Try searching for this. There was a discussion among the old stuff I looked through when I first got on here.
Think the min pressure needs to be somewhere around 14 psi just to keep the bead from moving relative to the wheel. This is what some folks reported.
Don't think you have a tire heatup problem with low pressures as is the case with car tires. Lower pressures allow more of the tire to imprint upon the pavement/soil, but limited by the bead-to-wheel contact necessary.
Ralph