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Bows by mass

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Badger:
       I started using mass on my bows several years ago and just in the last 1 1/2 years started trying to put it to some kind of formula. I had become pretty comfortable building bows in the 60" and 62" range but found when i stepped outside my comfort zone my performance started falling off or at best became inconsistent. Once you understand the logic behind it you really dont need a formula at all. I still prefer to use the formula because I predetermine how much my bow is supposed to weigh when finsihed and it just speeds things up for me a bit getting the mass to the right point before I start serious tillering. I like to know my mass ahead of time for several reasons, #1 if the bow is high in moisture I will find that out quickly as it will be too heavy, I will know to halt work before I have done any damage. If I am working on a backed bow it will tell me if I have a poor match of back to belly materials. It will also help me find the right design for my bows and pretty much assure me consistent performance.
       It is not one of those things you just read and you know it, more like work it into your bow building program and simply use it as another tool. Your understanding of it will grow as you use it.

basic formula,  bow length-  for every 2" past 54" add 1 oz of weight
                    nock position  for every 2" reflex add 1 oz weight
                   stiff handle and fade area- add 1 oz for every 2"
                     draw length- a zero value would be draw length equals working limb (handle and fade minus bow length divided by 2) add 1 oz for every 2" draw over that number or subtract 1 oz for every 2" under that number
                    draw weight 1 oz for every 5#.

  side notes, english longbows, subtract 1% from calculation for every 5# over 50#
  backed bows with well matched backings, subtract not more than 10% from calculated mass, 5% seems to work pretty well.
 
example:  74" english longbow drawing 30" and 125#

            74" = 10 oz       ( 74-54=20/2=10)
nock position 0=0 oz
handle and fade area 0=0 oz
draw length = -3.5 oz   ( 74/2=37-30=7/2=-3.5)
draw weight=25oz   (125/5)

Total=31.5 oz minus 15%   (1% for each 5# over 50#)
total =26.7 oz


       example stiff handled bow

64"  55#@26" 

length 64" = 5 oz
2" reflex= 1 oz
8" stiff handle and fade = 4 oz
26" draw = -2 oz
55# draw= 11 oz
Total, 19 oz

these are based on normal arc of the circle tillers, in the book variations in tiller are discussed and how to deal with them mainly by manipulating the handle and fade area calculation and also the draw length calculation in the case of stiff tips.
Being exact is not really an issue being close seems to help quite a bit as fas as consistency goes.

DBernier:
Badger, not to beat a point to death but, suppose the wood is a real snaky piece. Say the nock to nock is 70 inch's as the crow flies. Does the fact that "if" you were to "measure" the center line, along the length of wood,  NtoN, and got 80 inch's, would this be a variable for consideration? I keep thinking about it and I want to say no. The action between the nocks in a straight line is what matters. Then again you still have all this mass of wood to contend with. It seems the limbs would be "thinner" than if the bow was a straight piece. BTW I like your formula and will use it on my next bow.

Dick Bernier
Mebane NC

jamie:
yeah im interested to try it. i sort of get it although i do see variables for different risers especially seeing as i dont have any real grip on most of my bows. i think for the most part it means getting the outer limb  weight as low as possible. am i wrong?

Badger:
   Jamie, knowing the length of the stiff center area is more for the sake of knowing how much working limb there is. The shorter the working limb the wider it needs to be. Not about low mass bows at all. My bows are much wider than they used to be since measuring the mass. It is about mass placement, the outer limbs have to be narrowed down if they are not bending that much, but not extreme by any means. Long bows with shorter draws, say 72" bows drawing 27", will work best with a longer stiff center or at least semi stiff center area along with the stiff handle. Say you have 8" handle and fades but the bow does just barely bends comming out of the handle for 6" on each side, case like this you split the difference and use a calculation with a 14" handle and fade area. Once you start playing around with the weight of the bow you start to get a sense of where the mass is working best and when to leave it stiff or bending. It tends to give the bowyer a little more lee way also. I will do a build along next week when my staves come in, Steve

jamie:
i have one now im getting startied on. typical for me . 62" bend in the handle. roughly 1 1/2 " wide shooting for 55#. tiller would be full circle with last 6" or so bending. can you give me an example of your formula for that and then maybe this will sink in .lol

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