Sheldon Brown (July 14, 1944 - February 3, 2008) was an American bicycle mechanic and technical authority. He maintained an extensive website containing detailed articles relating to bicycle mechanics and maintenance, as well as a thorough glossary of bicycling terminology. His trademark beard and plastic helmet-mounted eagle, named Igor, were matters of frequent comment.
Sheldon Brown was employed as the "Parts Manager, Webmaster and general Tech Guru" of Harris Cyclery, a bike shop in West Newton, Massachusetts. He was an enthusiast for many old and unlikely forms of bicycles and cycling including Raleigh and their English three-speed bicycles, the Sturmey-Archer hub in general, tandems, and in particular fixed-gear cycles. He repaired cameras, and was an accomplished amateur photographer and his site is well illustrated with his own work.
With the assistance of Galen Evans and Osman Isvan, Brown developed a new method for determining and comparing bicycle gear ratios. His method, which, for any combination of front chainring, rear cog, wheels size and crank length, results in a pure ratio that Brown terms the "gain ratio." This method purports to be the only formula that takes into account crank length in addition to the more commonly considered chainring, cog and wheel size factors. 
Sheldon was a frequent participant in online cycling forums such as the rec.bicycles.tech Usenet newsgroup and bikeforums.net. His April Fool's Day articles were eagerly awaited by the online cycling community. On some groups, links to his site in response to technical questions were flagged with the abbreviation AASHTA (As Always, Sheldon Has The Answer).
In 2004, Sheldon was recognized for his contribution to cycling by the UK's Cyclists' Touring Club (CTC). Sheldon Brown wrote the "Mechanical Advantage" column for Adventure Cyclist magazine. His site is referenced in the technical library of the CTC.
In August 2007 Sheldon was diagnosed with primary progressive multiple sclerosis. After losing his ability to balance an upright bicycle to the disease, he was able to continue pedaling by using a recumbent tricycle. Sheldon died on February 3, 2008 after a heart attack.