Reynolds 531  Reynolds 531 (pronounced 'five-three-one') is a brand name, registered to Reynolds Technology of Birmingham in the United Kingdom, for a manganese–molybdenum, medium-carbon steel tubing that was used in many quality applications, including race car chassis, aircraft components and, most famously, bicycle frame tubing. It is one of a number of tubing types developed by Reynolds. Reynolds 531 "was the standard of excellence for many decades" among bicycle frame-building materials. And because of the availability of a wide range of butting, diameters and thicknesses of tubes, along with different stays and fork blades, it became the tubing of choice for most frame builders. Introduced in 1935 and for many years at the forefront of alloy steel tubing technology, 531 cycle tubing has been superseded by more complex alloys and heat-treatment/cold-forming cycles as Reynolds continues to compete with other manufacturers of steel for the bicycle industry.  531 tubing became the tubing-of-choice for most framebuilders at least partly because of the huge range of butting, diameters and thicknesses of tubes along with different stays and fork blades available - helped by the willingness of Reynolds to make special tubes for certain manufacturers. Reynolds also made up complete 'sets' of tubing for different cycling applications - for example 531c (Competition), 531st (Special Tourist), 531ATB (All Terrain Bike) and so-on. This flexibility made 531 still competitive even after the introduction of more advanced alloys. The widespread use of TIG and MIG welding in cycle manufacture became a problem as 531 reacted poorly to the higher temperatures produced and 531 has been gradually phased out as a result. The most common like-for-like replacements for 531 are Reynolds 520 and 525 - a Chrome-Molybdenum tubing with very similar characteristics, but in addition to brazing, can also be welded. The 520 tubing can often be seen on modern "fixie" bikes (urban "track" type bike frames), while the 525 with its thicker-walled seat stays is more for cyclo-cross and touring framesets. 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).