Design And Mass Theory

[Bayou Ben]

Bow Specs:
67.5" NTN
R/D style Maple back/ Elm Core/ Yellow Heart Belly
Stiff Handle Area ~14"
Stiff tips ~9"
3"+ of glued in reflex
Goal weight ~50 lbs, 29" draw
1.5" wide to .5" tips straight taper

I was at 20" and full brace when I decided to weigh the bow since it was the 1st time I used yellow heart in this design.  It came in at 20.9 oz.  According to my mass theory calculation the bow should have been 25 oz maybe even more.  So I thought that something had to be off and kept on tillering.  I kept getting set each inch out from 20" so I removed more wood from the inners and stiff tips trying to get more wood bending.  By the time I got to 24" I realized the bow wouldn't make to 50 lbs, so I decided to just finish the tiller out to 28" without removing anymore wood.  It ended up at 47 lbs and it developed frets right at mid limb.  It lost 4 lbs in tillering from 20" to 28". 
So long story short I didn't realize what mass theory was telling me until after.  I should have continued my 1.5" width out to past mid limb and that would have gotten me close to target mass and would have made a nice bow at 50 lbs.  This bow will be put in the corner with the rest of my experiments, but I'll sure be listening to what the theory is telling me earlier in the game anytime I'm trying something new.  Hopefully this helps someone else avoid this mistake. 

 


 

[Bayou Ben]

Frets

[simk]

my sincere condolences sir. that sounds like a cool trilam combo and a great loss.
2 out of three of my r/d attempts also failed midlimb, whre i had only a width of 1 1/4". my next ones also will be wider. midlimb seems very stressed with this long reflex. cheers

[Badger]

  Ben, that was an excellent report. I think starting off at about 1 3/4" wide and keeping that until past mid limb might have worked out better. I am going to go re figure the mass and see what I come up with.   I came out with just under 24 oz. but pretty close anyway. Handle is pretty hard to figure in so who ever is building the bow needs to allow for that.

[BowEd]

I can see why that would be hard to do using 3 different types of wood.

[upstatenybowyer]

Awesome report as Badger said, and many thanks for sharing your results. I think this is a great example of what a difference every little variable makes, especially when working on highly reflexed bows and/or using unfamiliar wood species.

Thank you also for helping us not to think of these outcomes as failures but rather important learning experiences that deserve some careful scrutiny. 

[Bayou Ben]

simk- Thanks but no condolences necessary.  It happens when you are trying new things.  You are right, the midlimb get stressed a lot in these designs.  There's really not much working limb even at this length.

BowEd- Yeah it's difficult to guess where I'm going to be mass wise during glue up with 3 species, but I should have known that I needed more parallel limbs.  This design has worked well for me with ipe or osage and bamboo backing.

Badger- I was hoping you would chime in.  As you say, there's a lot of "logic" necessary to come up with a mass number for these styles.  Would you mind showing the math behind your 24 oz so that I can check it against mine? 

upstate: No problem.  I don't mind these failures as I learn so much.  It's the really good ones that explode with no reason that hurt.  Plus now I get to work on other bows


 

 

[Badger]

I just finished one up yesterday very similar to yours. 50#  4" reflex and 14" stiff center as well as 8" stiff outer limbs. I came out with a projected mass of 27 oz. I sill took 1" of set but I am pretty sure my wood has not seasoned enough. I am going to dry it out for a month and re heat treat.

Badger's Mass Method Calculator            
            
            
component   statistic   Mass Figure      
NTN length (inches)   67   6.5      
nock pos'n (+=reflex, - =deflex inches)   4   2      
handle (non bending center) length   14   7      
Draw length (inches)   29   1.25      
Draw weight (lb)   50   10      
Is the bow bamboo or wood backed? (Y/N)   n   100%      
            
            
Total Mass (oz)   26.75         
Total Mass (gram)   758.34966         
            
            
I

[BowEd]

simk- Thanks but no condolences necessary.  It happens when you are trying new things.  You are right, the midlimb get stressed a lot in these designs.  There's really not much working limb even at this length.

BowEd- Yeah it's difficult to guess where I'm going to be mass wise during glue up with 3 species, but I should have known that I needed more parallel limbs.  This design has worked well for me with ipe or osage and bamboo backing.

Badger- I was hoping you would chime in.  As you say, there's a lot of "logic" necessary to come up with a mass number for these styles.  Would you mind showing the math behind your 24 oz so that I can check it against mine? 

upstate: No problem.  I don't mind these failures as I learn so much.  It's the really good ones that explode with no reason that hurt.  Plus now I get to work on other bows


 

 

It very well could be Ben that previously you used wood that could handle the shorter working sections better than just the latest used wood could,but making it wider like you said could eleviate things.

[Bayou Ben]

Thanks Steve.  Let us know how that bow turns out. 

Yeah you are definitely right about that Ed.