To decrown or not to decrown?

[WhistlingBadger]

Well, I got a half dozen decent staves out of this rocky mountain juniper log.  (And about killed myself getting it out.  Straight rocky mountain juniper trees, like big deer, tend not to live in convenient places)  This is the straightest one, so I'm going to start with it.

So, pretty high crown on this.  I'm going to do a pyramidish profile with static curves, sinew backed.  Chuck Loeffer, our resident juniper expert, has already weighed in that he wouldn't decrown it; he thinks a high crown makes the sinew work harder.  Anybody else have any experience with this kind of wood?  What do you think?

[WhistlingBadger]

If I decrown:  I've never done that before.  Any advice or pitfalls to watch for?

If I don't:  What kind of belly shape works for a high back?

[mmattockx]

If I decrown:  I've never done that before.  Any advice or pitfalls to watch for?

If I don't:  What kind of belly shape works for a high back?

Chuck is correct in a way about the high crown. The highest part will do the most work, so the center strip of sinew will be stretched the most and the areas away from the center will do less.

I've never decrowned, either, but it is essentially making a board out of a stave, so make it flat and smooth with no steps or rough areas. A flat belly is pretty much always the best shape to optimize the compression strength of your wood.


Mark

[TimBo]

I am just lurking at this point...but I do very much like your organizer boxes (Stuff and Things)! 

[WhistlingBadger]

Chuck is correct in a way about the high crown. The highest part will do the most work, so the center strip of sinew will be stretched the most and the areas away from the center will do less.

I've never decrowned, either, but it is essentially making a board out of a stave, so make it flat and smooth with no steps or rough areas. A flat belly is pretty much always the best shape to optimize the compression strength of your wood.


Mark

So that's why it's usually mounded toward the center, right?  More work needs more sinew.  Is there such a thing as too much crown?  I've just never worked with a stave this rounded before so I'm trying to visualize it.

I am just lurking at this point...but I do very much like your organizer boxes (Stuff and Things)! 

I need more.  "Pointy Things"  "Doohickies"  "Junk I'm Never Going to Use but Too Cheap to Throw Away"

[mmattockx]

Is there such a thing as too much crown?

Performance will increase until the point where a tension failure occurs. That point depends on the wood used and how well the sinew application was done and no one can really define it precisely.


Mark

[WhistlingBadger]

Is there such a thing as too much crown?

Performance will increase until the point where a tension failure occurs. That point depends on the wood used and how well the sinew application was done and no one can really define it precisely.


Mark

You know, that's the really hard thing about bow making, I think...The characteristics of natural materials are imprecise.  "Try it and see if it blows up" is not the funnest way to learn a complex skill.  ha ha ha  But...I'll try it.  I bet it won't blow up.

[mmattockx]

You know, that's the really hard thing about bow making, I think...The characteristics of natural materials are imprecise.  "Try it and see if it blows up" is not the funnest way to learn a complex skill.  ha ha ha  But...I'll try it.  I bet it won't blow up.

You're correct that this is one of the toughest things about working with wood for bows. I wouldn't say a failure point can't be calculated but it requires fairly detailed info on the bow materials and dimensions to be used. You can make some educated guesses and be conservative and probably be safe, if giving up a small portion of maximum performance.


Mark

[WhistlingBadger]

Well, thanks for the consensus, boys.  I'll leave her curvy on top and start carving on this thing tonight.  Stay tuned!

[bownarra]

Static recurves aren't best suited to a pyramid width profile. Too narrow out where the recurves start and you are likely to end up running into stability issues. Keep the limbs parallel wdth until about 10" from the tips then taper.
If you do the sinew job right sinew will never, ever fail on a wood bow - it can't. The wood can only compress around 1% of its length before getting compression fractures...the sinew can safely stretch 7% of its length....sinew is never working hard on a wood bow and therefore you need to strain it as much as possible (so it isn't 'dead weight', sinew/glue is 1.3 s.g.).

[George Tsoukalas]

It is not just   a matter of removing the crown. The bowyer has to take care to follow the vertical or lateral grain.
I usually leave the stave a few inches longer to counteract the stress caused by the crown. (Not the grain as I previously wrote)

Jawge

[WhistlingBadger]

Static recurves aren't best suited to a pyramid width profile. Too narrow out where the recurves start and you are likely to end up running into stability issues. Keep the limbs parallel wdth until about 10" from the tips then taper.

Ah, OK.  I was also thinking I'd make the curves narrow but deep, sort of a molle profile with recurves.  Maybe a sudbury profile would work better.
 How wide should the curves be to be stable?

It is not just following the  a matter of removing the crown. The bowyer has to take care  the vertical or lateral grain.
I usually leave the stave a few inches longer to counteract the grain.

Jawge

Could you elaborate on this, George?

[George Tsoukalas]

Sure. As you proceed make sure you see equal spaces on the lateral grain. That way you know there are no violations. Kind of hard to explain. How did I do.

[WhistlingBadger]

Sure. As you proceed make sure you see equal spaces on the lateral grain. That way you know there are no violations. Kind of hard to explain. How did I do.

No, I think I see what you mean.  You basically want the grain to run straight down the new back, with no islands, points, or other run-offs, right?

I think I'm not going to decrown this one, but if I ever do that chokecherry molle I want to try I probably will have to.  So that's good to know.

[George Tsoukalas]

Yes, straight down the back. But as you do it make sure the space between the grain lines is equal except for when the stave  begins to narrow. Sorry hard to explain. Not my favorite thing to do.I just leave the stave longer.