Author Topic: Mass movement, tiller shape, and width profile  (Read 2617 times)

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Offline RyanY

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Re: Mass movement, tiller shape, and width profile
« Reply #15 on: July 02, 2021, 11:52:24 am »
I've been thinking about the math necessary to model the strains and mass movement/efficiency of even a very simple circular tillered ALB, and it seems to involve multiple differential equations and other calculus in statics and dynamics, not to mention needing empirical data from the wood itself, such as the stress strain relation for the specific piece of wood.  That design may work out ok, but if the cross section of the bow is not rectangular, or worse if it gradually varies, and if the bow is sinew backed or Perry reflexed, the math will get really ugly.  Not necessarily impossible though.

The drawings make a lot of sense to me, and I will be trying that in my next bow.

Everybody wants a faster bow but no one wants to do the math.  (lol)

My pattern seeking/pattern finding brain wants me to think that mass distribution along the limb, moved efficiently, will also utilize that mass efficiently in terms of strain. Any thoughts on this?

Offline RyanY

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Re: Mass movement, tiller shape, and width profile
« Reply #16 on: July 02, 2021, 11:57:15 am »
Don, trial and error does little to explain why you succeeded. I also think we know enough to not have to build thousands of bows to find out. Thereís also the aesthetic preference which you arenít taking into account where there are infinite possibilities for bow shapes, draw lengths, draw weights, wood species. If we understand how bows work entirely we can maximize efficiency for every bow. With trial and error we can maybe build one maximally efficient bow,

Also, this thread isnít meant for you to find the answer to your question. So far you havenít contributed anything to the discussion other than complain that itís not what youíre looking for.

Offline Don W

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Re: Mass movement, tiller shape, and width profile
« Reply #17 on: July 02, 2021, 12:23:42 pm »
Then I hope you find the help your looking for.
Don

Offline willie

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Re: Mass movement, tiller shape, and width profile
« Reply #18 on: July 02, 2021, 07:27:45 pm »


IMO, it's never going to be all math. Take a stick and over bend it, it doesn't matter how precise you are with your calipers. It's and extreme example of process requirements, but I hope you get the idea.



Good point Don, the math/formulas/spreadsheets/programs do have their limitations.  They work well up to the point of set taking for wood. 

wood is a complex material. it has viscoelastic properties.  https://en.wikipedia.org/wiki/Viscoelasticity
these properties are visible at stresses that typically produce set.  When we have just unstrung set that disappears overnight, we are seeing viscoelasticity in action. The wood properties below this point can be predicted well with the programs. 

The process requirements you suggest, in order to improve the work of TBB et al, would have to not just categorize wood by species, density and moisture and point of set taking,  but also incorporate a method for the bowyer to evaluate the kind of set he is working with, ie. the viscoelasticity of his wood.

For bowyers not looking for the cutting edge of material performance (by not pushing stresses into this viscoelastic zone), woodbears sheet works well by simply specifying a reasonable strain. Woodbears spreadsheet also does a little more, it takes into account set taking, (if you do the bend test), something the other programs do not.

One useful aspect of using programs for design,  is stated well by leroy 
https://www.primitivearcher.com/smf/index.php/topic,65115.msg987024.html#msg987024
Quote
I used this program to make a some virtual bows, to see how a specific design works, and understand how it's spreading the stress along the limb. So for me it's an interactive version of the TBB where you can produce F/D curves and experiment with profiles.

It's much easier to make a virtual bow an inch longer or 1/8 inch wider than a physical bow :-;

I haven't used it as a "design" tool, but it's a valuable learning tool.
(as mentioned above, too many variables in real wood compared to laminations)

This mention of woodbears sheet and virtual bow is not to take away from Ryans work. Understanding basic mechanics of materials as lleroy mentions above and Ryans methods show, is what I find so valuable.

The most necessary addition to any "paper" bow needed to make an actual bow is as Mark stated,
You absolutely could do this but it would require measuring each piece of wood for it's individual properties and then making all your calculations based on that.

It does. It isn't too much work to measure MOE of your material, which is by far the most important number to have. I now do a bend test sample for every bow I make to get MOE and go from there.


Mark

Offline mmattockx

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Re: Mass movement, tiller shape, and width profile
« Reply #19 on: July 02, 2021, 10:26:28 pm »
Even using something like David Deweyís bow design spreadsheet is far outside the reach of the beginner.

It can be an exercise in frustration without some engineering background to understand the terminology and how David got to where he did and you have to get your head around the particular way David arranged the sheet and the workflow through it. He wrote up a small description of how he reverse engineers a bow with it and that was what turned the light on for me and got me really going with it.


As far as Iím aware weíre still trying to understand what would make any given bow as efficient as possible so those spreadsheets likely arenít complete.

Yes. This is a two part (maybe more) question. Don is asking for a design that will get everything available out of a particular piece of wood. You are looking for the most efficient design. These are not really the same things in my (designer's) mind. This is based on my understanding of what you guys are saying, so correct me if that is wrong.

1) Don is looking to get all the performance out of a piece of wood. To me that means straining/stressing it to the limits of the material but not beyond, regardless of the bow design chosen. This is absolutely something the various software programs can help with. You can pick a back profile and then calculate limb thickness/taper so that the drawn bow has stresses near the measured limits of the material but not over. This is largely what I have done a lot of in tinkering with different back profiles, reflex, deflex, recurves and so on to see how the different designs stress the wood and the draw characteristics of them.

2) You are looking for the most efficient/highest performance shape for a bow. That is much harder to figure out. It encompasses the dynamics of a shot, limb vibration and more.  It may even vary among different kinds of wood because they offer different damping characteristics and other fine details that will change the outcome at least slightly.

Is this a fair description?


Mark
« Last Edit: July 02, 2021, 10:31:53 pm by mmattockx »

Offline RyanY

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Re: Mass movement, tiller shape, and width profile
« Reply #20 on: July 03, 2021, 12:34:19 am »
2) You are looking for the most efficient/highest performance shape for a bow. That is much harder to figure out. It encompasses the dynamics of a shot, limb vibration and more.  It may even vary among different kinds of wood because they offer different damping characteristics and other fine details that will change the outcome at least slightly.

Is this a fair description?

Mark

Close. I am looking for the most efficient/highest performance for EVERY bow. How do we squeeze out the best performance out of any given design while maintaining the desired as aesthetic? Thatís my goal. Especially because 99% of people arenít trying to build a world record flight bow.

Anyone have any thoughts on my actual post? If not Iíll try and draw some diagrams to help show what I mean.