Speed Testing different pulley systems

Discussion in 'Slingshot Crossbows' started by RalphG, Jun 13, 2014.

  1. RalphG

    RalphG Member

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    For those who have read my previous posts its clear what this is about. For those who have not : I want to test different pulley systems for speed and see if there is a gain to be had. At the end 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!

    [​IMG]

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

    [​IMG]
    Spanned:

    [​IMG]

    The Arrows:
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    Draw length is 70cm and draw weight is app. 80lbs

    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: 346,85fps


    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 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 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 :D 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 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 crossbows 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 .
    I hope this encourages some others to try and experiment a bit with pulley systems - we are not seeing very much of that here. Jörg has about almost given up on this kind of concept but I think it still has something to offer.

    Best Regards
    Ralph
     
    Last edited: Jun 13, 2014
    Markus Oswald likes this.
  2. CEZ

    CEZ New Member

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    Awesome, looking foreward to more tests. I was always wondering if pulleys worked good with rubber like that. Even did some small scale tests simillar to your rig, seemed promising at the time but I didn't have a chrony to even measure if it has any effect so gave that up quickly.
     

  3. RalphG

    RalphG Member

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    Thanks for the encouragement :) Most people here just shoot slingshots, so I dont know if there is enough interest to show this, its nothing spectacular to see, no destruction test, no final weaponized system etc. etc. Its interesting for me and a few others at least.
    Maybe if I did my testing in secret and come up with a 400+fps crossbow this would rise some more eyebrows:D
     
  4. CEZ

    CEZ New Member

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    Nah, I'm sure lots of people here appreciate crossbows but they are harder to make:)
     
  5. Brazilviking

    Brazilviking Thread Hijacker

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    This is fuc$%nig scientific, Ralph!!! I like experiments!!!! Nice!
    Seems that ther is not a significant difference between the systems. I wonder...have you tested the "no pulley" system?
     
  6. onnod

    onnod Im from Holland, isnt that weird?

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    I am interested for sure! I love seeing these well performed tests, even just for the fact that they are well performed. But if you hit that 400 fps mark, please shoot something and film it ^^
     
  7. BoyntonStu

    BoyntonStu Member

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    571/223 = 62.9 fpe

    222fps for a 571gr arrow is a decent speed but not overly impressive so far.

    I disagree. That is very impressive, and with the right broadhead, capable of killing an elephant!

    Did you pull 80 pounds with a single moving pulley?

    If so, that is 160 lb on the rubber.
     
  8. RalphG

    RalphG Member

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    I still want more speed!!!:D:D:D

    400fps seems doable but with a lighter arrow I think.
    571gr at 400fps would require too much draw weight to be easily made and still nicely portable.
    Its 80lbs draw weight on the trigger. So yes, because of the pulleys the total rubber pulls at 160lbs.
     
  9. CEZ

    CEZ New Member

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    Well I think you are doing great anyway, you managed to put out 63 ft/lbs out of 80 pounds of draw! That's better than many crossbows.

    You could probably do 400 fps with a lighter arrow if you double the draw weight, change to TBG and perhaps make the rig a little longer? Your trigger looks sturdy enough to handle it, don't know about the pulleys.
     
  10. BoyntonStu

    BoyntonStu Member

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    Sometimes, what you think you need is not what you really need.

    My .177 springer shoots 1,200 fps. Big deal for crows. Not so much for deer.

    [ame]https://www.youtube.com/watch?v=KBtgJZ9F8ns[/ame]
     
  11. RalphG

    RalphG Member

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    Well, its only a test rig made from some spare wood I found in the workshop. The final build will be made from better materials. And since I am aiming for speed it will use TBG rather than tube. But I had enough of it so I use this. I am aiming for final dimensions not longer than 1 m- max 1,2m and not wider than 50cm. These restrictions I set myself to have something nicely portable.

    I will see if the 3:1 pulley system does any better than the 2:1. The 4:1 would require too much initial drawweight to get 160lbs on the trigger to make it practical in a easy build.
    The bearings I use now in the pulleys can take 120lbs combined static load without damage , so I can go a little higher than that but doubling that would damage them after a while. I will see if I can find some light small bearings that can take more weight.
    The trigger works fine so far and I am sure it will take much more without problems...


    Yeah, its already lethal enough, so there is not really a "need" to go faster but rather a "want" ;) because I want to see if its doable.
     
    Last edited: Jun 13, 2014
  12. CEZ

    CEZ New Member

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    Yep, it's a great experiment just to see what the rubber can do.
     
  13. deraNdy

    deraNdy Active Member

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    Big Hands for this test results. This looks like you found a way to take the rubber powered crossbows to the next level.
    What i would like to know, how long is the overal drawlength?
     
  14. RalphG

    RalphG Member

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    So I upped the power by adding two further loops of Theratube silver and secured the frontal rollers a bit better. Drawweight on the trigger is now about 120-125lbs. Draw length is the same (70cm) and I used the same arrow weights. The test rig begins creaking and groaning - so thats about as high as I dare to go with this contraption. A sturdier frame would take surely more weight.

    [​IMG]

    Unfortunately the 12g wooden arrow broke on the first shot so I made a even shorter Aluminium version which is a really close 11,9g in weight.

    [​IMG]

    [​IMG]


    Thr Results

    37g (571gr) Arrow

    1. 270,70fps
    2. 269.39fps
    3 268,57fps
    4. 269,11fps
    5. 269,18fps

    Average: 269,39fps

    [​IMG]


    The 22g (339,5gr) arrow

    1. 327,14fps
    2. 322,74fps
    3. 329.59fps
    4. 328,12fps
    5. 327,38fps

    Average 326,99fps

    [​IMG]

    The best for last :)

    the 12g (185,2gr) Arrow

    1. 415,75fps
    2. 415,75fps
    3. 408,67fps
    4. 418,38fps
    5. 414,89fps

    Average 414,68fps

    [​IMG]

    This time I also took some screenshots from the laptop so you see I`m not making this up :) I used a slightly larger magnet on the 12g arrow maybe thats why the graph looks different from the others.


    So 400+ fps is possible - so far only with light ammo, but definitely doable. I`m happy:):D:D:)
     
  15. RalphG

    RalphG Member

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    Did some rough calculation on my results:
    I increased the draw weight by 50%

    37g Arrow

    269,39 to 222,56 is a 21% increase in speed

    22g Arrow
    326,99 to 272,58 is a 19,9% increase

    12g Arrow

    414,68 to 346,85 is a 19,5% increase

    so its roughly 20% increase in speed If I do a 50% increase in draw weight. I dont know if the progression will be linear or if the curve will flatten out at some time.
    Assuming (totaly unfounded assumption at this lack of sufficient data;) it would be linear with an increase of 150% in drawweight we can shoot the 37g Arrow at 400+ fps. This would mean it takes 240lbs on the trigger to get it that fast.
    But with this drawweight the pulleys would have to be stronger and thus heavier robbing some of the speed so in the end it will have to be a bit more than 240lbs....
     
    Last edited: Jun 14, 2014
  16. CEZ

    CEZ New Member

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    That's an awesome result if you ask me! By the way, doesn't the line slip out of the pulleys?

    Are you going to make a stronger rig now?
     
  17. RegulusRubber

    RegulusRubber Junior Member

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    Because rubber has a limited contraction speed it would be interesting to see a roller system with a drawweight X at the string compared to a pulley system with the same drawweight at the string but double the rubberpower.
    How much more speed would it give?
     
  18. RalphG

    RalphG Member

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    Yes, I`m happy about it :D

    and no, the string did not slip even once from the pulleys.With the latest mod it cant slip from the frontal rollers and the moving pulleys also seem to be no problem.

    No I`m not making a stronger rig, I will use it to test the 3:1 pulley system. Then I will decide which one I will use in a nice crossbow build;)
     
  19. RegulusRubber

    RegulusRubber Junior Member

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    You don´t need max. rubberpower for testing wich system works best. I would prefer moderate rubber for proving the theory of speed enhancing.
    Oh, I think the third system is technical nonsence. It wouldn´t give an advantage. But it would be nice to see it working.

    Oh I´m wrong. It the 3:1 pulley. The drawing has confused me somewhat.
     
    Last edited: Jun 14, 2014
  20. RalphG

    RalphG Member

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    Well, I wanted to test the roller setup first but as I already said, it would have needed a redesign of my test rig - so I am doing the pulleys first.

    Nonsense? That would surely hurt all the riggers and treeclimbers out there who use these systems on a daily basis...;)
    Do a little google search on pulley systems and you will see that the 3:1 as shown is fully functional.

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