I found this a few weeks ago while wondering the same question. I'm not sure if US/UK Spec M235i have the same spring rates but the guys in the UK have done a bit of work to figure this out. Rates for rears are below, as is a link to the discussion on babybmw.net. It seems like the fronts are 120-180 lb/in depending on the model of your car or aftermarket springs you buy. If someone here knows better, please share! OP - Hope this helps!
H&R M135i Rear = 348 lb/inch
OEM M135i Rears = 383 lb/inch
Eibach Rear = 384 lb/inch
'AC Schnitzer' Rear = 384 lb/inch
H&R M235i Rear = 392 lb/inch
OEM M235i Rears = 417 lb/inch
OEM M140i Rears = 587 lb/inch (~same as OEM M3/M4)
https://www.babybmw.net/forum/viewto...44195#p1344195