Why do we use so many laminations in glass bows?

[Cupcake]

OK I have to add my two cents.

I have not made a bow yet but am studying and buying tools. (you should see my new Jet drum sander - shaa-wing).

Going back to my engineering school basics, the bow limb is a spring.  The lams between the fiberglass are there to get the desired moment of inertia.  The fiberglass is much stiffer than the wood core and determines most of the springs properties (from the moment of inertia).  Carbon is four times stiffer than fiberglass.  Ideally, the lams spacing the fiberglass should have no mass to slow down the recovery - that is why foam cores are used.

I am in the fewer lamination camp but could change my mind based on my soon to start experiences.

I hope to conduct a lot of these experiments.
Kevin

[lostshot]

In some bows I see today, they use a very thin paralell veneer on the back and belly of the bow,just under the clear glass. This wood while it may be very nice to look at gives little to the bows consitancy or performance. The inner core is Action wood or maple, red elm , some 'stand alone wood'. I just think alot of bows are like fish'n lures. Made to catch the fishermen.

[Orion]

Regarding Binghams advice to put a reverse lamination in the bow form before glue up.  Might it be to compensate for the small amount of springback one gets when taking the glued up bow out of the form?

Re single laminations.  Jack Harrison in his book says that one thick lamination of bamboo actionwood was 10% faster than three laminations of the same material making the same thickness.

[insttech1]

Hmmm....some good responses, yet some confusion persists.

Case in point--people were assuming that I meant strictly recurves, and therefore, I could possibly get away with using fewer lams.

That confuses me, because longbow limbs "travel" a shorter distance during the draw stroke, and don't "unfold" at the ends, like the recurve does.  

It would seems to make more since that the longbow limbs would suffer from less shearing force than the recurve limbs, and could, therefore, be a better recipient for a "fewer lam" recipe.

And yes, I know--go make a half dozen experimental bows and find out for myself...but unless someone coughs up about $1200 in materials, I'll see what responses come up here first...

Don't get me wrong--I LOVE the beauty of a nice 'curve or longbow with exotic veneers, and the master bowyers know their stuff...I just would like to know how it came about that more lams are absolutely required, as I would think they want to shave their costs for materials as well.

To me, the more lams you have, the more chances you have for epoxy/natural-wood oil contamination and therefore quality issues.

And I can also be a cheap schmuck at times, and if I can order $90 in lams instead of $140 and still get a reliable bow with 90+% shooting qualities of the more expensive alternative...well guess what...that $50 is going somewhere else, or to upgrade to veneers, etc....

Take Care,
Marc

[insttech1]

Orion,
Thanks for the Jack Harrison tip.  Any other tidbits like that in the book?  Was that comment pertaining to his longbows?

10% is an absolutely HUGE gain in speed, and is the difference between guys/gals paying $700 for a nice longbow, or $1200 for an ACS.

Thanks,
M

[Orion]

insttech, yes, he did make that comment based on testing with bows he built.  Keep in mind that actionboo or laminated bamboo already contains a lot of laminations for consistency across the limbs.  I believe he did adopt the single thick actionboo lamination as the core for the bows he builds.

I think laminating three or four pieces of actionboo together is over doing it.  Then you have a lot of glue joints going in two directions, 90 degrees from each other.  Makes for a very stable limb I suppose, but I think all the glue joints take away from the springiness and thus performance of the bow.

For the same reason, I think more than 4 laminations of any wood or bamboo is also overdoing it.  Just too many glue joints to detract from the bow's performance.  The joints do stiffen the bow, but I'd rather have the draw weight come from the wood and glass themselves, not the joints between them.  In the end, it probably doesn't make a lot of difference though.

If you go back a few years, to the 40s and 50s, most recurves contained two wood laminations plus glass, and most longbows contained two or three plus glass.  In part, that was due to lower draw weights.  Many, but certainly not all,argue that today's designs are faster, but it's difficult to determine if it's because of more laminations or just improved materials and design, or maybe all of them.

[Jeff Strubberg]

Re single laminations. Jack Harrison in his book says that one thick lamination of bamboo actionwood was 10% faster than three laminations of the same material making the same thickness.

No offense intended, but Jack came up with somne other very odd ideas that directly counterdict empirical testing, too.

[James Wrenn]

It would depend on the bow design.A straght longbow with a glued on handle would not show much difference if there were two lams in it or 6.As the limbs bend more when you are putting them in a form you will see some differance with more lams.Thinner lams bend around things easier and 3 or more thin lams will hold it shape more than a couple of thick ones.If it holds the shape better there is more resistance and they store more energy.They are also under less stress than a thick lam in areas like fadeouts or curves in a recurve limb.Another reson is multiple lams can be put in cross grained from eack other and will not be as prone to twisting because of the iregularities in the wood.Even lams that are not perfect or have twist can be used to calce out each other.Both ways will build a bow but if glued up right the one with muliple lams will give you a more stabile limb in most cases in a curved limb design.For me personally 3 lams in bows of 45-50lb range is plenty unless I am really making a curvy one.If the bow is to be heavier I usually go with 4 if I can.The bottom line is both ways work and if it is a good design you will have a good bow either way.I do think however that sometimes more can be better in some designs so I think about that before I start glueing.    jmo

[Orion]

Don't want to get side tracked, but good observation Jeff.  Among other things, I never understood why he is so sold on a continuous loop string as opposed to a flemish string.  Once shot in, there's no more stretch in one than the other, and the flemish provides twice as many strands around the nock, which is where they're most needed.  Of course, the high tech string material of today makes that a less important consideration.