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    Rubber as energy source - pros and cons

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    JoergS
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    Rubber as energy source - pros and cons

    Post by JoergS on Tue Sep 13, 2011 2:13 am

    Hello folks,

    I am excited to see that rubber based crossbows are now covered here as well. I have worked with "slingshot style" weapons a lot these past few years, built everything from micro slingshots all the way up to car sized slingshot cannons. I dare say that I have amassed quite some experiences with Charles Goodyear's ingenious invention.

    Therefore, I thought I elaborate a bit on the issue of rubber as an alternative energy source, compared to wood, steel or compounds.

    Pros:

    Cheap. You can make a powerful crossbow band set for under five dollars.

    Available. You can use gymnastics bands, spear gun rubber, even surgical tubing.

    Easy. You don't need expensive tools or skills to make a crossbow with rubber. No comparison to making a crossbow with steel or wood as energy source.

    Powerful. Rubber can be stretched far. About 5,5 times the relaxed length is usually achievable. This gives you a loooong power stroke, much longer than wood, steel or compounds would allow. This means you can achieve very high energy levels without much draw weight. The longer acceleration phase does that for you. A 45 lbs slingshot crossbow can achieve as much energy as a commercial high end 175 lbs model, if the tiller is long enough.

    Narrow prods. All you need is enough clearance for the projectile and the bands. Rubber based crossbows can be very compact.

    Easy "cable" change. Since rubber is usually not pre-stretched, exchanging the bands can be done in seconds, without any tools.


    Cons:

    Vulnerable. Rubber is a natural material, and will break at some point. You can get thousands of shots from a single bandset, but if you max out the performance, you may only see 50 shots before you got to repair the bands.

    Must be kept relaxed. Rubber looses power quickly in drawn out condition, so you should not cock a slingshot crossbow and then wait a long time before you shoot. Now this can be compensated by using heavy rubber, and also the rubber "heals" quickly (full force is back after 15 minutes if you relax the bands again), but this issue is serious for hunters.

    Long tillers are called for. The downside of the stretchiness is that you need a long acceleration. An easy slingshot crossbow can be very narrow, but is usually long, musket style I'd say. Now this can be compensated by bullpup constructions and rollers to remove the dead play, but a simple, straight forwards slingshot crossbow is a long thing.

    Cams don't work. Cams can dramatically increase the manageable draw, so compound crossbows are much easier to cock. The let-off effect eases the draw in fully cocked condition, which eases the pressure on the lock/trigger mechanism. But this does not work for rubber, as the "slippage" effect is dramatic. The bands will stretch very much between the "pouch" and the cams, making the cams useless. You can use rollers (= pulleys) to reduce the dead play in the bands, making the weapon more compact, but what you don't get is the "compound" effect.

    Uncertain legal situation for hunting. While in many countries (most of Europe anyway), hunting with any kind of crossbow is illegal, there are many countries where you can hunt with one. But often there is a minimum draw weight. Here we run into the problem that a slingshot crossbow is more effective than a conventional one,so the minimum "legal" draw is complete overkill for a rubber based model.

    Temperature sensitiveness. Rubber works best when it's hot, and does not work well in freezing weather. Not a problem, your slingshot crossbow will still pack a punch even in winter, but you need to adjust your sightings every time before you start the day.

    Reputation. Yes, people may chuckle, even crack jokes, when you show up with a rubber based weapon and declare that it is serious. The jokes disappear quickly when the shooting starts, though.

    No commercials models available
    . That's right, if you want one, bring out that old toolbox and start woodworking!

    Is this list complete? Certainly not. I just compiled it from the top of my head. If you want to add something, please post away!

    Greetings from late summertime Germany


    Jörg

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    Re: Rubber as energy source - pros and cons

    Post by jeep on Tue Sep 13, 2011 11:15 am

    Hi Jorg
    Glad to see you here!!! I was feeling a bit lonely . IVO created this section a couple day ago it is why my creation are not yet there (but IVO if you want put it here it maybe it right place) you can look at Bigjoe5 and spring powered crossbow. you'll see a lot of rubber but plainly of small one ,it will be interesting to see for the same draw weight if the projectile speed are different...
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    Re: Rubber as energy source - pros and cons

    Post by Ivo on Thu Sep 15, 2011 9:44 pm

    Hi Joerg,

    Nice post. I did my own comparison chart of different materials a little wile back...it's good to learn a few new things. However I got something for you to.Smile

    JoergS wrote:Cams don't work. Cams can dramatically increase the
    manageable draw, so compound crossbows are much easier to cock. The
    let-off effect eases the draw in fully cocked condition, which eases the
    pressure on the lock/trigger mechanism. But this does not work for
    rubber, as the "slippage" effect is dramatic. The bands will stretch
    very much between the "pouch" and the cams, making the cams useless. You
    can use rollers (= pulleys) to reduce the dead play in the bands,
    making the weapon more compact, but what you don't get is the "compound"
    effect.

    While I agree that direct rubber to cam connection will not produce the expected effect, there is a loophole and I would like to contest the statement about Cams. Smile

    It is essential to understand that a the compound setup was brought into play as a component designed specifically for energy distribution. Cables are a solid element of the setup that acts as a link of the energy distribution module to the energy storage module (bow limbs, spring, etc) with definite anchor points to each. No stretch is one of it's most vital function as it is responsible for proper cam function.
    If rubber is to be used in this composition, it can only play one role and that role is energy storage. When you try to make something play more than one role, it rarely works well. So...By connecting the rubber to cam via cable, cams gain back their functionality.

    Jeep,

    Glad you feel better about the new sub-forum. I did split the topic and create one with your project in it. >>>Link

    I hope you're fine with the title...if not, you can always click "Edit" on the first post and change it. Also I compacted your picture posts into as little number of posts as I could. You can post more than one picture in one post adn they will automatically resize, no matter what the original size is. Smile

    Good luck on your build, I'll check in a little later with some thoughts...I've been designing a similar in-swinger since my last post in theWW2 Big Joe topic. Wink

    Ivo




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    JoergS
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    Re: Rubber as energy source - pros and cons

    Post by JoergS on Thu Sep 15, 2011 10:35 pm

    Ivo, you are right about the cable idea, but by using cables, you run into more issues.

    As I stated above, rubber has the advantage that it stretches far and allows a lot of acceleration length. Now if you use cables between the cams and the projectile, then you either need a lot of it, or you give up that advantage by employing a transmission setup.

    If you give up the acceleration length advantage, you loose the most important advantage of rubber (but retain most of its disadvantages).

    If you use looong cables, you add tremendous weight that needs to be accelerated along with the projectile, reducing the energy of the shot. You also create a "lash", that snaps out far and with serious speed. Dangerous.

    All my tests with cables attached to a rubber energy source pretty much failed. I gave it up in the end.

    That does not mean it is impossible! It's just not very easy.
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    Re: Rubber as energy source - pros and cons

    Post by Oppenheimer on Mon Oct 29, 2012 10:01 am

    Have read about the WW2 Big Joe crossbow that had many smalldiameter rubberbands instead of one or a few big ones. The reason should be that it got a higher speed but noone knew why during the tests in WW2.

    Could it be so that many small springs/rubberbands are faster or has it maybe something to do that they produce or get rid of the heath better? Has anyone tried to compare the speed?

    JoergS
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    Re: Rubber as energy source - pros and cons

    Post by JoergS on Mon Oct 29, 2012 10:10 am

    It really depends on the rubber type. But thin bands are always having a better max retraction speed than thicker bands. If you shoot heavy projectiles, that does not matter much - that is the reason why scuba diving type spear guns use very thick rubber. The lighter the ammo, the thinner the bands.
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    Re: Rubber as energy source - pros and cons

    Post by Oppenheimer on Mon Oct 29, 2012 10:29 am

    Your answer is important. In fact I think it could be used in many speed projects. The negative part of a small springs short drawlength could also be avoided by using a thin and long spring instead of a short, thick and slow one. Many springs together in serie would work the same. In my latest inswinger I used two paralell mounted springs from a jumping-mat but I have nothing to compare with. 10 cm drawlength gave me 30 kilo each. Felt like a 30 lbs recurve-bow when compare on 10 m distance. It was a "Woodland survival flipper"-copy with 16 cm alulimbs strengthened with building-foam.
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    Re: Rubber as energy source - pros and cons

    Post by olrono on Mon Oct 29, 2012 2:14 pm

    Oppenheimer wrote:Have read about the WW2 Big Joe crossbow that had many smalldiameter rubberbands instead of one or a few big ones. The reason should be that it got a higher speed but noone knew why during the tests in WW2.

    Could it be so that many small springs/rubberbands are faster or has it maybe something to do that they produce or get rid of the heath better? Has anyone tried to compare the speed?

    Geeeezzz man what a beautiful avatar you have there! I would like to get a big framed poster of that one. Shocked Shocked Shocked
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    Re: Rubber as energy source - pros and cons

    Post by Oppenheimer on Tue Oct 30, 2012 1:39 am

    To Olrono: Just google-pictures on "greetings from the nevada testsite" and you´l find bigger pictures.

    Joerg wrote: Long tillers are called for. The downside of the stretchiness is that you need a long acceleration -I think it´s not a disadvantage since springs instead has to be longer to spread out the force on a longer dravlength/runway for the arrows. Both powersources needs long drawlengt to create speed=kinetick energy.

    Big Joe 5 had a short drawlength using small rubberbands and was efficient up to 60m. Should bigger rubberbands recuire a longer drawlength? If so, a way to smaller building sizes could be using many thinner rubberbands. A spring-manufacturer said the most efficient way to use a spring is to compress it to only one third of it´s length. Guess friction and inertia eats up a lot of the energy if you compress more. Probably rubber works in the same way? Thats why I guess it might be more efficient to use many small powersources and use only a third of their maximum drawlengt. This maybe also be a way arounde the disadvantage of the rubbers bad "memory" to return it´s full power after being drawn. Could be drawn a longer time and could have about the same power for the first three shots? Just my teories and I hasn´t tried them but maybe someone else?

    For example: How much longer should "Big Joe" have to be, to shoot the same arrow 60m. with the same precision, if Joergs bigger rubber bands were used?

    JoergS
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    Re: Rubber as energy source - pros and cons

    Post by JoergS on Tue Oct 30, 2012 3:19 am

    Oppenheimer, my experience is that draw force and draw length are the two most important factors. BJ5 had to employ a huge draw force to shoot "effectively" at 60 meters. The same arrow speed could have been achieved with about half the draw force and twice the draw length.

    As far as the hysteresis (=loss of power when the bands are kept in drawn out condition) goes, it is not a problem really. If you avoid the maximum stretch (5,5 to 6 times the relaxed length), then you can keep rubber bands stretched for days.

    They will initially loose some draw weight (about 30%, depending on the temperature of the environment) when they cool down. This cool down procedure will take just a few minutes, after the first minute 60% of the cool down is done. What you do is you simply compensate for that loss using 30% stronger rubber.

    The ideal draw length is 5 times the relaxed length. So 20 cm bands can be stretched to 100 cm. If you go above that value, the rubber will tear early and also can't be kept drawn out for a longer time. If you go below that value, you will loose efficiency, very significantly so.

    For conventional slingshots, a very good way to increase performance is to cut the bands in a tapered fashion. Means, they are more narrow at the pouch and wider at the fork. This means that the bands are stretched to maybe 700% near the pouch, which accelerates the shot based on the same draw weight. It also means that the bands will tear early, always near the pouch. For crossbows, this is not desirable, and since a stirrup allows cocking the crossbow with both arms there is no draw force issue. Very hard to draw out a slingshot 40 lb draw weight. Very easy to cock a crossbow at 150 lb.
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    Re: Rubber as energy source - pros and cons

    Post by Oppenheimer on Tue Oct 30, 2012 10:57 am

    Thanks. I have alot to learn about rubber and slingshots and now I understand the tappered rubber I´ve seen on your picktures.
    Silence must be another good thing about rubber. Thinking of the Woodland survival flipper.

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