I've been experimenting with three different diameters over the last year. I bought bulk rubber from Primeline in 9/16, 5/8, 3/4 in black and amber.

I found that the 9/16 bands are too light for my taste and will not be using them again. I guess that they are for people who want an easier time loading. I think it's easier and faster to load 2x5/8 bands than 3x9/16. Besides the power gain from going to a 5/8 from a 9/16 I don't see any drawbacks.

My dilemma is with the 3/4. Obviously there is more power in a 3/4. It is also more difficult to pull but that is not the main issue. It appears to me that when firing the 3/4 creates too much turbulence. It is as if it's fighting its own mass/volume to contract in the water. It creates much more of a burst of bubbles than a 5/8.

With two wraps of line 2x5/8 bands move a 9/32 spear to the end of the shooting line very well. I switched to 1x5/8 and 1x3/4. There is more power but the spear can only move to the end of the shooting line anyways. With both setups the spear moves straight to the end of the shooting line but it does so more smoothly with 2x5/8. I'm thinking of permanently removing the 3/4 diameter band from this and future configurations.

SE this was taken from another forum, don't remember which (perhaps DeeperBlue:scratchhead:)

The "set up gun" used for the test had a 90cm barrel which is also very close to the distance of the band stretch. Assuming that the most popular band size that is equipped on 90cm guns is 22cm or so, that makes it about a 300%stretch.

The Test
Each pair of bands was tested measuring the velocity of the shaft in m/s (meters per second) from a distance of 1.8 meters from the muzzle. The shots were taken in air, not under water.

10 shots were fired under these circumstances for each of the 14 pairs and types of bands. What we found was that the loading effort of a band is not a good indicator of power and of how good the band is.

This is actual data acquired from the tests. Let's say A, B, C, and D represent the different type or brand of bands (so I don't get into trouble):

Type and Thickness Maximum Velocity
V.MAX m/s
Kinetic Energy Loading Effort Energy/Kg
(Efficiency)
A-20mm 30.46 137.3 70 1.96
B-17mm 29.38 125.5 62 2.02
C-18mm 32.55 148.7 65 2.29
D-16mm 30.07 119.0 57 2.09

As you can see just because a band is harder to pull doesn't mean it will move the spear any faster. While the 20mm band was the hardest to load at 70 kg, it did not produce the highest velocity. Furthermore, if you divide the kinetic energy by the loading effort to find out how efficient the band is, you will see that the 20mm band places last. This is not to say that all 20mm bands are no good, just that the loading effort is not a good indicator of the speed of the shaft. (Editor's note: While static testing of band material isn't a good indicator of speed, it is still important to determine soak off.

here's another interesting site

http://rocknfish.com/Rubber_Test.html

I dipped into primeline yesterday, but they sell $100 min order,how much of that tubing did you buy?

SS, interesting info thanks. This confirms what I was thinking. I believe I'm going to stick with 5/8 bands with one change. Take a look at this post which I made previously in http://www.spearfishingplanet.com/showthread.php?t=2213Alright, here's the load down.

First a little background info. Dimensions for bulk rubber are specified by inner diameter + wall thickness. Hence a normal 5/8" band is 1/8 X 1/4, 1/8' being the inner diameter or hole and 1/4" being the wall thickness. To total it you do 1/8" + 1/4" + 1/4" (there are two walls, one on each side of the hole), this adds up to 5/8".

The rubber I suggested with no inner diameter is not possible. The manufacturing process requires a hole. For there not to be a hole it would have to be a different process called extrusion. Extrusion apparently produces inferior rubber so it is out of the question.

Now, the largest rubber now available at 1/16" inner diameter is 1/16 X 1/4. This reduces the inner diameter keeping the wall thickness the same, which means that the total diameter of the band is reduced by 1/16". This is still not bad because there is just as much band material but less space inside for water to accumulate.

A better yet option is to keep the inner diameter at 1/16" but increase the wall thickness to 9/32. This will bring up the total diameter back to 5/8" yet have a tiny inner diameter of 1/16"

This would be a custom order and requires a minimum purchase of 200ft. At $2/ft that's $400. If anyone wants to tag along I'd go in for $100. The 5/8 (16mm) bands is what I use primarily, I don't touch 9/16 and rarely 3/4 so it would be a good buy. I'd get it in black.I did previously make a $100 order. It consisted of a variety of diameters and colors. This time I'm looking to get one type of band material only, the one I'm talking about in the quote. It will be superior because water intrusion will be cut down and at the same time there will be more band material at the same diameter. I could do it as an investment, I think people will buy it, but I'd rather make a group buy. So again, is anyone interested?

I'm very curious about this myself. I've tried to reason my way through a little thought experiment, but got lost. Here goes:

The force exerted by a spring (or elastic band) is equal to K*X, where K is the spring constant and X is the length that it is stretched.

The kinetic energy contained in a stretched spring, however, is 1/2*K*(X squared).

In other words, stretching a band further (longer "X") increases the potential energy more quickly than it increases the loading force. Since force = mass x acceleration, less force = slower acceleration = less kick. Less acceleration does NOT have to equal slower shaft speed if the band has more time (i.e. greater % stretch) to act on the shaft.

So, it seems, stretching a 9/16" band to 350% might have the same loading force, but more potential energy, than stretching a 5/8" band to 300%. Obviously there is a limit, since you can only stretch the bands to the shaft fin, but you could start with a shorter band to get a greater % stretch.

The experiment that I would love to see is this. Three bands, of 9/16, 5/8, and 3/4" diameters. They are of different lengths (9/16" being the shortest) so that the loading effort to place the wishbone on the tab is the same for each of the three. Which would generate the greatest potential energy and shaft speed?

Does anyone have one of the measuring devices they use to clock arrow speeds? Does anyone know if it would work underwater?

Tin Man, I don't have the patience to the numbers work, but I did mess around with the bands quite a bit. I'd make them, use them, then shorten by a little bit, then use them again. I'd make them first, use them and only then measure the resulting ratio. Granted my research is not as extensive as what others have done but my feeling is that 5/8 bands at 3.6 stretch work perfect.

Does anyone have one of the measuring devices they use to clock arrow speeds? Does anyone know if it would work underwater?

would it be necessary to use it underwater, I would think not, but maybe I'm wrong- This would be interesting.

SE McMaster may have what you need in smaller amt, no minimum $$$order

McMaster-Carr sales Primeline. Also they have more sizes on their website then their catoLOG.

Does anyone have one of the measuring devices they use to clock arrow speeds? Does anyone know if it would work underwater?

I have a chronograph. I doubt it will work underwater unless you seal it up pretty good in a box of some sort with a clear lid.

How does it work? By that, I mean how does it detect the flight of the arrow/spear ?

It has 2 light sensors on it a certain distance apart. It just times the amount of time it takes for something to set off both sensors. Then it divides the distance by the amount of time it took. It is meant for taking bullet speed but works with paintball and arrows as well.

Here's some additional information on band strength... some of us can't do the math so we need a visual chart:

I have found that the 9/16 bands throw the spear out with less shock and kick. In multi band configurations I will use a combination of 5/8 and 9/16 bands. The 5/8 seem to provide a faster initial launch and the 9/16 "seem to" provide the longer throw.
The single most important aspect of banding is not to over power. Also shaft diameter, legnth and weight will need to be considered. There are a lot of different set ups for different guns.
I think another question for this thread would be what size bands work best on "Gun X" with a certain shaft size. "Diameter and legnth"

Cheers

This is good information. I did not know this about the exchange of energy to load and energy transferred at release. I would have though the 20mm would have done better, but the 16 and 18mm do seem to get off the block faster.

Speed is one factor, but one other consideration would be hitting power when the shaft makes contact with the fish. Speed is important with fast fish with soft flesh (mackeral, small to medium snapper etc.), but in my experience a single 14mm or 16mm band does not always have the punching power that a single 20mm band does for large bony fish with tougher penetration. I would be curious what you guys think?

I am not a big fan of shortening bands. It may add power but I feel it really throws my accuracy off.
I do like the black primeline way better than the amber. Amber 20 is impossible to pull in the same length as black.

I'm very curious about this myself. I've tried to reason my way through a little thought experiment, but got lost. Here goes:

The force exerted by a spring (or elastic band) is equal to K*X, where K is the spring constant and X is the length that it is stretched.

The kinetic energy contained in a stretched spring, however, is 1/2*K*(X squared).

In other words, stretching a band further (longer "X") increases the potential energy more quickly than it increases the loading force. Since force = mass x acceleration, less force = slower acceleration = less kick. Less acceleration does NOT have to equal slower shaft speed if the band has more time (i.e. greater % stretch) to act on the shaft.

So, it seems, stretching a 9/16" band to 350% might have the same loading force, but more potential energy, than stretching a 5/8" band to 300%.

I like your line of thought, I remembered Kd as the spring force, but forgot about the stored energy. Since we assume that the spear is going to be a constant mass, the stored energy should be the primary consideration. I'm also assuming that all the energy is transferred to the spear. Because of this, the initial velocity (of a bigger band) shouldn't matter, unless it can give more in a shorter distance compared to a longer distance of a thinner band.

Again, I wish there was a spearfishing magazine who would use science as part of their format and do a definitive study like this.

(That one site did have part of the equation, which was to calculate K and band stretch over time).

From a purely physics point of view, a thinner band (lower K) stretched longer should be superior for the final velocity, which is what matters when it hits the fish, be it soft,hard, etc.

As with everything it is a trade off. This previously posted link

http://rocknfish.com/Rubber_Test.html

is to an "experiment" by John Warren and while it doesn't give data for specific band diameters or manufacturors it highlights some of the important issues.

Tin Man is right, you gain force by increasing the stretch which results in a longer period of acceleration for the shaft. The downside is that the band is "damaged" and doesn't return to its original length and over time will wear out quicker. Another factor is the "soak time". When you load a gun generally you do not fire it right away and as time passes the force that the band will generate decreases. In their tests there was up to a 20% loss of force in twenty minutes.
They suggest prestretching the material before making the bands. ( I've never heard this before).

So the trick would be to find (for the material you are using): What is a good midway point that gives a good accelleration but doesn't shorten the life of the bands or lead to excessive loss of acceleration after "soak time".

The single most important aspect of banding is not to over power.I've come to realize this too.

Using a 9/16 band IMO would be acceptable where a combination of strictly 5/8 bands will result in overpowering or underpowering. Such as 1x5/8 being too little power and 2x5/8 being too much. In such a situation 1x5/8 plus 1x9/16 would be ideal. I can only see this being necessary with shafts lighter than 7mm. I will not use a shaft less than 7mm(9/32) so I don't think in my case I need to consider the 9/16.

As for the spring constant of the rubber (K) it is not constant but changes as you pull the band. I believe it decreases and slowly amd eventually approaches a constant but not in the range you would use it.

We did some tests at school on a tensile testing machine. It was for a project about a year ago. We only tested 9/16, but I cant find the results anywhere. The constant started at about 3 lb/in and then went to around 2 lb/in on that specimen.

As for the spring constant of the rubber (K) it is not constant but changes as you pull the band. I believe it decreases and slowly amd eventually approaches a constant but not in the range you would use it.

We did some tests at school on a tensile testing machine. It was for a project about a year ago. We only tested 9/16, but I cant find the results anywhere. The constant started at about 3 lb/in and then went to around 2 lb/in on that specimen.

Heh, so much for constants! In theory, it should be a constant- or an approximation of one over the normal range of pull. Is 'K' given by the manufacturer? I guess from your science work you were calcluating k- did it match up to anything published?

Looking at the hammerhead chart above, it appears that 'k' is constant for the first 300% stretch, which is more to our range?

In reality spring constants only apply for linear springs or the linear range of a spring which is very small. Since the crosssectional area of the rubber also changes as you pull you have many factors coming into play.

But, we could get a an approximation of k (I'm assuming that is what is done for any elastic material). I need to go to those manufacturers sites to see.