminary calculations show that well over 400 fps is achieveable
@420 gr, even more with advanced materials, I just can't tell which
is better: lightweight crossbow, long crossbow with low draw
weight or short crossbow with high draw weight(crank).

So, what are the most important
properties of a crossbow? If you could change one property
of a common crossbow, which one would you choose most?
Any opinion is welcome. Videos will be linked in as soon as
the thing is ready, in 1-2 weeks, maybe.

 

[MachineCrossbow]

I'd like to add one or two more questions:

Assume just for now that accuracy is independent from
-wind
-range (speed loss)
-moving target

Just taking arrow drop, is it OK to say that
if the same drop happened at X times longer
range, then you would perceive the useful range is
X times longer? It may seem to be a stupid question.

I was thinking about how increasing the speed affects
the range solely through less arrow drop.

The good news is: (arrow drop)= grav*(flight time)^2*0.5
But (flight time)=range/speed, which
turns the former to : (arrow drop)=grav*(range^2/speed^2)*0.5
that is vacuum arrow drop is inversely proportional to the square
of the speed at a given range.

k times more speed means k^2 less drop.

(Firearm owners may also answer this question.)

Second question:
How important is the traditional bow shape for you?
Bolts?

Thanks.

 

[SeekHer]

Please verify one small point for me--are you talking traditional limb or compound crossbows?

Also, intended purpose would be nice to know since you can get bows that will exceed 300 yds in flight but are inaccurate at that distance (plunge fire) and have no clean killing potential other then accidental and damned unlucky target/victim.

I shoot both kinds and in traditional we even have replica medieval wooden bows with wooden or fiberglass or steel limbs.

We have a 165# draw that will propel a 400 gr 20" bolt at 330 fps...We have a 175# with arrow speed of 340 fps and another one with 300 fps...A 185# will get me 345 fps.

A Darton Lighting has details of 365 fps with a 475 gr bolt from a 175# draw bow yet their Serpent produces 346 fps with 425 gr bolt at 180# draw and 354 fps with 400 gr bolt at 180# draw yet the Excalibur Equinox only produces 330 fps with a 225# draw.

Longer limbs will produce a quieter let off and usually a more accurate bolt but that can be compensated for by using more pulleys in a compound, with a shorter width but then tension resistance and friction over the rollers comes into play.

If you can get a 475 gr bolt to 400 fps then you'll extend effective/efficient/clean kill hunting distances to 75 maybe even 80 yds but that's the limit you'll get as momentum and energy drops so drastically after 60 yds but the added weight of the arrow will aid in penetration.

As to the poll, I wish you would have included an "All of the above" button.

I wish you good luck with your endeavour and please keep us posted as to the outcome.

 

[MachineCrossbow]

Yes, I meant compound ones. The purpose is diverse, not exactly
for hunting, but I don't want to exclude this, I wouldn't make a crossbow
with limited capability in any way. Then we can say it's for hunting, or target
shooting, because these require the most demanding crossbows, I think.
I checked those crossbows you mentioned (Darton Lightning, Darton Serpent,
Excalibur Equinox) on the net, I see, the bigger crossbows are more efficient,
especially with a cam: easier to cock and faster.
At first I wanted a "Please order these options by importance",
but didn't have any idea how to do it with this type of poll, so I stayed
with "Choose the most important one".
I included the "less drop with same range/speed" in the options because
I felt the drop might be lessened by constant in-flight lift.
Difficult, because of the conflicting factors of
lift and stability. But it would be very nice to see the arrow as it floats
away.
From your reply I guess the number one limiting factor of the range
is the drag, before the drop. It's possible to increase the sectional density
of the bolt while retaining the same weight, and it also positively affects
penetration. Of course, the projectile may or may not lose its arrow-like appearance, and wounding potential may also suffer.
I think the main point is to "ease the conflict" between wounding and
"less drag", to change them somehow independently of each other.
(I have no-BS ideas, but I'm scared like hell, because they're not patented.
But how to talk about them then? Stupid situation. Maybe
I should pretend to be a totally insane retarded idiot jackass. I'm going to
think about this.)

 

[earlthegoat2]

I would like to see a crossbow that is thinner and quieter. I would also like to see a crossbow that is not hard on bolts, cables, and strings and is not a general maintenence nightmare. If performance is sacrificed to acheive this then I will have no problem with that.

 

[MachineCrossbow]

Thanks for both of you. I appreciate these opinions, they will be
considered.

 

[janobles14]

i have to go with "lighter." someone who can get modern performance from a weapon of half to even three quarters of the weight will get my business faster than the bolt might fly.

 

[MachineCrossbow]

Originally, I thought bolts/arrows were highly suboptimal,
only speaking about sectional density (Light and wide or heavy and thin?),
but now I see that the launch force and required
stiffness determine a minimal diameter
for the shaft, and the point must cut the way for the shaft,
so the point must be at least as wide.
So I think in their common configuration, the arrows/bolts
are close to optimal, so that improving them by small, "smooth"
changes is not possible. (Except one little add-on, more about it later.)
(Manufacturers aren't fools.)
Improving them in other ways...must be strange.

 

[MachineCrossbow]

http://www.excaliburcrossbow.com/content/arrow_ballistics

 

[MachineCrossbow]

Luckily people don't touch the normal length.

 

[MachineCrossbow]

i have to go with "lighter." someone who can get modern performance from a weapon of half to even three quarters of the weight will get my business faster than the bolt might fly.

OK, losing weight seems relatively less problematic. Even with safety margins. But with
a different outlook. BTW what weights 6-9 pounds on a crossbow? I suspect the main
load bearing element or its equivalent, which holds the draw weight along the "track".
It has to be stiff against buckling.

 

[MachineCrossbow]

According to the poll, people favor more compact dimensions over
performance. A more compact one can be as powerful too, but
with a higher peak draw force -> slower and more tiresome to
cock. I can make the peak draw force variable. How?
Here we are again. Somehow No, not like that.
Somehow ))

I also saw that you could use more accuracy. I don't understand
exactly how the accuracy is the bottleneck. Would somebody
explain it to me? Target shooting?
(4 MOA at 50 yds is 2", too big for hunting?)
Or you'd like to exploit the greater range?
It's OK, if you feel "So there, feel better with greater accuracy."

 

[dprice3844444]

http://www.swivelmachine.com/html/rimfire.htm

 

[MachineCrossbow]

http://www.swivelmachine.com/html/rimfire.htm

Yeah, I checked it. Really good! Looks like
I have to achieve similar specs. I didn't find
the weight of the arrows, but the speed is
a convincing 435 fps.
There is a compressed air
powered version, too.
Interesting:
.22LR - disallowed for big game,
arrow powered by blank .22LR - allowed for big game.
The heavier arrow has more momentum and a tipped, sharp
arrowhead, those make better penetration.
Range is less than that with a bullet.
Accuracy is 2 MOA, that's better than that of most crossbows.
Then it's clearly the launching mechanism what makes the
difference, because they fire the same projectile.
+speed, closed barrel -> more accurate shot.
I still think I can replicate this performance,
+ would shift the focus more
sharply toward compactness and lightweight.
My original intention was to make a projectile
testbed, reaching or surpassing existing
options was just added for passion,
but these values seem doable to me.

Thank you for posting!

P.S.
128 ft*lbs corresponds to a 315 gr arrow weight at 435 fps.
Website says energy is 128 and speed is 435.
With a 420 gr arrow and the same energy, speed
would be around 376 fps.

 

[MachineCrossbow]

How it started: I saw one of Joerg Sprave's videos on the youtube,
got interested, wanted a universal launch machine, which is smaller,
lighter, and capable of launching small projectiles at over 666 fps (
no evil, 666fps~200m/s), but deals anything bigger as well. (It withstands dryfire).
Then I thought I could make a crossbow, still able to launch small
objects, and also heavier ones, attached to the end of a pusher "arrow".

 

[SeekHer]

I also saw that you could use more accuracy. I don't understand exactly how the accuracy is the bottleneck. Would somebody explain it to me? Target shooting? (4 MOA at 50 yds is 2", too big for hunting?) Or you'd like to exploit the greater range? It's OK, if you feel "So there, feel better with greater accuracy."

The problem isn't the accuracy of the tool but of the user.

Hunting accuracy and target accuracy are the same thing--hitting an exact point--the difference is in the intent of the shooter and to some degree the size of the target.

2" at 50 yds is not too big for hunting--most shooters would be ecstatic about that grouping as long as it was centre...When hunting you wish to de-oxygenate the prey by hitting it in the lungs--hopefully low enough that the projectile will go through the lung and hit the heart.,,Even if you don't hit the heart the lung is a huge organ and a tear at the top, middle or bottom will accomplish the same thing...So accurately hitting the middle of a 2" circle isn't the foremost thought on an archery hunters mind.

There was some tournament archer years ago giving demonstrations about his shooting prowess by hitting centre of the bull at 100 yds--which he did consistently but the problem lies then in the ability of the arrowhead to penetrate deep enough to stick in a target or, if hunting, of course hitting an organ to incapacitate...To do so you then have to increase mass and/or velocity (preferably both).

The problem then becomes; to move that mass at the velocity necessary to make the clean kill the bow has to be larger, probably wider (smoother release) and definitely heavier...The Law of Diminishing Returns comes into play here...If I'm using a tree stand and the longest shot I'll ever make is 35 yds why would I want something that would kill at 100...Sure, it would be nice, but do I want to have a 60" wide, 400 lb draw, 25 lb crossbow in the stand with me but that would be the only place I could use it as I'd certainly not relish the thought of carrying it on a stalk.

Medieval castles had what were called rampart bows--similar to the Roman ballista and quite capable of hitting a target at 200 even 300 yds but they were huge, heavy devices that required two even three soldiers to cock--rate of fire was probably 2 bolts per five minutes and they were fired from bi/tripods or the like...While effective deterrents they weren't very accurate and any kills were strictly luck--good or bad dependent on which side you were on! .

 

[TimboKhan]

Manufacturers aren't fools

That's a good thing to realize early on.

You want to tinker and see if you can improve on things, I say more power to you, but crossbows aren't exactly new technology, and you are smart to realize that there are generally pretty sound reasons why certain things are done a certain way. Still, it is fun sometimes to really dig into something and figure out why it works and if you can see a way to make it work better. I mean, without that kind of spirit, nothing would ever get pushed to the next level!

 

[MachineCrossbow]

The problem then becomes; to move that mass at the velocity necessary to make the clean kill the bow has to be larger, probably wider (smoother release) and definitely heavier...

OK, I see, I agree. I don't want to solve contradictions, i'd rather circumvent
them. I'm really grateful to you for giving insight.

Still, it is fun sometimes to really dig into something and figure out why it works and if you can see a way to make it work better. I mean, without that kind of spirit, nothing would ever get pushed to the next level!

Thanks! I mean it!
I think I'm going to start doing the detailed design, and I'll make a launcher next week.

 

[MachineCrossbow]

...

Hi there, I did most of the design.
Today I'm going to measure the mechanical
properties of the materials I have (sheet metal,
fishing line, bonds, etc.)

 

[MachineCrossbow]

self-moderated

 

[MachineCrossbow]

Back on accuracy:

Earlier we discussed the accuracy, and came to that it's already satisfying.
Now I've found a source where the main problem with
accuracy is that some crossbows can become very bad at it abruptly. (totally unusable spraying, 40+ MOAs). They say the reasons are abnormal hop-up or asymmetric cocking.

http://www.excaliburcrossbow.com/phpBB2/viewtopic.php?f=3&t=2517

 

[SeekHer]

Those posts were from 2002/2003--lots of changes have happened since then.

Have you checked out International Tournament Crossbows--The Crossbowman’s Den has some info on them as does Ausbow Industries--totally different beasts then hunting bows.
The National Crossbowmen of the USA
World Crossbow Shooting Association
International Crossbow Shooting Union

FITA - International Archery Federation -- Oversees all

For Regular, hunting bows try:
Parker -- What I shoot
Tenpoint -- What the wife shoots
Barnett
Darton -- site is down
Excalibur
Horton
PSE

NACF - North American Crossbow Federation -- Hunting bows mainly.

Blogs:
Crossbow Review

That's all I have, I'm not into crossbows much!

 

[MachineCrossbow]

I checked those sites out. I linked that forum conversation
in because I wasn't certain if those voted on accuracy wanted
more normal accuracy or more dependable crossbow.
Learned from the websites:
Crossbow accuracy is strongly affected by the steadyness of
the weapon during the shot, the balance of mass and recoil,
and they use Torque Flight Compensators (= +angular inertia (moment
of inertia)) to
address this. (+shooting from a rest)

Other thing I found:
At longer range, the degrading accuracy of range estimation
causes rapidly increasing elevation error -> more likely miss.

Robin Allen said on Crossbowman's Den that it takes up to a year to build a crossbow...

I hope he meant traditional ones
His idea of surface hardening metal parts comes in handy.
(He's a famous man! May he rest of peace. (Member of a Belgian
crossbow guild along with Custer?))

 

[SeekHer]

Other thing I found:
At longer range, the degrading accuracy of range estimation
causes rapidly increasing elevation error -> more likely miss.

Robin Allen said on Crossbowman's Den that it takes up to a year to build a crossbow...

Glad you enjoyed them...If hand building from wood the drying and layup would take the time but I wouldn't say one year to completion if you're doing it yourself--lots of guys have made them over a weekend...Now if he's talking a custom maker, then I could readily see that time frame.

Bolts/arrows at distance perform badly due to two things and two things only --even if you've used a laser rangefinder or a tape measure to get the exact distance and you're firing indoors where wind isn't a problem--but that's part of the first listed hardship below.

Resistance and Gravity!

Is the vane at the exact same angle on the shaft as the previous or latter bolts(s), exact same height, exact same density, is their a single strand or fibre or feather sticking out?
Is the shaft as well polished, any gouges, is it the exact same length and if it's synthetic material did it get any heat or cold the same way--inside of a bundle reacts differently from the outer edge?
Any chips out of the head--broad or target, is the blades at the exact same angle and weight, polished or oiled--even a minuscule amount?
Is the nock as deeply seated, is it smooth, any chips to the edge, exact same weight?

Every minute, little difference produces a slightly different variable to the flight resistance equation...Just as in BR/target shooting, what scores is consistency...The old adage of "Practice makes perfect" is bullcrap! If you're shown how to do something improperly, doing it more times only makes your error proficient but not correct.

If you take a 200 lb stationary crossbow with a 400# draw and bolted it to a concrete benchrest and used a remote firing device--no human error--to use to test velocity of different sized, materials and attached angle of the vanes and with different weights of shafts as well as heads, you'd be in for a surprise--variations of that, is how the arrow makers and fletchers proof their wares.

You would think that at fifty, seventy five and one hundred yards that all the same meticulously set up arrows would do a Robin Hood and split the arrow in half, yes it's happened, but very infrequently--reports of 0.5 to 1.2 percent are bandied about.

Nothing you can do about gravity! You can alter the flight's velocity/mass but all that does it make it fall a little further from or closer to you.

 

[MachineCrossbow]

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