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A device for shifting gears on a bicycle by moving the chain between sprocket wheels of different sizes.

[French dérailleur, from dérailler, to derail; see derail.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.


(Mechanical Engineering) a mechanism for changing gear on bicycles, consisting of a device that lifts the driving chain from one sprocket wheel to another of different size
[French dérailleur derailer]
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014


(dɪˈreɪ lər)

a gear-shifting mechanism on a bicycle that shifts the drive chain from one sprocket wheel to another.
[1945–50; < French dérailleur literally, a device causing disengagement or derailing]
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.


[dəˈreɪljəʳ] n derailleur gears npl (Cycling) → deragliatore m
Collins Italian Dictionary 1st Edition © HarperCollins Publishers 1995
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References in periodicals archive ?
Someone broke the derailleurs, braces, and stole pedals, TASR reported.
And so by virtue of my hobby, I have a decent grounding -- I can, for example, build and true wheels, measure and "break" a chain, pull cranks, and adjust brakes and derailleurs (though I've had to learn a lot to be able to do this well enough for paying customers).
I listen to the chain moving across the crank, derailleurs, and cogs; test the shifting of every gear; and listen intently for any sound from the bearings on the hubs and bottom bracket.
Obviously, AW is primarily in the business of selling bicycles, but it also sells accessories (such as bottles, bike racks, brakes, etc.), clothing (such as caps, gloves, jerseys, etc.), and components (such as brakes, chains, derailleurs, etc.).
Topics include preventative maintenance, flat repair, wheel bearings, cables and housing, handbrakes, chain and cogs, derailleurs, bottom brackets and wheel truing.
“Gates Carbon Drive simply makes sense on electric bikes because it provides a clean, grease-free and low-maintenance alternative to chains and derailleurs,” Scurlock said.
So he set out to bike up the hill, and remember, it was cobblestones back then, and one speed was it - no derailleurs. A crowd formed to watch, and Major Taylor went up - and down - twice in quick succession.
All early bicycles ran on a direct drive, first with the pedals cottered onto the front axle, then with a fixed gear spinning on the crank and rear hub; the Tour de France racers were stubborn to adopt the derailleurs that accommodate multiple gears.
Probably the most obvious change has been the introduction of gearing, both by means of hub gearing and derailleurs. The former were developed mainly in England and Germany, the latter in France and Italy.