Riding through Central/Balkanic Europe, Turkey including Anatolian Highlands and the Caucasus is doable in the winter.
I have done it twice, cycling ( and camping often ).
I am not saying it is easy or the best season to tackle that route ( if I would have had the choice I would have chosen to do it when it is warmer out ) but that it is possible.
Plus on a motorbike you can ride on highways which are almost always cleared of snow. On a bicycle you have to stick to secondary roads, and there the fun starts.
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