Originally Posted by
Retro Grouch
PSI means pounds per square inch. A wide tire has a lot more square inches. If you were to inflate it to the same pressure as a skinny tire, it would have a lot more force trying to push the rim flanges apart.
That's part of the reason. The other reason is that you can get away with it. One valuable function of tires is that they provide suspension. Getting pounded by rough pavement actually takes energy & slows a ride down. Skinny tires have to be pumped up hard to provide enough force to support the rim and avoid pinch flats, dented rims, etc. Fat tires, because of the same pounds per square inch, can support the same weight with lower pressure. They can avoid pinch flats at the same time they provide suspension.