The bow was trying it's best to bend sideways.
The two factors contributing to this are the string line being a bit off centre and the cross section being about as thick as it is wide and is some places even thicker! (One of the billets was rather narrow to start with)
I've got it flexing now better now and thinned it a bit ad teased it back to a reasonable brace. The tips are getting pretty narrow where I've tried to adjust the string line. It feels a bit unstable and was making some ominous ticking noises on the tiller, it sounded like cracking glue, but I don't think it's the splice failing.
I'm fully expecting it to explode as it come back further, I think the bug holes in the sap wood will cause it to explode, but you just don't know.
Making a bow is a road travelled in hope rather than certainty.
It's back to about 40# at 21" , I'll be sure to get the video running before I take it back any further.
Here's a pic of the break in the sapwood you can see the curve of the bug hole along the break and a neat round hole where it turns down into the wood...
I've run the next two billets through the bandsaw to take off the sapwood, but I may need to buy (wince) a bench mounted belt sander/linisher to thin down the bamboo I'm going to use for a backing.
Just noticed someone has ticked the 'boring' reactions box... LOL! Maybe they need to have a bow explode above their head to see how boring it is?