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.

[Kirkll]

First of all you need to define the term “Performance” . Even though the term performance is generally synonymous with arrow speed and hand shock, overall performance should include smoothness of draw and stacking point in the limbs. The string angle at full draw is another important factor. This part  is typically measured by doing a DFC chart, and then running the bow through the shooting machine using a chronograph to determine you arrow speed. After transferring that data . There are spread sheets available that will show the amount of stored energy, and KE and a percentage of dynamic efficiency results.

But….. different limb shapes, or limb designs, store energy differently. Some store the energy along a longer section of the limb. Others store most of the energy in a shorter working section of the limb. In my experience building glass and carbon backed bows is that these shorter working limb designs typically have higher performance…..

 But ……the key to getting that higher performance in the form of arrow speed is actually transferring that stored energy into the arrow shaft, and the only way to do that efficiently is by having enough string tension, or preload on the limbs to stop the forward motion of the limbs dead.

This is where things start getting debatable…. There are a lot of things to factor in that determines how much preload is needed to stop the forward limb travel. Manipulating the distance of the limb travel, and minimizing the forward motion is a big factor.  Just lengthening a string will do this a bit. But you’ll see a lot more results by pushing the working section of your limb out further on towards the tips. This is especially true on long bows and hybrid long bows. But it applies to recurve designs too.

Then you have to factor in the actual mass weight of the limb tips to the pre load. Generally speaking, this is why bows in the 50-60 pound draw weights or higher yield much better performance than a 35-40# bow across the board on design.

You can use lighter weight core materials or carbon fiber, and narrow the width profile to lower your limbs mass weight too. This typically helps performance.

Now where things start getting crazy is when we start talking about super curve designs, and these Asian bow designs. The mass weight of the limb tips are mitigated by shape of the limbs at brace, and that longer string curls around the limb tips. The actual forward motion of these limbs do not start until the string lifts off the belly of the limbs. I’m sure some of you that have studied these designs are familiar with the term “Lift off”. ….

This term lift off and actual string length touching the belly side of the curl applies to static tip recurve design too. But it’s a balancing act getting the stability you need as you increase the size of the hook and push your tip notches out further.

 But this is a totally different rabbit hole to go down, and one I’m not experienced in myself. So I cannot speculate further on these. The stability requirements to build these super hook bows typically require carbon backing belly and back, and they are not known to be real good for longevity. You guys can have that rabbit hole. Too expensive for this kid to dive into.

The Asian horn bows have incredible performance, but you need to be a ninja just to get a string on them…A different class of their own….   Kirk

[willi]

Thanks for commenting, Kirk. Sounds like you have done quite a bit of chronowork with various designs.

But you’ll see a lot more results by pushing the working section of your limb out further on towards the tips. This is especially true on long bows and hybrid long bows. But it applies to recurve designs too.

Has the "working section further out"  held true with your shooting tests of flight weight arrows ? (5 GPP)
Looks like your adjustable riser is set to 52" NTN in the pic above, but can go longer. I presume you shoot with the test riser.

[Crooked Stic]

I have also found moving the working part of the limbs closer to the tips is a plus for performance. And preload at lower brace will also improve in manners.

[willi]

I have also found moving the working part of the limbs closer to the tips is a plus for performance. And preload at lower brace will also improve in manners.

plus for performance is clear.  What do you mean by "manners" ?
I would think less early draw weight would improve getting an arrow to come off the string smoother.

[Crooked Stic]

Because you have more preload with shorter working limb and the Limb goes back to brace after the shot is likely not to have as much vibration (manners).
The design in a recurve that I know of now is a Predator.
They no longer make.

[Kirkll]

Thanks for commenting, Kirk. Sounds like you have done quite a bit of chronowork with various designs.

But you’ll see a lot more results by pushing the working section of your limb out further on towards the tips. This is especially true on long bows and hybrid long bows. But it applies to recurve designs too.

Has the "working section further out"  held true with your shooting tests of flight weight arrows ? (5 GPP)
Looks like your adjustable riser is set to 52" NTN in the pic above, but can go longer. I presume you shoot with the test riser.

Yes, that aluminum riser can,  and has been used in a shooting machine, but isn’t designed for shooting by hand. It’s mostly used for its adjustable limb pad angle at different lengths on the tiller tree for preload and tillering purposes. That bow was 60” AMO length and I believe those are medium length limbs.

My design testing parameters have been for bows from 56-66” in length, wit( 26-33” draw lengths.and rarely shot them below 8-9 gpp. Most of the bows I build are for hunters that want heavier arrows for higher KE . If I can stay at 190-200 FPS on hunting weight arrows between 9-10 gpp those are the numbers I want to see. But it’s very difficult to break 200 fps at 28” draw length at 10 gpp. At 9 gpp at 30” draw it’s much easier to do.

I haven’t pursued flight bow limb designs shooting ultra light arrows. Those would  have totally different characteristics to them. Very short working section using more composites in the core with lighter weight tips that are durable enough to hold up to dry firing. Tough combo to come up with… flight bows are not known for longevity. though.  But to answer your question, yes… that working section would be closer to mid limb and designed with a very short limb travel distance at crazy high preload levels.

The draw weight would be measured at shorter draw lengths using much shorter arrows. For example 50 @18” draw in the 50# class shooting 15-16” needle sized arrows off an over draw. To get into the competition seriously you would need to start hitting 300-400 fps to get the distance I would think… but I’m just throwing numbers out there. Alan Case is the guy to get that info right. He’s devoted 15 years into it…. It’s a whole different ball game flight shooting.

I do know his foot bows at 300# are shooting 110 grain arrows at 800 fps.

Kirk

[Kirkll]

I have also found moving the working part of the limbs closer to the tips is a plus for performance. And preload at lower brace will also improve in manners.

plus for performance is clear.  What do you mean by "manners" ?
I would think less early draw weight would improve getting an arrow to come off the string smoother.

Nope Willi… you have exactly backwards… The only thing you get with less early draw weight is more hand shock, and much lower arrow speed.  The higher preload limbs with more tension on the string at brace, stops the forward limb travel dead and eliminates the hand shock completely. All that energy goes into the arrow instead of staying in the riser while your limbs are flopping around.

I’ve got some high speed video I took when testing different bows with different limb designs that show exactly what happened to those limbs once Elvis has left the building.

I can’t find the clip I’m looking for, but here is one I did on paradox arrows that have a good slow motion of my Sasquatch SS design at the end of the film. Keep in mind. This is a high preload limb design…

I’ll see if I can find the other one comparing it to other bows and shows tests of my hybrid design too.

https://youtu.be/rqO6WkXJ87A?si=YSAklbfh8PbYb-vM