So again print out bits, this time though had a little experiment with hollow parts and some with infill.
The hollow part is only a few grams lighter but way too flexible to be of any use.
A spot the difference showing some modifications made.
I've also printed a new top tube as a test (which probably means i'm doing another front triangle).
So under 100grams for the core will be good to monitor how much weight is added as we go.