I rode to work yesterday. It was -21 degrees C / or -6 F. The shimano hydros start to get stiff at those temps. I also rode for about 1.5 hours on singletrack last night around -16C.
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1 Super Record bike, 1 Nuovo Record bike, 1 Pista, 1 Road, 1 Cyclocross/Allrounder, 1 MTB, 1 Touring, 1 Fixed gear