Just curious how the BETD linkages slacken your bike without lowering the bb or altering seat tube angle ?
I can se how they could slacken ha the same as offset bushings though.

I’m struggling to imagine how what you describe would work. You could have BB height or ST angle but not both as you are constraining it at two points.

Just checked BETD, no claims of ST angle being preserved.

Sorry I just read that on a thread here. So how does it slacken ha without lowering bb?

It will be the way the linkage was designed and the positioning of the bearings in relation to the shock pivot. We made linkages for the old Spesh FSR Enduro’s where we increased the travel from 90mm to either 100mm or 130mm and even 120mm to 150mm and again the BB height was kept the same. We do this with CAD/CAM work. I don’t know the full in’s and out’s as to how the design guy did them as I only worked on the CNC lathes. Regarding the seat tube angle I have no idea if these changed but I’m sure I could find out.

It can’t. Surely.
You’re pivoting around the front axle. If you change the head angle, you change the seat angle, as they’re linked by solid bars between then (unless they’re one of those bonkers DJ bikes with the shock in the TT…)