Here's the story. Will follow up with pics and analysis.
A couple of years ago I made a bunch of board bows for a youth archery camp. Some were hickory and some red oak. One of the red oak bows had some grain run off but we had more youth than bows so I crossed my fingers and tillered it out to 45# @ 28" with about 1" of string follow. It shot well for a couple of weeks (maybe a dozen arrows a day) and then the second day at the camp a sliver started to lift up on the back where the rings ran off.
A few weeks ago when I got all that elm, I debarked and planed a strip of it down to about 3/16" thick and let it dry. Then I sanded down the back of the damaged red oak bow and glued the natural ring elm back on it with about 4" of reflex using TBIII.
I let that dry for a week and tonight began to tiller it.
Everything seemed to go well, better than I expected in fact until the time came to string it. It was pulling 40# at 18" of tiller string when I decided it was safe to string and continue tillering.  I was wrong.
Below are some pics of the failure and my ideas about what went wrong.

CJ

The profile of the bow:
 

Handle section:
 

The back:
 

And the close ups of the break:

This one shows possible compression failure:
The belly is crushed more than the picture shows.
   

 

I think this one shows a possible glue failure:
 

And this one has a suspicious knot in the back where it splits.
 

The belly was getting mighty thin in places too. My back was uneven because it was natural and the belly was a cut board.
I think the too thin oak belly was too weak for the too thick elm back coupled with the high stress of the large reflex and possibly a poor glue line caused this one.

What do you all think?

Thanks
CJ

I think what you think...  I tried gluing a red oak board into reflex w/ a bamboo backing on my 3rd bow.  Perfect grain.  Blew up almost the exact same place as yours.  Same problems.  Belly got too thin, glue let go, and the oak couldn't take all the compression.  The bamboo stayed in one piece on mine, though.  A generous trapping could possibly have saved yours.  As much as I was asking mine to do, I don't think there was any hope for it...

You need to use super thin backs with red oak. I have a sweet little hickory backed oak R/D bow that shoots really nice at 42#. My back is under .100 thick after finish sanding. I made a simple sled to allow my planer to go below its minimum. My bet is compression failur. I just LOVE this stuff because there is so much to learn from $8 worth of wood! The glue may have been the culprit as well, its hard to tell what happened for sure though.

Yeah, the experiment involved using the natural outside of the tree as the back of the bow.
To get any thinner, I would have had to de-crown or quarter saw the back. (That may be my next experiment) But I think you're exactly right. The back was too thick.

CJ

My Ash backed Ash bow #2 blew up just like that. I think my back was too thick. keep this in your mind as a learning experience. Thanks for sharing!


Pearl what type of sled did you make for your planer to get thinner than .125 backers?Got pics? please PM me or start a new thread.
Thanks

4" of reflex seems a little much for red oak.

I'd say that belly failed first.

Good experiment though...nothing like tinkering any finding out what works and what doesn't.

Wouldn't be the self-bowyers we were today if ppl didn't try out new stuff.

The bow I described above had 4" of reflex glued in and 2" of defex. If the back is thin enough the oak will take the compression.............sometimes. Oak isnt my first choice.