FBinNY:

Thank you, once again, for giving an informed and fact-based response ;o)

As a technical journalist, I've had the privilege of visiting two tire manufacturers (Dunlop in New York and Continental in Hanover). They were fascinating experiences, but, that's another story. I went away from both experiences with great admiration for those company's dedication to facts, dependency upon realistic testing, and engineering competence.

A tire is a practical thing. Yes, it depends upon engineering prowess, but -- a tire's (diesel truck or bicycle) design is evidence/scientifically driven. That is: theory is derived from facts (testing). I have participated in such testing and I can assure you that that the conclusions tire engineers make are based on demonstrable facts.

Continental and Michelin (perhaps more) agree that the 'correct' pressure for a bicycle tire is such that the tire's height is diminished by 15% under load (that is: with your fat-ass on board ;o)). Because of my experience with tire manufacturers, I consider this a reliable starting place for 'proper' tire pressures.

15% compression is for the entire tire height, including the bead. 20% is a more convienient measure as that allows for the 'hidden' tire bead inside the rim.

A typical road bike has a weight distribution of 40% on the front wheel and 60% on the rear, both my working bikes have this distribution. Because I weigh some 225 pounds, I use larger tires; I also run a smaller tire on the front than the rear to stay within the recommended 15% compression under load. This typical load distribution dictates that the front tire needs two-thirds the pressure of the rear to achieve the tested/fact-based recommendations of tire manufacturers.

Joe