Thickness of different bow profiles

[DaveMac]

Hi I am currently part way through tillering my first ash self bow, and I'm totally hooked on bow making. My mind is already racing onto the next project.

So I wanted to confirm something. My current project (I don't want to call it a bow yet ) is a 70" pyramid 1.75" wide tapering to 0.5" thus far it's pretty much 0.5" thick the whole length of the limb.  I understand that this is because the front taper takes care of most of the tillering. If I was to try a different front profile next say 68" length 1.5" wide parallel  to mid limb then tapering to 0.5" at the tip, how would that change the thickness? I'm assuming that it would need to taper, but would the taper be even along the  limb or would it just taper when the front profile doesn't?

[Echatham]

yes it would taper, but don't worry about that.  getting tiller right dictates the taper, not the other way around. wood is not always consistent, and so some parts of the limb may have to be thicker in order to be the same strength as another part.  proper tiller shape varies by design, a pyramid design should have a circular tiller, and the design with limbs parallel for half the length should be more elliptical, but don't worry about that yet, that's bowyering level 2 in which i am just starting to dabble.

[DaveMac]

Thankyou for the quick reply. I understand that thing will sort themselves out in the tillering process. However I would like to understand the theory, its just the way my mind works.

   

Both A and B taper, I would just like to know which one would be correct.

To put the question into context it is asked in relation to another thread I posted regarding bamboo flooring

   Bamboo flooring questions

So a rough idea on how the taper should look would be helpful in terms of the layers/laminations.

[Sam Harper]

B is correct.  A will just give you a hinge.

[Echatham]

Well sam... I agree with you if it had that sudden angle like that.... But i think that answer might lead him astray.  Bottom line davemac, is that the limb is under the most stress closest to the handle... And so there has to be more wood there.  Removing thickness has a much greater impact than removing width.  A pyramid profile of uniform thickness will be close to correct tiller.  A parallel limbed profile of uniform thickness will bend to much out of the handle because the limb is roughly the same strength for all its length, but is under more stress near the handle.  So you have to remove thickness toward the tips to make it bend evenly.  Make sense.  Dont try to tiller by calipers on a wood bow.  Do a search for "tillering gizmo"  you'll be all set.