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Stretch factor

Thread Status: Hello , There was no answer in this thread for more than 60 days.
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New Member
Jul 19, 2004
Have any of you measured your spearguns 'stretch ratio', and how did you do it.?

I came up with a method but don't know if it is the same as used by others. The type of gun which was measured uses a continuous loop, not screw in plugs.

I measured the overall length of the loop and subtracted the wishbone length. IOW, the length of the rubber between tie-offs was measured. Thus, a rubber band which has 26" overall length has an effective length of 24". I measured the length of pull on the gun less the wishbone length. Thus, a gun with 42 inch nominal pull has an actual pull of 39 inches when the pull is measured between the muzzle and the rubber's tie offs. This number, 39 inches, was multiplied by two. One inch was added to account for muzzle width. The resulting number, 79 inches, was divided by 24, the effective rubber length. The quotient was 3.3 which looks to be the correct number for the stretch ratio.

The gun is the Biller 'Carribean' mentioned in the previous thread. The bands are Riffe rubber, 5/8.
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man...you need to get out and hunt more...it sounds like youve got too much time:D Guess all this weather is getting to all of us.

Since each batch of sling rubber will be diffent, even the same size tubing, I start with a typical band length for whatever gun Im shooting...say 27 inches on my bluewater gun and if it feels a little too loose I shorten them up by a half inch to an inch until I am satisfied with them. I know Mori out in cali tests rubber batches and published them with the lot #'s as well as % of power lost over time stretched. Surprise me the variance between batches of even the same brand. I will try to find his site.
Amen, rig diver. It was just that thought that had been running through my head. There was a town that had been listed as THE BEST place in the USA to live/retire. If my investments had not gone in the toilet I would have already been a resident......of Punta Gorda.

When I'm not diving I like to putter in my shop, actually a large diving locker with work bench and some tools. Routinely, I work on military surplus air compressors. It's amazing what military power can do on a weight/output basis. I sell one now and then. I also help others. Recently, I sent some parts and advice to a guy in Turkey, gratis. Lately, stimulated by a lively discussion of 'euro' guns, I've taken a second look at my stuff to understand some things and reason out whether I am up to date, or keeping up, at least. I suddenly had this thought that a custom builder might ask me "what stretch ratio do you want?" Rather than blurt out "DOH"!!, I'd like to have a bettter answer in hand.
Murat, thanks. I'll never use 3/1. I know some people do. However, it is possible to go overboard. My Riffe island is 3.8/1 at about 200 lbs each. I took another look at Riffe standard rubbers. Theyi also sell a Riffe 'special'. You can guess what the power is on that one. Too much for me. Rubber is not linear and there is probably a reward to effort ratio which is different for differing rubber types. I can tell you that the arrow that flys out of the Riffe muzzle is 'carrying the mail' as the old timers used to sayi.I will settle for 3.3-3.5 from now on.
Murat, I made an obvious mistake in my method of measurement. Maybe somebody caught it. It turns out that my guns furnished with 5/8, Mori, hi mod rubber are actually set up with a 3/1 stretch ratio. One could say it was by accident or coincidence. Actually, it was the maximum weight that I wanted to pull and that's just how they came out. So, you were right. That does appear to be the average, practical ratio for heavy rubber, for me. I apologize for the error.
hovewer few cms further strech of bands gives much more power. May be its better to shorten the bnads 1-2 cms acccording to the needs.
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