In their unquenchable need to sell more product, bicycle manufacturers have managed to create a false and unquestioned mythology; namely, that to traverse paved roads it is necessary to possess a narrow-tired "road bike," and to traverse dirt roads it is necessary to have special fat-tired bicycles with shock absorbers called "mountain bikes." The time has come to banish the fat-tire myth. Bay Area cyclists have been riding skinny-tired bikes in the dirt for decades. The area has long been host to a cyclocross racing circuit, a skinny-tire sport where competitors ride dirt trails over streams, cliffs, and log piles. Road racers in the foothill hinterlands, meanwhile, have been exploring obscure dirt roads without giving a single thought to polyurethane-dampened suspension systems, add-on chain-slack-inhibitors, or other such mountain-bike nonsense. The best way to put the lie to this hoary myth is the old Mount Tam railroad bed, billed locally as a mountain bike trail to the summit. But because of its mild grade -- it was built to be scaled by steam engine -- it's equally doable by 14-speed road bike. A 42-23 low gear will get you to the top just fine. From the Golden Gate Bridge, take the Sausalito bike trail into Mill Valley, then follow East Blithedale Avenue to Blithedale Park, where the Old Railroad Grade begins. Given that road bikes are usually lighter than fat-tired ones, and that the road rider's position is more ergonomically efficient, you'll find yourself zipping by dozens of sweating mountain bikers on the way up. You'll get a breathtaking view of San Francisco and the bay, too. Once at the summit, buy yourself a soda and a hot dog at the refreshment stand, lounge on a park bench, and watch the mountain bikers straggle in. Once rested, you can head down the Panoramic Highway, across the Golden Gate Bridge, and home -- around three or four hours all told. It's a bracing, beautiful ride, and long enough to reminisce about those halcyon days when skinny tires ruled the trails.