Pulley system -speed testing

First topic message reminder :

Rubber has an inherent speed limit which  for Theratube silver for example is at 55-60m/s.  If you use more rubber you can shoot heavier ammo but it will not go any faster. Very fast speeds can be had with extremely tapered bands and butterfly draw but these do last only a few shots. 
To get faster you need to employ some kind of mechanical advantage like levers or pulleys. But some of the MA is eaten away by inertia mass and friction from the added parts this will take.
I want to test different pulley systems for speed and see if there is a gain to be had. If I find something reasonable that works I want to build a rubber powered crossbow comparable in draw weigh and no larger in size than  commercially avaliable high end crossbows and hopefully get it as near as possible to contemporary arrow speeds. Lets see what rubber can do! 



I omitted the roller setup because with thicker rollers at front the string would have been riding too high over the rail. This would have needed a redesign of my test rig so I will do this last. I just build a simple stationary test rig from some scrap wood I had in the shop. Nothing fancy or elaborate  just enough to test some pulley systems on it.

I started the test with the 2:1 pulley system, here is the setup:


Spanned:



The Arrows:


Draw length is 70cm and draw weight on the trigger is app. 80lbs I use coil chrono to record the speed.

Results:

The 37g(571gr) arrow (equals a 21mm steel ball)  was shot 5 times and the speed recorded:

1.  221,66fps
2.  222,31fps
3.  222,46fps
4.  223,16fps
5.  223,24fps

Average: 222.56fps


The 22g(339,5gr) Arrow (equals a 17,4mm steel ball)

1.  272,73fps
2.  273,01fps
3.  272,84fps
4.  270,77fps
5.  272.56fps

Average: 272,58fps


The 12g(185,2gr) Arrow (equals a 14,3mm Steel ball)

1.  346,46fps
2.  348,28fps
3.  346,45fps
4.  345,83fps
5.  347,25fps

Average: 346,58fps




I am using Theratube silver. According to other peoples findings its max speed if used directly is about 55-60m/s =180-197fps 

So this shows that ist possible to get some more speed from a simple pulley system.
The gain in heavier projectiles is as expected - not so very much  but you can see that using lighter arrows the speed gain is noticeable.

This indicates that to shoot a large 37g arrow we will have to increase the draw weight considerably from what its now. But if the system was already maxed out or pulley systems would not work at all it would not have shot the light arrow at higher speed.
Try shooting a 14mm steel ball at 346fps using theratube silver directly- not possible I think  even if you could do a butterfly draw with it.

You may also notice that I did not stretch the rubber to its maximum length - I only get about 170-180% elongation from the slightly prestreched loops here. 

222fps for a 571gr arrow is a decent speed but not overly impressive so far. But keep in mind that high end crossbows  usually range in between 180-200lbs in draw weight to shoot  very fast arrows.  We are nowhere near that here.


It would be nice to test this setup with 200lbs and see what it can do then, but unfortunately  the ugly test rig is nor sturdy enough to handle such draw weights. I am hesitant to try  because bad things may happen.
I will probably increase the draw weigh  about as much as I dare but  will do the other tests before.

The 346fps on the lighter arrow was nice but now we need to do this with the large one At the moment with this setup its more like the "raptor" slingshot - only able to shoot light ammo fast. But I think we can shoot even heavier ammo much faster if done right.

So there is room for improvement. I can use higher draw weight, shorten the string or the rubber a bit(there is still some room) or I could use Theraband gold which is reportedly faster than slow silver tube. 
But first I will do some more tests on the other pulley setups to see if we can get better than the 2:1 without having to resort to insanely high drawweights.


Using pulley systems with hand drawn slingshots is  in my opinion not a viable way to use but employed in a really powerfull (150+ lbs) crossbow I think they can really increase the speed .
These are my first findings on the subject of rubber*pulleys and I am curious what the next one will do  


Best Regards

great tutorial on lathe-less pulley making, thanks

rolynd wrote:Well, I actually have tried it , it looked like this:


Maybe a little more if the draw length were the same but  the increase is not as great as I expected.

OK. Problem is those fixed router pulleys. This is not the Ming set up. The front pulley should be moving backwards - that is what gives the leverage.

Your set up above needs longer string and the resulting fast moving extra mass is ruining the efficiency.

Fix the fatter rubber with pulleys to front and thin ones back. Run the string only through the moving pulleys. This way the string will be shorter & lighter.
This naturally means some 15 cm idle length in front of the system, so you may want to move the bolt track back to get full power stroke.

Yes, I get you, its not exactly the setup  as in the chuanzi nu because of the frontal rollers.

This is what you talk about I guess: Green= string, red = rubber



It would do away with the frontal rollers and uses a shorter string.  

Well, it does not work on my test rig because with this setup the distance between the frontal attachment points is too small. It would work if  I could widen the frontal "fork".  If I use the attachment points of the frontal rollers  what happens is  that the pulley wheels are pulled to the middle until they touch. It looks then like some sort of X  with the pulleys touching in the middle.

It has merit though and is closer to the original setup  but I would need a wider "fork" in the front and as you said it wastes a bit of stock length in the front. But it lightens the whole setup for a few grams and does away with inertia and friction of the frontal rollers. Should be faster but how much I cannot tell without testing.

Testing is on hold because I accidentally shot my trusty chrony and destroyed the back ring.   (note to self: check alignment of chrony after bumping into it!) I am constructing a new one now using photodiodes amd IR LEDs  so I can do away with magnetised tips...



Thers always a little setback but first results were relatively promising in overcoming the rubber speed limit so  I`ll carry on - but slowly at the moment....