This really is not the time of year for a Minnesota adventure, especially if you are not much into cold and snow.

The route from the border to Grand Marais has been improved so there are shoulders and less traffic (but also less services) but from Grand Marais south to Two Harbors it is without shoulders for much of the way , though they have been working on road improvements and I did not go up the shore this summer beyond Two Harbors. There are some bike trails following the road for part of the way but they are unlikely to be plowed in winter. This is the only way up the shore so there is a lot of truck traffic, which I would find worrisome especially in the winter when shoulders may be not accessible or nonexistent.

If you camp the state parks may be open on the route for winter camping but the water and electricity is off entirely so no showers and only pit toilets. You would have to bring quite a bit of equipment and appropriate clothing. Plowing of the camping areas may or may not be done. The sun will set before 4:30 and rise about 8:00 so your days are short. In northern Minnesota the average low temperature is below zero at the end of December and the high in the teens. The record low for I Falls in December is 42 below zero.

I hate to be a party pooper but frankly I cannot imagine a bike trip in northern Minnesota that time of year. It is possible you would be fine (given the warmer winters of late and December is better than January or February) but it is also very possible that you could end up with bad weather that is dangerous and you face difficult routing choices.

Today in Grand Marais it is 30 degrees, relatively warm for this time of the year, with ice crystals in the air. It is snowing in International Falls today. I just left Minnesota and am on my way south. There was about 8 inches of snow accumulation on the ground when I left.