Wood bow logic and common sense and words of wisdom

[Badger]

    I have no doubt that the beam theory is correct but an experiment I did a few years ago surprised the heck out of me. I made several extra wide bows like 3" wide for 50# osage bows, basically more than  twice as wide as I normally would. The thickness should have been about 25% thinner than normal for the same weight. The thickness actually came out at about 33% thinner than normal and the overall mass which should have been higher came in at about 20% lower than my normal narrower bow. I didn't do enough tests to declare anything as fact but it really made me believe that we are straining the wood even more than I previously thought we were. Since that time I have made plenty of osage bows between 1 3/4 and 2" wide. Even at the extra width my mass seemed to come in at about what I expected. Assuming the beam theory formula is correct this could only mean we are underestimating strain on our bows.

[bushboy]

A bit off topic but I've been toying with a design.bend though the handle,gradually changing to a flat bow at mid limbs and down to Molly levers.from what I've seen,it makes for a fast bow with little set.thoughts?

[Jim Davis]

    I have no doubt that the beam theory is correct but an experiment I did a few years ago surprised the heck out of me. I made several extra wide bows like 3" wide for 50# osage bows, basically more than  twice as wide as I normally would. The thickness should have been about 25% thinner than normal for the same weight. The thickness actually came out at about 33% thinner than normal and the overall mass which should have been higher came in at about 20% lower than my normal narrower bow. I didn't do enough tests to declare anything as fact but it really made me believe that we are straining the wood even more than I previously thought we were. Since that time I have made plenty of osage bows between 1 3/4 and 2" wide. Even at the extra width my mass seemed to come in at about what I expected. Assuming the beam theory formula is correct this could only mean we are underestimating strain on our bows.

Steve, the formulas for beam strength are based on homogenous materials, which wood is not. So for wood, we have to expect and watch for variation. That means approaching the dimensions the way we do--cautiously.

[Badger]

A bit off topic but I've been toying with a design.bend though the handle,gradually changing to a flat bow at mid limbs and down to Molly levers.from what I've seen,it makes for a fast bow with little set.thoughts?

  A little bend through the handle is a good strategy for lowering stress on the limbs, just a little flex can give you about 4" additional draw, most of the set we pick u usually starts around 24". So, yes I like the idea and sometimes use it myself.

[bushboy]

My logic is a d shape in the handle bending and taking no set.gradually panning out to a rectangular cross section to distribute the load and taking a little set and stiff outer limbs for performance.thanks for your response Steve!will test this model more extensively this summer.its kinda like a Sudbury but not quite.

[bushboy]

Bow logic?I really can't stress enough how much more enjoyable bow building has become after I ditched the t stick!it stressed me to the max when I used one.now. Floor tiller and exercise and touch up by brace shape is my go to.takes some time but touch ,feel and sight win the day imoh!

[Springbuck]

Badger:  "I have no doubt that the beam theory is correct but.........."

    I apply it to limbs of course, but really where I use this rule is in stiff sections and handles.  If I have a 1/2" thick X 2" wide limb and want to narrow in a 1" wide handle, it better be 1/2" thick PLUS 1/16" to bring the stiffness up even with the limb, plus at LEAST another 1/16" to make sure it bends less than the limb, plus a tad more as fudge factor, or I know I don't have enough wood for a narrowed handle, even with an added block.  Then I need to rethink the design

You gotta know what you can get away with and what you can't.

Bushboy: "A bit off topic but I've been toying with a design.bend though the handle,gradually changing to a flat bow at mid limbs and down to Molly levers."   I have not done this, exactly, but the principles are sound.  What I have done is a bendy handle where the limbs flatten considerably (maintaining their width), through the mid-limb where they bend the most, and then narrow considerably in the "Eiffel tower" style toward rigid tips. 
Baker's Pecan bow from the TBB'S is kind of like yours, but with a fairly long, rigid middle, and barely bending inner limbs.......

[H Rhodes]

Good stuff in this thread!  I don’t have much logic or wisdom to impart.  I would suggest to a newcomer the importance of shooting a bow a good bit before putting a lot of time and effort into finishing work.  Some pieces of wood just won’t make a long lasting, durable bow.  Save yourself the emotional kick in the groin and shoot a new bow a hundred shots before you put twenty coats of true oil on it.

[gfugal]

Good stuff in this thread!  I don’t have much logic or wisdom to impart.  I would suggest to a newcomer the importance of shooting a bow a good bit before putting a lot of time and effort into finishing work.  Some pieces of wood just won’t make a long lasting, durable bow.  Save yourself the emotional kick in the groin and shoot a new bow a hundred shots before you put twenty coats of true oil on it.

+1 I would even add that 100 times is probably not enough. I'm planing on shooting a bow 500 times before I call it safe