calculating poundage at X drawlength

[Aksel]

Hello, I thought of a something Iīd like to hear your opinion on. Maybe a stupid question but here we go:  Letīs say I am working on a straight stave flatbow and it is currently pulling #40 at 24" and it isnīt taking any set. I am not aiming for a very specific draw weight (this is the way I make most of the bows I am making for myself: experience guiding my decisions juggling limb width/draw length/wood type- quality/ length of bow, set taken etc = aiming for something I like to shoot, usually around 45 lbs +/- 3). This way of making bows might be a bit backwards, stabbing in the dark, but is there a way to calculate how much it will pull at, letīs say at 28""?

[bradsmith2010]

50 ish

[Will B]

I usually see about a 3# increase in draw weight per inch of increased draw length, so I would predict that bow to come in around 52# @ 28”.

[Pat B]

I think about 3# per inch.
 If, as you are tillering your bow only pull it to your desired draw weight when you reach your draw length you should have your draw weight and you haven't over strained your bow.

[bentstick54]

I’m the same way. I have checked a dozen of my selfbows and they all gain 2.5 to 3# per inch.

[bradsmith2010]

I do this weird thing where I dont even pull to target weight but under,,,till I get close to full draw,,I calculate where it will be,,so I havent pulled to full weight till I get to end of process,,

[Aksel]

Thanks for replies. I suppose length of bow must also play a part in weight gain per inch, right?

2-3# per inch sounds reasonable and I think I am heading for about #50 in this case, but Iīm working on a bow which is 8 inches longer than I usually make so I find it slighter harder to assess..

[Del the cat]

40/24 = 1.6667
multiply that by 28"
1.6667 x 28= 46.66

I've tried making this simple calculation more accurate by just using the actual draw (e.g draw minus brace) but it doesn't make much difference.
Simply draw length divided by draw weight gives a very usable approximation of pounds per inch that can then be extrapolated to any reasonable draw length.
Del

[George Tsoukalas]

I think that procedure is pretty good, Aksel. I do something similar. That way there is much less stress on the bow. I roughly figure 3 inches per inch also. Jawge

[Aksel]

Del, thatīs brilliant and it actually turned out pretty accurate in this case. The bow finished at 45 at 28 (it was #39 @ 24 to be precise). Then I piked it 1 inch on each end. Now at 48#.

[Albert]

What Del said, with power stroke instead of draw length gives this:

draw weight at full draw = (draw weight at intermediate draw length)*(power stroke at full draw)/(power stroke at intermediate draw length)

in your case:

brace height = 6''
intermediate draw length = 24''
power stroke at intermediate draw length = intermediate draw length - brace height = 24''-6'' = 18''
draw weight at intermediate draw = 40 lbs
power stroke at ful draw = full draw - brace height = 28'' - 6'' = 22''

draw weight at full draw = 40 lbs * (22'')/(18'') = 49 lbs

Bert

[Selfbowman]

Del that’s s close on my hundred pound bows also