Bicycles for girls and bicycles for boys are no different in the way they function, but there has long been a difference in their appearance. In a side-by-side comparison, bicycles for girls often have a significantly lower top tube (crossbar) than boy's. A bit of historical background makes it clear how this difference in frame design came about.
Long Dresses and High Bars Don't Mix
In the late 1800s, when cycling became a popular mode of transportation and recreation, social customs dictated that women had to wear long dresses in public. A long dress hiked over a high crossbar was out of the question, and modifications to make cycling clothing more masculine was quite shocking at that time. Since it wasn't possible to make the clothing more bicycle-friendly, the logical step was to make girls bicycles more clothing-friendly. The result was the frame with the low top tube that could accommodate the dress.
The Lower the Bar, the Weaker the Bike
The traditional diamond construction of bicycles for boys has remained essentially unchanged since the late 1800s, a credit to the structural integrity of the design. Dropping the top-tube to accommodate a dress weakens the frame. This loss of strength doesn't have any significant effect on recreational riding. But, as bicycles became more specialized in the 1990s, more women found they needed the strong frame.
Diamond Shapes are a Girl Biker's Friend
With social dress rules a thing of the past, bicycle designs for girls are dictated by function. Some, like mountain bikes, are often nearly identical to their male counterparts. The top tube is raised to a near horizontal position, which gives the frame strength for aggressive rides. The frames have a diamond shape for greater strength and safety.
Traditional Frames Endure
The dropped top tube design has not gone by the wayside. Check out any store where bicycles are sold and you'll see there's still plenty of demand for traditional girls' frames. These bikes are generally intended for recreational use, leisure riding, and transportation around town. Riders with those goals in mind have no issue with any decrease in frame durability.