A Look-up Graph for determining Selfbows Performance

[Archer Bunta]

well I like your idea, thanks for sharing
I have been shooting bows through a chrono since the first mojam,,and shot some there,,
I have kept quite a few records of different bows,,,
your bow 42#,, it is in the range of reality to shoot the 420 grain arrow 138fps( must have some string follow or moisture issue
I would say even with a b 50 string, it should shoot a 500 grain arrow at least 142fps,, or the 420 grain 150 fps and up as badger stated,,
I think when you look at the mojam speeds ,,, remember most bows were shooting b 50,, and most if not any were not heat treating the belly, I dont recall anyone doing that at the time,, anyway,, most experienced guys here can tell you about how fast the bow is gonna shoot,, before you put it on a graph or shoot the chrono,, or what it should shoot if tuned correctly,, as stated there are alot of factors that would be hard to put into the equation,,, I think release being one of the more difficult ones,,,

Thanks Bradsmith.

My bow is a hickory board bow. The limbs were not of the best design. My taper did not start until a little bit above midlimb thus more mass. In addition,I tested my speed during spring which was more humid so my hickory bow was "very soggy" so to speak with about 1.5" of string follow. I used B55 with a flemish twist that is most likely too heavy. Not the best bow out there. You build, shoot and learn.

I have a more recent hickory pyramid bow that feels a lot faster but haven't gotten the time to actually test it out.

[willie]

Its a shame that with all the work done at mojam, a f/d curve for each bow was not recorded.

AB-
certainly you have given quite a bit of thought to the subject of efficiency losses. Do you have any thoughts on the nature of, or various components of efficiency robbers that have been categorized as
hysteresis?

willie

and..... welcome to the forum

[Archer Bunta]

          Bunta, I don't think you are calcualting efficiency right. Your bow would measure closer to 50% efficient. The only way to calcualte efficiency is to measure work in. You measure the draw weight at 1" increments starting with the first inch all the way to full draw. You total up those numbers and divide it by 12 foot pounds of stored energy.
You then calculate how much kinetic energy is in your arrow based on chrono speeds and divide it by the stored energy.

        There are so many factors that effect efficiency and performance that rely on a bowyers skill that any kind of graph that attempts to predict it would be useless.

  A group of 50# bows all shooting the same arrow will have a variance from about 135fps to 175 fps.

You're right I did calculate efficiency wrong. I was a factor of 2 off.

It is probably not ideal to be predicting efficiency of a bow because of those factors. The scatter is quite large in the actually data set. Again its just a reference based on principles of physics on data that I found. Nothing concrete.

   There is an interesting graph you can create that can somewhat predict the speed of various weight arrows. It uses a method known as virtual mass. The only problem with using this methos on self bos is that hysterisis comes into play and it is not linear. The hysterisis increases as bow speed increases.

Interesting. I'll have to look into that. I'm probably overkilling primitive bows by over/doing it with all of these unnecessary analysis .

[Badger]

  Willie, About a year ago I came up with a series of measurements and tests for the purpose of isolating hysteriis losses from other losses. To be honest it is so involved it is really not worh fooling with. I was just kind of obsessed with knowing for my own purposes. The great majority of your losses are in limb vibration or distorsion in the limb before it releases the arrow. The mass of the limbs has a lot to do with this as they carry momentum and can take on a life of their own to some extent. The amount of working limb also affects this, it gives the limb more opportunity to distort.

[bradsmith2010]

unnecessary analysis is part of the fun,, we all do it,, just enjoy your process, we all get to the same point in the end,, shooting the arrow,,

[Wooden Spring]

You might appreciate this:
http://www.primitivearcher.com/smf/index.php/topic,54520.msg741055.html#msg741055

It is a graphical method for designing a bow of any wood to any draw weight and draw length. It is taken from several sections of the book "Archery: The Technical Side."