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Rubber length vs performance?

Thread Status: Hello , There was no answer in this thread for more than 60 days.
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Hey everyone,

Don't forget that inertia, momentum and acceleration play a big part in this. Some of the single thick band setups work great on thin shafts because they overcome a large percentage of the shaft's inertia right off the bat. The larger tuna guns use the thinner multiple bands in order to maintain acceleration on the longer, heavier, thicker shafts as the inertia is overcome throughout the course of the shaft leaving the gun. I believe Def, Bill and Dave somewhat cover this.

Also, the "snap" (change in acceleration over the change in time) is a big factor here. The large single band most likely has its punch at first but never really has the ability to increase the rate of acceleration throughout the distance of the band-pull. The smaller softer bands have the characteristic of being "smoother" (in thicker shaft scenarios) because of the opposite effect. They DO have the ability to increase the rate of their acceleration over a larger part of the band-pull than the former band/shaft scenario.

I believe one reason why Euro styled guns have the single band setup in parallel with the shaft is due to the fact that the strongest part of the pull is immediate. Having more of the shaft on the gun when it's fired in this situation makes it more subject to muzzle lift. Having a paralled band pull corrects this. Not many American styled guns use one single, large band because (IMO) the multiple thinner band setup gets the shaft out farther before the muzzle lift has the chance to drastically affect the flight of the shaft.

Now, only if the manufacturers had a listing of optimum band lengths for spearlengths, spear dia. and band pull length of the gun. :hmm

Re: Rubber length vs performance? (long)

I want to clear up more confusion I created in this thread that lead me to make a fundamental error. I do not want anyone else to follow my erreoneous ways.

I said, "So if k is one unit for each 50# it does not matter how long the band is."

That statement is absolutely correct if each of the bands in question has a K of 50#. The trouble is that K is not a true constant. Something I did not know about is Young's modulus (Y). Young's modulus is also called the modulus of elasticity. Y is a characteristic of the material a band is made of and is a true constant that is independant of any dimensions. K, however is dependent on the initial length of a band and its cross sectional area. So if the same brand and diameter rubber were to be used in making longer and longer bands, while Y would not change, K would.

As an example: if a fat band 1 unit long had a k of 100; a fat band (made of the same rubber and the same diameter) of 2 units length would have a K of 50. The impact of this is that Bill's descriptions of the real world behavior of elastic bands is way more correct that mine. I was wrong in not believeing that band stretch is proportional for a given brand and diameter with a given force on it no matter the length. I know this is so because I did the experiment I proposed to Bill and found that, for a given rubber with a given weight attached, the stretch was always proportional no matter the length of the rubber. And as I suggested, you can't fool mother nature. That includes me! So with mother nature staring at me I did some more research and found:

An alternate formula for k is:

k = ((Y) (A)) / (Li)

Y = Young's Modulus
A = Area of the band
Li = Initial length of the band.

So you see that K is inversely related to the inital length of the band and directly related to the cross sectional area of the band.

What this all means is that my discriptions and examples of non-proportional band stretch are wrong while the others are correct. But, the general conclusions are still correct. To sum it all up:

1. A longer thinner band will have more Potential Energy than a shorter fatter band when both require the same force to pull them back to 3:1.

2. Use the longest bands possible at about a 3:1 ratio. You will get the highest PE possible that way and not quickly destroy your bands.

3. If you have trouble loading one or more fat band(s) use a higher number of thinner ones. If the cross sectional area of all the thinner bands you use exceeds the Cross sectional area of the fat band(s) you will have even more PE to do the job.

3. A roller gun has a significant advantage over a typical gun if the force necessary to load is the same for both so long as the roller uses longer rubber of the same brand and diameter.

While all of these rules seem like common sense now, I needed to go through the process to understand the mechanics of speargun power. It has helped me and I thank you all for helping me get on the right track.


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A lot of "in the weeds" physics theory here. My short answer which I explain below is; all other things being equal, higher stretch multiples will travel greater distance and force x distance = work/kinetic energy so same force v. longer distance 9higher stretch multiple) means more energy. In the end a best theory is no good until the data come in. The right question and experiment design was proposed so someone should do it. How long could it take switching rubbers on a gun and using depth penetration or distance as a ranking measure.

In support of my answer I like using Einstein's approach of gedanken Versuche. He essentially thought about the infinite extremes. So if we consider a infinite barrel length and then on it an infinity short (say 0.005 mm") v. infinity long (say 1 meter) band both loaded at 100 pounds to me "intuitively" I can't see how a spear moving say 0.005 mm v. 1 meter will travel same /have same energy. Again my vote is for the higher multiples.
Hi Guys
I followed most of what was in the thread, I think :)

I have a RA converted 900mm roller that has 450mm x 16m RA progressive bands which stretch to 400% (reactive next time)

My next project is converting my RA 1100mm to an inverted roller (non rear roller) for pelagics and I was wondering what would be the best setup for the rubbers?

I found this http://www.rollerspearguns.com/product/inverted-roller-speragun-rubbers/ which makes sense - 18mm reactive x 1 to give it the throw and 16mm progressive x 2 to give it the punch, can anyone tell me what length they need to be?

To me it looks like 300% would be about right... So on an 1100mm use a 365mm set. 400% would only be about 275mm which seems too short to be real.

What do you think?
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