how to make conclusion from testing or experimental bows

[ekalavya]

It is many times mentioned that making smaller maybe half sized or quarter sized
testing or experimental bows would help in designing a full scale bow or allow
conclusion about a full sized bow.

Up to now I am not sure how the bows should be build and how the conclusion
are made. Tim Baker mentions in The Bowyers Bible part one a standard bending
test which would make those testing bows unnecessary, however the theory behind
is not fully understood by me.

My assumptions for a half sized bow are .. (e.g. around 33" )
compared to a full sized bow (e.g. 66" )

      + draw weight remains the same for full sized bow
      + draw length is half
      + brace height is half
      .  bow width could/should be half or bow with should be same

It would be undoubtly easier if the bow width of the half sized bow would be only half of that
of the full sized .... however the "crown" would not be comparable with a smaller diameter stave.

Tim Baker mentioned that edged-ringed or as i call them standing ring boards are only
10% or a tenth worse. Maybe this would make comparison easier .... but until now i just do not know.


                       

[Del the cat]

It's a good question as things don't scale linearly. E.G If you half all the linear dimensions of a cube the volume doesn't half it goes down by a factor of 8.
Anyhow, back to the question:-
Work on scaling everything down linearly, half the length, half the draw length, half the width.
The thickness may need scaling differently as half the thickness won't give half the draw weight.
I think making a miniature is as much about feel as measurement, will it with stand a tight radius bend, does it feel lively etc?
Dunno what draw weight you should aim for...
Just have a play and see what you get.
A 2/3 scale would probably be more representative of the full sized bow.
Here's one I made earlier:-
http://bowyersdiary.blogspot.co.uk/2012/03/finished-miniature.html
Del

[willie]

the bend test used by Tim baker tested for relative stiffness and a how much force it took to create a predetermined amount of set. the test was fairly simple but actually tested the wood at the single point that the wood bent over a fulcrum.

 bows can be modeled with spreadsheets using Baker test data, although I find it useful to just make a kids bow and scale it up in the spreadsheet.

[Badger]

   When I had to build the giant bow for the Da Vinci project I simply scaled up. it did give me an accurate representation. The one thing that does puzzle me about scaling is the molecular structure of the wood which is something we have no control over. I have always wondered if that is something that would need to be included if using scaling for test purposes. I built the Da Vinci bow at 4 times width and thickness and length. I started with very conservative measurement to minimise the chance of failure.

[PlanB]

Well, the mass principle sure wouldn't scale linearly if a bow was true half size, because as Del the Cat pointed out, the bow mass would reduce as the cube of the scale, in other words an eighth in the case of a halfer bow.

If an elm bow weighed 20 oz for a 66" length, using the same elm for a strictly proportional 33" bow would give you a little shooter that weighed 2.5 oz.

Since the recommended adjustments to the table of values in TBB4 are linear, they probably won't work for this great a reduction in scale. They may be a good (and easy to use) rule of thumb for most full size bows. But I bet when you get down to this level, the adjustments need to be exponential.

[Badger]

PlanB, yes the bow mass theory works pretty well within a window of common size bows but starts wandering off if you go to large or too small. Thats largely because it is not an exact science as much as it is a rule of thumb.

[Selfbowman]

Heck just build the bow design . If it is not what you expected build a different design. At least you have something that can be used by someone. I promise as many bows as Tim builds or built one more is a drop in the bucket. Most of the bows he built I feel where test bows. I would love to meet that man and bend his ear. He's full of bow knowledge that is for sure.  Arvin

[Badger]

  Arvin, if you ever come to Los Angeles I will take you by Tims house and you can bend his ear for a while.

[ekalavya]

Making the right conclusions is important. For the mentioned miniature bow I have to say, unless
it was not for it was for , it wasn't it.

When building a bow, one not always or hardly never uses a scale, but depends on how it
feels like when you draw. So as i observed many people build first a 25lb and then they always
ended up with a 25lb and one reason might be , when it felt too strong to draw they thought it might
break or so.

So I think the draw weight should be the draw weight you want for a full scale bow... everything
else may vary or be different.

Also it's quite a pity that edged-ringed board bows are easier to scale and calculate while
for bows out of saplings with flat rings and natural crown things are a lot complicater than 10%
making the mentioned 10% edged-ringed board bows are worse neglectable.
 

[willie]

if you want full weight but small length, then you might have to scale thickness to bow length, and only draw to the same bend radius?

[ekalavya]

@willie

what i had in mind (after some time)  was just increasing draw length and bow length ....

like 14" draw at max. for the 33" bow ->  28" draw for the 66" bow
12" draw for the 33" bow -> a 77" bow would have 28" draw

just a theory which might be far from reality

[Selfbowman]

  Arvin, if you ever come to Los Angeles I will take you by Tims house and you can bend his ear for a while.

To meet you and Tim personally would be worth the trip . Maybe in the next year or two. Arvin

[Badger]

  Arvin, if you ever come to Los Angeles I will take you by Tims house and you can bend his ear for a while.

To meet you and Tim personally would be worth the trip . Maybe in the next year or two. Arvin

   Scaling is something I have had a lot of interest in for some time now. If you like to experiment it makes good sense. Several years ago I started working on an experiment to do list. The idea was to lay out all the experiments I wanted to perform on bows and wood and then try to create a comprehensive pattern of building that would give me more linear results. The list grew and grew and grew to the point it looked more like something a crazy person would do. Pages after pages. I doubt I could do it in one life time. The comical part is that even if I were to spend an entire life time doing all those experiements I would only gain very little from what we allready know. Wood is one of those things that is never the same from piece to piece, it is similar enough to give somehwat predictable results but if you are looking for that 2% margin of error wood is not the right medium to be working with.

    I have come to the conclusion that we have a lot of very good bowyers right here on PA. If we watch what they are doing we have access to the best pool of information since the begaining of mankind. Thats about as good as it gets.

[PlanB]

And then there's the fun factor. In the Bent Stick, Paul Comstock talks about making tiny 12" bows, just to explore the wood. And if you are making them out of different woods seems like it could be useful that way because scaling is the same across them. But it also just seems like it would be fun, too. 

[Selfbowman]

So true Steve. That poor old dude three thousand years ago had it rough. No power tools , I don't remember when steel came into the picture but maybe not that to harvest tree or shave with.  They had heat to bend the wood with . We could go on and on . I am pretty sure lazy old Arvin would have eaten grubs and wild berries.    Arvin