poundage increase per inch

[adb]

A few days back, there was a post with discussion regarding how much the draw weight increases per inch of draw length. I said 3-5#s per inch, and I was told by more than one person I was full of it. So... I thought the next bow I finish, I'll measure it when I'm done.
I finished a heavier draw weight bow today. It's a maple backed yew warbow, 74.5" NTN, 95#@30".

Here's the draw weight breakdown per inch:

20" - 55#
21" - 59#
22" - 63#
23" - 67#
24" - 72#
25" - 76#
26" - 80#
27" - 84#
28" - 87#
29" - 91#
30" - 95#

Hmmm... came out to 3-5#s per inch. Largest increase was 5# from 23" to 24" and smallest increase was 3# from 27" to 28". Pretty much bang on at 4# per inch. I don't know about everyone else, but this is what I see on the majority of my bows, especially on draw weights >50#.

[Joec123able]

Yes but most of us aren't building 95 pound bows. How many pounds per inch is easy to determine you divide your draw weight by the draw lenth and tht gives you a close number a 50# bow at 20 inchs is about 2.5 pounds per inch. Again how many pounds per inch depends on the draw weight of your bow

[adb]

I wondered about that. Does it? I think I'll throw one of my 55# bows up on the tiller tree tomorrow and see what it says. It would definitely be interesting to know. Comparing a longbow and a recurve would also be interesting.

[Joec123able]

Wondered about what ??

[adb]

Whether heavier bows have more draw weight increase per inch?

[Joec123able]

Well I think so think about, it a 90 pound bow at 30 inch draw and a 50 pound bow at 30 inches the 90 pound bow would have to increase more per inch

[adb]

No... I disagree. The bow I just finished increased its draw weight 40# in 10". Just because it ended up at 95#, doesn't mean it has to increase more. This is about poundage increase per inch, not final draw weight.

If a 55#@28" bow started at 15#@18", it would still end up at 55#@28" if it increased it's draw weight 40# over 10".

[lostarrow]

I don't know adb, but  it sounds about right to me. I figure on 3 for a rough guide. My guess is that you'll find that  if you have a 6" brace and draw 30" you are left with 24" of string travel. 0 lbs force at 0"travel and 95 lbs. at 24"  .
 
 95lbs divided by 24" is pretty close to 4lbs/ inch.
   
 50 #@28"  =22" string travel  50lbs divided by 22" gives 2-1/4

 I'm no mathemagician ,but I'm guessing this is how it will work out. Although we are talking about different weight bows,and different length draws, at 0" of draw they are all equal......0 lbs. of pull, from there you have to get to the draw weight that the bow is designed for at that length. I would also hazard a guess that recurves will not show any different. The shape only allows the energy to be distributed  differently.The recurve acting as a lever ,just as a siyha does to get the last bit of bend out towards the tip of the working section,therefor allowing for a shorter working limb.
    I will be following your post to see what revelations appear! I'm interrested.I'll measure  what I have here. I have a 65#@28" FG. recurve to test as well as some 50 -60 lb molly-ish.

[lostarrow]

I don't recall seeing anyone post this kind of info before. Have you?  this might dispell some long held myths  about high initial string tension!?!?!?    Pandora's box?

[adb]

Awesome! I look forward to seeing your results! I agree about the recurve... I doubt it will be any different.

I'm going to grab a bunch of my bows and see what they say on the tiller tomorrow.

[adb]

I don't recall seeing anyone post this kind of info before. Have you?  this might dispell some long held myths  about high initial string tension!?!?!?    Pandora's box?

Nope. Just another accepted 'myth' in bow making. I'd be interested to see a lot of hard data and come up with something a bit more based in fact. It might lead to performance design changes. It would be really interesting to see if different wood species offered up different results.

[Gordon]

I generally see about 3-4 lbs per inch with my bows.

[adb]

I generally see about 3-4 lbs per inch with my bows.

Is that all different designs and draw weights? Would you be willing to measure some of your different types of bows and see what you come up with?
I totally agree with you however... I consistently see 3-5# per inch, not the 2-3# most people say.

[lostarrow]

Are you usually making heavier weight bows though?  I'm not going to speculate further.I made my prediction, based on my own( however  flawed and broken) Lets get some data together! I don't have anything that heavy  but can test what I have . Pretty busy ,but I should be able to post tomorrow night.

[JackCrafty]

It will be interesting to see how this new data will compare with the large number of existing f/d curves for various bows.

There is data showing higher initial draw weight bows increasing poundage in a linear progression.  Low initial draw weight bows increase in draw weight more drastically per inch toward the end of the draw.