I noticed one of my bows has a layer of glass in the middle of the lams.  what would the pros and cons of this approach be?  Has anyone else tried it?

Prolly core tuff. They used to use it to stabilize. There is also uniweft (a woven glass for that also.

Pro: nice Colour accent, maybe a Bit torsional stability
Con: adds weight to the limb

The lightest weight glass product you can use for torsional stability in the core is Stable Core. its only .015 thick. Seems to work best right under the glass on back side of the limb though... If its dead center of the core it's pretty much a cosmetic  racing stripe.

A thin twill or bias weave carbon works better for stability. For a rock solid light weight recurve i've used .015 twill over stable core on the back and got incredible torsional strength in a 30# radical recurve limb. It felt as stiff as a 60# limb torsionally.   Kirk

I don't see the point in that from a mechanical standpoint at all..  Fiberglass is denser than wood and the only point in adding that mass to the limbs is to provide strength.  The bending stresses are at a minimum in the center of the limb, so putting it there is pointless, and putting it at the back or belly would give the same torsional benefit.

It has me scratching my head. The bow in question is a toelke pika. Another member and I were talking about them and he says his doesn't have it. Mine does though and looking at their website it looks like most of them do. And yes it is right in the middle, like it is in line with the riser fade.

Here's a good article on the subject that might shed some light or start a debate...

https://sunsethilllongbowsandleather.blogspot.com/2024/05/the-core-glasswhy-secrecy.html?m=1

Quote from: evgb127 on May 12, 2025, 07:43:42 PM
Here's a good article on the subject that might shed some light or start a debate...

https://sunsethilllongbowsandleather.blogspot.com/2024/05/the-core-glasswhy-secrecy.html?m=1

After reading that article about the only thing I agree with is how it could help a short riser block transition into the fades stronger without extending the fades, and possibly eliminate the need for a tip wedge in some cases.

When you have a bow with glass backing, belly and back the same thickness. The neutral plane will be dead center of the core. Heavier glass on the back than the belly will shift the neutral plane .  Adding a piece of glass to the core doesn't change that... it's nothing but a heavy racing stripe, and unless you are using a woven glass product, it does nothing for torsional stability. Even using a bias weave carbon in the core only has minimal effect on stability. The belly and back glass or carbon  is what controls that stability.   IMO.   

Quote from: evgb127 on May 12, 2025, 07:43:42 PMHere's a good article on the subject that might shed some light or start a debate...

https://sunsethilllongbowsandleather.blogspot.com/2024/05/the-core-glasswhy-secrecy.html?m=1

I see Donato Milesie (Italy) is mentioned in Nate's blog.  I've been interested in his bows and discovered he also uses glass in the middle. I'm mainly interested in Mr. Milesie's bows because he builds them in the true Hill style, similar to Nate - and he is still building, so you can actually get one. Nate's not building anymore and it is super rare to see one of his bows come up on the used market.

Kirk he's putting the glass under the first lam under the back- even though both belly and back glass hold backset the back glass holds a more significant amount- he doesn't mention that in his article but my testing verifies that it does.

Quote from: Kirkll on May 13, 2025, 07:39:14 PM

Quote from: evgb127 on May 12, 2025, 07:43:42 PMHere's a good article on the subject that might shed some light or start a debate...

https://sunsethilllongbowsandleather.blogspot.com/2024/05/the-core-glasswhy-secrecy.html?m=1

When you have a bow with glass backing, belly and back the same thickness. The neutral plane will be dead center of the core. Heavier glass on the back than the belly will shift the neutral plane .  Adding a piece of glass to the core doesn't change that... it's nothing but a heavy racing stripe, and unless you are using a woven glass product, it does nothing for torsional stability. Even using a bias weave carbon in the core only has minimal effect on stability. The belly and back glass or carbon  is what controls that stability.  IMO. 

Exactly my thoughts when I read it. Typical bowyers latin 
I'm 100% with Kirkil.

QuoteWhen you have a bow with glass backing, belly and back the same thickness. The neutral plane will be dead center of the core.

I think that's true to a degree but also think that the wood immediately under both the back and belly glass is going to behave as it would without glass namely more effected by compression.  The neutral plane cannot be dead center.  That's more relevant to a thick cored hill style than a wide flat shorter RC limb where the wood is a lesser percentage of the stack.  It's evident in hill style bows where string follow is inevitable unless design mitigate it.

Quote from: Longcruise on May 20, 2025, 11:15:44 AMI think that's true to a degree but also think that the wood immediately under both the back and belly glass is going to behave as it would without glass namely more effected by compression.  The neutral plane cannot be dead center.

Kirk is correct that a bow with the same weight of glass on back and belly and a solid wood core will have the neutral plane dead center of the stack. The strength of the wood compared to the fibreglass is irrelevant. This assumes the core lams are all made from the same wood with the same stiffness (Young's modulus).


Mark

Mark, what happens to the neutral plane when you add a lam of fiberglass directly under the wood lam of the back- say you have 4 wood lambs with glass on back and belly?

Quote from: Stagmitis on May 23, 2025, 06:02:12 PMMark, what happens to the neutral plane when you add a lam of fiberglass directly under the wood lam of the back- say you have 4 wood lambs with glass on back and belly?

So, going from back to belly:

FG
Wood
FG
Wood
Wood
Wood
FG?

If this is correct it would shift the neutral axis towards to back. How much it shifts can be calculated if we have all the lam thicknesses.


Mark

So then would there be more tension in the back and less compression on the belly or the other way around since compression starts earlier? Wish I could do the math brother. I just build them and see the effects. Very time consuming lol

Quote from: Stagmitis on May 19, 2025, 08:00:14 AMKirk he's putting the glass under the first lam under the back- even though both belly and back glass hold backset the back glass holds a more significant amount- he doesn't mention that in his article but my testing verifies that it does.

With a deep core long bow i can see it holding the reflex and helping with string follow to a certain extent, but the added weight of the glass seriously effects performance and increases hand shock depending on the draw weight of those ASL designs. The higher draw weights (60-70#) and up have more preload or string tension at brace and that helps stop the forward limb travel better after Elvis has left the building.

But... If you skip the glass in the core and use thinner wood lams in your stack, and more laminations, it will help your backset or reflex just as effectively with lighter limb mass....The added glue lines add a wee bit of mass. but nothing close compared to glass.

I've done a ton of testing years ago with using carbon under veneers and clear glass on several limb designs. i used uni carbon on long bows to get a bit more HP, but had to use ultra thin clear glass to get any benefit at all. But putting it directly on the back and it lowered my stack height and increased performance quite a bit. Putting carbon in the core is worthless. An expensive racing stripe...Period.

On recurve limbs Uni carbon is worthless unless it's directly below the bias weave carbon on the back. But even then, the increase in performance is minimal.

With all this said, and i apologize for the carbon side track there.... The type of materials you use in a deep core longbow can make a huge difference on performance and feel of the draw. For example if you use 100% Bamboo cores you'll get a smooth feel to the draw on one end of the spectrum.  Now build the same bow using 100% Hickory core stock, or a combo of Zebra, or paduke, or wenge. What you'll find is a much stiffer limb with more draw weight at the same stack height, and a pretty good jump in performance... But...... those brittle woods in the core will stack up sooner in the draw cycle too and can seriously effect longer draw length capacity..... Sooooo... You cant really say cores dont matter, and it's all on the belly and back material....

Now build one using rock hard maple cores.... Much smoother draw than the brittle woods, but not as nice as the bamboo feel..... Building recurve limbs the perfect combo i've found is bamboo back, and maple towards the belly.

My .02 cents worth....   Kirk

Quote from: Stagmitis on May 25, 2025, 08:35:23 AMSo then would there be more tension in the back and less compression on the belly or the other way around since compression starts earlier? Wish I could do the math brother. I just build them and see the effects. Very time consuming lol

Shifting the neutral axis towards the back will lower stresses in the back and raise them in the belly.


Mark

Makes sense Mark- So that was what Nate steen /sunset hill longbows was trying to convey in his blog- changing the neutral plane by adding glass under the first lam under the back to cause more compression in the belly hence increasing performance.

Quote from: Stagmitis on May 26, 2025, 04:58:04 PMMakes sense Mark- So that was what Nate steen /sunset hill longbows was trying to convey in his blog- changing the neutral plane by adding glass under the first lam under the back to cause more compression in the belly hence increasing performance.

I wouldn't expect that to benefit performance. You will get the most energy storage with a limb that has a balanced lam stack with the neutral axis on the centre line. That keeps the stresses equal in all the limb material, which means there is no weak area that will limit it (until it all blows together).

Performance is gained by stressing everything more, typically by using bigger hooks (on a recurve), more reflex, and narrower/thicker limbs.


Mark