# Just made a new Band-Calculator. Need some help for measurements.



## squish

Hi,

I've reimplemented tobses Band Calculator in JavaScript, so it's now useable as a Browser App. Here's the link:

http://squish.111mb.de/slingshot/blob.html

However, tobse made a mistake while calculating the volume of the rubber, so my calculator still needs to be calibrated to get some meaningful result regarding the resulting speed of the projectile. It would be nice, if someone, who owns a chrony, could provide me with the resulting projectile speed of several band configurations. The calculator assumes, that you pull the bands to 500% of their original length, therefore you should add a free hanging rope with a knot at the release point to the fork, so that the results are more consistent.

Another feature I'd like to add is another field, which tells me the draw force of the different bandsets. For this, I'd need a few measurements I can't do myself, because I don't have the tools ready. I'd need a diagram of the pulling force by stretch distance of several standardized bands. If someone here could do that, it would be great.

You would need a hanging scale to measure the pull force and some workbench to attach the band to. For example, I'd need to know how much a 2x20cm untapered band (don't forget the attachments) pulls, while it's stretched over various distances. One measure point for every 10% stretch over the distance of 700% - 750% stretch distance would be reasonable. I'd need the same for the various taperings (1:1.5, 1:2, 1:2.5, 1:3) for the whole calculator to work. The exact width of the band would depend on the scale, because it should make use of a significant part of the scale's range, in order to reduce it's imprecision.

In any case, the performance of rubber depends on the ambient temperature, so this factor should also be noted.


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## pgandy

I see what you are innocently asking and I could give you figures. However, I think they would be misleading to say the least.

I know of no substitute for a chrony and they are not cheap. My chrony has taught me many things about my slingshot and blowpipe. Mainly neither are mechanical devices and various outside influences can affect the results such as posture or the particular batch of rubber I use. I also find that slingshots are not an absolute science and find variation from band set to band set. I understand that TBG's formula has been changed. I rarely use it but noticed the last batch that I bought on Amazon was different than the previous stuff. I blamed the vendor for playing switch. The information on velocity that I can provide would be meaningless for anything more than the roughest approximations.

But not to ignore your request completely: Over the last 2 year period using a double pseudo type 1745 band set with a 3.5" loop and tail each and drawing to 34" I've had and extreme spread of 195.6-160.2 fps using ½" lead ball and both black and natural rubber. The 5 shot group averages would be 192.1-163.5 fps, with a maximum fpe average of 14.17. Not really the type of info that one would use as a base for future calculations. The deciding point for me is the average of a 5 shot groups over time in relation to band life, both of which I now monitor along with accuracy. I am convinced that there is variation in rubber from batch to batch which is not surprising especially with natural rubbers as they are depended on climate conditions that change from year to year as well as differing growing locations.

For draw weight I recommend you buying a scale. I believe this in the one that I bought http://www.amazon.com/Portable-Digital-Hanging-Luggage-Fishing/dp/B005JA5U88/ref=sr_1_4?ie=UTF8&qid=1415640350&sr=8-4&keywords=fish+scale and it looks like they are have a special at $3.30 including shipping, but I did not completely read the ad. Which one you choose, opt for one that will lock and hold the reading until you have your info. That and a metre stick should give you more information than anyone will provide.

I find testing the various combinations, which appear to be limitless, the better part of the fun. I know this is not the answer you were hoping for but I believe it is the way to go.


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## hainfelder

well - a well made calculator would be nice

fed with the right data ...


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## Rolex

Great :bowdown:

Please also for tubes ...


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## squish

Hi,

due to the fact, that most answers I've got were from krauts, the discussion will be continued in this thread:

http://www.theslingshotforum.com/f7/ich-hab-nen-neuen-bandrechner-geschrieben-brauche-noch-hilfe-36155/

the thread's official language is krautspeak. If anyone needs a translation to english, feel free to ask.


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## pgandy

My German is not the best but BeMahoney is saying essentially the same as I did, I think. Wishing you luck.


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## unkraut

Ahoj, weitrer unten in Deutsch..... blättert hinunter...

google Translate:

Moin moin,

I've Tried it once implement something ...

But that is nothing !!!

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Why?

Pretty much as soon as the ball speed decreases when one considers the band a little longer, so diminishes the traction !!!

My luggage scale will beep for a moment, if there is no big change is more, and displays the value ....

If you at the same excerpt, the measurement is repeated a few times, there are other values, respectively.

If you keep the numbers as far as the eye, so it looks all the time ... you can see that. Within 2..3 Sec the value of about 100 gr.

was tested TB Black gives way ....

At low strain, it is hardly measured bahr, so to neglect, but in our areas, if we understood the band pull up to the limit, or beyond .... is it too much!

I try to hold the trials in the clip, let's see if I can upload it later on yt ....

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Furthermore - practice, or error in reasoning with you !!!

With a tape rejuvenation, the tape is not easily changed ....

But if one has for his statement, and his bullet weight, the ideal amount of tape is at the Pouch taken away what ... this has to be then added thereto at the fork side again ... so sqmm latex remains the same, but the pull resistance is then also the same?

My attempt so far with 20 20 20 he band has just so strong deviations in the measured wiederhohlten shown, I would say .... hardly detecting bahr !!!

Try it just once with TBG.

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So, either someone else confirm my observation ... or are you doing the same ... or another, myself ...

Or come drop by, and we meet, meet for shared: and test ....

Approximately 2 hrs. Drive from you ......

Moin moin,

ich habe es mal Versucht etwas umzusetzen...

Aber das wird nichts!!!

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Warum?

Ziemlich genau so schnell, wie die Kugelgeschwindigkeit nachlässt, wenn man das Band nur etwas länger hält, so lässt auch die Zugkraft nach!!!

Meine Kofferwaage, piepst nach einem Moment, wenn keine grosse änderung mehr ist, und Zeigt den Wert an....

Wenn man bei gleichem Auszug, die Messung ein paar mal wiederholt, gibt es jeweils andere Werte.

Wenn man die Zahlen soweit im Auge behält, also, die ganze Zeit drauf sieht... kann man sehen, das innerhalb von 2..3 Sec. der Wert um ca. 100 gr.

getestet wurde TB Black, nachgibt....

Bei geringer dehnung, ist es kaum messbahr, also zum vernachlässigen, aber in unseren Bereichen, wenn wir das Band fasst bis zur Grenze ziehen, oder darüber hinaus.... ist es zu viel!

Ich habe Versucht die Versuche im Clip festzuhalten, mal sehen, ob ich sie später auf yt hochladen kann....

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Desweiteren - Praxis, oder Denkfehler bei dir!!!

Bei einer Bandverjüngung, wird nicht einfach das Band geändert....

Sondern, wenn man für seinen Auszug, und seinem Kugelgewicht, die Ideale Bandmenge hat, wird an der Pouch was weggenommen... dieses muss dann an der Gabelseite wieder dazugefügt werden... qmm Latex bleibt also gleich, aber bleibt der Auszugswiederstand dann auch gleich?

Mein Versuch bis jetzt mit 20 20 20 er Band, hat halt so starke abweichungen beim wiederhohlten messen gezeigt, das ich sagen würde.... kaum feststellbahr!!!

Versucht es halt mal mit TBG.

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Also, entweder, jemand anderes bestätigt meine Beobachtung... oder du machst die gleiche... oder eine andere, selber...

Oder komm mal vorbei, und wir treffen uns, zum gemeinsammen treffen, und testen....

Ca. 2 Std. fahrt von dir......

PM zu mir, Berichte übersehe ich hier schnell.....

Ach ja, mein Kordelstop, hat einen Grund, wenn ich einen Stärkeren einbaue, könnte es sein, das es mir als Armstütze - hilfe - ausgelegt wird.... darum.... nee, Danke, bleibe ich lieber dabei....


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## squish

Hi,

first, I'd like to thank you guys for your input, it's greatly appreciated. I've posted the link to the other forum earlier, because I've got more responses from there, which also were mostly by german speakers. I thought it easier, if the communication takes place in a single point, because this way I don't have to maintain several different groups of people. However, you guys seem to prefer this forum, so I'll write here too.

I know, that you have some considerations regarding the computability of rubber, and it's true, that the physical properties of rubber are different, depending on a variety of factors. Where it's from, how old it is, how it has been stored, which weather it is, and so on. It's true, that all those things change its performance, I'm aware of that. But this only means, that the computable result is somewhat imprecise. I think, that a imprecision of +-10% would be a achievable result for such a calculator, which wouldn't be too bad, if you ask me. After all, everything people use to build things have their tolerances, including wood, steel and concrete, and last time I checked, houses and skyscrapers were still standing. Or think about bungeejumping - it's possible to calculate the fall of someone precise enough, so that the elastic rope securely stops the fall of that person a few feet, before he hits the ground.

Today I've measured the draw of several different bands, and I was unable to reproduce the problems mentioned by unkraut. Quite the contrary, even though I had a different approach. I think, that unkrauts inconclusive results may have been caused by various factors. First, the scale seems to display a static result after a length of time or after the measurement doesn't change more than a given amount within a given time. It appears, that rubber changes its draw force over a short peroid of time, until its molecular structure reaches some kind of equilibrium, from which on the pull remains static. As far as I understand, rubber heats up, while it is stretched, and it takes some time, until its balls of molecular strings have reached a stable configuration, and so on. However, this doesn't mean, that its impossible to measure, but that the scale is a bad choice for this project.

To find out, if that's the case, you might try another experiment: Try to attach one side of the rubberband to something elevated, and attach a bag to the other end, so that the bag hangs freely. Then you can put a bottle of water into the bag, wait until the rubber doesn't bounce around anymore, measure the distance between the attachments, put another bottle into the bag, measure again, take one bottle out, measure again, put it in again, and so on. I did this the better part of this day, and I had very little variance, while measuring how much the rubber stretches by a given weight. A had the problem, that the rubber sometimes slightly slipped through my attachments, so it hat a slight tendency to physically become longer, but the majority of measurements stabilized within a range of less than +-2mm, even when the rubber had been stretched to 600% of its original length and more. If you'd be able to reproduce this result, it would mean, that there is a way to approximate the energy stored within the bands, and that would be a good starting point for further calculations.

This was my (hardly improvized) setup:
http://666kb.com/i/ctke920vy1p017fa9.png

This kind of attachement tended to slip:
http://666kb.com/i/ctkebb8ltdtc3xu75.jpg

Therefore, next time I'll use something like this, but with less space between the constrictor knot and the loop:
http://666kb.com/i/ctkebh7i6cfgv5ogx.jpg

My method was to put steel balls into the bags until the rubber stretched 10cm more than the last time, separate the bags from the rubber, weigh them, put them back on, check the distance, fill more steelballs into the bag, etc.. The only problem so far was, that the rubber tended to slip through the attachments, if there was no pull on it, so I hung in a bottle of water, while the bags were on my scale.

At the moment, I'm trying to acquire enough data to make the following calculation:

http://666kb.com/i/ctkdu5bblu5zy05k1.png

Blue -> Draw Weight
Red -> Variable Draw Length
Green -> Total Stored Energy

The idea is, to calculate the total energy stored within the bands in order to find a relation to the energy transfered to the projectile. The first part is easy, and the second part will be some calculation I don't know yet, which will result in the efficiency of the energy transfer. I've got a few theories, but in order to find out what works best, some standardized tests will be neccessary.


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## unkraut

Ahoj,

https://www.flickr.com/photos/[email protected]/sets/72157649306682311/

Alles zu meinen Versuchen.... alle Daten, auch gescheiterte....

Ich hoffe, man kann erkennen, wie die Zugkraft abfällt.

Die Frage ist jetzt, welche ist Interessant?

Die Kraft, mit der man wirklich, das Band nach hinten ziehen muss....?

Oder, wenn man es zum Zielen, hinten etwas gehalten hat?

Oder, die, wenn man die Kugel loslässt..?

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Bei deinem Versuchsaufbau, mit Wasserflaschen, oder Kugeln, wird nicht der Kraftaufwand berücksichtigt, dehn man wirklich benötigt, um das Band nach hinten zu ziehen..... Den du misst ja erst nach einem kleinem Moment! Also, wenn das Band sich berühigt hat, und nicht mehr schaukelt!!!

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Bei meinem Versuch, wird von anfang an gemessen!!!

Frage, was bringt jetzt mehr?

Was ist für die Praxis - also beim Schleudern, wichtig mit einfliessen zu lassen!

Ich hatte es so verstanden, das man herrauslesen soll, wenn der Bandrechner das mal kann, welchen Kraftaufwand Mann bei dem Band aufwenden muss, um es Spannen zu können!!!

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Hausbau, Druckfestigkeit von Steinen, und Stahl!

Ja, es ist bekannt, deswegen, wird hier ja auch ein unterer Grenzwert genommen, in der Norm, heißt es meistens, das der Stein mindestens, einen Druck, von xxx aushalten muss!!! Welche Aussage, hätte das bei uns mit dem Latex? Mindest Zugkraft? aber es kann auch sein, das du 30% mehr aufwenden musst? Oder müsste man hier dann max. Zugkraft angeben?

Ansonsten, vermerke das in deinem Bandrechner.... und weise auch gleich auf die Toleranzen hin, und wie viel es sein kann.... bloss welchen Zweck, hat der dann noch???

https://www.flickr.com/photos/[email protected]/

Gruss


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## unkraut

Moin moin,

wie Jörg geschrieben hat...

gut, 40% ist verdammt hoch gegriffen... könnte man erreichen, wenn man ganz dolle Chargen, Temperatur unterschied, und langes Halten zusammenzieht....

Aber 15 - 25% Also, ein viertel, sind schon drinn... und ob man ein viertel mehr Kraft aufwenden muss, um etwas ziehen zu können, ist schon ein Unterschied!!!

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Das würde ich auf jedenfall mit berücksichtigen!!!


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## squish

Hi,

for those interested, there is an updated version of the calculator.

Behold: http://squish.111mb.de/slingshot/v1.1/blob.html

It's now able to calculate the draw weight as well as the total energy stored within the bands. I'm confident, that, at least for theraband gold, those two values will be quite usable.


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