Stack height and limb width on heavy HH Design

[Jaroslav]

Hello guys,
time has come I make me HH longbow. Some of you know I can pull up to 150#, 125# quite comfortably.
Most of HH builds and designs I see here are somewhere into 60#, they are quite narrow.

Assuming I go for full 2" width and 72-73" lenght and 31 1/2" drawlenght, can somebody experienced advise me on how to stack the lams? I have Ipe, Hickory, black locust, garapa.
What I dont really dont have is time to experiment much. My workload is rather brutal and what I can spend on my own projects is really limited. Please help. Jaro

[monterey]

Well, at two inches wide, you have left Hill country.  They are typically 1" to 1 1/8" wide.

But it will still be a good design.  The length puts it beyond my stats for a stack formula.  What weight do you want to hit.  What will your riser length be.

[jsweka]

Another problem you'll have is that most fiberglass comes in 72" strips which doesn't leave you any room for string grooves.  You might need to buy the rolls of Bear Paw fiberglass that Kustom King sells, but that's a whole lot of fiberglass if you aren't planning to build many bows.

[mikkekeswick]

I suppose you could cut the back glass in the middle (riser) and splice a small piece in for some extra length. However 72 inches is still probably enough for your drawlength. I haven't actually made a heavy bow with glass but i'm sure its more than elastic enough to handle the weight at a more normal width for a Hill bow.
In a glass bow the wood for the core lams isn't massively important other than not being too dense. Out of your woods I would go for the locust.

[Jaroslav]

I have longer glass than 72". That is smallest of problems.  I shall make riser no longer than 15". I need about 125#/31 1/2" or some, thus the extra width.
I assumed that if I go with typical "HH" width - like 1 1/8"  for 60#, and attempt to double the weight I will end up with a bow too thick, thus getting shear core failure.

[Jaroslav]

I should perhaps clarify my experience - I have made about 1000 bows, some up to 140#, even laminated wood ones, but I m out of my depth when it comes to glass.

[jsweka]

In my design for a 68" length and 1 1/8" width, I'd use 0.525" total stack to get 68# @ 28".  Considering your extra length and extra draw length, my best guess is that 0.675" stack would get you to 125#.  I don't think that is too thick.

[monterey]

John is correct, I think.  There are plenty of very heavy Hill style bows around and they are doing well.

[Jaroslav]

Thank you guys I shall work from that.

[KenH]

Remember that glass provides 88% of the strength of a bow and the wood provides only 12% percent.

[Jaroslav]

OK. I assume that is for tension and compression side, but what about shear?

[Jaroslav]

Jsweka - with my glass that is 1 mm thick, that would mean 2 mm for glass and then say 5 lams by 3 mm, all tapered to about 2 mm from riser to tip ?

[jsweka]

I needed to do some math to convert from metric to english units.

1 mm thick glass is equivalent to 0.040" glass.  I'd go with 0.050" glass (1.27 mm) if you can get it.  The total stack would be 17.15 mm (0.675").  I use a 0.005 total taper rate in my lams (0.127 mm taper rate).  So, with a total length of bow of 72"(1829 mm) that 36"(914 mm) from mid bow to string grooves, your total thickness would run from 17.15 mm (0.675") down to 12.75 mm (0.495") at the string grooves.

Four or five lams probably won't matter much as long as you keep about a 0.005" per inch taper rate (that would be the same as .005 mm per mm taper rate).

[mikkekeswick]

I've just checked my HH style form and it's stack numbers.
I'd say Jsweka is spot on!
Don't worry about sheer - no problem.
Post some pictures when you have made it  

[Jaroslav]

Many thanks guys, I sure will.

Jaro

[Jaroslav]

One more question - how does tapering from exact mid of the bow play with glued on riser?  I would assume and I planned to leave the area under the riser untapered and grind the tapers from past the riser fades?

[monterey]

You can do that, but you can also lay a matching taper layup in reverse on top of the form and under the lams.  That makes your lam stack flat for the riser.

So, if you have, say three tapers off.002 in the stack, put three under the stack with the thick ends at the end of the form.

[Jaroslav]

I see, this didnt occured to me.