HHB Double Void Experimental Bow (3/4 finished)

[Hamish]

Hey Jim, That's funny I think I saw one of these bows, on a Star Trek episode. Maybe it was from a Borg?

[simson]

Philip, spectacular looking bow!
I think that this design can only be done with special woods like HHB, Elm, or Hickory.

I studied the forces on this design, but I cannot see any advantage - maybe Steve's theory about vibration compensating.
I'm really curious how that bow will shoot. So thanks for doing the work and the interesting posting - keep us updated!

[Pappy]

Cool looking bow either way, hope it works out and shoots well for you.
 Pappy

[Jan de Bogenman]

Great experiment, and great bow!
Made me think of the bow of David A Jenkins, 2000AC. If I understand the concept correctly, it has holes in the limbs too. Claims a benefit of 10%! So there you go. Patent...http://www.google.com/patents/US6536421

[willie]

Interesting reading, Jan. especially the prior art and some of the patent citations.

Wonder how Philipp A's  HHB Double Void Experimental Bow is coming along?

[Philipp A]

Hi Jan,

Thanks for posting this, this appears to confirm that there is reason to my madness . Most things that people come up with someone else has already thought of before. I certainly would have never thought checking any patents to see whether there is one  similar to my bow design! Fortunately I am not into this hobby (more properly described as obsession) for commercial gain. The invention described by David Jenkins is indeed very similar to my bow. It doesn't appear though that he flared the voids into the tip and the handle and strangely he does not seem to keep the width of the side limbs the same throughout, but shows the slot as being straight. In my design I maintain the width at 7/8" throughout until the side limbs meet up towards the tip and the handle. However typically in patents they show only the stylized schematics without some of the details included, so his design at the end could look very similar to mine.  I am curious wether David Jenkins has built any of these bows and whether there are any pictures on PA of the same. Do you know?

I am now almost finished tillering the bow and I will be likely shooting it before the year is out. I will post full draw pics as soon as I have shot the first few arrows and have finalized the tiller if satisfied with the results. The bow does draw pretty smoothly but is still too stiff for me. I am aiming for ~55-60# at my draw length of ~26".

Happy New Year,

Phil

[Badger]

  I don't think that bow ever made it to the point where anyone actually heard anything about it. I wonder if there are any working models floating around.

[Philipp A]

Hi Badger,

I have looked Professor Jenkins up on the University of Florida website. He is a Professor of Engineering. I will contact him once I have finished the bow and get his input and see whether he has any pictures from the the similar bow design he has obtained a patent on. I am sure it will be an interesting discussion.

Happy New Year,

Phil

[avcase]

The design like in the patent are sometimes called “shape shifters” because of how it uses the change in cross-section to change the stiffness of the limb as the bow is drawn. I’ve built a couple other variations a number of years ago. One was simply a top limb that twisted 90 degrees one way to the nock, and the bottom limb twisted 90 degrees the opposite direction. As the bow was drawn, the limbs straightened and it flattened out the force draw curve for the last part of the draw. It required very careful tillering.

Alan

[Badger]

  Allen, how was the performance?

[avcase]

One of the problems with the bow in the patent is that it is very sensitive to grip pressure or slight tiller imbalance. One limb will tend to “break over” or flatten out suddenly before the other. It depends on how how deep a “V” the split limbs are made too. Greater shape-shifting effect is usually less stable between the top and bottom limb. This is probably why this design isn’t seen much.

Alan

[avcase]

I didn’t really carry it much past half size working bows.  It seemed to shoot pretty well, but I went another direction before takin get it to full size Flight bows.   I was just surprised to see how it affected the force draw curve.  It really takes a commitment, time, and energy to develop these things into something that works reliably.

I should still have one of these twisty bows laying around somewhere that I can take a picture of. The limb width was not tapered like a normal pyramid bow.  Width was only trimmed from one side of the limb, the side that twisted to the bow belly. This approach seemed to work best and left the edge forming the bow back less violated. I made them from carefully planed wood with very uniform properties.

Alan

[Philipp A]

@avcase,

how did your bow compare to my HHB double void bow from a design perspective? I have both sides to the void stay same width and I match the slope of the belly with the curvature of the crown on the back of the bow. I do flatten out the slopes as I get closer to the outer limb so I have a smooth transition to the flat portion.

[avcase]

There’s was an article about that Florida Professor’s bow (Jenkins) published at least a dozen years ago. I may have a copy somewhere.  I was playing around with designs at the time that had a U-shaped cross section that was supposed to flatten out but I think was extremely difficult to get to work reliably. I felt the Jenkins approach was better and might have some potential for a center-shot crossbow where the bending moments are identical on both limbs.

Alan

[Philipp A]

@avcase,

I have found today this article from the University of Florida about Professor Jenkins' bow design. Is this the one you were referencing?

http://news.ufl.edu/archive/2004/04/uf-engineer-redesigns-classic-archery-bow-to-shoot-farther-easier.html

I will let you know how mine shoots once I hopefully get to the point of shooting a few arrows over the next couple of days. I still need to take a bit of draw weight off before I shoot it, hopefully I will get to it tomorrow morning.

All the best,

Phil