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Does Dankung not use their posted shipping fee?

"The start shipping fee is $16 for THREE items. Therefor you will be charged $16 when you order one,two or three items."
They have two shipping methods. The "per item" method is much cheaper, but a bit slower. They don't do a very good job of advertising it, but if you start the checkout procedure you will be given a chance to compute the shipping. Per item shipping for 2040 is $2.92 for one 10 meter length to my US address.
 

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Discussion Starter · #184 ·
Wow! Did not know that. High shipping has always kept me from ordering. thanks for the info.
If you don't mind the longer delivery time, Dankung is, I believe the cheapest place to buy tubes. Truly Texas has great service and I have had good experience with them, but Dankung's delivered price is a lot cheaper and TT doesn't carry 2040.
 

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Wow! Did not know that. High shipping has always kept me from ordering. thanks for the info.
If you don't mind the longer delivery time, Dankung is, I believe the cheapest place to buy tubes. Truly Texas has great service and I have had good experience with them, but Dankung's delivered price is a lot cheaper and TT doesn't carry 2040.
[/quote]

The longer delivery time doesn't bother me either. I just move up the reorder point. The problem I found with Truly Texas two fold. One he misrepresents his policy saying that he will ship anywhere in the world. A quote "We ship anywhere in the world via USPS Priority Mail ". He refuses to ship to me directly saying he doesn't ship to Costa Rica. I have to have it sent to my US address and then forwarded, more time and money. Also he was posting a photo of the hemmed pouch but I kept receiving the unhemmed, cheaper pouches. So I switched to his supplier at a cheaper price, although since Dankung has increased their prices but it is still cheaper from me. My experiences with Dankung have always been polite, helpful, and courteous. I just move up my order point, no problem.
 

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Some more data:
Did some testing with my preferred setup,tapered 1745 tubes about a week ago in about perfect 24C conditions.
I whipped up a board cut with long forks. By tilting the forks forward I gained another 15cm draw to my usual butterfly.
Over the chrony it was doing about 15-20fps faster than a standard butterfly with OO buck.
Consistent readings of 320-330 were obtained,max of 350.9.
This yields at least 12ftlbs with ALOT of penetrative capacity.
Take home message:roughly 1cm draw=1fps at these speeds.
I haven't been able to match these results with 1842 or 2050 tubes.

Witness the pine board taking hits at 10 metres with rounds nicely buried!
 

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I did some quick calculations. I couldn’t find the density of lead off hand so I used the fact there are 8 #00 buck/oz which is about 54.69 gr/ball. Using 325 fps as average that works out to be 12.8 fpe. The one shot at 350.9 fps had 14.9 fpe. I recently got serious with 1745 by using a double loop. I’ve used a single loop for some time and is my favourite for general shooting. The one double 1745 band that I’ve tested gave an average of 13.6 and 12.8 fpe during the two tests at different times. My pull length is 34”. I am still on that band set with 250 shoots testing for life before checking out variations with that tubing. I think the 1745 rubber will be the one that I’ll stick with as I can reduce my rubber inventory while obtaining good performance, and I am hoping for longer life. The double 1842 gives almost as much energy at a slightly less pull weight making it more comfortable. Since doing my PT I am pulling the double 1745 with no problem now and is becoming my favourite.
 

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View attachment slingshot3pdf.pdf Recently I've reported on a physical model of a 2-section (pseudo-tapered) slingshot and its predictions. In the most recent discussion I gave results showing that velocity at constant draw (anchor) distance and pull force was better for a 2-band per side slingshot than for a 2+4 band (per side) pseudo-tapered slingshot, but worse than for a 1+2 band per side pseudo-tapered slingshot. (In my terminology, a long strand shaped into a loop with both ends connected at the pouch is considered to be 2 bands or strands.) Further analysis of this situation exposed new issues, and, I think, one approach for optimum slingshot design.

First of all, while the 1+2 band per side pseudo-tapered design seemed an improvement over the 2 band per side design, the next question was what would be the optimum length of the bands in the 2-band section of the 1+2 band design. Well, the result was that the optimum length would be zero-- which is to say a single 1 band per side (= UNtapered) slingshot is best. So, the overall drive seemed to go in the direction of minimizing the mass of the rubber bands, all the way down to one band per side. But here's the problem: the designs with 1 band per side, or 1+2, have excessive stretch factors (total length of a stretched strand divided by its unstretched length). These stretch factors at rather modest pull forces exceed what seems to be considered the maximum safe value of perhaps 5.0 or 5.5.

It turns out that one can show that the optimum slingshot design for velocity, when constrained by a maximum stretch factor, occurs at a pull force that causes the stretch factor to be the maximum allowed stretch factor. For example, if you have a 30 inch draw distance with unstretched band length of 7.5", untapered, you are operating at a Stretch Factor of 30/7.5=4.0, which is below the maximum stretch factor of (say) 5.5. A better design would use reduced unstretched band length of 30/5.5 = 5.45 inches. Such a design benefits both from allowing a larger pull force, AND lighter bands (because of their shorter length), both of which increase projectile speed.

When considering pseudo-tapered designs, calculations always show that the optimum length of the stiffer section approaches zero for highest velocity; i.e., it is best not to have the stiffer section. So optimum designs constrained by Stretch Factor will be UNtapered, and the choice of 1 or 2 or more strands of "rubber" per side will be set by how much pull force the user is comfortable with.

In the table in the attached pdf file, I show various cases illustrating these points for a 30 inch draw (from slingshot frame) shooting a 3/8 inch diameter stainless steel ball. The rows of the table highlighted in yellow give optimum designs having pull forces of 10.8, 13.8, 17.3, and 21.6 lbs, with corresponding velocities of 208, 230, 254 and 278 ft/s, all at a maximum stretch factor of 5.5. The corresponding configurations (per side) are 1 band of 2040, 1 band of 1745, 1 band of xxxx (=unknown) rubber having 1.6 times the 2040 rubber density (in g/inch), and 2 bands of 2040.

The attached pdf file also goes into my experimental determinations of the stiffness constants k1 (previously referred to as ko), and rubber density (g/inch) for 2040 and 1745 bands. I found k1=1.20 lb/inch for 2040 & 1.52 g/inch for 1745, while densities were 0.247 g/inch for 2040 and 0.317 g/inch for 1745. A priori, one expects the stiffness ratio of 1745 to 2040 should match the density ratio. In fact they do match: 1.27 vs 1.28. (For example 2 bands of 2040 in parallel would have twice the stiffness and twice the density if they were considered as a single effective band.). This means that one can probably skip the pull force vs draw measurements for types of bands other than 2040 and 1745, and just do mass & length measurements to get the g/inch of the other rubber types, from which you could infer the stiffness relative to 2040 or 1745. (But you need to weigh long lengths (e.g., 30 feet) to get accuracy.) If anyone has such density data for other rubbers, please share.

Using my measured force vs draw (i.e. k1 extraction) and density data for 1745 rubber, predicts a velocity of 199 ft/s for the conditions and setup that "Rayshot" used (earlier in this forum) when he measured 196-199 ft/s at "30 - 31 inch draw" (I used 30 inches in the calculation). So our physical model gives good predictions.

See the attached pdf file for more details.
 

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Discussion Starter · #192 ·
When considering pseudo-tapered designs, calculations always show that the optimum length of the stiffer section approaches zero for highest velocity; i.e., it is best not to have the stiffer section. So optimum designs constrained by Stretch Factor will be UNtapered, and the choice of 1 or 2 or more strands of "rubber" per side will be set by how much pull force the user is comfortable with.
I can't argue with what calculations show, but in the real world tests I made with pseudo tapered 2040, a 1:1 ratio of double to single produced higher velocities than either singles or doubles.
 

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Henry--
The concern is that your pseudo-taper worked better because you were either exceeding a maximum stretch factor on your pseudo-taper or not being at maximum stretch factor for your untapered design. To get at this, you need draw lengths, rubber type (2040 or ?), and lengths of each section of your pseudo-tapered or untapered designs, and the number of bands in each section. I totally agree that one should not be trustful of untested theory, on the other hand, theories can be a lot easier to design with than experiments once they are established. To establish a theory, one makes predictions and then everyone tries to shoot it down. Happy hunting!
 

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I'll sell you the tubesets and slingshots to prove your theories... I'd love to see the results match up with the theories.
 
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Interesting work Boby.
I tend to agree that with pseudotapers the single strand component may well be excessively stretched beyond your "safe maximum" of 5-5.5x. I don't see this as a problem however as tubes are very robust and seem to tolerate these high stretch ratios with ease.
I just suspect your missing something in your data as your conclusions aren't borne out by the chrony. My results have shown up to a 20% increase with pseudotapers. Why this occurs I'm not entirely clear. Maybe there is a compounding effect with an acceleration component of the single strand(eg 200fps) with an additional acceleration(eg 100fps) from the double strand(highly simplistic,my apologies).
Whatever the causative factor the pseudos have always been profoundly faster. Using them in a extended draw setup gives super fast speeds.
 

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As I said on the previous page theories are fine within limits. Bumble bees will testify to this, otherwise they would be grounded because theoretically they cannot. Your work would have more credibility if you published your test results, I am eager to see those results, rather than what theoretically should be happening, which is not in accordance with field findings of those reporting on such bands. As you Americans say, the proof is in the pudding.
 

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Discussion Starter · #197 ·
Henry--
The concern is that your pseudo-taper worked better because you were either exceeding a maximum stretch factor on your pseudo-taper or not being at maximum stretch factor for your untapered design. To get at this, you need draw lengths, rubber type (2040 or ?), and lengths of each section of your pseudo-tapered or untapered designs, and the number of bands in each section. I totally agree that one should not be trustful of untested theory, on the other hand, theories can be a lot easier to design with than experiments once they are established. To establish a theory, one makes predictions and then everyone tries to shoot it down. Happy hunting!
OK, I think I've got it. My real world tests don't fit your theories, so I must be doing it wrong. Thank you, and please get back to me after you've done some actual chrony tests.
 

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It will be nice to see some actual tested theories for a change. We've just been wandering through this hobby with no guidance or theories to what to shoot. I use old condoms for bands because I have no idea what I'm doing.
 

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I don't have a chrony, just soup cans. I do know that psuedo-taper goes POW! Double taper goes pow but harder pull.
Singles are good fun, too. Theories make my head hurt. like POW between the ears
 

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Discussion Starter · #200 ·
It will be nice to see some actual tested theories for a change. We've just been wandering through this hobby with no guidance or theories to what to shoot. I use old condoms for bands because I have no idea what I'm doing.
Don't use the ones you find on the beach. The sand is bad for them.
 
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