A question about reducing the thickness of glass lams on a longbow.

[rich k]

A question about reducing the thickness of fiberglass laminations while increasing the core wood thickness in a laminated longbow.
I have built about one dozen laminated longbows but I no longer have a shop so it is something I am unable to try for myself. I always wondered about this but never got around to testing it. Suppose you have a laminated longbow of 62 inches in length and 45 pounds. The core wood is laminated bamboo and the fiberglass lams are 40 thousandths. If you made a second bow with the exact same design and limb thickness but used 30 thousandths fiberglass lams and increased the core thickness the same amount what would be the result? Would the draw weight decrease? If so, by what amount? Also, what about performance? Would it cast an equal weight arrow at about the same speed? As the physical limb weight would decrease that would suggest possible improved performance.
I recently reduced the weight of one of my longbows from 44# to 38# and it seems to shoot the same arrow at almost the same speed. The arrow has a 150 grain field point and weighs 420 grains. I am getting right around 160 fps at my 28” draw. I reduced the original 40 thousandths glass lams to obtain the 38#. Unfortunately, I didn’t chrono the bow before reducing the weight and have lost my build records in my last move. Of course, I couldn’t increase the thickness of the core wood to make up for the thinner glass.
Any thoughts/ideas will be greatly appreciated.
I sure miss my shop.

[Kirkll]

Yes... the draw weight will decrease with fiberglass at a rate of about 1# for every .002 of glass. So your bow would likely be 8-10 # lighter in draw weight. But... you are on the right track with increased performance due to less limb mass weight. But there are trade offs....

The problem with building with .030 glass is that you do not have a lot of room for sanding glass when balancing your limbs. The durability of the finished product using glass under .030 is what you need to take into consideration... The thinner glass has a tendency to split easier.  Carbon fiber is stronger with less mass weight, but has its own issues...

If your goals are higher performance, your best bet is to get into measuring your preload and adjusting your limb pad angles and string length until you find your sweet spot on your limb design.  Then adjust  limb shape or geometry using different taper rates until you've reached your goals.... Good luck... this can be a long process of trials and adjustments.    Kirk

[rich k]

Thank you for your reply. My goal wasn't to increase performance. I wasn't looking for more speed. The design I used, after comparing notes from a dozen or so bowyers, is actually a combination of all of their ideas. The bow turned out to be very quiet and also smooth at my 28" draw. The speed was respectable for my goal of creating a practical longbow for hunting whitetail, and still comfortable to shoot all day at a trad shoot. It has a reverse handle and seems pretty forgiving.I was just curious as to how far I could reduce the thickness of the glass and still have a durable bow. You answered that question that having too thin a glass might cause a split.
Rich

[Kirkll]

I use .040 glass on everything from 30#-60#  maybe even 65#.  But i go to .050 on heavy draw weights from 70-120#'s

Another thing about building heavy draw weight bows is using a thin layer of G-10 on the butts of the take down limbs that extend past the riser 1/8" to 1/4". I've seen the glass fracture at that pivot point on bows over 80# even rounding over end of the riser.

That being said.... I highly encourage all beginning bowyers to pay close attention to knocking the edge off the end of your take down bow risers with a very slight round over before mounting the limbs and checking draw weight.... I cant count the number of times i've forgotten to do that and had to sand out a line left by a sharp edge. I have ruined several limbs doing that too.

Good luck on your adventures...   Kirk