Reynolds 531 (pronounced 'five-three-one') is a brand name, registered to Reynolds Cycle Technology of Birmingham in the United Kingdom, for a manganese–molybdenum, medium-carbon steel bicycle tubing.
Introduced in 1935 and for many years at the forefront of alloy steel tubing technology, 531 tubing has been superseded by more complex alloys and heat-treatment/cold work cycles as Reynolds continues to compete with other manufacturers of steel for the bicycle industry.
The approximate alloying composition of 531 tubing is 1.5% Mn, 0.25% Mo, 0.35% C, and is similar to the old British BS970 En 16/18 steel (EN 16 is similar to grade BS970 605M36). Its mechanical properties and response to heat treatment are broadly similar to the AISI 4130 standard alloy steel, also used for bicycle frames, motorcycles, as well as aviation and motor-sport. This material was used to form the front subframes on the Jaguar E-Type of the 1960s.
The nearest available stock material is BS4t45 to Bs5T100 in accordance with BS6S100 conditions  (T45)
- Reynolds Technology FAQ, retrieved 23 October 2011, http://reynoldstechnology.biz/faqs/materials/2
- Jaguar Enthusiasts' Club, "How Safe Are Your Frames?" retrieved 23 Oct 2001, from http://etypefabs.com/enthusiast.htm
- Sheldon Brown's bicycle glossary, http://www.sheldonbrown.com/gloss_ra-e.html.
- Pro Formance Metals Ltd, http://www.proformancemetals.co.uk
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