Some help please?...

[Charl]

Hi everyone,

I decided to try and make a reflex deflex longbow as my next bow after the recurve and went to Ausbow to buy my supplies.

I asked John what he would recommend to get a 45 - 50 pound draw weight at 28 inches in a 68 inch bow.

He suggested that i use:

2 x .050 fibreglass laminations
3 x pairs of 100/1 Canadian rock maple laminations
1 x pair 90/2 Canadian rock maple laminations

And that that would get me in the ballpark.

Now I am learning as I go but this bow has turned out to be nearly 80 pounds!! Almost double what I was trying to achieve.

I have done some more research since then and at the thickest part the limb is .434 of an inch thick and according to Binghams that equates to around 75 pounds.

Did I get led astray or is this something I have done wrong in the making of the bow?

And here is a pic of the bow.

 

[Charl]

If you own a r/d longbow in the vicinity of 45 - 50 pounds would you be so kind as to measure the width of the limb at the end of the fade and then at the nock points for comparison please?

[bamboo]

to reduce draw weight on that profile I would start by narrowing from the fades out to about 12"
start with 1/16"removed from the width--throw it back on the scale--if that doesn't get you down to the low/mid 60's take more --that would be my target--then aggressively trap it--keep an eye on tiller too--good luck!

[KenH]

I'm not gonna say you got mislead.  But it's a good bet.  

Same thing happened to me when asking a supplier about lam stack thickness. I ended up with 70-80# when I wanted 30#!

Bamboo has the right idea.  Start by shaving 1/16" off each side, then re-scale it and see what you get.  

What's your width at the ends of the riser and at the nominal tips?

[Swissbow]

Reducing the draw weight from 80 to 40-45 is a lot. I'm not sure if it can be done. If you reduce the width of the limbs to much they might get instable and start to flip/twist to the side as soon as you string her up or draw her. I would finish her and start a new one with the correct stack. Take it as a learning experience.
__________
Andy

[fujimo]

you didn't by chance measure the wood lams, then forgetting to add in the glass thickness- that's an easy thing to do- especially if you are grinding your own lams!

is that a long string on there- I would shorten that up so that as you reduce poundage- and check for tiller- that you are getting true readings  

[Charl]

Thanks Bamboo, I will take this onboard and see how I go.

Ken H, the bow is 37mm wide at the fade and 15mm at the tip.

Swissbow, Thanks for the advice. I know I am not going to get to target weight but I still want to try and reduce it a bit.

Fujimo, I did not measure any of this. I went to a "reputable" Bowyer and asked for the appropriate material for my goal and this is what he told me to use after looking into his own records.

So all in all I ended up with a bow not at the target results but as you guys have said, I can use it as a learning experience and add the data into my records for future reference! Not a total loss, just a bit dissapointed with the situation.

[bamboo]

charl
with rigorous trapping of the belly and rounding on the back edges then sanding the glass you could end up closer than you think

my read on the brace shape tells me its carrying a fair amount of weight in the butts [.oo3" taper]
 .434 on a 68" bow has a lot of room for adjustment--maybe not #45--but I wouldn't be surprised at #55---never give up!!

[KenH]

Drop me a note and I'll send you a copy of B. Jansen's Lam Stack Calculator.  It's an Excel spreadsheet that has been available here at TG, but there have been problems with getting it to download.  If you have the measurements of a given bow, you use it to calculate the stack specs for the draw weight you want of the same design.

You need to know the total stack thickness (to the nearest hundredth (.01) is good.  And the measured draw weight of the current bow (to the nearest pound is good)

You plug in the current total lam stack, the current draw weight, and the desired draw weight, and shazam you get a new total stack for the desired draw weight.  How you divide that stack thickness -- glass, and various lams -- is up to you.

After my own experience similar to yours. I decided it was worth my time and the cost of cheap materials to build a Prototype of a new design, not really caring what the draw weight is, and then use the calculator to get the specs for the bow I really want.

These days I built a prototype of the design I wan with the cheapest black glass (.03 or .04 for $9 a 72" stick from Binghams); and a cheap lam or two (I often use 1/8" pine lath from the Big Box store, or 1/16" ripped from a 2x4), and a piece of 2x6 for the riser if I'm making that kind of bow. So the Prototype sets me back maybe $30 in materials.  

I don't really bother to finish it out.  Just clean the junk off.  I saw  some simple pin nocks on the tips and use a piece of #36 Mason's line and make a bastard string to get the approximate brace height, then hang it from my tillering tree and put the scale on it.

Then it's off to the calculator to get the specs for the draw weight I really want.  Now I can drool over the woods I really want and all the final details, knowing I'll be pretty darn close with the second one I build.

[LittleBen]

FWIW I think that stack would probably make 40# in hickory and ipe ... thats a thick stack for a glass bow ...

send it to someone tough enough to shoot it ... i.e. not me

[Robertfishes]

The total that the bowyer gave you adds up to 0.490 inch in the center of bow. If that's what you used then A wild guess for your total stack for 45# @28" would be 0.380 inch. Assuming that your bow is 80# @ 28" now with a .490 stack, and you get 1# for every 003 of stack..then you were around .110 too thick on your total stack..again assuming that your gain is 003 per 1# draw weight. I have came out  5-10 #s heavy more than I'd like to admit.. I just bought a new set of digital calipers and a set of feeler gauges to check them against..

[LittleBen]

Robertfishes calculation leads me to believe it was supposed to be .390 including glass and not .390 plus the glass. that would account for .100 thickness difference ... very near what he calculated.

[Robertfishes]

I saw on another site that the bow is 92# @ 28"...if thats accurate then you might have to go to 0.355 total stack in center of bow..I don't know, I'm not sure I understand exactly what the taper is..did you use three 001/inch tapers and one 002/inch taper?..I have concerns about the original stack height being 0.490..was the 0.434 you measured at the end of the fades? If it was then add 005 for every inch from end of fade to the center of bow..if your riser is 18" then the stack in center would be around 0.480 inch. I am hoping that the more  experienced guys will continue to add their experience to this thread. The first bow off of a new form can be/always be a guessing game. Accurate measurement's of materials before glue up are really important for the next bow..

[Charl]

KenH, pm sent. I love the idea of making a prototype and will definitely be taking this up!

LittleBen, I don't know anything about making pure wood bows so will have to take your word for it!

Robertfishes, it is the same bow but with all the fiddling it has been gradually losing some weight.

I will be taking this as a learning experience and really appreciate everyone's input so far. I have decided to finish this bow as is and move onto the next one.