Author Topic: Stress and performance  (Read 10199 times)

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Offline Kidder

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Stress and performance
« on: June 02, 2022, 01:21:05 am »
Is there any correlation between design stress and performance? In other words, all else being equal, will a higher stressed design bow result in greater performance? One example of what I’m thinking is a 50# 66 inch bow versus the same 50# bow but only 60 inches overall. Assuming that is the case, will the performance degrade over time where the performance of the low stress design overcomes that of the high stress design after extended usage? Thanks for entertaining my wandering curiosity.

bownarra

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Re: Stress and performance
« Reply #1 on: June 02, 2022, 01:39:37 am »
Every piece of wood has its own set of properties. How elastic is it? How able to resist compression? How close are its resistances to stretching and compressing? Plus others!
Your job as a bowyer is to 'listen' to the wood and hear what it is telling you.
The more skilled/experienced the bowyer the closer he can get to optimal strain on the wood.
One reason I always suggest people should trace the outline of the back before ever bending the stave - during tillering you can placed the bow back against the original outline and see where/how the set is appearing.
Also get your head around 'tiller logic' as soon as possible....once you understand how width taper is married to thickness taper and that thickness taper determines 'tiller' you can make any bow in any design and KNOW how it should bend to optimally strain the wood. 

Offline BowEd

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Re: Stress and performance
« Reply #2 on: June 02, 2022, 05:52:50 am »
If you give wood enough width to handle the strain in the areas it needs it.It holds up over time.There is no certain script that fits all bows and designs.All things are not equal then.All woods are not the same either in density/shapes of backs/elasticity and the list goes on.Evaluating the wood and knowing it before design and tillering is started.Width's and thickness's are different then too.If the design is going to be more stressed you need more wood to handle it.
The cleaner the wood the more stressed design it can handle.Even then it's a fine slow line to walk and not always accomplished.
To me that's the beauty of self bows or sinewed bows for that matter.
It's one of the reasons I think why these type bows will out shoot FG bows as most all those are scripted.There are FG bow makers out there though that do not conform to the script,but they have their limits too.
Even the highly stressed bows will shoot quiet as a mouse with or without silencers [I like the looks of them] if they are tillered correctly/settled in and take years and years of shooting/hunting seasons and still shoot with the same performance.





« Last Edit: June 02, 2022, 07:37:06 am by BowEd »
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Offline superdav95

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Re: Stress and performance
« Reply #3 on: June 02, 2022, 08:10:25 am »
Well said mike, Ed.  Sometimes our own thoughts and ideas or ambitions get in the way of listening to the wood.   
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Offline RyanY

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Re: Stress and performance
« Reply #4 on: June 02, 2022, 09:32:36 am »
I think there’s likely a sweet spot for performance. Too stressed will obviously result in set and worse performance. Understressed May mean more mass or wind resistance in the limbs. I think there’s still a question of mass given Badger’s experience with wide limbed bows that we’re not necessarily heavier than narrow limbed counterparts.

Offline mmattockx

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Re: Stress and performance
« Reply #5 on: June 02, 2022, 10:41:21 am »
Is there any correlation between design stress and performance?

Yes. You will get more performance as stresses go up until you go too far and the wood starts taking set or otherwise failing due to the high stresses. As the others have noted, the fundamental problem with wood is its inconsistency in material properties. You never know quite what the limits are for the piece of wood you are working on and you can't tell the peak until you have gone past it.


Mark

Offline Jano

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Re: Stress and performance
« Reply #6 on: June 05, 2022, 07:05:13 am »
Firstly I would like to thank Alan Case for publishing his valuable work for other bowmakers and then recommend to read his comments /see the graphs/ here ( and several previous pages too ) :

 http://www.primitivearcher.com/smf/index.php/topic,69529.345.html     .

I ( as not very experienced bowmaker ) do not know other way to do asked task, than to test ( similarly to avcase's method, but preferably using adviced 4 point bending setup ) on appropriate samples of wood from prepared stave, determine the proportional limit and decide what set is still acceptable along different parts of limbs for your bow design. Then use bendmetering at the final part ( at least ) of tillering to not overcome the desired set ( in every stations along the limbs ).
    To resolve the problem mentioned by Mark - "you can't tell the peak until you have gone past it" - I suppose one could make the bow intentionally thicker/stronger/ from the beginning and after overcoming the peak ( evenly ), you will remove damaged belly wood without changing the tillered shape of the bow and by such way achieving desired draw weight and performance.

Offline Selfbowman

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Re: Stress and performance
« Reply #7 on: June 05, 2022, 10:08:03 am »
Ok I have some questions?
Is it possible to reduce mass to prevent set in a computer program?
My thinking is if you know the property’s of the wood . Which we can get close on doing bend test and float test. Let’s say the test reveal the wood is 30 percent stronger in tension. Would it make sense to reduce the back of the bow in width 30 percent to let the belly’s compression catch up in strength as not to fail to compression. Trapping the bows back. This would also use the mass to its full potential would it not. Just asking.I would like to see some of the guys playing with computer designs to try this. This may be a good time to figure out perfect diminishing mass using 8 oz on a 28 inch limb. Maybe not. You smart guys please ponder this ! Arvin 
« Last Edit: June 05, 2022, 10:36:28 am by Selfbowman »
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Offline organic_archer

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Re: Stress and performance
« Reply #8 on: June 05, 2022, 10:53:35 am »
+1 to what everyone else said. The TBB performance chapters have detailed tests on power stroke as well. The same bow of 45-48 pounds (if I recall correctly) was tested at low 20’s draw and shot 135 fps. It was retillered over and over to 45-48# out to 28” in one inch increments, and it gained 20+/-feet per second in the process. Power stroke is also important.
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Offline mmattockx

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Re: Stress and performance
« Reply #9 on: June 05, 2022, 11:53:10 am »
Ok I have some questions?
Is it possible to reduce mass to prevent set in a computer program?
My thinking is if you know the property’s of the wood . Which we can get close on doing bend test and float test. Let’s say the test reveal the wood is 30 percent stronger in tension. Would it make sense to reduce the back of the bow in width 30 percent to let the belly’s compression catch up in strength as not to fail to compression. Trapping the bows back. This would also use the mass to its full potential would it not. Just asking.I would like to see some of the guys playing with computer designs to try this. This may be a good time to figure out perfect diminishing mass using 8 oz on a 28 inch limb. Maybe not. You smart guys please ponder this ! Arvin

Hey Arvin,

To answer your questions in order:

1) A bend test doesn't tell you how much stronger your wood is in tension or compression, it simply gives you the stiffness of the wood in bending (the modulus of elasticity) and the stress level where set starts to occur (assuming you test like Alan Case did in your other thread linked to above). You could figure the tension/compression balance out by trapping bend test samples different amounts and examining how they fail until you find the one that fails nearly simultaneously in tension and compression. You might run out of wood before you find this point, though.

2) The effects of trapping are not linear like that at all. Removing 30% of the back width will not raise the tension stresses by 30%. The change in stresses is based on how much you shift the neutral axis of the limb cross section and that is dependent on the back width along with how much of the side is removed and the final cross sectional shape.

3) I would say the perfect mass distribution is what you get with a quasi-pyramid back profile and constant limb thickness. The constant thickness keeps the strain/stress the same throughout the working limb length and the width changes to optimize the amount of wood used to the minimum required to keep the strain/stress the same everywhere.


Mark

Offline Selfbowman

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Re: Stress and performance
« Reply #10 on: June 05, 2022, 12:07:46 pm »
Mark I’m thinking removing the edges of the back of the bow will reduce the tension possibility’s. Not add to. Reducing the mass should also make increase speed in the limb returning to brace. Redneck thinking.🤠🤠
Well I'll say!!  Osage is king!!

Offline bradsmith2010

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Re: Stress and performance
« Reply #11 on: June 05, 2022, 01:08:12 pm »
as you make the bow shorter,, there will be a point of diminishing return,, where the perfromance will go down,, even the draw weight is the same,,

Offline mmattockx

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Re: Stress and performance
« Reply #12 on: June 05, 2022, 01:09:45 pm »
Mark I’m thinking removing the edges of the back of the bow will reduce the tension possibility’s. Not add to. Reducing the mass should also make increase speed in the limb returning to brace. Redneck thinking.🤠🤠

Trapping will help balance the tension and compression capacity of the wood and let you get closer to the maximum performance the wood can achieve but it isn't magic.


Mark

Offline Selfbowman

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Re: Stress and performance
« Reply #13 on: June 05, 2022, 01:14:06 pm »
Ok Mark thanks for the answers.
Well I'll say!!  Osage is king!!

Offline simk

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Re: Stress and performance
« Reply #14 on: June 06, 2022, 01:39:38 am »
For the same reason we round the belly on yew bows I think...because with yew the weaker side is the back, imho.
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