hysteresis in tension and in compression

[willie]

I have followed in a few discussions about engineering aspects related to bow design. Most recently in a  thread about crowned backs. I have also seen where frequently, some of the posters here have discussed the effects of hysteresis in the bow limb and are looking for ways to minimize the losses associated with hysteresis.

After recently looking at the bow "cold snake" by adb (very nice bow, btw),

http://www.primitivearcher.com/smf/index.php/topic,45616.0.html

I have to wonder what are the virtues of having a bow that is designed to stretch more on the back, than it may compress on the belly. (I assume that may be the case with this maple backed ipe), or vice versa?

my presumption has always been that a bow needs to have it's back stretched in equal proportions with having it's belly compressed, but that presumption has no actual basis other than intuition. I suppose that it might make quite a bit of difference whether the back or the belly is working harder, if there was a difference in the hysteresis realized by a wood stretched in tension and it's efficiency as it snapped back to normal, and the hysteresis experienced by compressed wood returning to its normal dimension. Some how I want to believe that a wood reacts quicker in tension than compression. Maybe I am just thinking about other elastic materiel like rubber or steel. I know that I would move further away from something that was being stretched to a dangerous limit in tension, than I would if it was being crushed to its limit in compression ,and could be a danger if something let loose.

any thoughts as to whether the best performance come from a back working to it's max, or a belly working to it's max in a bow?

[Hrothgar]

'Some how I want to believe that a wood reacts quicker in tension than compression', with absolutely no scientific reason why, I tend to feel the same way. Bow making is a hobby for me, so I don't often ask why. But I believe that compressed wood stays compressed, thus string follow, and I suppose the same conclusion can be made for wood under tension that is repeatedly stretched until its ability to return to form is diminished.
Guess this is why superb bows can be made by applying sinew to the back of a compression-strong wood such as osage, allowing them to be 'over-drawn' without failure.

[Badger]

  The problem with stretching the back is that wood doesn't stretch well. Once it starts to stretch it will fail quickly.

[PatM]

I don't think it's true that stretched recovers faster than compressed.  Besides both are happening at the same time. So it's the average of both that would matter.
 The fact that the compressive abilities of wood are so highly sought out tells you which side is more important.
 

[willie]

badger-
 you have had a lot of experience with flight bows and your observations might be instructive in looking at this question.

I have read about what I think is called the "one bow wonder" I will assume that these are bows that perform well for a few shots. Do they generally go down hill by taking set?

 Compare the "one shot wonder" to a bow that is properly shot in and survives as a good performer for reasonable life. Have you observed whether this longer lived bow tends to fail more in tension?

Or put another way, do you have any reason to think the better bows are stressing the limbs differently?

willie

[Badger]

  Willie, thats an excellent question. I no longer believe in the one shot wonder bows. I do only in the sense that they might blow when they hit brace becuase an arrow is so light but not in the sense that they are built to very low mass and highly stressed. My ideas on how far to stress a bow have gotten a lot more conservative. I believe that as long as we put the mass in the right places we don't pay any penalty for it and the bow will last longer. Making tips scary skinny will add speed and possibly shorten the life span of a bow and possibly coulb be construed as a one shot wonder but speaking for the working part of the bow I keep the stresses as low as I can now. I have a little 54# bow I am hitting speeds of 272 fps with and have several dozen shots out of it with no signs of taking set or breaking down in anyway. I took it back 1" further this morning to about 57# and it showed no loss in weight. I didn't get a reading as my chrono is tempermental but I am certain it hit over 280 fps. Arrow weight was only 160 grains. I can see where a fiberglass bow might push the limit on stresses more but with wood I loose too much in histerisis when the wood starts to breakdown.

[bradsmith2010]

thanks Badger,, now I have  a new dream speed to think about 

[DC]

280!! And it doesn't have even tiny little wheels?? Amazing!

[Badger]

 Not so amazing arrow was very tiny, lol. I was happy with it though. I am happy anytime I get over about 240 something.

[willie]

badger

when you speak of the one shot wonders blowing, do the backs break first or do the bellies collapse?

is that54# the osage with the wide inner limbs you are telling us about a few weeks ago? It sounds like a winner.Did you ever post pics of it?

[Badger]

   I think I have pictures of that bow. The bow I posted a few weeks ago that came in too light I cut off to 57" or so and retillered and ended up back at 35#. I am rebuilding another bow like it from scratch as we speak. I have it out to about 18" @ 50# and maybe will get it braced up tomorrow. Taking my time on it. The fast bow I am talking about now is a 56" flight bow set to draw about 25" now I have it out to 26". Just a little bamboo backed osage with small recurves. It wouldn't do so well in the broadhead class because of the shorter draw.

   The one shot wonders we sometimes talk about will either brake when they hit brace or sometimes they just pop the back or go badly out of tiler all of a sudden. The one I posted last week that I was getting some real good speeds out of started to collapse a bit in the belly because I brought th bow out to full draw without excersizing it. Risky thing to do that sometimes pays off more often not. I still have a very strong 35# bow for my efforts so not all is lost.