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Discussion Starter #1
Puny humans,

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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The Evil Genious
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Nice one
Make an new... With 100mps
 

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Discussion Starter #4
Now I got it. You asked me to rewrite the calculator, so you can shoot faster projectiles. No problem:

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

According to this, your projectiles are approaching the speed of light, if you cut your bands as described! Attaboy.

However, if you want something more realistic, feel free to provide me with the requested data, after which I can model my calculation.
 

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I have my own, based in numerical integration, a simulator run of one of my setups, agrees closely what I have measured

input data --------------------
band fork width (cm)= 2.00
band pouch width(cm)= 1.50
fork width (cm)= 5.08
band lenght (cm)= 31.00
ammo weight (grams)= 4.00
pouch weight (grams)= 1.50
pull force (kgf)= 6.00000
band(s) per side = 1

output data -------------------
bands mass (grams)= 7.39992
elongation *** (cm)= 115.72881
elongation ratio = 4.73319
stroke (cm)= 115.81106
draw (cm)= 146.70682
ammo speed (m/s)= 97.41067 [ 319.58880(FPS)]
ammo energy (joules)= 18.97768
efficiency (%)= 62.01590

*** for comparisons between cases, elongation must be equal
this model don't takes account on heat looses, aerodynamic drag
real speed will be a bit lower

Arturo
 

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Discussion Starter #9
Hi,
I'd like to announce, that I've updated the calculator. Now it's able to display the draw weight as well as the total energy stored within the bands. More updates will follow.

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

And thanks for your measurements, arturo. If you have more like those, I'd be greatly interested.
 

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Hi,
I'd like to announce, that I've updated the calculator. Now it's able to display the draw weight as well as the total energy stored within the bands. More updates will follow.

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

And thanks for your measurements, arturo. If you have more like those, I'd be greatly interested.
Hi squish, may I ask if have you measurement chart of the force of a straight cut (no taper, in example 1cmx10cm or any other dimension) up to 6x elongation for the new TBG ?
I don't understand nor reliable read the measures you wrote in the notebook (picture), I would like to run my simulator with your measures
Arturo
 

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Discussion Starter #11
Hi Arturo,

I've tried several different cuts. If you look at the first graph, you can see the ratio in the legend. Norm1:2.95 means, that the tapering of this peace of rubber was 1:2.95. Roughly 1:3, but it made sense not to disregard the fact, that the band wasn't attached at it's very ends, at which the 1:3 ratio would have been correct.

As I've mentioned in the long german text, my measurements weren't perfect, because the rubber tended to slip through it's attachments, so the data will be slightly too low. The Measurements in my notebook indicate, how far the rubber stretched under which weight. The first column is weight in grams, the remaining six are the rubber's length in mm. The first row shows two values, which are the rubber's length at start and after finish. You'll still need the width, which I don't have at hand right now. If you wish, I could send you a link to the .CSV files I made, and also notify you, if I get more and/or better measurements.

Another thing is, that you could simply use the coefficients of my source code. Then you'd simply have to calculate a polynom. Those are:

COEFFICIENTS = [ 0.01156734, -0.11579321, 0.57734283, 0.07410897];

That function would provide you with a model of what a 1mm³ of rubber would do. I assumed, that my theraband was 0.75mm thick, which may be wrong, but you'd have to multiply the result with 0.75/1. Additionally I considered it preferable, if the function starts at 0, so the stretch-factor is always -=1. Stretching the rubber to 200% is 1, 300% is 2, and so on. In order to get the draw weight, you need to multiply the result with the width of your strip in mm.
 

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Discussion Starter #12
oh, by the way, I'm sure you've already noticed, that my speed approximations aren't reliable. They are just a hack based on a slightly flawed calculation tobse made in his calculator, which, interestingly enough, gave correct results within some limits. So far only draw weight and total energy are based on physics.
 

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The Evil Genious
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Zuggewicht pro Seite oder für beide zusammen?
Kraut: Rotkraut Sauerkraut (want some English guys to say both)
 

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Discussion Starter #14
Lars, the model calculates the draw weight of one piece of rubber, the calculator multiplies this by two. So the calculator tells you what the draw would be on your slingshot. Due to the less than perfectly measured data, this value may be ~10% too low at the moment.

Also, this is an english thread, so please don't use krautspeak here. Use this thread, if your kraut-heritage won't allow you to do otherwise:

http://www.theslingshotforum.com/f7...rechner-geschrieben-brauche-noch-hilfe-36155/

Kraut.

Kraut. Kraut. Kraut.
 

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The Evil Genious
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Thanks uncle squish
Kraut...
 

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Hi squish, I am about to test your polynomial coefficients but I need some clarifications before,

is this expression correct ?
Y = (0.01156734*X*X*X) + (-0.11579321*X*X) + (0.57734283*X) + 0.07410897

I understood this polynomial applies for 1mm3 of the new TBG rubber,
now if the coefficients are placed OK (above) then what is Y (result value) and X (input value) ? what are the corresponding units of X and Y (force, elongation)
thanks in advance
Arturo
 

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Puny humans,

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...rechner-geschrieben-brauche-noch-hilfe-36155/

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

Yes, arrogant. How about you drop the racial nicknames as well as pull your head from your backside. Trust me, when it comes free, you will hear a 'squish.'
 

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Discussion Starter #18
Dear Will,

please stop inhaling your own farts. It's not healthy, because the high concentration of methane may damage your brain and makes you a smug.

yours sincerely,
kraut.

P.S. Kraut Kraut Kraut.
 

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Vashikaran Gold Medalist
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Puny Humans!

Be still!

How dare anyone poach in Super Baba´s realm?

Puny humans! - Thou shalt not shout in wrath!

Baba is the wisdom - and the reference when it comes to "gases of life"

Envy to Baba is unknown! - But to Baba, the fine smelling fragrance deriving
from honourable Will is attractive like the clear voice of the nightingale after.
I mean after the squirrel has been freshly mated.

So behold your manners, young friend squish! - If not you are desperate to be
able to tell the mighty, brute, squishing power of SuperBaba after you tasted
Baba´s own squish and odour of enlightment being pushed out of Baba´s hottest spot!

And holier-than-thou shalt not banter about the "Kraut" - It´s way underestimated!
It´s healing life-giving nature is miraculous!

So behave, young man! - And work on your glow!

pfaaart !



Baba
 
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