2.8s on a hardtail mtb. Locally on dirt singletrack with some tree roots for obstacles, I've run as low as 15 but usually end up at 20. (About 170lbs rider plus gear). Rode a bit in Moab earlier in the month, ran 23-25 psi and felt I could have gone lower. Plenty of opportunity for rim dings there but never got one. My riding isn't super aggressive though.