Wider tires with an aggressive tread
If you use smooth, narrower tires during the warmer months to achieve higher speeds, remember that riding on such tires is not safe in winter, as there is a risk of skidding and accumulated mud will make traveling on the road more difficult. Owners of mountain bikes and fat bikes are prepared for all weather conditions, but on a city, road, trekking, cross, or gravel bike, you should necessarily change the tires to a version with an aggressive tread and, if the frame and fork clearance allows it, also choose wider ones.
Mudguards
You probably do not use mudguards in summer, and if you ride singles and forest trails on your mountain bike in winter, you will not need them either. Commuting to work on a city bike is a different matter. Because of wet surfaces and mud, without mudguards you cannot expect to arrive at your destination with clean clothes.
Reliable lighting
Let us return to the issue of safety. In winter, darkness falls much earlier, and its arrival will surely take you by surprise more than once. That is why you should definitely consider bicycle lighting. At the front, you will need a lamp illuminating the road with a light output of at least 100 lumens for the city and at least 400 lumens in less well-lit areas.
At the rear, there must be a clearly visible position light—mandatory bicycle equipment at any time of year. It is worth placing reflective stickers on the helmet, frame, and tires.
Winter drivetrain lubrication
The chain lubricant you use during the warmer months will probably not work well in winter. When puddles and mud are abundant outside, you need to choose a thicker product if you do not want to spend an hour laboriously cleaning the drivetrain after every ride.
Brake inspection
Before winter, it is worth taking additional care of the brakes. If the brake pads or linings are already close to being worn out, now is a good time to replace them. With disc brakes, it is worth degreasing the surface of the rotors even before every ride. When riding in below-freezing temperatures, regularly check whether ice has accumulated on the brakes.
Are you unsure whether more demanding winter riding is a good idea for you? Perhaps it would be worth investing in a city electric bicycle, which will ease the strain on your muscles somewhat thanks to its electric drive while also providing a convenient way to get to work, university, or the shops?