laminates grain direction?

[DonLee]

is there a preferred grain for laminates when making a glass bow? im thinking of a ASL bow, but recurve really as well.  quarter sawn vs plain sawn ? has anyone keept a record of not only thicknes but grain direction? Im thinking yes.

[Crooked Stic]

Edge grain for core will be best such as vertical laminated bamboo.as far as veneers go being so thin crazy grain gonna look better.

[Kirkll]

On an ASL design you would be better with vertical grain rather than flat sawn material for the highest strength in tension.


But..... I just spent a couple hours on the phone with Alan Case the other day talking about heavy bows and compression issues ive been having. He told me that he did some compression testing using flat grain maple and pushed it till it blew, then put it under a microscope to see the difference from vertical grain maple.

Believe it or not, the flat grain held up to extreme compression much better than vertical grain.  He said he has a test going right now using thin book matched flat grain lams that are stacked 180 degrees so the opposing grains are equalized. He expects much higher compression ratings with the two thin lams and an epoxy joint than a thicker maple lam.     Food for thought on that subject...   Kirk

[Crooked Stic]

You may be right about maple. If that's what you use for core. That the guy who concentrates on flight bows.

[Kirkll]

Yup…. Alan has been chasing the world record for 15 years now. shooting arrows over a mile is pretty incredible. All you need to do is get 110 grain arrows shooting about 800 fps, and be able to draw 300#.

Here is an article….


https://www.opb.org/article/2024/11/16/beaverton-mechanical-engineer-archer-chases-record-longest-arrow-shot/

[Kirkll]

Rock hard maple has been an industry standard for many years due to its longevity characteristics. For mass weight to strength properties, you will be hard pressed to find anything better in the natural wood class.

With popular draw weights between 50-70# draw you can realistically use pretty much anything for core wood with fiberglass backing and have it hold up well. Especially building long bows…. 

But once you get into building high performance static tip recurves, those limbs are storing energy in a much shorter portion of the limb, and the dynamics going on in the core of the limbs are much more extreme. Once you get past 70# it’s the compression of the core next to your belly glass that fails. Using thicker glass is the ticket on heavy bows.  Using carbon on the belly typically shears the bond to wood cores even quicker. I’ve tested using carbon over glass before and it’s still tough to get it to hold up long…. The best longevity I’ve come up with so far, and highest compression rating is using thinner maple lams on the belly side.

But I’ve never tried a flat grain laminated approach before, but it makes sense that two .030 lams with epoxy would be stronger than an .060 wood lam by itself.


I’m currently testing this Fossilized bamboo for extreme compression properties on the belly side  in hopes of getting some 80-115# limbs to hold up longer. These guys drawing them from 29.5 to 31.5” isn’t making it easier ta boot…. I’m going to mix in some carbon in the core too on the really heavy set. I’m still using 050 glass on the belly though.

Who knows… I may come up with a suitable flight bow before it’s all said and done.

Kirk

[willi]

Who knows… I may come up with a suitable flight bow before it’s all said and done.

Kirk

have you ever tried two lams of glass on the belly and one on the back?

[Kirkll]

Who knows… I may come up with a suitable flight bow before it’s all said and done.

Kirk

have you ever tried two lams of glass on the belly and one on the back?

The last seriously heavy set of limbs I built I used stable core right under .050 black glass, over maple and it still sheared the maple lam after a period of time. I believe he had several hundred arrows through them before they finally failed. 

So this time I’ve decided to use .032 carbon under the glass and either run a thin maple lam or this fossilized bamboo next, then stable core, then bamboo under the back glass.

 When building these heavy draw weight limbs, the actual mass weight of the cores isn’t a big deal. The string tension at brace height is over 200# and has no problem stopping the forward limb travel. I’m having .strings built from BCY Mercury  material.

The  interesting part about building these heavy limbs is getting a string on them safely. What I really should do is remodel my old compound bow press to accommodate these longer bows. Testing is done with a shooting machine. Lord knows I can’t draw these things by hand.     Kirk

[Stagmitis]

Kirk how exactly are you measuring string tension Aka preload at brace?

[Kirkll]

Kirk how exactly are you measuring string tension Aka preload at brace?

I use an inline scale with two short sections of string. I built these strings on an endless loop jig and factored in the length of the scale. Determining the brace height and measuring exact string length is important. Sometimes i need to twist a string up a bit to fine tune that...

Believe it or not, your string tension, or pre load weight increases as you lower the brace height. 

This is kind of mind boggling because your brain tells you as you shorten the brace and put more bend in the limb the string tension should increase, but it doesn't. It's the opposite.

Granted... i don't do a lot of exact preload measurements using a scale anymore. But it's a valuable tool when prototyping new limb designs and trying to maximize the preload at a preferred brace height. Generally speaking, the perfect preload for a high performance bow will have twice the draw weight in string tension at brace. So a 50# bow should be close to 100# on the scale....  Different limb designs will give you different numbers, and different amounts of pre load.   But once you have determined the preload with a scale on a certain design, the ratio of draw weight to preload is pretty consistent  on different draw weights.

With that being said, i can safely say that my 100# bow has about 200# of preload tension on the string. It's no wonder it's a work out trying to string these heavy bows.

Kirk

[Kirkll]

Trying not to go completely sideways, and hijack the thread here.

But It must be noted that when using an adjustable riser and changing your limb pad angles and string lengths to get an optimum preload for a new limb design, the maximum string tension is not necessarily the best preload setting.

Read that last very long sentence again....

As you lengthen your string and increase the preload with a lower brace, there is definitely a point where you start losing performance if you go too far. Typically the shape of the limb at brace gives it away.

look at this photo of when i was getting my pre load dialed in on my SS RC design years ago. look carefully at the limb at brace and full draw with two different settings.  Once that limb Flattens out like the photo on the left you have gone too far even though the preload was higher in that photo.  I believe what is happening at that point is that the performance is lost due to limb occelation in the working portion. So the setting on the right side performed much better...The flatter limb wasn't as stable vertically either.

Of course you have to play with string length and limb pad angle a bit to optimize the limbs performance. But you get the idea of what i'm saying that higher preload tension itself does not automatically give you better performance. It's a balancing act... Get the limbs dialed in first... Then measure back to your riser block for your preferred brace height, or deepest part of the grip location.    Kirk

[willi]

So the setting on the right side performed much better.... But you get the idea of what i'm saying that higher preload tension itself does not automatically give you better performance.

How are you determining performance?

There are a couple of explanations for high string tension. More energy being stored in the limb and string angle.  I am not sure if string tension due to string angle actually adds as much peformance as stored energy.