are fat bands more powerful?

Hi Jtkwest i think there is misunderstand somewhere in this info you have seen because this is against physics and specially Hoks law about springs which is applied here. The true in the above info is that you will get the force that you will put to a rubber but the question is if you bring two rubbers one 16 mm and another 20 mm with same length 20 mm will be harder to pull and therefore the force you are putting to pull the 20 mm will be more so the power produced by 20 mm will be more. Hooks Law saying F= K(x-x1) where F is the force K is the constant of the spring and x-x1 is the displacement of the spring (In our case it is rubber) if we consider F to pull 20 mm is the same F for pulling 20mm with same displacement x-x1 then we are saying that K =K and this is right only for same diameter rubbers.

This is the scientific answer that i believe is right in case i understand you correctly...

Apples and oranges. You are using work and force interchangeably. Force times distance equals work.
If the rubbers are the same length, the fat one will have more force when stretched the same distance and more power. If they are stretched different lengths, a lot will depend on which part of the power curve they are on. Comparing 13 and 15 mm rubber, stretched the same distance, you have about 16% more diameter and 33% more rubber and power (if you stay on the linear part of the curve).
As a comment on real experience, when I switched from the standard of 3 x 9/16 to 4 x 1/2 of the same length, the only differences were easier and a little longer to load. Depending on where the rubber is stretched on the curve, you'd get the same affect by using four 9/16 about 25% longer. The biggest difference will be about a dollar more in cost.

No way. The 5/8 will have to stretch almost 50% more to get the same force as the 3/4. It will produce more power than the 3/4 (same force for a longer distance/time). Actually, if you cut a 9/16 about 15% shorter than the 5/8, it may provide even more power but will probably wear out the rubber a lot faster.
If you use a fisherman's scale and some weak surgical tubing you'll be able to see the force/stretch distance increase dramatically as you near the breaking point. The ideal length is just before that increase.

jtkwest said:

to clarify my question and position: everyone has a different amount of strength, if i can manage ,(lets say) 250lbs of force, when i attempt to cock a band .if the band is as tight as i can possibly manage to pull back, it wont matter what thickness the band is(within 5/8" through 3/4" parameters) it will release with 250lbs if that is what it took to pull it back. therefore a 5/8 band IS equal to a 3/4 band if it is proportionately shorter. you can only get what you put in, no more no less. regardless of the size of the band . that is my position. my question is : is this right ? seems like it has to be.

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Power band rubber from around the world has different properties and different soak off times depending on percentage elongation of the specimen length. The modulus of elasticity even varies from batch to batch from the same balk rubber supplier. There can be no generalization here that will apply to shaft impact force without knowing all the gun specs.
Cheers, Don Paul

I find this thread very interesting. I've often wondered as jtk has if a short thin band and a longer thick band stretched with the same force would be equally efficient.

I note that many big blue water guns with multiple bands use 14mm bands where as single band guns use bands at least 16mm and often 18mm or even 20mm in diameter.

The graph shows some typical results for stretching a 9/16th (14mm) band and a 5/8th (16mm) band. Force in pounds is on the left vertical scale and the degree of stretch compared to the original length is shown along the bottom axis.

Note how the thinner 14mm band (lower curve) "tails off" as it is stretched between x3 and x4 times.

For 80lbs force the 16mm band has to be stretched 2.2 times and the 14mm band has to be stretched 2.8 times. At 100lbs of force the 16mm band is stretched 2.7 times and the 14mm is at it's limit on a 4 times stretch.

As pointed out the different thickness rubber bands give you more flexibility in setting up a gun and more options for loading it. Bands come in different qualities as well as diameters. Bands also do not behave in a linear manner as you can see with the graphs being curves not straight lines.

So despite all of the very good points made the original question is would a spear fire at the same speed and with the same range with the 80lb force applied by a 16mm rubber stretched 2.2 times as with a 14mm rubber stretched 2.8 times?

I don't know.

Dave.

Power band rubber from around the world has different properties and different soak off times depending on percentage elongation of the specimen length. The modulus of elasticity even varies from batch to batch from the same balk rubber supplier. There can be no generalization here that will apply to shaft impact force without knowing all the gun specs.
Cheers, Don Paul

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Hi Don

I can give you an example that you can apply and see if you can find a law for this rubber or no. First of all you need to bring the below:

1- Weight of 2 kgs 5kgs and 8 kgs.
2- Bring 3 rubber bands same length and same diameter but from different suppliers i.e different stretching behavior.
3- A bench to hang the rubber vertically.
4- you need meter tape for measuring the length.

If you connect the rubber from one side and put a weight of 2 Kgs from the other side vertically the rubber will stretch in specific X value. Going back to Hooks law F = K(X-X1) and as you said we will find that each brand will stretch in a small difference, but still we can apply this law since the stretch value will be different and the constant K therefore will be different and this proves what you said K is different from one factory to another depnds on how they make the rubber. Lets take it like this K will define the power of the band so if we have weight of 2 Kgs which will produce a force of F= mass x Gravity Then F= 2 x 9.81= 19.62 N If the power K= 5 for Brand A and K = 6 Then it will be like this:

Case A K = 5 and F= 19.62 = K(X-X1)= 5 (X-X1) then (X-X1) = 3.924
Case B K = 6 and F = 19.62 = K(X-X1) = 6 (X-X1) then (X-X1) = 3.27

As we can see when K was more which is the rubber band power was high we got less stretching (X-X1)

You can do it on different weights and see the difference while applying different weights how it will affect. Hooks law will be applied till the spring lose its elasticity, and here till it reach it is maximum point of stretching.

So despite all of the very good points made the original question is would a spear fire at the same speed and with the same range with the 80lb force applied by a 16mm rubber stretched 2.2 times as with a 14mm rubber stretched 2.8 times?

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Not always right 100% as bill says work = force x distance so if you take the values for the same gun it will be right because same stretching distance will be there approximately(Shaft will be same), but for different guns it will be different since the shaft length and weight will make the difference.

What we want is energy (work), and not force. The potential energy stored in the bands is the area under each of Old Man Dave's curves. Mathematically, the potential energy (PE) of a spring is equal to 1/2(x-x1)^2. It's that "squared" part that gets you.

So a thinner, shorter band, stretched further, would likely contain more potential energy at first than a thicker band pulled with equal force. But only if the spring rate (k) were constant, and only if it didn't bleed force when held cocked for long periods. The problem, as I understand it and Bill pointed out, is that bands generally bleed retained force rapidly when streched much beyond 3.5:1.

So small bands stretched further looks good on paper, but I think that the limitations of the rubber get in the way in real life.