It's been like almost a month since I last posted this so I guess I'll bring it out again:
http://www.bikeforums.net/showthread.php?t=229546
Originally Posted by Psimet2001
Formulas:
Only the intercepts change for tires between 20-28 widths. Larger widths and the slopes change. Obviously these are generalized and assumed to be linear. While not 100% correct they will give you a great place to start.
Tire Width=20: Pressure(psi) = 0.33 * Rider Weight in lbs + 63.33
Tire Width=23: Pressure(psi) = 0.33 * Rider Weight in lbs + 53.33
Tire Width=25: Pressure(psi) = 0.33 * Rider Weight in lbs + 43.33
Tire Width=28: Pressure(psi) = 0.33 * Rider Weight in lbs + 33.33
Tire Width=32: Pressure(psi) = 0.17 * Rider Weight in lbs + 41.67
Tire Width=37: Pressure(psi) = 0.17 * Rider Weight in lbs + 26.67
Example: You are 150lbs running 28's
Pressure (psi) = (0.33*150) +33.33 = 82.83psi (rear)
Front Pressure = .9*Front Pressure = .9*82.83psi = 74.55psi front
You could start by running 85 rear 75 front and adjust from there. For your weight I would step down to 23's which you could run at 92F - 102R. That would be nice and comfortable and have less rolling resistance than the 28's.
These equations are linear extrapolations of the recommendations that are on Sheldon's site. The real curves of recommended pressures would probably not be linear so keep that in mind. Basically I'm saying that these equations will get you into a good starting point. Adjust as needed for your specific tires, ride quality, weight, etc...