The crossbars on bicycle frames give them added strength. On a man’s or boy’s bike the crossbar extends straight across the top of the frame, just below the seat. On a woman’s or girl’s bike the crossbar is attached to the seat tube at an angle, far below the seat. Because of this structure, women’s bikes are not nearly as sturdy as men’s bikes.
When bicycles were first built, women didn’t wear pants; they always wore skirts or dresses. The low crossbars on their bikes allowed them to get on, ride, and get off with dignity without showing their underwear! The design of bicycles for women and girls, then, is based on a long-standing tradition and still offers the advantage of easier mounting and dismounting. But today, women and girl bicyclists wear pants or shorts when riding and can easily use bikes designed for men. As a matter of fact, serious female bicyclists who do a lot of riding or travel through tough terrain and need bikes with sturdier frames buy those made for men.
But along with the comfort of air-filled tires came the frequent task of filling them up. The rubber that is used to make bicycle tires is thin and porous, which means that it has tiny microscopic pores, or holes, through which air can escape over time. Air that is pumped into bicycle tires is pressurized, meaning it is compressed into a much smaller space than it would ordinarily occupy. Without pressurized air inside, a bicycle tire would not have its firm shape. Air under high pressure, like all gases, moves or migrates to surrounding areas that have lower pressure, traveling even through fairly solid materials. Air in a bicycle tire naturally tries to escape through the valve stem that is used to fill it and the inner tube that holds it. So even bicycles that don’t undergo the wear-and-tear of frequent use eventually end up with flat tires.