A step-through frame (aka low-step frame) is a type of safety bicycle frame with a low top tube (crossbar), in comparison to a diamond frame, or no top tube at all. It is most commonly found on utility bicycles.
Traditionally, bicycles built around a step-through frame were referred to as "Women's (or ladies', girls', etc.) Bicycles", since through the late twentieth century such designs were offered primarily in consideration to riders wearing skirts or dresses. Conversely, diamond frames were referred to as "Men's (or boys') Bicycles". The two different frame styles are now distinguished mainly for their distinct technical advantages, and the contemporary terms which make no reference to gender are often used. Still, many manufacturers continue to offer bikes described as "Men's" or "Women's".
- less risk of stretching or ripping clothes when mounting the saddle
- the rider can wear a skirt (also requires a skirt guard and possibly a chain guard)
- very quick to mount and dismount, so is suitable for delivery bicycles, or any journey with many stops
- suitable for elders or others with restricted agility
- potentially safer than a high crossbar; a rider who loses balance can step through the bicycle without becoming entangled
- potentially heavier, and therefore slower. The lowered top tube must be longer, and so simply has more material. The frame may also be substantially thickened and strengthened to compensate for the loss of the primary triangle in the frame geometry.
- potentially less stiff, and therefore less efficient. If the thickening and strengthening is insufficient to compensate for the loss of the primary triangle in the frame geometry, the frame becomes too flexible and therefore slightly less efficient.
- fewer places for accessories, eg. a pump or water bottle.
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 greater standover height of a step-through frame bicycle while avoiding some of the additional stresses the step-through frame bicycle places on the seat tube.
A direct appropriation of the French word meaning "mixed" or "unisex", "mixte" is pronounced "MEExt", although the usual North American bicycle industry pronunciation of this loan word is "MIX-ty". Both pronunciations are widely used.