Unusual bow behaviour

Bede · Post by **Bede** » Sun May 03, 2009 10:58 pm

This photograph was taken with a camera in a phone. The scanning of the image by the camera created a time lapse between the upper half of the picture and the lower half.

Some comments:

The upper limb is shown in two stages of the release, at full draw and moving forward nearer the grip.
The lower limb is shown on the rebound after the arrow has left.
The bowstring is shown oscillating on the lower limb taking up energy left over from the shot.
The arrow is oscillating after leaving the bow.

Stephen Selby · Post by **Stephen Selby** » Mon May 04, 2009 12:59 am

Alternatively, could this be 'bow-shock' in action? That is, that the bow is rebounding after the string has reached full tension following the shot?

Bede · Post by **Bede** » Mon May 04, 2009 3:40 am

Stephen,
I think you are right, but there is absolutely no kick in the hand. I hold the bow firmly and I think that if there is any imbalance in the limbs I would feel it. However, the left over energy is certainly re-bending the limbs. Perhaps there is a slight asymmetricality in the bow which is preventing hand shock.

Incidentally, the draw was exactly 28 inches and the bow is 40-45 pounds at that draw. The target was a plastic bottle on a bamboo stick at 40 metres. I rarely hit it at that distance.

Jos?Dom · Post by **Jos?Dom** » Tue May 05, 2009 4:19 am

Very strange, looking at the picture I guessed it was a very low poundage bow because of the string behavior not a 40-45 pound bow. Looks like a commercial Korean style bow without rigid siyahs , Am I right?

Bede · Post by **Bede** » Tue May 05, 2009 4:39 am

Jose,
Good guess. It is a 25 year old Korean modern laminated bow similar to the modern Samick bows. Its nominal draw weight is 50 pounds but I think it really draws 45 pounds at 28 inches. I usually use longer arrows with this bow but I was shooting at a special target and had shorter arrows with the right type of heads.

When these were first sold, there were no handles or indications of where they should go. The current handle is two bands of serving thread with a leather wrapping between them for the hand. I glued on a small leather arrow pass. I replace the arrow pass leather every 8 years or so.

The limbs are narrower than the current models of this profile. The handle is set back; the recurves are working recurves.

rfz · Post by **rfz** » Tue May 05, 2009 12:36 pm

I am going to hazard a guess to what we see here: the arrow has gripped the string so that when the arrow (the nock) first pulls the limb closer after first being pushed by the limbs straightening and then releases it we see the inertia of the limbs as they can not take up the slack as quick as it is made available with no arrow to slow it down. So it is sort of a spring back of the string from the arrow nock.

rfz · Post by **rfz** » Tue May 05, 2009 12:38 pm

Of course it could also be the limbs just oscillating...

Kyudohund · Post by **Kyudohund** » Tue May 05, 2009 5:19 pm

Hi,

on a Yumi. These pictures were made with 1/4000 second. The white line in the last picture shows the point where the nock (keyhole) left the string.

Peter

Jos?Dom · Post by **Jos?Dom** » Tue May 05, 2009 8:19 pm

As interesting as the other picture. Our bows perform in curious ways beyond the depth of our eyes.
By the way, excellent form, Peter.

Paul Slawik · Post by **Paul Slawik** » Tue May 05, 2009 11:52 pm

Bede and Peter,

Thank You for sharing these fascinating pictures!

robno · Post by **robno** » Sun May 10, 2009 8:03 pm

I'm far from very knowledgable in physics, but a plausible explanation comes to mind:
Most (well-built) bows are slightly assymetric - either in shape or in strength of the upper and lower limb. The more pronounced this assymetry is, the more one limb will pull on the string before the other reacts. This should cause and oscillation.

Bede · Post by **Bede** » Mon May 11, 2009 11:50 am

Due to the nature of the human hand and the position of the arrow on the string, event a geometrically perfectly symmetrical bow will have some asymmetry in the way it behaves. However, the degree of the oscillation depends on the amount of the energy left by the arrow in the bow.

I have seen high speed movie scenes in a German documentary on Japanese archery that showed similar string movement. Even comparatively light strings retain enough energy to oscillate. One of the advantages of contact siyahs is that the amount of string left to vibrate is smaller.

Stephen Selby · Post by **Stephen Selby** » Tue May 12, 2009 1:57 am

Actually, you only need to listen to the sound to know that a basic sine wave and harmonics are being produced. What is not so clear is whether the string is resonating and the bow amplifying, or the bow is resonating and the string showing a visual image of the wave, or both...

Tapio Manner · Post by **Tapio Manner** » Sat May 16, 2009 4:46 pm

If one had a good enough FEM model of a bow, phenomena like that should come out. One of these days I'll have the time....

TLM

ginni · Post by **ginni** » Sat Aug 08, 2009 12:48 pm

A frined took this last week. Is a korean laminated hwarang and definitely it does not look like that in real life

Unusual bow behaviour

Unusual bow behaviour

is happening again