Increasing stack thickness

[buckeyebowhunter]

What's up fellas. Hope everyone is doing well. Been a minute since I've posted been busy with life and trying to do some hunting in my spare time so haven't been building any bows lately.

However I recently had someone reach out about building them a heavy set of limbs. 65#ish. I have mainly built bows in the 50 to 55 lb range. So my stack has been pretty straight forward.

My question for you gentlemen is when increasing thickness by quite a bit say 20 thousands where are you adding that thickness? I'm inclined to add it to the taper being that it is my bottom lam, and then maybe some to my parallel but do you guys ever increase glass thickness or always use the same? So far I've only used .040 glass.

I build recurves by the way, pretty much a bingham design.  When I asked Bingham's about this a long time ago they just said stay away from extremes. Not much help lol.

Thanks for any thoughts and input 

[Mad Max]

I think us guy,s do it this way. Glass needs to be around 25% of the stack
.300 stack x 25% = .075,  2 pieces of .040 glass would work

[buckeyebowhunter]

Thanks Mark 

[mmattockx]

Glass needs to be around 25% of the stack

Can someone elaborate on this number? Why do we need glass to be ~25% of the stack and where does that come from?

If OP doesn't want the semi-hijack here I can move this to a separate thread.


Mark

[Mad Max]

I’m building a low pound recurve and going to use.030

[Crooked Stic]

I think .030 up to 40lbs. .040 40lbs. to mid 50s then .050
For me all my bows are less than 55lbs I use .040.

[Longcruise]

   they just said stay away from extremes. Not much help lol.

They told me the same thing a long time ago.   

As far as stack goes,  bjansens calculator is very close.  I don't have a link to it but someone here will help out with it.  I've found it to be useable with my own stacks.   

I like to use a bit thicker glass on the belly.

[bjansen]

I would stick with .040 glass for the 65# bow you are building. I agree that extremes will call for different glass thickness (like a 30# bow is much better suited for .030 glass...and maybe a 90# bow would call for .50 glass on the belly, or belly and back ) but for most hunting weight poundage bows, I really think .040 is just right.

[Mad Max]

x2

[jess stuart]

One of the guys at Binghams once told me .050 glass made for a sluggish spongy bow.  This was back when they still carried glass up .060 thick.  I have always used .040 for just about everything.
I too would like to know where the 25% rule came from.  I have never heard a concrete reason for it.

[Mad Max]

KennyM said it
I think he said 23 to 28% and I just rounded to 25%, Glass will handle more than normal R/D and Recurves.

This static recurve is 58" AMO 56.5 NTN , Just bracing it bends the glass a good bit and will drw to 30"
44#@28" .264 stack with .030 glass---.264 x 25% =.066---.033 glass


27"

[Flem]

Lots of guys are making all wood bows that need no glass. The 25% rule is to compensate for bad grain alignment and or fancy wood grain that has marginal integrity. Use wood suitable for an all wood bow and you can minimize the glass thickness.

[mmattockx]

I too would like to know where the 25% rule came from.  I have never heard a concrete reason for it.

KennyM said it
I think he said 23 to 28% and I just rounded to 25%, Glass will handle more than normal R/D and Recurves.

Yes, the FG will handle strains that destroy the core wood. Has anybody ever had the glass fail on a bow, especially a bow that used thinner than typical glass lams? My guess is all the failures will be on the belly side and will be in the form of set or a delamination of the lam off the core, but no actual failures in the glass itself. Is there a thread that people post pics of their failures in? That could be very helpful in avoiding future failures.


Mark

[kennym]

Ahhh I didn’t make any rule, just did a poll of bowyers a long time ago and 20-25 percent was what folks used ... 

[kennym]

On longbows , recurves will prob have a higher percent due to thinner limbs ...

[Mad Max]

Thanks Kenny

[Mad Max]

Yes, the FG will handle strains that destroy the core wood.
Mark
[/quote]

Describe destroy of any bows you know of? 

[jess stuart]

So is the 25% rule hard and fast or just something that has been accepted for years.  I know bowyers who commonly sand glass thickness to reduce weight and time their limbs.  Obviously you can't sand so much if that you violate the longitudinal glass fibers.   I am just wondering if is one of those accepted best practices but not necessarily based on any hard facts.

[mmattockx]

Ahhh I didn’t make any rule, just did a poll of bowyers a long time ago and 20-25 percent was what folks used ... 

Ha! This is starting to sound like the story of why grandma always cut the end off the roast and how her daughter and granddaughter did the same simply because that is how they were taught without knowing why.

Describe destroy of any bows you know of? 

What I meant is you can bend a limb far enough to cause failure of the core and the FG lams will survive it just fine. When a bow takes set that is a compression failure of the wood in the core, just not a big one. The core can also fail in shear, which would be a big event. Not as spectacular as a back failure in tension, but still catastrophic.

The very best bow woods can take ~1% strain in tension, less in compression. The FG limit is around 2.5%.


Mark

[kennym]

Quote from: kennym on Today at 04:34:32 PM
Ahhh I didn’t make any rule, just did a poll of bowyers a long time ago and 20-25 percent was what folks used ... 

Ha! This is starting to sound like the story of why grandma always cut the end off the roast and how her daughter and granddaughter did the same simply because that is how they were taught without knowing why.

I wasn't trying to lead anyone to doing 20-25%  , just thought it would be of interest.  Folks can and will do what they want to...