Hollow Limb Design and the Mass Principle

[Holten101]

I agree with onebowwonder...I dont think improved aerodynamics is an issues worth exploring...any lateral deformation will be too slight to have any effect (I generaly dont belive air resistance on the back impacts much on effectivness in wooden bows).

Testing wether the hollow limb design is more effective should be rather easy. I would suspect that taking any relativly high crowned flatbow and gradually make the limbs more hollow while recording weight, draw weight and arrow speed, would reveal alot.

If you are right that the hollow limb design is more effective (and there by circumventing the mass principle) then that design should soon dominate competetive flight shooting. I remain sceptical until I see more evidence (Im still in awe of Simpsons work though).

Cheers

[Del the cat]

Fist I admit I couldn't be bothered to read through all the posts so I may be talking out of turn.
A lot of the analysis is IMO flawed. We are not after stiffness with a bow we want flexibility.
Tubes, cylinders I beams etc give good stiffnes but have nasty failure modes.
An I beam may be great for a lever on the end of the bow, but I'd guess not for the limb.
I can get impressive failure modes without looking for less stability !
But hey what do I know I'm a cat
Del

[Slackbunny]

It wouldn't circumvent the mass principle, it would just require a new parameter be taken into account. I would guess that the new parameter would be the average moment of inertia of the limbs.

[Buckeye Guy]

I think you folks need to stop the flapping and get to scraping if you want to know the answers to your questions !
But what does a worthless old nut know anyway.
Have fun
Guy

[Holten101]

It wouldn't circumvent the mass principle, it would just require a new parameter be taken into account. I would guess that the new parameter would be the average moment of inertia of the limbs.

Yeah....the "circumvent" note I regret now...please ignore it:-)

Cheers

[Onebowonder]

I think you folks need to stop the flapping and get to scraping if you want to know the answers to your questions !
But what does a worthless old nut know anyway.
Have fun
Guy

...but I'm ever so much better at flapping! 

OneBow

[Josh B]

First off, I haven't made an HLD bow yet and I'm certainly no physicist.  This is just my conclusions, hypothesis, mental flapping or whatever that I have come to after much deliberation and contemplation of this design.  150,000 miles in the saddle every year leaves a lot of time to think.  I believe that it works so well because the limb doesn't know its arched.  If the basic principle on which a bow works is the opposing forces of tension and compression, then surely the HLD is mostly a wider flatbow thats arched.  Meaning the entire outer surface is under tension, while the entire inside of the arc is under compression.   This would put more wood to work than if it were simply a high crown flat belly design.  I got out pen and paper to attempt to illustrate what I mean.  The arrows in the neutral zone are representing the way I perceive the opposing forces of the different cross sections.  If my thinking is correct, the ten percent of thickness that carries the tension and the ten percent that like wise carries the compression is drastically improved with the HLD.  In effect, your taking an inch and a half wide sapling and getting the effect of a much wider flatbow.  If you add in the structural rigidity added by the HLD, you are talking significant improvement over the old high crown flat belly design.  Just my opinion of course.  If there are holes in my explanation,  please feel free to challenge it and enlighten me.   Josh

[Badger]

  As a flight shooter you can be assured I am looking real hard at this. I am also cussing! Just when I think I ran out of things to try one more pops up, it just keeps happening over and over and over LOL.

[simson]

UUUh my!

That is an interesting discussion, always wanted to initiate this. I just have answered some questions in the elder HLD thread, don't want to do that here again.

Measurement: I will do that tomorrow and post it on the Elder HLD thread.

I have read all the replies, but haven't understood all. Have to translate some things with google ....
Please excuse me for now, I'm coming back tomorrow.

For Steve: I always thought of flight shooting. Here in Germany flight shooting isn't an item. Can you tell me what determines a bow as a flight bow. Too bad you are not here in Germany ...

[Holten101]

First off, I haven't made an HLD bow yet and I'm certainly no physicist.  This is just my conclusions, hypothesis, mental flapping or whatever that I have come to after much deliberation and contemplation of this design.  150,000 miles in the saddle every year leaves a lot of time to think.  I believe that it works so well because the limb doesn't know its arched.  If the basic principle on which a bow works is the opposing forces of tension and compression, then surely the HLD is mostly a wider flatbow thats arched.  Meaning the entire outer surface is under tension, while the entire inside of the arc is under compression.   This would put more wood to work than if it were simply a high crown flat belly design.  I got out pen and paper to attempt to illustrate what I mean.  The arrows in the neutral zone are representing the way I perceive the opposing forces of the different cross sections.  If my thinking is correct, the ten percent of thickness that carries the tension and the ten percent that like wise carries the compression is drastically improved with the HLD.  In effect, your taking an inch and a half wide sapling and getting the effect of a much wider flatbow.  If you add in the structural rigidity added by the HLD, you are talking significant improvement over the old high crown flat belly design.  Just my opinion of course.  If there are holes in my explanation,  please feel free to challenge it and enlighten me.   Josh

With all due respect, but the perception of the forces at work is wrong Gun Doc.
The force planes (tension, neutral and compressiv) are decided, not by the cross section of the limb, but by the direction of the overall bending force. If you tip over an I-beam (making the cross section like this: I--I) and bend it,  then the upper flanges take tensile deformation and the lower will compress and collaps....the middle wont be stressed til the flange collaps happen. Im sorry I cant explain it better with my limited english:-(.

Cheers

Edit: God I sound like a twat:-(. I dont know for sure how the forces behave when bending a half pipe...I dont have the anwsers or knowledge to settle this in any way. And im sorry if I come across as impolite.

[Badger]

  Simon, I will just give you an opinion here. One of the biggest challenges to a flight bow builder is mainating efficiency with very light arrows while still storing sufficient energy. Longer bows with longer draws store more energy, shorter bows tend to be more efficient. This is where I may be wrong but I dont hink so. I believe that the slightly longer bows of around 62 to 64" will eventually prove out to be the best light arrow bows when made from all wood. The biggest single loss I have identified from efficiency comes from hysterisis. The best way I have found to reduce that is by reducing set or damage to the wood fibers that happen when we bend the wood. I can get my short bows very efficient but they just seem to lack the energy storage I need.  I also believe that very light outer limbs with lowest possible string angles reduce momentum in the limbs that is exagerated by light arrows. Everything relates to giving the arrow more leverage to control the limbs with. A 200 grain arrow has very little mass to excert on the limbs durring acceleration so it needs all the help it can get from better geometry and lower mass limbs.

[IdahoMatt]

This is one of the most thought provoking topics that I have read on this sight.  Another way to do a test would be make a wide flat bow and bend it lengthwise over a pipe, with another half pipe over it as a form.  You wouldn't have to make a full scale test just some thing to get a good idea.  I might just have to try something like that.  If you made the limbs thin enough.  Wouldn't you get more of the flattening out of the limb.  My mind is kind of hurting thinking about these things because I am not too into physics but this is really intriguing.

[hammertime]

This is interesting and i think gun docs picture makes sense but wood is not a manufactured product and any knot or twist,or wiggle in grain could affect performance.just my two cents -Hammertime

[Onebowonder]

OK - ...so I can't stop thinking about this idea of an HLD laminate bow.  I realize this may be a distraction from the main topic here, so ignore me if need be, ...but if I don't write what I am thinking/dreaming about I might just burst. (...which would not be pretty!)

What would be the potential of building a long tapered half conical or cylindrical form to lay real thin flexible lams up on to build an HLD bow?  You could select the wood species for the lams based on the suitability to withstand the compression and or tension strains anticipated.  If the Lams are thin enough, getting them to wrap around a mold and glue-up in the HLD shape shouldn't be too tuff. 

Hmmmm... Pondering...  I guess you basically wind up with an engineered 'plywood' bow.  OK, now the idea sounds much less exciting after typing that last line.  I can move on now.

OneBow

[George Tsoukalas]

Some FG  lam bows have been made in that design. I can't remember which. It will come to me...or not. Jawge