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What type of rubbers?

Thread Status: Hello , There was no answer in this thread for more than 60 days.
It can take a long time to get an up-to-date response or contact with relevant users.
GreekDiver said:
Bill your 350% is probably 250% stretch!
Click to expand...

You're right, but it depends on your precise wording. Mine are stretched 250% longer than when relaxed, but that makes them 3.5 times as long as their relaxed length. Right?

When I talk band stretch, I'm referring to the ratio of the stretched length to the original length, while some people seem to refer to how much additional length the bands are stretched. Its confusing.

Just to be perfectly clear, here is the way I calculate band length.
____________________________________
1. Measure the distance from the middle of the
band slot to the center cocking tab.

2. Double the distance ( for both sides of the gun )

3. Subtract the length of the wish bone. (Alexander and Riffe's are
approx 6", Daryl Wong uses about 3".)

4. Divide the number by 3 1/2.

5. This is the length of your powerband if you are using a standard 3 1/2
: 1 stretch ratio. This is a measurement of the powerband from knot to
knot. For actual rubber length, add 1" to this length or 1 1/2" in you
are tapering the ends of the powerbands.

If you have separate holes for each band as with the newer Aimrite/Wong muzzles, then rather than measuring from the center of the band slot (you don't have a slot) to the middle tab, measure from each hole to the appropriate tab, and you will have a different band length for each hole.
 
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Precent Elongation= Final gage length Minus original gage length divided by original gage length times 100.
Cheers, Don
 
Don Paul said:
Precent Elongation= Final gage length Minus original gage length divided by original gage length times 100.
Cheers, Don
Click to expand...

Fine, but I didn't use the term "percent elongation." I'm talking final length as a percentage of relaxed length.

Let me try one with the math simple enough for me to understand. Lets take a 12" piece of rubber and stretch it to 24".

The increase in length was 100%.

But total length is 200% of initial length.

The rubber is 24" long, but you chose to say how much it increased in length, and I choose to say how long it is as a multiple of original length.

It got 100% longer, but its 200% of original length.
 
Bill I wasn't addressing your formula, I knew what you were using as a yardstick. I was using the standard I use when testing metal yield.
Cheers, Don
 
When you load your guns, do you draw to the first or second notch?
The reason I ask is this; I ended up calculating my band length using a method I saw online (link below). It advised measuring the length from the muzzle to the first notch and multiplying by 2/3rds. For my Seatec Snake 90, this made the band 47.5cm. So I allowed for the wishbone and cut at 52.5cm. The finished band is around 50cm. When I asked Seatec for advice on what single band I would need to fire the 6.5mm spear they said use 1 x 50cm 20mm rubber'. So I figure the equation above works ok BUT, I can only draw the band to the first notch and there are three notches. This is not because I am not strong enough (grrr), it really does not feel like it wants to stretch any further, so now I am curious. Would there have been any advantage to measuring to the second or third notch and ending up with a band that can be pulled further down the spear? If I had calculated to the third notch, the band would be 60cm.

Here the link to the formula I used:
● Calculate Speargun Band Size : Spearitco.com, The worlds largest selection of Speargun Bands and Do-It-Yourself Spearfishing Supplies

PS. If you have never cut and tied your bands I would wholeheartedly recommend it! An immensely satisfying experience! I even bought a dremel to round the edges!
 
Dave there is no one answer. Different sorts of rubber have different properties. The idea is to strech the bands to their optimum length. For some bands that's 3 times their natural length and for some bands it's 4 times. Longer stretching also requires more strength. In Terry Maas's book he has some interesting info on testing rubber and how and why etc.

Back to the plot. Most rubbers commercially available are designed to stretch an average 3.5 times for optimum performance. My standard Beuchat gun with standard rubbers has a 3.2 times stretch to the first notch and a 3.5 times stretch to the back notch.

Unless you have some super rubber then I'd go for a 3.5 times stretch and that would be to the back notch.

Dave.
 
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20mm bands are a bitch to pull back!
The longer distance the band stretches the better - that's why a 110 has more power & range than a 100.
so yes definitely better to load to the back notch in the performance stakes but probably not necessary.
3 0r 3.5 x band stretch is a guild line as different rubbers behave in different ways.
 
Thanks for this! So you think make a longer band and pull to the third notch or just pull this band harder?
 
Many thanks!

I have just calculated the band length using the three different methods in this thread:

All to the last notch:
First method: 61cm
Old man Daves: 55cm
Bill: 52.85

My last band was 55cm and I never got it past the first notch! Now I see what Foxfish means when he says 20mm are a bitch to pull!. I think I will cut another at 60 and see how that feels!
 
Dave if its any concellation i put a 20mm @ 60' on my new woody gun and its a bitch to pull back but with some good technique and loading in the water it will be no prob, very powerful.
 
Reactions: Mr. X
On my Omer XXV Gold 90cm gun the bulk band length I am using at present is 60cm total.

The working length is shorter when you allow for tying in the wishbone. I use longish tapers on the band ends and a short wishbone. The XXV has a notch right at the end (back) of the spear making for a long stretch. All this works out to around 3.5 times stretch.

DB member Mundial uses a 57cm total length band on his XXV 90 but maybe his tapers and wishbones are different to mine. Standard bands are 18mm diameter.

New bands are a hard pull early on in the season but practice and "bedding in" of the bands makes it easier as time goes on.

I ran 20mm bands on a big beuchat and tried them on other guns. I found the guns were overpowered by the bands and while powerful were inaccurate (except on a woodie). 18mm I have found to be good but 16mm work well for me too. I prefer accuracy to range.

Dave.
 
Reactions: Mr. X
Dave is on holiday at the mo but he uses the standard spear that came with the gun, 6.3mm but not sure on the lenth however he is always going on about getting a new sharkfin spear.
 
Could only find my USA imperial tape measure but my 90cm XXV spear is 49 & 3/8" long, which is 125cm -- 5cm shorter than normal. I believe the same is true of my 75cm XXV - the Omer XXV's spears are 5cm shorter than normal. I like that. It means that my 90cm XXV is shorter (as well as slimmer and lighter) than my RA90 - so it acts as my 80cm/Medium-size gun, sort of (actually the 82cm version would have made more sense in hindsight). And the 75cm guns acts as my 70 & 75cm gun (I reckon it would be comparable to an RA70 for power & handling).
 
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