Questions for you Pyramid Guru's ?

[Stick Bender]

This is a project I have been thinking about for a while & got inspired by Tim Bakers recent post & I need a lighter project for summer , but only have made one pyramid style bow , I have a couple questions for you pyramid gurus , I'm planing a 67 in. Pyramid bow ,Tri lam with hickory back , bamboo core, & osage belly , 8 in. Riser with the osage belly lams running the ramps , I have all lams ground except the hickory backing still deciding over all stack dimension, I'm fixed now with belly lam parallel 0.150 & core lam 0.150 with 0.001 taper  , the combined taper is 0.001  ,I'm shooting for 47 lb @ 30 1/2 in  with a front profile 1 3/4 width to zero at the tips , I'm planing on inducing 4 in. reflex during glue up in the outer 1/3 , my hopes are that I can achieve 90% internal tiller as with only 0.150 on the belly doesn't leave me much room ,I'm also trying to decide if I need reverse tip wedges with ending at zero ? any body have any thoughts or design ideas that I'm missing  , I relies the stack thickness is going to be Swag but always have side tiller as a option ?

[leonwood]

Not really sure about the exact thickness needed but don’t make your core lam too thick or you will end up with a heavy bow.
Once glued up remember that you can also tiller from the back and the sides if your belly gets too thin. Works great with pyramid bows which don’t need that much taper.

[Stick Bender]

I think I'm going to error on the lite side with the reflex because if it holds half of it , that's the wild card in lam thickness , thanks for the input it's always a guessing game on these lam bows !

[bushboy]

I would leave the bamboo out and make a simple hickory backed Osage..if it must be a trilam,maple would be my choice.

[Stick Bender]

Maple would have been a great choice the last 2 bows I have done I have used maple core lams I love maple but I wanted to try boo for a switch and my thoughts with the tri lam was it holding more of the glued in reflex !

[Jim Davis]

I would leave the bamboo out and make a simple hickory backed Osage..if it must be a trilam,maple would be my choice.

+1

Bamboo is wasting its talents as a core. I have made a few hickory-Osage pyramids and they work fine.

[Stick Bender]

Thanks Jim could you comment a bit more ,did those bows have any reflex ? My thoughts with bamboo is a lighter core then hickory and a thinner lam to hold the reflex better ? Plus this bow is going to be a long stretch to 31 in.

[Jim Davis]

I just did  a little research and found that the specific gravity of bamboo varies from .5 to .8, so it might, or might not, be lighter than hickory, which is in the mid sixties.

I only glued in reflex on one of my HO bows, and it was only about a half inch. The limb lost that and took another 1/2 inch or so of set.

[Bob W.]

Ive done them both ways and it will hold reflex just fine with just a Osage and the Hickory backing.

[Bayou Ben]

Bamboo as a ground down core will be lighter than hickory and will work fine as a core in a tri lam.  The heavier SG is held in the power fibers on the outside of the bamboo as when using it for a backing.  The SG published for bamboo is an average of the entire thickness.  The power fiber SG is more than likely higher than .8. 
Ritchie, I would rather something a littler thicker for your belly, but since you already have it cut and ground you are doing about the best that you can with the lams you have.  You may be able to get to your target with your long draw, that's if you don't mess much with your thickness, but even then you will be cutting it close.
I would make your hickory back around the same thickness .15, and taper it maybe .001.  I would say go thicker but then you'll have trouble getting it to bend around your riser.  If you don't have to make it bend, then I would do about .175" on the hickory and like mentioned you could do some back tillering if needed.
 You could add tip wedges but it will put more stress on your inner limbs which at your draw and 67" bow will be worked pretty good already.  Up to you if you think you can get away with it, it will perform better with them, imo.  I haven't made a bow with that long of a draw.   
I do my tri lams straight pyramid 1.5" fades to .5" tips.  I don't reduce the width until I get to the last few inches of tiller and it hasn't taken much set.  Kinda in the lines of Badger's mass principle.
I think even at 1.75" wide you will need a little more taper, that's why I said .001 on the back.  I'm not sure about that though, just what I would do.  I normally have .006" taper on mine and they are mild pyramids.  Hope it helps.
-Ben

[Stick Bender]

Ok thanks Ben I appreciate the info great thoughts in what was said just a couple of points on this bow, I'm going to glue in about 4 in. Reflex in the outer third with no deflex, I'm looking for arc of a circle tiller hence the 0.001 taper and I'm going full pyramid at going to zero and knock at the thinnest point I can get away with ,the hickory back is a single lam , I don't have your grind set up so I would have to cut it grind the 0.001 and skiev joint back together unless I did it by hand ,some thing to think about , the riser I was going to grind the fades from 2 in. Back 1/8 1in. at 1 in. 1/16 and paper tips , if I went tip wedges I was think pretty thin at 0.005 taper just enough to stiffen but your giving me pause on that

[Bayou Ben]

Oh didn't realize you were going to zero on your pyramid.  I would leave your hickory with no taper then.  You can always tiller in the taper if you aren't getting the arc you are after.
This one is the closest I've made to what you are doing.  Osage belly .19 thick, maple core .19 thick., hickory back .14 thick, all tapered 0.002".  6" wedge tips.  Gives an arch-ish tiller with the 0.006" total taper.  Came in a little under weight at 40 lbs.

[Stick Bender]

Nice looking bow looks quick ? I also wanted to mention that the riser is strait so are you still thinking 0.175  hickory back ? total  0.475 ?  I'm looking  45-48 lb @31 usually if I hit 40 lb. @ 28 I can make that Aprox weight  ,Ben what was your glued in reflex on that bow ?

[Bayou Ben]

Yeah I would go close to .175 on the hickory and don't touch your belly at .15.  Do all of your thickness tillering (if you need it) from the back and sides.  You will be heavier with only .001 taper compared to my .006, but I'm not sure by how much. 
I didn't chrono that bow but they all are pretty quick.  It's just an efficient design. 
I don't remember exactly, but my form blocks were close to: 4" center post, 3" mid limb, 6" tips post.  I change them all the time.  That's what makes these fun for me. 

[Stick Bender]

Ok thanks I will make a mock up & see where I'm going to knock at ntn before I decide but I greatly appreciate the insights you gave me a lot to ponder, it's always a guessing game on these bows but have a better picture of where I need to be with the fixed belly numbers makes me consider every thing else but that's all I could get out of that osage that clean !