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Join Date: Apr 2005
Location: Minneapolis, MN, USA
Posts: 1,152
Bikes: 1990 Burley Bossa Nova, 1992 Paramount PDG-70, 1993 Specialized Stumpjumper, 2005 Jamis Dakar XC Pro, 2007 Rivendell Bleriot
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They are _designed_ to break if your rear derailleur gets knocked inwards.
It saves your frame from getting bent instead, which would be a much more serious thing on an aluminum frame (in fact, the frame might be ruined).