Scaling up question

[stuckinthemud]

I need to increase a bow design by 12” in length.  I can change width and thickness but the draw weight needs to stay the same.  Any ideas how I can calculate the initial dimensions based on the original bow?

[mmattockx]

You have more questions to answer first. Just offhand these come to mind:

1) Same draw length?
2) Same wood species?
3) Is max performance a priority?


Mark

[stuckinthemud]

Same wood (yew), same draw length, needs to be a rugged, not fast, heavy ELB

[willie]

within the same design, one would increase weight by increasing width, but thats not being asked for
within the same design, one would increase drawlength by increasing length, but ditto

if you want someone to estimate how much thickness needs to be increased when adding 12 inches, one would need to know if you are making a 24" bow into a 36" bow or making a 60" bow into a 72" bow, in other words a proportion.
and if the bow is "arc of circle" bend in handle, or has non working areas like handles or levers

[stuckinthemud]

I’m going from a 95 cm 250lb bow with a 30cm draw to 115cm 250lb bow with 35cm draw length, yew ELB, slightly stiff across centre 2 inches but basically working through the centre, arc of circle tiller.  Yes it is a crossbow, but a bow is a bow is a bow.

[Badger]

   If same draw length then you are not scaling up, what kind of wood are you going to be using, weight? and length. and draw length.

[stuckinthemud]

Not quite the same draw length, it is increasing by 2”.  So, here is how I was guessing at things.  If you pike a bow you gain about 5lb per inch, so by increasing string length by 20cm/8” I’ll lose 40 or 50lb but then the extra draw length will counter most of that.  Working on 1mm of thickness being worth 10lb, I was thinking of increasing thickness by 3mm and adding 5mm to the width then chasing weight as normal.  Any thoughts?

[Del the cat]

I'd scale the width up by the same proportion, but scale the thickness up by much less.
E.G. if the bow is 6% longer, make the thickness just 2% thicker... that should make it over weight so it can be reduced to give the correct draw weight.
If you increase thickness pro rata (e.g 6% as per example it would be way too stiff)
Del

[Del the cat]

I'd scale the width up by the same proportion, but scale the thickness up by much less.
E.G. if the bow is 6% longer, make the thickness just 2% thicker... that should make it over weight so it can be reduced to give the correct draw weight. (just my guess)
If you increase thickness pro rata (e.g 6% as per example it would be way too stiff)
Del

[stuckinthemud]

Oh, so length will increase by about 23 percent: a 3mm increase would represent a 10 percent change, the “model” being 29mm thick at centre and 15 at the tips, so dial it back a little? But, width at centre is currently 40mm, which means increasing that to 50mm?  Tapers would remain unchanged?