Feedback on a theory...

[Eric Garza]

I've been pondering the relationship between a bow's efficiency and cast over the past year. I recall from other posts as well as from discussions in the Traditional Bowyer's Bible series that shorter bows are more efficient, that they deliver a larger percentage of stored energy into an arrow. I also recall a conversation here on the PA forum where someone, I can't recall who, said that hysteresis was internal friction caused by there being lots of non-working wood between the outer fibers working in tension and compression.

I wonder what role the reduction of hysteresis plays in increasing the efficiency of shorter bows? In my head I've convinced myself that shorter bows enjoy greater efficiency because their limbs can be thinner for a given draw weight and draw length then would be the limbs of a comparable bow that was longer. These thinner limbs have less non-working wood between the working back and belly fibers, so there is less internal friction and therefore greater efficiency. The observation of thin-limbed bows being more efficient seems to hold with modern fiberglass bows, which have wide, thin limbs compared to many wood bows, and tend to shoot faster.

I wonder, then, if someone made a wooden bow with limbs wide enough that the limbs ended up the same thickness as those of a fiberglass bow of comparable draw weight and length, would that bow achieve greater efficiency and cast? What do people think about this idea?

I'm currently working on a hard maple pyramid bow that I purposely made far wider than I normally would, over three inches wide at the fades, and am interested to see how it turns out. I'll let folks know how it turns out.

[Badger]

   Eric, a book could easily be written on just the topic of efficiency. Problem is no one seems to understand it well enough to write a complete book on it. Lots of theories floating around, hard to say how accurate or inaccurate they really are. I have my own theories that I have tested to some degree but working with a material like wood tests leave a lot to be desired.

   When it comes to efficiency hysterisis is not the biggest factor, in glass bows it is so small it is negligible. In wood bows it is substantial. In the past couple of years I have done quite a few experiments just aimed at tracking the energy losses in wood bows and trying to figure out how much control we have over them.

   My tests indicated that hysterisis is not as inherent to wood bows as previously thought. The majority of it is induced by damaging the wood cells during the building process. The damage cannot be detected solely by monitoring set but it can be monitored by measuring weight losses in the bow that occur when the draw distance is increased.

    I built some extra wide bows as you are doing just to test hysterisis. I decided to ignore the excess mass and just see what I could come up with. To my amazement the bows came out lower in mass than their much wider counterparts. This can only be explained away by accepting the bows were enduring much more damage than I had previously thought.

    The majority of efficiency losses are lost in the way of limb vibration and distorsion, hysterisis will usually account for about 10% loss, Vibration accounts for the other 20 to 25% losses. Shorter limbs vibrate less thus more efficient. Shorter bows and shorter limbs also store less energy so they are not necessarily faster. I find well made longbows on the longer side will usually outshoot the shorter ones, recurves can be a bit shorter and reach their peak.

  There are tests you can do that will isolate the efficiency losses in a bow and let you know how much is lost to vibration and how much is lost to hysterisis but they are very tedious and time consuming and require a number of repetitions to establish any kind of useful patterns. You can use a no set tillering technique to monitor the condition of the wood and determine how well you executed your design and this will help to reduce the hysterisis to manageable levels.

[PatM]

 But won't the wider limbs then be more prone to vibration?   Seems you have to take your losses somewhere.

[Badger]

But won't the wider limbs then be more prone to vibration?   Seems you have to take your losses somewhere.

  Pat, I am not sure how much difference the width makes, the length seems to make the most difference. The bows I made with wide limbs I also used very short working areas. Very hard trying to isolate things.

[bradsmith2010]

after reading everything I can and making bows for 20 years,, I am starting to understand a little,,,

[Eric Garza]

   My tests indicated that hysterisis is not as inherent to wood bows as previously thought. The majority of it is induced by damaging the wood cells during the building process. The damage cannot be detected solely by monitoring set but it can be monitored by measuring weight losses in the bow that occur when the draw distance is increased.

    I built some extra wide bows as you are doing just to test hysterisis. I decided to ignore the excess mass and just see what I could come up with. To my amazement the bows came out lower in mass than their much wider counterparts. This can only be explained away by accepting the bows were enduring much more damage than I had previously thought.

Thanks for the response Steve. I was hoping you'd chime in.

I'm a little perplexed by the above sections of your post. You say that you can monitor the damage to the wood by measuring weight loss in the bow that occur when the draw is increased. If a bow loses mass when it's drawn further, where does the mass go? How does it leave? Are microscopic bits of sawdust falling off the bow? I guess I'm struggling to understand what you're saying.

You also say that "To my amazement the bows came out lower in mass than their much wider counterparts." Do you mean their much narrower counterparts?

Thanks!

[Badger]

  Eric, I wasn't very clear, when we talk about weight loss in bows we are talking draw weight loss, We usually refer to physical weight loss as mass.

 Yes I meant narrower counterparts.

[jayman448]

Would not a bow made so wide to copensate for so thin have so muh wind drag that any theoretical gain from the thinness would be lost in wind drag to an extent?

[PatM]

Swing a paddle and a golf club and decide. 

[jayman448]

Well thats what i figured

[willie]

very interesting thread.

These thinner limbs have less non-working wood between the working back and belly fibers, so there is less internal friction and therefore greater efficiency.

 Something to keep thinking about for sure.

Just a quick question for Badger about some of your newer "thin-limb" designs. When you isolate the short wide working limbs from the rest of the bow, do you try to keep a thick thin handle and tips as light as possible? Could some of the mass reduction your reporting, be from the non-working part of the bow as much as from the working limbs themselves?

thanks
willie

[Badger]

  Willie, I made the outer limbs that were slightly working about the same dimensions that I would on any bow. If I remember right it seems like they finished up about 1/4" thick. I gave a couple of them away but still may have one, I will check tomorrow.

[Del the cat]

I think "theory" in bowmaking is fine up to a point. It's V useful if you are using modern materials, less so if you are using laminated timber and only good for guidance if you are using staves.
It's usful if it enhances your understanding of how the wood is working, but I think the basics of sound bow design are pretty much there.
You only have to read the flight bow section in TBB to see how the wood like to ignore theory
Del

[Lukasz Nawalny]

 In selfbows high hysteresis is inescapable but in wooden laminated bows can be very low. I have interesting observation - low hysteresis is not necessarily connected with thin limbs, there is few more factors. I have made few experiments to understand this - for example bow with over 60 % hysteresis - 50 lb shoot like 20 lb. Now Im arround 85 % eff in wooden laminated bows but I have few more ideas how go over 90% . I dont write for now about details, first I must check it.

[George Tsoukalas]

I like my bows longer (64-66" for a 26" draw) and quick to draw with no stacking so I have very little experience with short under built bows.

I believe Comstock in "The Bent Stick" did research on overbuilt vs under built and found the over built bows to have more cast.

Jawge