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Unread postAuthor: JoergS » Tue Aug 04, 2009 10:06 am

What I mean is that I use 8,5" long bands (knots and pouch not counted, just the active length) and draw them out to about 47" in my "Half Butterfly" style.

Then I get max power from the black tubes.

With the silver tubes, I can maybe draw those out to 30" or so. The black tubes at 550% have a lot more power than the silver ones at 300%.

I am already a "bad boy" on the Trumark slingshot forum, especially as my last video is pretty critical about their S9FO product (weak, poorly designed). So I doubt that they would tolerate such a thread there. But we can try!

Jörg
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Unread postAuthor: JoergS » Tue Aug 04, 2009 10:30 am

Oh, Stu, one thing.

If you want to achieve supersonic speed, then do not use a .177 pellet. Use a lead or steel BB.

Too much pressure on a pellet will cause the projectile to tear at the thinnest point. The front part will fly out, the funnel shaped rest will be pressed firmly against the barrel wall. A nightmare to scrape it out.

That won't happen if you use a BB.

Will try to get a fish scale, Wall Mart gave up in Germany a few years back so I have to find another source.

Jörg
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Unread postAuthor: boyntonstu » Tue Aug 04, 2009 10:51 am

JoergS wrote:What I mean is that I use 8,5" long bands (knots and pouch not counted, just the active length) and draw them out to about 47" in my "Half Butterfly" style.

Then I get max power from the black tubes.

With the silver tubes, I can maybe draw those out to 30" or so. The black tubes at 550% have a lot more power than the silver ones at 300%.

I am already a "bad boy" on the Trumark slingshot forum, especially as my last video is pretty critical about their S9FO product (weak, poorly designed). So I doubt that they would tolerate such a thread there. But we can try!

Jörg


Joerg,

I purchased the Silver on your recommendation.

47/8.5 = 552% of its original length.

However, according to Thera-Band the elongation is 452%.

Both Thera-Band and you indicate that one should not go beyond 350% for extended use.

If I pulled the Silver to 400% elongation would it not have more power/speed than Black at 452% elongation and live a long time?

Bottom line: Black or Silver?

BoyntonStu

P.S. Any forum that would ban a user for presenting a honest factual review is not worthy of the reviewer.

Stay here and you will be accepted, appreciated, and admired.

I also recommend: http://www.airgununiverse.net/community/
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Unread postAuthor: JoergS » Tue Aug 04, 2009 11:18 am

Stu,

the silver tube is perfect for your plan (crossbow like design).

The black tube is perfect for my slingshots. The silver tube is too strong for a slingshot.

At the same draw factor, the silver one is MUCH better.

The Hygienic guys are very careful with what they recommend. Understandable in a world with more lawyers than teachers.

And it is true, if you stretch the tubes to the max (which really is 550%) then they do not last very long. I do it anyway, life is short so what.
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Unread postAuthor: boyntonstu » Tue Aug 04, 2009 11:49 am

JoergS wrote:Stu,

the silver tube is perfect for your plan (crossbow like design).

The black tube is perfect for my slingshots. The silver tube is too strong for a slingshot.

At the same draw factor, the silver one is MUCH better.

The Hygienic guys are very careful with what they recommend. Understandable in a world with more lawyers than teachers.

And it is true, if you stretch the tubes to the max (which really is 550%) then they do not last very long. I do it anyway, life is short so what.


OK.

How long is "do not last very long"? 10, 100, 1,000 pulls?


Silver it is.

BoyntonStu
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Unread postAuthor: JoergS » Wed Aug 05, 2009 3:01 am

Well, that involves a lot of factors.

First, you have to make sure that the rubber never touches anything. Even the slightest contact with any part of the setup (wood, metal, the likes) will cause chafing and the rubber will tear at that point. Therefore, a Hi speed camera like the one I use is very handy as it allows you to optimize that.

Second, the binding is very important. You are already using the best method I found ("Knot in tube"). However, sooner or later the rubber will tear at the knot position. You have to find the right string material. A very dense string will make a very hard knot, and the rubber will break earlier.

If the string is too soft, the binding will eventually come loose. So it takes some experimentation. Once you found the best available string, buy a whole lot of it.

If you always stretch out to the max. then the black rubber will last about 150 shots if you have optimized the above points. The silver rubber is much more stable and will probably last much longer. However, I have no reliable data on the silver tube.

Jörg
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Unread postAuthor: boyntonstu » Wed Aug 05, 2009 6:49 am

JoergS wrote:Well, that involves a lot of factors.

First, you have to make sure that the rubber never touches anything. Even the slightest contact with any part of the setup (wood, metal, the likes) will cause chafing and the rubber will tear at that point. Therefore, a Hi speed camera like the one I use is very handy as it allows you to optimize that.

Second, the binding is very important. You are already using the best method I found ("Knot in tube"). However, sooner or later the rubber will tear at the knot position. You have to find the right string material. A very dense string will make a very hard knot, and the rubber will break earlier.

If the string is too soft, the binding will eventually come loose. So it takes some experimentation. Once you found the best available string, buy a whole lot of it.

If you always stretch out to the max. then the black rubber will last about 150 shots if you have optimized the above points. The silver rubber is much more stable and will probably last much longer. However, I have no reliable data on the silver tube.

Jörg


Joerg

Very good information.

I learned a lot about stretching yesterday using the fish scale.

After a point close to 300%, additional stretching adds almost nothing to the force.

Without the scale it seems that you are gaining power, but it is not happening.

My guess is that you are drawing the rubber too far.

Try a scale and you may be surprised.

BoyntonStu
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Unread postAuthor: JoergS » Wed Aug 05, 2009 6:55 am

Stu, I don't have to use a scale as I own a chrony, so I can measure the speed of my bullets.

The silver tube is indeed very thick, and therefore you may be right. But the black tube has to be stretched to 550% for full power.

Maybe that is only the case because the acceleration process is longer, though.

What kind of force results (in pounds) did you get from the silver tube?

Jörg
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Unread postAuthor: boyntonstu » Wed Aug 05, 2009 7:10 am

JoergS wrote:Stu, I don't have to use a scale as I own a chrony, so I can measure the speed of my bullets.

The silver tube is indeed very thick, and therefore you may be right. But the black tube has to be stretched to 550% for full power.

Maybe that is only the case because the acceleration process is longer, though.

What kind of force results (in pounds) did you get from the silver tube?

Jörg


'Maybe that is only the case because the acceleration process is longer, though.'

I thought of that.

A chrony test:

Pull the black 350% and measure speed.

Repeat at 550%.

There may be a point of diminishing return.

If you go beyond that point you get unnecessary wear and tear.

For powering a slingshot, it matters less.

With my crossbow, additional rubbers could be added to achieve the desired power.

BoyntonStu
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Last edited by boyntonstu on Wed Aug 05, 2009 9:38 am, edited 1 time in total.
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Unread postAuthor: JoergS » Wed Aug 05, 2009 7:56 am

Will do the test today, Stu. My gut feeling: Big difference.

Jörg
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Unread postAuthor: JoergS » Wed Aug 05, 2009 12:25 pm

Stu,

I did not have a setup with the black tube ready, but had one with the weaker green tube.

350 % 44 Meters per second, 550% 62 Meters per second.
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Unread postAuthor: boyntonstu » Wed Aug 05, 2009 1:05 pm

JoergS wrote:Stu,

I did not have a setup with the black tube ready, but had one with the weaker green tube.

350 % 44 Meters per second, 550% 62 Meters per second.


Almost a 50% difference!

Wow!

Next, we need some pull data.

How many pounds of pull at 350% and at 550%?


Fish scales are cheap and they are always on Ebay.


To increase the capacity, use a dual pulley for 100 pounds, etc.


Another way to measure pull is with weights and a pulley.

Add a weight and measure elongation.

BTW Rubber mounting: Would a smooth rod with a smooth groove and a constriction knot help to prevent a hard knot from pulling through?

Another idea for your knot in rubber method is to cover the knot with a piece of plastic tubing

Thanks,

BoyntonStu

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Unread postAuthor: JoergS » Thu Aug 06, 2009 2:27 am

Stu,

a smooth hard object works fine, but in a slingshot, you try to avoid anything that weighs. Means, there is little enough power. Also, hard objects are a serious danger in case the rubber tears.

In your design, I don't think the weight matters, and you have put on a shield for your face anyway. So for your gun, this would work.

Covering the knot in rubber would work, but then the assembly is tough because of the friction.

Here is a picture that shows how rubber typically breaks:

Image
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Unread postAuthor: spud downunder » Thu Aug 06, 2009 3:08 am

hi joergS, i am a member of the usa slingshot forum, im snapgoestheslinshot. ive seen some of your videos and slingshots on there, there great, and i like stus slingshot/crossbow desighn here. keep up the good work guys
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Unread postAuthor: JoergS » Thu Aug 06, 2009 5:03 am

Have converted the system for quick-change bands.

The green ones are really light, about 90 Meters per second, great for training.

The silver ones... my oh my, I can pull them and the power is enormous, but aiming is really hard.

Will put an aimpoint red dot on today, for what it's worth.

Jörg

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