New stack thickness

[Mckenzie]

I'm curious if any of you can help out with calculations/approximations for the stack thickness of a bow.  I'd like to end up with a 68" bow in the range of 55 to 60# at 28 inches.  The press is built for a 68" bow up to 1 3/4" wide with a mild reflex/deflex design (18" riser).  One bow has been made on the press but it was for my son, so it was shortened to 58".  His bow is 32# @ 28" - 0.030" glass on the back, 3 tapers (0.085" butt thickness with 1.5 thousandth per inch), and 0.030" glass on the belly.  The outer tapers are maple and the center taper is bubinga.  The width of the bow is just a bit under 1 1/2" with 1/2" tips.  I suppose the calculations would be a bit more accurate if I wasn't changing the length and the stack thickness.  

I'm guessing that my son's bow would be around 25# if it were to be full length (not 58").  So I'm thinking of using 0.040" glass on both sides, increasing all three tapers to 0.100" (which would be 0.015" per taper or 0.045" total wood increase).  I haven't fully decided on limb woods but they will be the density of maple or bubinga.  I may make the center taper of yew.  

I'm working under the assumption that increases of 0.010" of wood will increase draw weight by about 5 to 6# and 0.010" of glass will increase the draw weight by about 7 to 8#.  I know that I'm asking for estimations and guesses, but I'm betting that you all will have some experience behind your guesses.  Thanks in advance, Mark

[Bradford]

Is this a long bow?

[Mckenzie]

Yes, I'd add a photo of the first bow but I can't figure out how to add it.

[Mckenzie]

Here the address to a photo of the press.  

https://sites.google.com/site/imjatse27n86e/home/misc
 

[Bradford]

Check out this site:

http://go.professormax.com/swissbow/swissbow_eng/download_eng.html

Swissbow has a full layout for stack thickness for long bows.  I think the answer you are looking for is there.

[Mckenzie]

Great - I appreciate your response.  I checked out the information and it gave me a reasonable place to start.  Thanks . . . Mark

[**DONOTDELETE**]

anyway you look at it it's going to be a crap shoot.
you've got too many changes going there to accurately come up with anything. the easiest way is to take a SWAG and build it. then you have a basis for getting closer on the next one.

[Mckenzie]

Hi Kirk!  Agreed - There are too many changes in the wood, length, and glass.  It has been good to go over the responses to at least get a start.  It seems as if it might be good so slightly overbuild it and reduce weight if necessary, as it can't ever be added back.  

I think I'm going to construct some method of testing weight before too much time is spent shaping the bow.  I suppose I will at least have to bring the limbs down to their approximate width first.  There was a fabricated tool one this site that was built out of 4 washers that could be used to grab the limb tips to test the weight before the grooves were cut.  I'll look around.

[Bob Sarrels]

On my longbows I normally get 1 lb from .oo3 stack change, up or down.  Mine are fairly highly reflexed.

[GREG IN MALAD]

How much reflex are you gaining between 58" and 68"? You will gain about 5# for every inch of reflex.
If it were my bow I would use a .489 stack with .040 glass. This should get you close to 55# depending on the amount of reflex(subtract .015 for every inch of reflex). If it comes in too light you can cut it shorter, if it's too heavy you can narrow it from 1 1/2" to 1 1/4" wide and drop 15% draw weight.
Kirk's right, it really is a crap shoot. So roll the dice and hope for the best.

[Mckenzie]

Thanks for the advice!  I'm going to make an educated guess then go from there.  There are an amazing number of factors that affect performance!  The build is going to start this week.  mk