Compound twinbow pistol (posts vs pulleys)

First topic message reminder :

Cheers,

I'be been thinking, on-and-off for the last 35 years or so, about building a small crossbow, guess this was triggered by a souvenir xbow from San Marino ages ago...
Anyway, the direction my mind is currently wandering is sort of inspired by the "Twinbow".
Being the first build I naturally started off with the fun parts without having a clear picture of the final design.
I used some discarded kitchen drawer handles I had lying around, they act as the stock and arrow rest, with a classic roller nut as release.
Tried it out with some rubber bands and everything seems to work fine (with that comparatively low load).

I'll assume the twinbow is familiar enough - since my draw is pretty long for such a short bow I'd like to add some kind of compound system - however since pulleys are a bit pricey for decent ones I was thinking of doing a POC using posts instead of pulleys.
Any ideas as to how much this will affect performance? (hence the thread title)

Build so far: Total length 32cm, probably using 30cm prods from spring steel 2.0x35mm (tapered ends to 15mm), riser fixed in the middle approx where the trigger is.


Am I way off in my thinking below, about 24cm draw giving roughly 4cm flex per prod end?
Prods almost straight at rest, middle post/pulley on the fixed riser. My thinking is that string would be approx 165cm total, draw 24cm should "consume" 50cm forcing the posts closer, so at each "crossing" the string needs to be (50/6=8 ) cm shorter, = 4cm per prod end. Good/Bad thinking on my part?

If so, it should suffice fixing the prods approx 6cm out from the body, keeping the width at 17cm.
However, that might be bad due to "compound angle" being too steep - thoughts?
BTW, that's the brass posts in the pic, threaded through with M4.


Any thoughts or feedback welcome //M

Mooki, about twenty strands to make a cable. Twist it good to compact the strands. Should only have to serve where the string nocks. Also ,that's where you join the cable. If you serve good and tight about 8 to 10 inch overlap should work.I just now seee your drawing. I thought you were continuose wrap. Terminal posts would make it easier to do.

Thanks for your time and effort 

Yupp, a continuous loop would've been nice, but I think a "classic" cable type with loops at each end should work - if I can come up with a decent attachment to the body (in the rear), a post or similar.

Hmm, 20 strands of B50 equivalent?
I was thinking 18 strands of #2.0 Dyneema, roughly half the diameter and strength of B50, should meet the recommended strength for a 35-45lbs x-bow - but might be a bit small in diameter...

My rough outline for making the cable was originally:
2 poles, 9 complete turns => 18 strands.
Set tension to 90 lbs and serve the loops.
Twist 50 turns (66" cable length), increase tension to 250 lbs over night.
Release tension, close the loops with serving.
Set tension to 90 lbs and measure length, adjust with twists.
Serve the middle, tight as I can.

Mooki, one thing to remember with using two seperate cable anchors is, both sides have to be matched perfect. It's no different than cableing a two wheeled bow. The pully wraps have to be tune perfectly identical for good performance.Your setup sounds right on, and don't forget you can adjust the length some by twisting.

Some progress, and a few setbacks...
I finally got my arse in gear and made a string, ended up with 28 strands so slightly thicker than originally intended yet in retrospect way too thin despite double-serving the part where it contacts the arrow..

Restrung the pistol and went out in the garden with my Chrony, brought 3 arrows of different weights.

Arrow 1, 18g (278 grains)
Shot 1 registered 49.17 m/s (161.3 fps)
Shot 2 registered "error" and the arrow is gone forever, pretty sure where it landed yet can't find it.

Arrow 2 and Arrow 3, only got misfires, tried 10 shots each yet not a single succeeded.
I'm pretty sure that the string is too thin and slides underneath.
The groove is 5mm and the shaft 8mm, I'm thinking I need to find thinner shafts (6mm) or possible reshape the nock so it catches the string.

Also found that I need to redesign the pulley holders, currently they cause the limbs to twist when the pistol is cocked. I've ordered some BB pulleys and will try to come up with a better mounting for the holders.

Anyhow, the one successful shot I got off gives around 22 Joules (16 foot-pounds)
I have to say I was at best hoping for a more constant output energy, but it seems I also gained quite a bit of power with the string change. Although just one arrow isn't much data to go on...
Kudos to c_sitas for the string suggestion, I'd have never thought...

Hopefully the BB pulleys, new pulley holder mounting and adjustments to the arrow nock (or thinner shafts) will give me a bit more data and possibly even a tad more power.

Not sure how clear it is in the picture below, but the string loops are aroud a single post behind the bolt holder.

Does the string skip over the bolt or under it? Does the bolt shaft sit in the groove at a depth where the string is centered on it? I'd definitely try half-mooning the butts first.
I hate to say it, because the overall design of your rails and release mechanism is quite elegant, but a single, central tooth to hold the string without initial contact with the bolt invites just the problems you're having now. A traditionally wide nut with the bolt nestled between two  teeth, butted up against the string, would be a lot more reliable. That would be a lot of rework, so I hope you find a solution to your current design. I don't see how thinner shafts would help- if anything, that would be an even smaller target for the string to strike squarely.
Good luck and keep us posted,
Gnome

Thanks for the reply!
Unfortunately the bolts sit rather high, the groovs is 5mm wide and the shafts are 8mm.
I'm pretty certain the string slips underneath. I've started halfmooning a couple of shafts to see if it helps, won't have time for a few days to do anything more though.

True about the nut, I went quick and easy but should have made the body much wider to accomodate a wider nut. I've contemplated re-doing the upper parts from scratch, and will if I can't get rid of the misfiring which is really annoying.

My thinking with a thinner shaft is that the arrow would sit further down in the groove, I'll see if I can find anything with 6mm diameter to try it out with and see if that helps - probably will need the half mooned butts even then.


A really dirty fix would be to add a vertical fin in the rear of the bolt that extends down in to the groove, it would at least make certain that the string can't slide underneath...
If the fin is sturdy enough and line with the butt end, it just might work... Think I'll add that to an arrow or two just to test it.

sorry I'm late to the game . what kind of string lock did you settle on.Do you have the same setup as your earlier pictures show. I'm thinking your hold down for the arrow has the pressure point way to far forward. Most string catches have a double finger holding the string . The arrow set between the fingers , and tight to the string. In your case I think the hold down is too far forward and maybe it doesn't push the arrow down hard enough. I would change that so that it would set barely clearing the release, and hold the arrow firmly down to the track.Curving the back of the arrow is a good thought, That hold down should cover the junction of the trigger finger and the arrow quite firmly. My thought is , with the two finger grip on the string, the arrow can set between the fingers , tight to the string.  Not possible with one finger. One crazy thing just entered my stupid mind.Does your string ride hard on the rail? If so , try moving the arrow about  two inches or so in front of the string finger.  Also be careful when playing like this , WHO KNOWS WHAT CAN HAPPEN. I made a trap for my experiments. It just amounts to firing the bow in a confined  box. The more I think about this, I would try moving the arrow forward before trying anything else. You have to use your own head with things, if it don't make good sense , don't do it. If it still misfires that way, it shouldn't be any worse than it has been.   Please keep me posted

Cheers c_sitas, I think I'm on a bit of a time delay
I didn't change anything (yet) so it's still the thin roller nut. I'll see how far I get without any major rework, but might build a second pistol or redo this one depending on the results.
My idea when using the kirchen drawer handles for the body was that the handles also formed the rails, any arrow needs to be thicker than the distance between the rails so as not to fall down between. Not sure how to combine that with a dual finger roller nut, unless I thin down the butt end of the shaft. Time for some contemplation with a cold 6-pack...

Interesting, hadn't thought of the arrow holder. Due to the vanes made of tape the shaft sits slightly above the rails so increasing the "slip under" tendency of the string. 
I'll see if I have any better/stiffer material for the arrow holder, the current one is made of rather soft steel sheet that is too easily deformed.

Indeed the string does ride the rails, the pulleys are slightly below rail level currently but when I redo the pulley holders they probably will end up slightly above the rails. Can't raise them too much or the roller nut won't lock on to the string properly.
I'll try your idea of moving the arrow forward a bit, along with the half moon butt and see if anything changes. Not sure about the weekend, I might get a bit of play time to make new pulley holders, the new BB pulleys are apparently on their way.

You can try making the bolt clip out of a steel hose clamp or a piece of PVC.

Maybe even just double  up the material for the hold ,just to try something. Most two finger string nuts are about 1" in width. I make mine like that with about a good 5/16" hole in them  for the arrow nock . I use the nock archery fashion and snap right on the string.I don't reccomend letting the string ride above the rails if your not using the snap on nock and a front mounted arrow rest, which I use also. I use the wisker biscut mainly because I can shoot from any angle and worry about my arrow falling off.I think your string release should work fine if we can just figure out how to catch the arrow.This just isn't a common way , therefor not a lot is known about what we are trying to do. New ground to me , I am 76 and still trying figure things out. I 'm slightly thinking that the extreme pinch angle might have something to do here.Just to try something , a person could just take one arrow  and instead of having a flat end on it, make it kinda match the string angle.Just shooting in the dark  here but, you have to try something. Who knows we might even learn something here.

How about use a d loop? I feel like that would solve your problem.

See, now people are staring to think. A good suggestion cause you can make it any width. AND, most of all , it is easy to try.

First off, thanks a lot for the continued support!
Excellent batch of new ideas to try, I had to google the D-loop but that sound like it could work without any other adjustments apart from making nocks for the arrows. A better arrow holder is also on the list of things that should be changed, since I'm not happy with the current one, a piece of PVC and a heat gun sounds doable...

Just a thought here ..... suppose the thickness and possible residue  from the
      duct tape fins  as it goes around the shaft of the bolt cause a friction on the rails 
      and become a drag. The front part of bolt rides on the rails the rear rides on the
      thickness of the duct tape .... the fins should come directly off the sides of the bolt shaft
      thus eliminate the possibility .............. maybe ?

       Chaz