rubber energy

  • if I have a 2 rubbers set same diameter with different length ,50 cm /80 cm ,and each stretch to 300 % (150 cm / 240 cm) ,each will have same stored energy ,if we check separately that energy for example by pulling scale ?the formula in spearit show energy for different cross section but what about the length ?it seem like length factor won't be considered ,is it true ?:/

  • The amount of calories you expend loading the bands should be an indication of the potential energy stored in the elongated band. Longer pull length means more effort to load.

  • band length should not matter, as long as %elongation is the same. They will both reach 0% elongation a the same time and have the same power stroke. ( Im assuming these bands are in the same muzzle hole but anchored on different loading tabs)

    This is in a vacuum, however.

    In practice. a longer and heavier band will throw more weight forward and probably have a bit more drag.

    I like to stick with the least amount of band mass possible using small ID rubber achieves this.


    In my opinion the most efficient set up on a traditional multi band gun is allowing your bands to have the longest power stroke possible. having your bands and your loading tabs as far apart as possible.

  • In my modest opinion

    the most effective stretch (K) for 14mm diameter should be 5 to 6 ( 500 to 500% )

    the most effective stretch (K) for 16 mm diameter should be 4 to 4,5 ( 400 to 450% )



    for example 50cm band should fit with about 100 cm length (from muzzle gun to the last fin of the shaft)

    ....

  • The question came to mind after I make 2 speargun 15cm difference in total length (115/130) cm ,I used same spear 150 cm 7mm but overhange is 20 to 35 cm

    Rubber 3:1 16 mm so in both cases even the hole to shark fin is difference the power is same ,am I right?

  • I could be wrong but if the shaft is the same length of both the 130 should have more power. The 115 is propelling the same shaft mass as the 130 but it has 15 cm less of band stretch. At least that’s how I understand it

  • If the question is which band setup will propel an identical shaft further, yes the longer power-stroke with the same stored energy will be faster off the sling.


    But if the question is the amount of stored energy, they should be the same with the same % elongation.


    Look at rifles, you can have two rifles chambered in .308. If shooting identical ammo, the ammo has the exact same stored energy. But if one rifle has a longer barrel, the bullet will have more time to reach max muzzle velocity................... wait what forum am I on?:D

  • If the question is which band setup will propel an identical shaft further, yes the longer power-stroke with the same stored energy will be faster off the sling.


    But if the question is the amount of stored energy, they should be the same with the same % elongation.


    Look at rifles, you can have two rifles chambered in .308. If shooting identical ammo, the ammo has the exact same stored energy. But if one rifle has a longer barrel, the bullet will have more time to reach max muzzle velocity................... wait what forum am I on?:D

    this confusing me ,i think in spearit formula they not added length of rubber as a factor in the calculation of force ,so how this will be different ?

    the design of wood speargun shall keep band holes as far as possible and contact notch or shark as near as possible to the trigger .

  • The most efficient rubber powered speargun (or any speargun for that matter) would have a band stretch (% elongation) as long as the speargun itself. This is the reason roller spearguns, and pneumatic spearguns are so "powerful". They utilize nearly the entire length of the gun stock.


    For a "conventional" circular rubber, screw in rubber, or tie in rubber speargun, the length of the band at 0% stretch is "wasted".


    The spearit calculator calculates the "optimal" band length based on the distance between your muzzle and your loading tab.

    The number you input is the distance between muzzle and loading tab (D)

    The number in yellow in the black box is your optimal band length (total circular rubber)

    This is based on elongation which can be inputted


    Their formula is L=2D/(E+1)

    ex: L=2[100cm]/(2.0+1)

    L=66.7cm or 26.2" this E+1 is elongation or %band stretch so (200%+100%) the (1) is the untensioned rubber


    They explain it as 0% being an untensioned rubber but in the formula an untensioned rubber is 100%, anything beyond that is tensioned.


    Instead of using the spearit calculator as the answer to your band setup questions, use it more like a guide line. Sure the calculation may work well on one speargun, but there are too many factors to consider, mass of the gun, mass of the spear, mass of the bands, wishbone length, type of track, balance of gun, fit of the handle, etc...

  • When i started trying to calculate band lengths for different guns I use and build. I started a little simpler. I basically measured the barrel length (on pipes) or the length between the mech and the muzzle holes (on wood).


    I then divided that number by 3


    and multiplied that number by 2.


    ex: 100cm/3=33.3 33.3cm X 2 =66.7


    simpler equation with similar result


    I find that this band stretch is really smooth for pipeguns utilizing a 7-7.5mm shaft and two 16mm rubbers or 14mm small id.


    I have modified the equation for larger bodied wooden guns to

    barrel length / 3.15 x 2


    this gives the shot a little more power.


    but this is all subjective, and based on your own opinion and feel. I know guys who run bands long, for a smoother shot, and i know guys who run bands short for more power.


    I hope this makes a little more sense than the spearit guide.

  • ...you can have two rifles chambered in .308. If shooting identical ammo, the ammo has the exact same stored energy. But if one rifle has a longer barrel, the bullet will have more time to reach max muzzle velocity................... wait what forum am I on?:D


    I see that as the bullet having to overcome friction and back pressure over a longer period of time and consequently having less energy when it leaves the barrel.

  • Dan,

    using the rifle analogy;

    the cartridge (powder/primer/case) is the rubber,

    the bullet is the shaft

    the barrel is the track

    and the rifled is basically a REALLY well tuned speargun set up for the cartridge to be fired.


    if there was not enough powder to overcome the frictional forces of the barrel, sure it would slow the bullet.

    But these rounds have so much power behind them, when the expansion of gasses occurs upon firing, the bullet has no where to go but straight out the muzzle of the barrel. With so much pressure, the bullet will continue to accelerate until it leaves the barrel at its terminal velocity (muzzle velocity). After leaving the muzzle, the bullet will have no accelerating force upon it and will continue with the momentum it has achieved in the barrel, slowing down with the friction of the air until it reaches its target, or drops to the ground.


    For the barrel to slow the bullet down it would have to be comically long, or the bullet would have to be chambered wrong (too tight). most likely resulting in an exploded receiver.


    This is basically the principal behind the argument for enclosed track spearguns. When overpowered, the enclosed track acts like the barrel of a rifle, enclosing the shaft, and directing all forces forward. So even if the enclosed track presents more surface area to cause friction, the overpowering of the bands should overcome these forces.

  • A discussion about this very topic https://boards.straightdope.com/sdmb/showthread.php?t=286084


    From post #8

  • Latex tubing is not the best material to apply mathematical equations too. Its stretch resistance is inconsistent when fresh and degrades at varying rates over time. I just cut them a little long to start and trim back as the material relaxes. Making a target and doing some testing in the water will help you find the sweet spot where you get both power and accuracy.

  • Exactly,


    I doubt most rifle barrels are the "optimal" length for muzzle velocity, but rather cut shorter to improve maneuverability, as well as to keep the barrels relatively thin, but still stiff.


    back to spearguns,


    Ladvr is correct, however I'd use the equation to find a starting point, and begin to tune your speargun from there. When you find this sweet spot between accuracy and power you'll be golden. In most cases, you arent shooting fish from 20-30ft away (unless you're George Steele), so when your bands start to degrade it shouldnt affect your shot too much, and when it does, you know its time to replace your bands.

  • this sketch for same spear

    Rubber strech same 380

    Rubber difference is 5 cm for same rubber

    As I understand there are more energy in the longest one (57) even both of them in 380% strech


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