Hi everyone,
after picking up traditional archery as a hobby this year (focusing on 3D) and being someone that just has this inner drive to build and tinker, I am currently planning my first bow build for the upcoming holidays. I am not an active bowhunter, but experiencing the vast amount of bowyer knowledge present here, I just had to register. I hope it's okay to join your ranks under these circumstances.
The build I am planning is a 62", 40#@28", glass laminated longbow with moderate R/D. It is based on a guide available online. It provides me with some great references, especially regarding bow profile/shape and stack thickness. I probably should just stick to the recommendations, but I just can't stop myself from "making it my own" at least a little bit.
And that's where the questions come in. Since most of my deviations are based on "optical design ideas", I wanted to make sure I am not compromising the bows function/performance with them. In the end form should follow function.
For my planned build the guide is suggesting a stack thickness of 0.36". It's advising 20-25% glass, so I chose to use two 0.04 strips of glass, resulting in about 22%. This results in a core thickness of 0.28".
My currently planned stack layout back to belly is:
- 0.039 / 1mm Bearpaw Powerglass Crystal Clear
- 0.022 / 0.55mm Bubinga veneer
- X" / mm vertically laminated bamboo
- Bubinga riser
- Y" / mm vertically laminated bamboo
- 0.039 / 1mm Bearpaw Powerglass Pure Black
The included build along is describing a symmetrical stack, which in this case would result in (0.039" * 2 + 0.022" = 0.1; 0.36" - 0.1" = 0.26"; 0.26" / 2=) 0.13" / 3.3 mm for X and Y. Though looking at different bow designs online, I really like the look of the "main" part of the core being on the back of the bow and would like to build it with X = 0.196" / 5 mm and Y = 0.064" / 1.6 mm. I also feel like that it would make the glue up easier, since it results in a thinner laminate that I have to force into the curvature of the fades.
The first question resulting from this change would be: Does this substantially change the bow's behaviour? Since this would not change anything in regard to the overall limb material distribution, I would expect it to stay mostly the same.
If I'm building it with 0.196" / 5mm bamboo on the back, would you advise me to use a single piece of bamboo, or maybe rather take two pieces at half the thickness? My concern with a single thick laminate stems from the bow keeping its shape based on the tension between the single laminates being fixed in relation to each other by the epoxy. Could a single thick laminate overpower the tension of the other way thinner laminates and cause the bow to deviate from the template's shape after glue up? Probably not a concern for the deflex in the area of the riser, but could I lose some of the limb's reflex? Would it make sense to "pre bend" the thicker laminate with heat/steam before the glue up, to decrease the laminates tension in its desired/unstrung shape? I somehow just like the idea of having a "reduced" buildup (fewer laminates/components) and would accept a more elaborate preparation for it. Though if it severely affects the bow's performance/behaviour, there is no need for me to pursue this idea.
Another change I made in regard to symmetry is that I plan to use Bearpaws Crystal Clear Glass on the back and the Pure Black on the belly. I already received the glass strips and while handling them, I felt like they were not showing an equal amount of stiffness. To test this I let them protrude a defined amount (19.7" / 50cm) over the edge of a table and placed a weight on them right on the edge of the table. The black glass is sagging slightly more (2.17" / 5.5 cm) than the clear one (2" / 5 cm). To make sure this is not caused by a difference in weight of the composition of glass, resin and dye/color, I also weighed the strips. Both strips being 72.5" / 1.84 m long, there is a difference of less than 0.1 oz / 3g. So the black glass seems to be slightly softer.
The initial idea was to only use the black glass for the belly of the lower limb and go with the veneer + clear glass for the top limb. But using softer glass for the lower limb of a bow with symmetrical limb lengths and a shelf above center seemed counter productive. So the plan changed to both limbs receiving the black glass. But while researching I came across the topic of the back of the bow overpowering the belly side and thus resulting in compression creases on the belly side. This was of course the case with selfbows or laminated wooden bows. I guess creases will not be an issue with a glass bow, but does it cause other undesirable issues if the belly is slightly softer?
Last but not least I have some questions on tapering the laminates. The guide is proposing a limb taper of 0.004 (0.005 to have a real D-Shape when braced, to be longbow "conform" for certain competition rules). Just to make sure my calculations are correct: That means I will reduce the thickness of the limb by 0.124" / 3.15 mm ( 0.155" / 3.94 mm for a 0.005 taper) over a 31" limb, correct? For me as a not-yet-bowyer, that "feels" like a lot. Especially considering the limb profile gets also quite narrow towards the tip.
Are there any rules on how to orient the tapers (or the single taper on the thick bamboo lam?) when glueing everything together? Thin part towards the string groove, that's obvious.
But what about the belly/back orientation? Should the angled side of the laminate be oriented towards the glass, or maybe to the core of the limb? Or is this (as far as I read it) just as negligible as the grain for core laminates out of wood?
Thanks in advance for your input!
Topic: Beginner questions regarding stack composition, order and taper (Read 444 times)
