Monday, November 28, 2011

On bands: The Break-In-Effect

In order to further clarify the advantage of tapered bands, I re-watched the slomos done with Destin's camera.

I noticed that the elongation of the rubber close to the pouch was a lot higher than the factor 5,5 (which is the way the band was cut). It was more like 7.

So it seems possible that the extreme overstretching is responsible for the speed advantage, as that does not happen on untapered bands.

In order to test this, I made a stretching stick. I put on a 2cm wide strip of TB Gold and markered the 10 cm length point. Then I attached a hand grip, with a karabiner in order to draw with the scales.

The first interesting finding: TB Gold can be stretched to the factor 8! That is right. But the draw weight increases steeply between 7 and 8.

Second interesting finding: The relaxed length increased. After stretching it out to the 80 cm, the relaxed length was 12 cm instead of 10...

Of course there is hysteris, means, if you keep the rubber drawn out it looses power swiftly. But it recovers, and fast. After 10 minutes, the relaxed length was 11 cm.

Third finding: Stretching the rubber that far breaks it in, means, changes it forever. The rubber did not go back to 10 cm. It stayed at 11, even after a few hours.

Fourth finding. Once broken in (stretched out to 80 cm and held there a bit), the rubber permanently looses draw weight.

See the attached graph - after the break in, 80 cm draw can be achieved with the draw weight needed for just 70 cm in unbroken in condition.

What does this mean? Well, the next experiment must be to cut a much shorter and thinner band set than usual, without tapering, that can be stretched to factor 8. The chrony will tell what the effect will be.


  1. Does this mean that tapered bands will lose strength faster?

  2. The limpness that occurs after a band has been overstretched is apparent to anyone who has used sling-shots for any time. One learns to not overstretch to avoid this. There seems to be a specific point where max rebounding is achieved wihout destroying the rubber and it's more apparent in colder weather. Extreme tapering will also destroy the rebound ability at the most tapered end very quickly as it tends to overstretch the tapered end too easily thus leading to ruining the band and premature tearing of the tapered end. Finding the proper balance of tapering at that point of max rebound ability without going over that point has been my quest and problem until your calculator came along. Since I have used your calculations, I have been satisfied with band life and rebound ability, but I think there might still be some need for experimentation such as what you have started. I await most eagerly your further data. I have lacked instruments such as your chrony and slow-motion camera to give valid evidence of what I have observed and 'felt' so perhaps you will understand my eagerness for your continued experiments. All the very best to you!

  3. Well, I think that maybe the way to achieve better performance is to "overstretch" the bands.

    What if an overstretched, straight cut band is faster at the same draw weight than a tapered band with a lower stretch?

    We have to explore that route, maybe just for the peace of mind.

  4. One experiment I have done is to make two halves of one band to test the speed and power. The only time I did so left me with inconclusive results but with your chrony and your slowmotion camera, we might be able to see the difference. It 'seemed' to be faster and with an easier draw, but unfortunately, my tapered ends tore rather quickly and I gave it all up.

  5. Your information is so helpful. Have you written a book on the subject?

  6. This is totally badass. I never realized how much I missed sling shoot when I was a kid. I love it. More, More, More!!!

  7. You should also show the contraction cycle, which is more relevant. You may use my charts, Jörg.

    What I really want to know is a chart of the area under the force-contraction curve against various elongations and the elongation subsequently.

  8. Sorry to restart this, but if it helps: all polymers have a Yield point.
    Stretching a ploymer is a non-linear business. The polymer will stretch linearly to a point, but from then, the extension per unit force will decrease, until the yield point.
    At this point, the polymer will stretch quickly and easily, and extend a relatively great distance. This is permanent.
    Interestingly, though, any extension until this point will return to its original size, or close to it. If the stretch is in the linear phase, it will return completely.
    Therefore, and scientifically proven, if you want your slingshots to fire repeatedly, stop stretching when the draw weight increases. Past here, you will lose energy on any subsequent firings.
    Look up 'Polymer Force/Extension graphs' online, see here:
    Hope this helps explain in some way :)