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Studded tires are awesome.
I have an adventure jacket with a liner that works pretty well down to the mid-20's (°F).
Colder than that and I'll either wear a hoodie or protective vest under that.
Heated grips.
Dualsport helmet, w/ balaclava if it gets really cold.
Heated grips and snowmobile gloves.
My legs don't get cold, so I'm usually fine with jeans and a pair of waterproof overpants.
My snowmobile boots work well for the cold days.
Remember to rejet for the temp. You'll need to add fuel to account for the increase in air density as the weather gets cold.
Coldest i've ridden in on the street is -16°F. Worst part of it was having to stop every few minutes to scrape ice off my visor.
