Elb design features for high efficiency

[lonbow]

I´m wondering about some design features for a very efficient elb. I´ve read about two design features which might contradict themselves.

1.The deeper the bow profile of an elb the quicker the bow.
2.For a quick bow, the main bending sections should be kept short in order to reduce limb vibration. That´s an elliptical tiller profile  with a relatively stiff grip and stiff tips.

It´s certainly not possible to make an elb with short bending sections and a deep bow profile. The bow would have a lot of set or break. Am I right there? In order to keep the set as little as possible, the bow should ether have a deep profile and long bending sections or a flatter profile and shorter bending sections. Which is the better solution in your opinion?


lonbow

[Del the cat]

You are right, many of the requirements are contradictory... it also depends what the projectile is... a flight arrow or a 1/4 pound war arrow.
For flight bows I personally favour short limbs, but with all the limb working, on a shortish bow, but then, that limits the available draw length.
the other big factor is longevity, there is a world of difference between a high stressed flight bow and a long smooth ELB target bowor a heavy weight warbow.
Depends how you defend efficiency too!
Even limb cross section isn't an easy choice, a rectangular (but lightly rounded) 5:8 depth to width qualifies as an ELB and is lower stressed than a deeper section, but the deeper section being higher stressed stores more energy for less mass.
It's all a compromise... and don't even start on adding a bit of deflex or reflex!
Del

[Badger]

  I am stiil experimenting with the elbs. A few of my elbs hold world flight records including the unlimited weight class. The Elb we broke that record with was only 90#. On that particular bow I actually got more handle area wood bending that I would normally and was able to keep the inner and mid limb wood a bit stiffer than usual. Over all I prefer a slightly bending center and slightly elliptical full draw profiles that aren't too far off from arc of a circle. My profiles are always right on the edge of the 5/8 ratio rule.

[Del the cat]

  I am stiil experimenting with the elbs. A few of my elbs hold world flight records including the unlimited weight class. The Elb we broke that record with was only 90#. On that particular bow I actually got more handle area wood bending that I would normally and was able to keep the inner and mid limb wood a bit stiffer than usual. Over all I prefer a slightly bending center and slightly elliptical full draw profiles that aren't too far off from arc of a circle. My profiles are always right on the edge of the 5/8 ratio rule.

Interesting, agrees with my thinking (or vice versa  )... I think the difference between arc of a circle and the slightly eliptical is a lot more subtle than some people think.
Del

[lonbow]

Thank you for your great answers.

So you both prefer a sligthly elliptical tiller profile. Do you mean a tiller profile that distributes the load evenly on the whole bow? That sounds like long bending sections. Am I right there?

How stiff do you keep the thips?

Do you prefer the 5/8 ratio so you can make your bows as short and light as possible without overstraining the wood?

[bradsmith2010]

I think what Badger described,, is one of the best ways to not overstrain the wood,,which translates into world records,,
its more complicated than it sounds,, 

[bownarra]

An elb will taper in thickness quite significantly. This equals an elliptical tiller. However you seem to think that an elliptical tiller should always mean stiffer tips? Well on an elb the tips section is still thinning therefore it can and should bend further than the thicker wood next to it.

A 4x4 can't bend very far before it breaks.
A shaving can bend full circle and spring back.
This is the principle you need to keep in your head. As it allows you to look at the taper rate and know what tillr profile will give you best performance.
It is very easy to end up with a poorly shooting elb by not following the (almost predetermined by the thickness taper) tiller. The centre should move a little with every section you move further towards the tips bending progressively further. Elliptical tiller. Just for a bit of fun I knocked up two hickory/ipe longbows the other day. I should get them both to perfect tiller by this afternoon and will take pics to show what I mean.
My elbs generally shoot around 175 @ 10gpp, sometimes low 180's.

[lonbow]

I´m looking forward to seeing your bows!

The design you described puts the wood under the lowest stress possible. So your bows can probably be made very narrow and light. And you can also make the bow tips very light, right? There´s probably no part of the limb that has to accelerate too much mass.

But I´ve often read that stiff tips lower limb vibration. This is often stated, but I´ve never actually seen testings. Can you say more about this?

[bradsmith2010]

Maybe it said a lighter tip will reduce vibration?
A stiff tip will reduce string angle

[PatM]

You sound a bit conflicted on the bow type you actually want.  At some point you have to pick the style and not try to morph it into a design which it does not even fit.

  Longbows however were and are made  with shorter bending sections and stiff tips.

 A deep narrow profile does stress the wood more which is why longbows of this design use better compression resistant woods.

 Victorian longbows seemed to feature this design.

 The link below shows a longbow working hard in short sections. 

[PatM]

http://www.bentoncountymuseum.org/assets/images/uploads/exhibitions/Thirty/2001_Gilman_Keasey.jpg

[bradsmith2010]

ok Im old school, just start making long bows,,, you will have a better understanding about the design as you go,, reading can only get you so far,,
its kind of like reading about playing guitar,, at some point you have to start practicing and playing to understand what you are reading,,

[JW_Halverson]

Funny you should throw in the photo of Gilman Keasey. I just bought a piece of yew that he had cut and never got around to using.

If there is any mojo in there I can only hope I don't screw it up.

[Del the cat]

I've taken the liberty of drawing approximate circles on the Keasey bow.(the two circles are the same shape and size)
I think it shows that the tiller is pretty much arc of a circle over the working section and the tips are not left stiff (other than the last inch or so, but that is due to lack of leverage and can never bend).
 Interesting that the circles intersect an inch or so below the arrow pass just where the bow is supported
There is some slight flex over the riser section, but this is to be expected as it is still slim enough to be gripped in the and not like a heavy solid recurve riser.
Del

[bradsmith2010]

very interesting bout where the circles intersect