Why do bike tires look flat on cold winter days than hot summer days?

Answer 1

Bike tires look flatter on cold winter days because they ARE flatter. The scientific mumbo-jumbo: That is because gases (i.e. the air in the tire) expand when hot and contract when cold. Air contains molecules of stuff like oxygen and nitrogen, all floating around as far from each other as they can get. When the temperature falls, those molecules huddle closer together, and when that happens the air fills less space. The empty space where they had been becomes a sort of vacuum, pulling the tire in a bit. What you should do: All that means that to keep the same tire pressure in winter as in summer, you have to pump in more air. It works best if the air you are pumping is the same temperature as the air outside, and thus air pumps at garages are generally installed outside. If you own a car: This is also true for car tires, and is thus a good reason to check tire pressure regularly as the seasons change and temperature rises and falls. A cold tire with too little air cannot maneouvre as well and cannot stop as easily when you apply the brakes.