Step-through frame

A step-through frame (also known as open frame[1] or low-step frame) is a type of bicycle frame, often used for utility bicycles, with a low or absent top tube or cross-bar.[2][3]

This article is about a type of bicycle frame. For step-through motorcycles, see Underbone.
A Triumph
Woman with a step-through frame bicycle in the 1890s

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 (cross-bar), 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
  • 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 elderly and others with restricted agility
  • potentially safer than a high top tube; a rider who loses balance can step through the bicycle without becoming entangled
  • compactness provides a popular starting point for folding bicycles.

Disadvantages
  • 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 monocoque frame construction. These structural elements may add weight or cost over a traditional diamond design.[4][5][6]
  • Inattention to structural design can lead to excessive flexing, resulting in lower pedaling efficiency and reduced frame life.[4][5]
  • 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
A Peugeot mixte frame bicycle

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 (lateral tubes, or lats) 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 (pronounced [mikst]) is a direct appropriation of the French word meaning "mixed" or "unisex". The usual North American bicycle industry pronunciation of this loan word is /ˈmɪkstiː/.[7]

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.[7] The FNCRM (Fédération Nationale du Commerce et de la Réparation du Cycle et du Motocycle) calls this style a sport.[8]

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
A Dahon folding bicycle with a cross frame

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.[9] [10]

  • Image from an 1889 advertisement for a ladies' safety bicycle
  • Postal delivery by bicycle in Cologne, Germany
  • Cyclist in Cologne, Germany
  • Folding bike
  • "Albany" bike
  • Damenrad from 1900
  • Itera plastic bicycle
  • Kia mixte frame
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See also

References
  1. "Nimrod road tests the Jack Taylor touring bicycle". Cycling. March 16, 1960. Retrieved 2013-04-02. their range of seventeen models includes a woman's open frame bicycle
  2. "Top Tube". Sheldon Brown. Retrieved 2010-02-07.
  3. Oxford English Dictionary (2nd ed.). Oxford University Press. 1989. cross-bar, n. 1. a. A transverse bar; a bar placed or fixed across another bar or part of a structure. spec. The horizontal bar of a bicycle frame
  4. Van Der Plas, Rob, Bicycle Technology, San Francisco: Bicycle Books (3rd ed.), ISBN 0-933201-30-3, ISBN 978-0-933201-30-9 (1995), pp. 60-2
  5. Peterson, Leisha A. and Londry, Kelly J., Finite-Element Structural Analysis: A New Tool for Bicycle Frame Design: The Strain Energy Design Method, Bike Tech, Bicycling Magazine, Vol. 5 No. 2 (1986)
  6. Wingerter, R., and Lebossiere, P., ME 354, Mechanics of Materials Laboratory: Structures, University of Washington (February 2004), p.1
  7. Brown, Sheldon (April 19, 2010). John Allen (ed.). "Mixte". Sheldon Brown's Bicycle Glossary. Harris Cyclery. Retrieved April 4, 2011.
  8. https://sites.google.com/site/hirosemuseum9/216
  9. Brown, Sheldon. "Cross Frame". Sheldon Brown's Bicycle Glossary. Harris Cyclery. Retrieved 2011-05-04.
  10. Cross Frames at rijwiel.net http://www.rijwiel.net/kruisfre.htm
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