Why do tires slowly lose air? From what it seems, they are sealed and shouldn’t really be letting anything in or out, but they still need to be refilled every now and then.

Question

I understand pressure changes as heat changes, but even in the same season you will still lose a slight amount of pressure.

In: 141

Answer 1

Same reason balloons lose air over time. Balloons are just thinner so they lose it more quickly.

Rubber is not really “airtight.” It is great at creating *pretty good* seals, but it is not leakproof and over time the air inside escapes.

The higher the pressure inside the rubber container (balloon, tire, etc) the more the air molecules “want” to be outside it. Over time they find their way out, either through places where soft material (rubber tire) meets hard material (steel rim) where it couldn’t create a perfect seal, or even through the rubber itself.

Also keep in mind that tires are not solid rubber. They are basically a mesh of kevlar (for bike tires, idk about cars) or some similar fibers that are encased in rubber. So now not only does the rubber have to fit tight against the metal rim, but it also has to “contain” these fibers which essentially live in little channels in the rubber. Air could feasibly escape by passing between the fiber and the rubber, *especially* when the rubber is stretched or deformed (e.g., when you go through a pothole)