From cane creek and the like. If they slacken the ha by 1.5 deg what diff would it make to bb height. I’m guessing lower it but anyone have an idea as to how much? Ta.

It would lower it but the amount will be teeny tiny. A couple of mm at the most I’d have thought.

as above, very little and most bike I’ve seen slackened with cups run dual crown forks anyway so have enough adjustment to keep the BB exactly the same height if need be.

As above, a 500mm fork, slackened from 69 to 68deg woudl lower the head tube by 3mm, so the BB would drop by arround 1mm.

The science bit:

sin68-sin69 * 500 = 3mm

BB is aprox 2/3 of the way back allong the bike (assuming 16.5″ chainstays and a 45″ wheelbase)

Cheers – saved me starting an expensive expt.