Ok gentlemen. Now back to the serious stuff. I want to build a bow that will come in around 40#@28".  I have the specs for a build that came in at 37#@27" but was only 63"NTN.  Here are those specs:
glass front & back    .40
1 bamboo core           .25
riser    18x 1.5 x 1.5

I would like to go 66" possibly 68" NTN, if I go the same with the core but use .50 'glass strips would that bring the weight closer to what I want or would the 'glass overpower the core? I have never built one of these and don't want to waste money on a bow I can't shoot. Maybe just add 1- .125 bamboo core to bring it up and leave 'glass @ .40?

You can't just plop another .125" lam in there. Alone, it would likely increase your draw weight by as much as 40 pounds or so. Even if you factor in lengthening the bow to 68", your draw weight would still go up a net 20+ pounds.

Where are these lams coming from? Are you buying them from a supplier? Grinding them yourself? Can you get/make whatever you need? The direction I go with my advice hinges on those answers... such that... if you can acquire anything you need, ground precisely, I would advise you to keep the glass the same thickness, lengthen the bow, increase the number of lams in the stack, and increase the overall thickness of the stack. I'll explain why below.

If, on the other hand, you're doing this yourself with limited means and are locked in to using the .250" bamboo, I would advise you to lengthen the bow to 66" and change both pieces of glass from .040" to .050".

There are so many variables it's hard for another bowyer to give accurate advice for hitting weight on someone else's design. This is why each bowyer should keep detailed 'spec sheets' on every laminated bow he makes. He should also try, at least initially, to alter only one thing at a time so that he can accurately gauge the effects of that change, and document it for future reference. Eventually, you'll have a list of 'constants' for that design that will help you accurately hit weight, even when changing more than one variable at a time.

I've never made a bow with a single bamboo lam between glass, and I don't know what design you're using, so I don't know if my notes will be of much help to you. But I'll kick a few 'constants' out there that are results of my mild d/r design longbows... at least the ones that seem relevant to what you're doing. Generally, these bows have 4 wood lams, 3 tapered and 1 parallel, with a 16" handle from fade to fade. Don't pay too much attention to the fact that mine have more lams than yours just yet, because generally, strength is determined by overall stack thickness, not the number of lams.

Shorten bow 2" o.a.l. = +7 lbs

Increase glass .010" = +5 lbs

.015" difference in core thickness = 5 lbs

Here's how I see your situation.

I would keep the glass on the next bow at .040" and change core thickness.

If you would pull your 37# @ 27" bow to 28", it would be close to 40#, which is what you want, so you need to focus on the length change. When you make it longer by lengthening limbs, you're going to lose weight. If you lengthen the bow 3", you might lose 10 lbs, so you need to make that up in core thickness. A .030" increase in core thickness should make up for a 3" increase in overall length... maintaining your 40# at the longer length.

That said, assuming your single lam in the previous bow was tapered, I would split the core of the next bow up into 3 lams... 2 lams each .120 tapered @ .0015/1" and placed on the front side of the riser, and a .040" parallel placed on the back side of the riser. This makes for a total stack of .360". Your previous stack height was .330". There's the .030 difference that should make up for the difference in length in this next bow.

The main reason more lams are better than a single one in a glass bow is because if there are any deficiencies in the core material, the effects will be offset by the good, homogenous wood in the lams to either side of it. The nodes in bamboo are such deficiencies. A greater number of thinner lams, with nodes offset from one another is a safer bet. I think it would make a better bow this way, if you have the means.

Bowjunkie, the design I stated above used all parallel lams. I would defintely buy my lams, probably from Kennym.
What makes tapers better than parallels? Not sure what makes them more desireable.

Bowjunkie's right - lots of things can influence what draw weight you get out of a laminated bow.

Here's a link to a lam stack calculator Brad Jansen (bjansen) posted a while ago.  It's an Excel spreadsheet you can download and has helped me out a great deal.  This is really geared toward the situation where you want to keep the design the same, but hit a different weight, but it also helps in some educated guesses when altering the bow length a couple inches for a given design.  

 http://tradgang.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=125;t=007699;p=1#000000  

Taper's aren't necessarily "better" than parallels.  They just do different things. More taper = more bend in the limbs at full draw farther from the riser.  If you don't have enough taper your bow will flex too much close to the riser.  Different designs call for different amounts of taper and each bowyer has there own recipe.  For example, in a straight Hill style bow, I use one 0.002 tapered lam and 3 parallels.  In my mild r/d design, I use two 0.002 tapers and two parallels.

John covered that question well.

Not enough taper can make a bow work too much inner limb and progressively less the farther out the limb ya go. This can also increase the amount of recoil or handshock felt, depending on design.

Too MUCH taper could potentially not work enough of the limb, and impart less energy into the arrow.

Recurves need less overall THICKNESS taper than longbows because they lose more material per inch of WIDTH than a longbow does.

Like John said... it depends on your design, and on what you want it to do.

If we assume that you want to build the same bow, but 3 or 5 inches longer I would advise you to do the following. Depending of the profile of your bow you'll need 3 - 5 lbs/inch to compensate the longer limbs. Add another 3 - 5 lbs to make sure you hit the target draw weight. On your fist bow the stack was 0.33" ( 0.04 + 0.25 + 0.04 = 0.33 ). To get 40@28 with a bow length of 66" I would increase the stack to 0.385". With this stack the bow should end up slightly above 40@28 giving you some room for tiller.

----------
Andy

I understand that piking/shortening the bow will increase the draw weight but the lam calculator states that on a takedown bow, incerasing the riser lenth will increase draw weight and shortening the riser will decrease draw weight. If this is correct, my brain needs to be re-calibrated.

If the overall length stays the same, then a longer riser will increase the draw weight, because the working length of the limbs will get shorter. But if you use the same limbs on a longer riser then the draw weight will slightly drop, because you got a longer lever.

----------
Andy

Thanks guys I've been trying to work out what all the different lams do great explanations many thanks good luck critman  hope you hit the weight on your next bow

Swiss..............

My thoughts exactly, thanks for clarifying this for me.

James