Compound twinbow pistol (posts vs pulleys)

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

I can't claim any particular knowledge about pulley-bows versus posts, but I will suggest the great advantage of pulleys over posts would probably be the reduction of friction exchange for greater weight.  Geezer.

Thanks for the reply.
Decided to go ahead and see if posts are good enough - short answer = No!

Definitely a lot of losses with posts, way more than I expected.
Quite noticable when spanning, only the forward prods flex voluntarily.
It's necessary to manually compress the rear prods to get some kind of balance, and making it at all possible to cock.

Started off without tapering the limbs, so the bend was noticably focused around the middle.
A quick trip to my workshop and some angle grinder time fixed some taper, but I should probable have used a more aggressive taper since the bow shape could be prettier.
As a side note, access to snow helps for cooling the metal when cutting to avoid overheating tempered steel 
Also added a retention for the bolts, and ran into a bit of trouble with it being in the way for my stubby fingers...
A couple of shots with some chop sticks as bolts, I really need to get some 8mm staves to make some better bolts...
Haven't measured draw weight yet, also some kind of spanning aid feels more or less necessary, but most important would be some pulleys for performance.
The way it is now, it's more of a kid's toy - which admittedly should suit me.


Measured the distance between the front and rear posts, at rest and when cocked.
Rest: 180+180mm (front+rear)
Cocked: 140+145mm (front+rear)
Change: 75mm (compounded 6 times equals 450mm, draw is 21cm)
Calculations give 443mm which seems close enough (to 450mm) considering measuring errors.

I like it!

Thanks 

It's now disassembled (again) waiting for some 25mm "Bemalon 1000" round stock so I can carve out some pulleys.
I noticed that the (rather flimsy) aluminium prod-support shows signs of deforming, hardly surprising since all the pressure is transmitted through an M4 with a small washer.
I'll probably place the prods on the outside of the support when assembling again.

I've also discovered that it works quite well as a Nerf gun, been popping off those little bolts all around the living room, surprisingly not every family member is a fan 

I know I have a Chrony M1 somewhere, once I find it I'll do a comparison between posts and pulleys to get some data. Might not matter to anyone but myself, but I find such things quite interesting...

Mooki, I'm pretty intrigued by what you're doing here. I didn't notice at first but this is a sort of "reverse" Twinbow, you're drawing back through the length of the bows instead of away from it, making the whole contraption more compact. I'm not too sure your construction methods will hold up to any serious tension, but as a testbed project, this is very interesting!
Gnome

Thanks Gnome.
Since the thread has gone a bit astray I changed the title, hope nobody minds.

I had actually forgotten that I changed the draw compared to the Twinbow, this build is rather aimed at being compact so I felt it was a natural mod.
True about the construction, I hope that moving the prods outside the support, possible also trying to spread the pressure a bit, will help out.
During the weekend I've made 4 pulleys, ate least 4 more to go - but I'm thinking I might drop the midway-pulleys all together. Shouldn't really matter as long as the string is clear of interference.
Also used the angle grinder on the prods, adding a little bit of taper and extending the tips, now at 25mm to allow for better "pulley mounts".


Current prod dimensions are:
Length: 300mm
Width: 32mm
Thickness: 2mm
Center: 25mm (un-tapered part)
Taper goes from 32 to 14mm where the tips start
Prod tips: 25mm long


Will update with some more pics as I stumble along...   
Not very pretty pulleys, but hopefully functional.

So, some updates.
First of all, posts really suck compared to even fugly pulleys like mine.
Using the pulleys, flex is evenly distributed throughout, also the shooting is much "snappier" so performance seems highly affected.

My little manufacturing setup for the pulleys below:


After reaming out the center hole to 5.5mm for each pulley they were mounted where the posts previously were.
I decided that skipping the middle pulleys was probably a bad idea, so pulleys all around.
Looks like making the shafts 25cm each should work well, I'll probably do something bodkin like for the points, not sure about fledging - if I'm gonna go bare, 2 or 3... I'll probably try a bit different styles and see what works.

Looking better all the time! The pulleys look great, a lot better than my first go at making them.
I say yes to fletching. The way it is set up now, two would be easiest, I think. It looks like your rail would clear for a third vane, but your bolt clip would squash any that were angled up that way unless you narrowed it down to less than the width your bolt. I bet your shooting will get a bit snappier still when you remove all the unnecessary weight from your pulley mounts.
Gnome

Thanks Gnome, always good to get some feedback  
You're right about the bolt clip, it was intended to narrow down but I forgot before riveting.
If I narrow it a bit I should be able to use vanes up to 12mm height (if 3 vanes).

Currently it's all in pieces, again...
Ran into a bit of trouble with my roller nut, I built the trigger system using 2mm steel I had available - however the distance between the rails is 5mm, recipe for disaster anyone...?
Worked well enough before the pulleys, I think I now get some more draw, so the trigger components are misaligning due to force - spontaneous ejac... sorry... release.
Good thing I was still using dull plant supports.
Currenly making a 4mm thick roller nut, probably also the release arm will need to be redone.
Mean while I'm trying to come up with a simple bolt design.

And, some pics of course - no thread is complete without loads of pics  

Used some scrap pieces I had laying around, some 25x4mm steel and 10mm copper tube.


Soldered the copper to the steel after a quick visit to the bench grinder.


Voilá! Not totally happy though, depending on which method I use I get F.O.C. 24 or 30, quite high...
Some vanes might improve it a little bit, not sure where I want it, but quite probably way lower.
I can only imagine using a bodkin tip would increase it even further, I'm unexpectedly fond of the look of this one.
Hopefully thinning it a bit is enough to get F.O.C. down, together with adding vanes.


Old and new roller nut.


Had to remove the stop from the nut, still works quite well. I currently use some 0.8mm 316LSi TIG wire for springs, seems to work well.
Some work remains to get the angles of the mating surfaces right, but it's getting there.

Since the weekend is upon us very soon I thought I'd add some more pics.
Re-did the trigger system a bit, removed one of the springs and changed placement of the remaining spring.
The result is much more secure thanks to the new roller nut, no tendencies to the pistol being "trigger happy".



Weird thing with the arrows (at least to me having more or less zero experience with these kind of things). To me an arrow point should improve penetration - but when shooting just a plain shaft it blew through the box and kept on going skipping around, while the complete arrow got stuck in the middle only penetrating one side of the box.
I suppose thinking about it this could be due to two things, the arrow head requires a larger hole to pass through, and the increased weight exceeds the optimum quite a bit - so less energy is transferred to the arrow.
My long term plans will be to try different arrow weights and measure the energy, the weight with most energy should be best suited for this particular bow. Thoughts about this?
So far I've tried 4 different weights.
Short shaft, 6g (sharpened tip)
25cm shaft, 8g (flat tip)
Arrow 1, 16g
Arrow 2, 17g
Best penetration I achieved with the 25cm shaft, so I guess this means that 6g is too light, and probably 16g is too heavy.
Measuring speeds and calculating energy should give more detailed information.

*** Edit to the above ***
I found that increasing the pre-draw (is that even a word?) made all arrows just blow through my cardboard box like it was rice paper. I should probably find something to put over the nick one of the tipped arrows made in the floor before my SO takes notice...


Below the 'super duper penetrator shaft' and the regular bolt.


Decided to trim the arrow head a bit, and also try a new one from 5x15 steel and brass tube - although that is probably the wrong way to go, my third try will be from som 2x15 steel.

Below the 4x25 with copper pipe arrow head (trimmed down) and the 5x15 with brass tube arrow head.
The weight is almost identical, 9g for copper pipe arrow head and 8g for brass tube arrow head, the shaft by itself weighs 8g.
Totals 17g for copper pipe (=262 grains) and 16g for brass tube (=247 grains).


So, back to the infamous F.O.C. I mentioned in my latest post - I've seen 3 different ways of calculating this.
Method 1: Measure the complete arrow from nock to tip (=27cm)
Method 2: Measure the length of the arrow shaft without head (=25cm)
Method 3: Measure the arrow to the point where it enters the arrow head or where the insert starts (=24cm)
They all use the same way of measuring from Rear to Point of Balance (19cm for brass, 19.5cm for copper)
The different methods give widely differing results.
The most popular (or maybe most widely accepted) seems to be method 2, but to my mind method 1 seems most logical.

Brass, method 1: F.O.C. = 20.4%
Brass, method 2: F.O.C. = 26%
Brass, method 3: F.O.C. = 29.2%
Copper, method 1: F.O.C. = 22.2%
Copper, method 2: F.O.C. = 28%
Copper, method 3: F.O.C. = 31.3%

So, I have for Brass a F.O.C. of 20.4-29.2%, for Copper a F.O.C. of 22.2-31.3%
Leaving me very confused... 

Still trying to figure out where my fishing scales are hidden, until then I don't really know how to measure draw weight.
This weekend was partly spent manufacturing some bolts with different weights and head designs.
But, first I took the two finished bolts and tried the out on a piece of plank I found in the shed.



At least they got stuck, but only if I hit with the head parallell to the grain, otherwise they bounce.
I think the tips need to be lighter, thinner and possibly more pointed.
Found the cut-offs from the prods and thought they looked useful.
Unfortunately I didn't get the time to try them out, somehow family keeps getting in the way of pursuing hobbies...


I fixed the tips using hot glue, and reinforced with some fishing line coated with epoxy.
After weighing the new arrows they are still a bit on the heavy side, so I also made some with just copper pipe points.
So far I have arrows ranging from 6.5g to 19g, different heads and gaffer-fletchings.
I'm not managing to get the gaffer-tape quite centered, not sure how much the offset will affect flight, at least they're parallell to the shafts.

Mooki, you are on the right track. Just keep looking for ways to raise your power stroke. Looking at your design , there might be ways.If they would be enough remains to be seen. Keep up the good work, you seem very handy with the tools you have.

Thanks for the cheery comment
Been a while since I had anything to add, so I have refrained from updating my post.
This weekend I finally located where my Chrony had hidden, I'd apparently left it at my parents cabin a few years back when I was testing different pellets in my air rifle.
Took it home with me and did a (very) quick check in the backyard.
Seems it's more of a toy than I thought.
First of all it's extremely uneven in performance, possible due to the pulleys.
With the same arrow I got the series [8.5, 4.7, 4.7] Joules.
I much prefer the first shot, unfortunately I missed the target so couldn't check penetration.
Another arrow gave me [1.2, -, 4.1] Joules (one shot didn't trigger the Chrony.

My aim was to try to map which arrow weight gave my the best energy transfer, but with the uneven performance I'm not sure it will be of any use.
I didn't have the time to test all arrows thoroughly, so I'm still lacking a lot of data, but so far it seems the heavier arrows retain more energy - unfortunately their flight characteristics are similar to those of a rhinocerous 

I'll see if I can figure out why the performance is so uneven, and also make a more complete measurement series.

Mookie, I know you haven't been around lately but, I thought I'd see anyway. I'm wondering if you ever figured out why your speed was so up and down with your chrony?

Hi c sitas,
Sorry about the late reply, I haven't totally forgotten but other projects keep getting in the way.
As far as I could tell it's the pulleys themselves - seems my free hand center drilling capabilities are sub-par, a couple of the pulleys rotate unevenly due to the holes being off center.
Did manage to get my hands on a small wood lathe that I'm thinking of using to try to get some better aligned pulleys. Might as well redo them all from fresh stock.
Somewhat thinking of adding some small ball bearings as well, not sure - but I imagine it would be better than without.

If you're planning to make wooden pulleys, see if you can get some lignum vitae.  It was by far the preferred wood for ship rigging blocks in the 17th century.  Strong, hard, self-lubricating.  Geezer.

Mooki, The first place I would  look would be to replace the para cord your using with something else. The shot figures you give makes me think that. With para cord ,the more you use it , the more it stretches. Any form of nylon would be noooooo good. Dacron bow string would be a better choice. Dynima bow string would be perfect.That is what all the modern high perf. stuff use . Absoulutely no stretch.I really do think the cables are to blame.The fact that the first shot, after a long rest, is that much higher is what I base my thoughts on.If you get time pm me please.

Try fast flight that stuff has absolutely no stretch.

Yes: fastflite is Kevlar. It's the best available. Geezer.

It has some draw back also. For a noobie it has a steep learning curve to handle the stuff. Also for a one off shot it's expensive. Ordinary knots won't hold either. In this racket it's kinda like dammed if you do ,and dammed if you don't. I been using dynema  for bow cables and strings ever since it came out.It's an industry standard now.Both regular compounds and crossbows.One thing to remember is don't ever just try to pull and break a single strand, you'll cut your self right to the bone. Fishing line is bad that way also.

Braided fishing line I think people also use.

I'll tell you a story about braided fishing line. That stuff is almost like steel.  I've seen several fishing rods broken because of a real hard yank, such as trying to free a snag. But, on the same note , while fishing in rocks and get a snag, just a slight jerk across a rock edge , and bango, gone.   Can't cut the stuff with scissors but gods creation will do the job. It will work to make a bow string though .

Light pound test monofilament will slice your fingers or arm up like a knife, I've been trout fishing enough times to be able to tell you that. Braid would make monofilament look like nothing. I would not fish with braid to be honest unless if I was wearing gloves and long sleeved shirts and pants.

Loads of good feedback, cheers and thanks.
Didn't even consider the string, but it makes perfect sense.
Incidentally one of my other hobbies is fishing, so I've got a bunch of braided dyneema.
Hopefully it will allow me to verify that the paracord is the culprit.
Sound like I'll be twisting some string soon, saw some tuts on u-tube that should see me started 

Seems I inadvertently caused a bit of confusion, I intended to use the wood lathe to shape the pulleys from the same round stock as before (nylon-ish material), a bit of a PITA using the bench drill so was thinking the lathe would be more suitable.