Main Discussion Area > Bows
Bows by the numbers - modelling a bow before cutting timber
willie:
--- Quote from: Aussie Yeoman on September 08, 2024, 11:37:17 pm ---
Yeah I've done heaps of bend tests. ..I've done a few different bend test styles - simple cantilever, simple supported beam, three point cantilever... But the one I want to try is a four-point bend test. This spreads the load evenly between the two inner supports, and so better simulates a bow's limb in that part of the test sample.
--- End quote ---
I have done conventional 3 and 4 point testing in the past but for a future bow I am considering a simply tapered minibow made from a ripping off the plank in question, putting a string on it and drawing until it retains set, and then "reverse" modeling it in virtual bow.
--- Quote ---The working strain is what I used to calculate the dimensions of these bows. In this case, the working strain is about 0.75%.
--- End quote ---
that works good for a few of the local white woods here. maybe 70% for less set. thanks
sleek:
Reducing set is the goal, not getting a lower draw weight. I'm asking g what the program would show had you built the bows to the same dimension but with lower draw weight.
--- Quote from: Aussie Yeoman on September 08, 2024, 11:37:17 pm ---
--- Quote from: sleek on September 08, 2024, 02:15:10 am ---Thanks that's perfect. If you had made them 11 to 15 % less in draw weight what does your calculations say you should predict for set? Per mine, 11 to 15% less the ideal range for no set for white oak.
--- End quote ---
Sorry Sleek I don't quite follow - is your objective to get less set or to have a lower draw weight? The thickness of the 40 lb and 50 lb bows is pretty much identical - the difference between them is the width. 40 lb is 80% of 50 lb, so the lower draw weight bow is 80% of the width.
--- Quote ---deliberately designing to 1-1/2" of set and coming in at 40mm is great. I wished some of mine came in that close. Guess it is all in the test data. Have you considered or tried a simple bend test to obtain a working strain?
--- End quote ---
Yeah I've done heaps of bend tests. And I got a copy of al the bend test data that Tim Baker did decades ago for TBB1. The working strain is what I used to calculate the dimensions of these bows. In this case, the working strain is about 0.75%.
I've done a few different bend test styles - simple cantilever, simple supported beam, three point cantilever... But the one I want to try is a four-point bend test. This spreads the load evenly between the two inner supports, and so better simulates a bow's limb in that part of the test sample.
--- Quote ---Do you have any pics of the bow unbraced, that show where the set occurred? 40mm is a good result in my books too, from an oak board stave.
I find oak can take a lot of set, unless it's very dry or heat tempered.
--- End quote ---
Stand by and I'll take some pictures.
I have never tried heat treating, but I may for this 50 lb job to see what happens. I'll have to brush up on my HT knowledge.
--- Quote ---Not to sidtrack the design part of this thread, but should one prefer more or less set with any particular kind of wood? ie do white oak bows work out the best with a moderate ampunt of set (40mm)?
--- End quote ---
I'm not sure on that one. I haven't done the testing. Designing these bows to have this level of set is what allowed me to get 1 lb per mm or width at this length without an outrageous amount of set.
--- End quote ---
sleek:
--- Quote from: Woodbear on September 09, 2024, 03:15:41 am ---
I have a few questions:
You state that the thickness is within 1 mm of the design dimensions. Does this mean you tillered to get the desired weight at the desired draw and then measured the thickness for comparison to the model?
Dave
--- End quote ---
I just want to point this out.. Thats exactly how folks should be doing it, you reduce the thickness of the bow to reach your draw length, not to reduce the draw weight. Too many people think wrong when tillering and that's what leads to set very often. The bows over all width is what makes the bows draw weight. I'm sure you knew this already, but felt it's important enough to highlight and not let it be glossed over.
Aussie Yeoman:
So, what I did was cut the bow's limbs out just oversize (in thickness, anyway) and then tillered as normal, seeking out the target draw force while keeping to the projected tiller shape as much as I could. The limbs ended up being just slightly thinner than the calculated dimensions.
The computed sections are all rectangular. The real bows have limbs that are ever so slightly rounded around the corners, and the handle more generously so. Especially in the handle, I figured the change in deflection was small enough to disregard.
I've not yet shot mine, nor have I measured F/D curves. A project for a different weekend. But I love data so it's something I should do. :D
The two sets of nocks at each end are for the bowstring and the stringer. My stringer is just a long length of paracord with a loop at each end.
The version of VirtualBow I have doesn't calculate/project set. I just fiddled with the design so the model had a working stress that I knew should theoretically produce that much set. I think your spreadsheet is probably the most advanced of all of us who have such programs in modelling bows. Actually Alan Case's SuperTiller might do it too... But mine definitely doesn't and neither does VirtualBow.
--- Quote ---Do you have any setup for, or intention to measure the dynamic properties of the bows for comparison to the model?
--- End quote ---
I'm not sure I can. Could you elaborate?
A dedicated 4PBT setup is something that's been percolating in my mind for a few years now. When I build one I will definitely share.
Aussie Yeoman:
Indeed. When I teach my classes I tell students that the width determines the draw weight, and the thickness determines the draw length.
--- Quote from: sleek on September 09, 2024, 03:45:40 am ---
--- Quote from: Woodbear on September 09, 2024, 03:15:41 am ---
I have a few questions:
You state that the thickness is within 1 mm of the design dimensions. Does this mean you tillered to get the desired weight at the desired draw and then measured the thickness for comparison to the model?
Dave
--- End quote ---
I just want to point this out.. Thats exactly how folks should be doing it, you reduce the thickness of the bow to reach your draw length, not to reduce the draw weight. Too many people think wrong when tillering and that's what leads to set very often. The bows over all width is what makes the bows draw weight. I'm sure you knew this already, but felt it's important enough to highlight and not let it be glossed over.
--- End quote ---
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version