A step-through frame is a type of bicycle frame, often used for utility bicycles, with a low or absent top tube or cross-bar. Traditionally, bicycles with a step-through frame were known as "Ladies, "Women's", or "Girls, mainly for their advantage to riders wearing skirts or dresses. Bicycles with a high top tube, known as a diamond frame, were known as "Men's", "Gents, or "Boys. As a result of changing clothing styles since the late 20th century, descriptions that describe the frame style, rather than the presumed gender of the rider, are becoming increasingly common.
Advantages
less risk of stretching or ripping clothes when mounting the saddle
Heavier. Compared to a traditional diamond frame consisting of two near-triangles, open or step-through frame designs must be designed with thicker gauge tubing, the use of additional gusseting members, and/or monocoqueframe construction. These structural elements may add weight or cost over a traditional diamond design.
Inattention to structural design can lead to excessive flexing, resulting in lower pedaling efficiency and reduced frame life.
Fewer places to mount accessories, e.g. an air pump or water-bottle.
More difficult to carry around off the ground due to the sloping tube near the bicycle's center of gravity, e.g. carrying it up stairs, or lifting to hang it for maintenance.
Variations
Mixte
One particular type of step-through frame is called a mixte. In a mixte frame, the top tube of the traditional diamond frame is replaced with a pair of smaller tubes running from the top of the head tube all the way back to the rear axle, connecting at the seat tube on the way. The normal seat stays and chain stays are retained. This provides the lower standover height of a step-through frame bicycle with a strong diamond-frame geometry. Mixte is a direct appropriation of the French word meaning "mixed" or "unisex". The usual North Americanbicycle industrypronunciation of this loan word is. A variant on the mixte uses a single, full sized top tube running from the upper head tube to the seat tube, but retains the middle set of stays. The FNCRM calls this style a sport. Other named French styles of step-through frames, in addition to mixte and sport, include berceau, Anglais, jumele, col de cygne and double col de cygne.
Cross
Another type of step-through frame is called a cross. The cross frame consists mainly of two tubes that form a cross: a seat tube from the bottom bracket to the saddle, and a backbone from the head tube to the rear hub.
