Author Topic: Stress and performance  (Read 3076 times)

0 Members and 1 Guest are viewing this topic.

Offline Aussie Yeoman

  • Member
  • Posts: 99
Re: Stress and performance
« Reply #15 on: June 23, 2022, 07:25:59 am »
Quote
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 Im 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.

When I've previously said things along the lines of what I say below, I have attracted criticism for my expression of this craft as being too divorced from the romance of bowmaking by feel. However, given your question I feel an answer such as the below merits this sort of info. Using methodologies from materials science and mechanical engineering allows us to answer questions such as yours, and has helped me to personally teach maybe 180 students how to make dependable, high performance wooden bows.

Yes, there is a correclation between stress and performance. If the stress is very (very) low, it will be because the stress is being distributed across a whole lot of wood. Either through very long or very wide limbs (or both). If your bow is very high stress, it will be because the bow is very short or very thick (or maybe both).

In the former case, all that wood across which the stress is distributed presents a whole lot of excess mass. More of the energy stored in bending the bow has to be expended on accelerating the heavy limbs.

In the latter case, the bow gets dangerously close to having stress that either causes high amounts of set (which diminshes performance) or breaking (which summarily ends all performance). A bow which is tillered to a high level stress and repeatedly exposed to that stress is likely to deteriorate in performance over time. I understand that it's not uncommon for wooden flightbows last a few shots and then break down (or break up).

So the aim of the game is to find that amount of working stress that walks the fine line between sufficient wood to withstand the stresses over time, and of sufficiently high stress to minimise the mass. In your example of the two 50# bows, ideally they'd both have the same amount of stress. There may be some who don't believe that, but mechanically it's true. A piece of timber with an ideal amount of working stress should be made into bows that experience that amount of stress regardless of the length or drawn shape.

While it is true that the variability of wood's mechanical properties can be quite broad, it is also the case that this phenomenon doesn't matter much so long as the stave is of sufficient size to extract a sample for bend testing first. There are probably many bowyers who would argue that this isn't 'listening to the bow/wood', but I'd argue that it is in fact listening very carefully after having asked the wood some very specific questions.

It is entirely possible to measure the mechanical properties of a piece of timber and from those calculations make a dependable, high performance bow. I've done it for years. What's more, I've outsourced the calculations to spreadsheets. I've written a reasonable amount about it here: https://ozbow.net/phpBB3/viewtopic.php?f=34&t=5450 and here: https://ozbow.net/phpBB3/viewtopic.php?f=34&t=13765. You can do the same with free software and engineer your own bows.

If you've the patience to go through that first link, you'll see its possible to make a bow with perfectly even stress and strain throughout the entire length of the bow, and that you can achieve this regardless of the particular drawn profile of the bow, be it English Warbow or short Mollegabet. If you go through the second, you'll see how I record bow properties in shorthand. Of note, by combining my bend test library with Tim Baker's, I found that while a wood like Rock Maple might vary in its mechanical proprties between samples, it is very often the case that the variation is sufficiently small not to matter a great deal.

I suspect the practice of rounding the bellies of yew bows is more a matter of convention than engineering best-practice.


AY
Articles for the beginning bowyer, with Australian bowyers in mind:

http://www.tharwavalleyforge.com/articles/tutorials

Offline hoosierf

  • Member
  • Posts: 491
Re: Stress and performance
« Reply #16 on: June 23, 2022, 11:35:08 am »
Paul Comstock spent quite a bit of effort analyzing the subject of highly stressed bow designs in The Bent Stick.   Id venture to say youll find lots of useful info in that short highly informative paperback.   
« Last Edit: June 23, 2022, 02:24:20 pm by hoosierf »

Offline bradsmith2010

  • Member
  • Posts: 4,837
Re: Stress and performance
« Reply #17 on: June 23, 2022, 04:10:05 pm »
ok here is a good example,,
take a 40 inch self bow,, and draw it 20 inches to 50#,, shoot it through a chrono with 500 grain arrow,, I would say 140 fps would be good,,
now take a 66 inch self bow 50# draw and draw it to 28 inches ,, not so strained ,, shoot the same arrow,, 170 fps is achievable,,165 being more achievable,, ;D
« Last Edit: June 23, 2022, 04:17:20 pm by bradsmith2010 »

Offline Aussie Yeoman

  • Member
  • Posts: 99
Re: Stress and performance
« Reply #18 on: June 24, 2022, 07:07:16 pm »
Bradsmith, the change in speed and disgance of the arrow in your example is due to the power stroke, which changes how much energy is stored and then released into the arrow.

Ideally, both bows would be designed and built to have equal strain/stress.
Articles for the beginning bowyer, with Australian bowyers in mind:

http://www.tharwavalleyforge.com/articles/tutorials

Offline bradsmith2010

  • Member
  • Posts: 4,837
Re: Stress and performance
« Reply #19 on: June 24, 2022, 07:10:55 pm »
yes but one bow is stressed more than the other,,

Offline Aussie Yeoman

  • Member
  • Posts: 99
Re: Stress and performance
« Reply #20 on: June 24, 2022, 07:19:12 pm »
No, it shouldn't be. That's what I'm trying to say. No matter what the design, profile, drawn shape, draw length or draw force... a piece of wood should be used so that it has the same maximum stress.

It probably is the case that people doing it by feel often make some bows with higher or lower stress depending on the dimensions and whatnot. But in an ideal world, and with the help of some scientific and engineering principles, a crossbow prod pulling 200 lb at 12 inches should be subject to exactly the same working stress as a 72" longbow pulling 45# at 28".
Articles for the beginning bowyer, with Australian bowyers in mind:

http://www.tharwavalleyforge.com/articles/tutorials

Offline bradsmith2010

  • Member
  • Posts: 4,837
Re: Stress and performance
« Reply #21 on: June 24, 2022, 07:29:08 pm »
ok

Offline Bob Barnes

  • Member
  • Posts: 805
Re: Stress and performance
« Reply #22 on: June 26, 2022, 11:09:00 am »
No, it shouldn't be. That's what I'm trying to say. No matter what the design, profile, drawn shape, draw length or draw force... a piece of wood should be used so that it has the same maximum stress.

It probably is the case that people doing it by feel often make some bows with higher or lower stress depending on the dimensions and whatnot. But in an ideal world, and with the help of some scientific and engineering principles, a crossbow prod pulling 200 lb at 12 inches should be subject to exactly the same working stress as a 72" longbow pulling 45# at 28".

So, if you have the formula, and if it's not a lot of work, could you explain using Brad's 2 bow example from the above post?  I'm just guessing that your formula involves coming up with the ideal dimensions for a given bow/bow wood...?  thanks.
Seems like common sense isn't very common any more...

Offline sleek

  • Member
  • Posts: 6,616
Re: Stress and performance
« Reply #23 on: June 26, 2022, 12:54:22 pm »
Mass distribution is more important than mass alone. That gets into surface area and pounds of stress per square inch. For a few years I have been on and off working on getting a formula for it. I have one that works well, but isn't exactly where I want it to be. I'm still trying to improve it.
Tread softly and carry a bent stick.

Dont seek your happiness through the approval of others

Offline Badger

  • Member
  • Posts: 7,976
Re: Stress and performance
« Reply #24 on: June 26, 2022, 05:05:03 pm »
    I think because all species and even individual samples of wood vary to such a large degree it is almost impossible to predetermine stress levels based on design beyond the standard rules of thumb we have come up with. You can fine-tune that a bit by monitoring set which is the ultimate practical limits of the bow anyway. But Ideally, I would agree that stress limits should be the same for any bow of any weight and design. Without engineering details on every piece of wood any practical useful chart to establish widths would be nothing more than the same estimates we use now.

Offline Aussie Yeoman

  • Member
  • Posts: 99
Re: Stress and performance
« Reply #25 on: June 27, 2022, 04:16:52 am »


So, if you have the formula, and if it's not a lot of work, could you explain using Brad's 2 bow example from the above post?  I'm just guessing that your formula involves coming up with the ideal dimensions for a given bow/bow wood...?  thanks.

Yep, absolutely can do. Stand by for a day or two and I'll pul something together.

    I think because all species and even individual samples of wood vary to such a large degree it is almost impossible to predetermine stress levels based on design beyond the standard rules of thumb we have come up with. You can fine-tune that a bit by monitoring set which is the ultimate practical limits of the bow anyway. But Ideally, I would agree that stress limits should be the same for any bow of any weight and design. Without engineering details on every piece of wood any practical useful chart to establish widths would be nothing more than the same estimates we use now.

Totally agree Badger. Fortunately, doing a bend test needn't take long. And the time it saves is immense too. I can make a longbow from a board that pretty much comes off the bandsaw tillered to within a couple of pounds of design.



Articles for the beginning bowyer, with Australian bowyers in mind:

http://www.tharwavalleyforge.com/articles/tutorials

Offline Badger

  • Member
  • Posts: 7,976
Re: Stress and performance
« Reply #26 on: June 27, 2022, 09:18:43 am »
  Aussie, 1st of all I wanted to publicly thank you for converting the mass formula to an excel spreadsheet for me many years ago.
 
  As for a bend test, I was never able to convert a bend test to usable information that I could apply to making a bow. I think if you could lay out the method for doing this, if it is not too complicated it would be a huge step in making higher performing bows. I came out with several tests that were useful in comparing wood but I was never able to directly apply it to a design. Hopefully you can send us in the right direction here.

Offline bradsmith2010

  • Member
  • Posts: 4,837
Re: Stress and performance
« Reply #27 on: June 28, 2022, 04:26:38 pm »
the answer to your question is yes,, no graph or formula needed, just make the two bows and see for yourself,, its that easy

Offline Bob Barnes

  • Member
  • Posts: 805
Re: Stress and performance
« Reply #28 on: June 28, 2022, 04:35:34 pm »
the answer to your question is yes,, no graph or formula needed, just make the two bows and see for yourself,, its that easy

Brad, I agree somewhat. but if someone can tell me that the shorter bow needs to be (maybe) 3" wide and 3/8" thick to be strained the same as a longer bow that is "1 1/4" wide and 3/8" thick... it would be interesting and a good place to make 2 bows just to see how they compare in reality.  I guess I'm a visual learner....
Seems like common sense isn't very common any more...

Offline bradsmith2010

  • Member
  • Posts: 4,837
Re: Stress and performance
« Reply #29 on: June 28, 2022, 05:05:11 pm »
me too thats a good point,, I would be thinking is the mass of the 3 inch bow gonna effect the performance,,
I would just have to make it,, shoot it through a chrono and see,, I dont know the answer,