If you get the bicycling-specific cold weather clothing, you can go riding comfortably in temperatures well below freezing. I used to go in 15-20F and the only part of me that got cold was the only part that wasn't covered, and that was the tip of my nose.
Those clothes cost a fair amount of money though.
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I'd think on a motor-bike you would have cheaper clothing options, since you (presumably) aren't pedaling and sweating as much.
Basically you want all-synthetic or wool clothes, do not wear ANY cotton stuff at all, not even cotton socks or cotton underwear. The cotton absorbs sweat and gets cold.
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