I understand pressure changes as heat changes, but even in the same season you will still lose a slight amount of pressure.
In: 141
They are mostly sealed. It’s really really hard to create a perfect seal between a tire and a rim. We can come pretty close, but it’s not going to be a perfect seal. Keep in mind there’s literally around a thousand pounds of pressure on each tire all the time, they are in motion a good bit of the time, and they are constantly heating and cooling throughout the day and as the vehicle is used. And remember that the materials that make up the tire and the rim expand and contract at different rates as they are heated and cooled. All of these things make it extremely difficult to create a perfect seal between tire and rim. But considering modern tires can often go from months without having to add air, the engineers do a pretty good job with the materials they have and the conditions those materials endure.
The easy answer, even though the tires are sealed and made of rubber, the material in the tire isn’t truly leak proof. The gas inside the tire, while being contained well, is still able to get through the rubber. The rubber is just a fraction bit porous. Slowly, over time, the heating and cooling of the tire leads to the loss of the gas inside the tire. It just happens really slowly because the rubber isn’t as porous as a sponge.
The surface of the rim may not be perfectly smooth, it could have corrosion or dirt, allowing a partial seal but not a very good one. The valve where you fill the tire is designed to open/close so of course this is another thing that is *supposed* to seal but it can get dirty or just wear out with time and usage.
A good tire shop should clean the rim as part of the process of installing a tire, and should possibly replace the valve with every tire change. But with tire pressure sensors built into many newer valves it is more expensive and complicated to do that. This is a common cause of a leak that is faster than you’d expect from a normal tire, but not as fast as a giant hole in the tire.
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)
When you think on a microscopic level, the elastomers in the tire actually have holes between the molecules — large enough to let some air molecules through.
Balloons are a great illustration of this. A balloon full of air will “shrink” slower than one full of Helium, which has a much smaller molecule.
Decades ago, for an outdoor event I was at, bought a tank of hydrogen to fill balloons. It worked great — but leaked through the rubber fairly quickly. Half an hour after inflation, they wouldn’t hold themselves up.
A side note. A “Helium Leak Test” is often performed on critical metal instrumentation/components. If there are faulty welds, the helium will leak through the tiniest of spaces and is easily detected.
Companies that make liquid products in plastic containers routinely do stability studies to figure out how much product is lost to the atmosphere over time. Even though the volume of lost material is small, it’s not zero, and for some products can be enough to affect the chemical makeup/potency/concentration etc. of the product. Plastics and rubber are porous, just not very. This is one reason for expiration dates on drug products. In addition in the case of a tire there’s also a sealing surface that isn’t perfect either.
Air molecules are really really really smaller, oxygen and nitrogen molecules are made up of just two atoms.
So from our “giant’s” perspective, yea, the rubber looks like a totally perfect solid seal that nothing can get through, but on a molecular level there are still gaps between all the tangle of rubber molecules that the tiny little air molecules can slowly but surely get through.
Gas molecules sneak past the seal between the rim and the wheel. No seal is perfect. A flaw wide enough for a single molecule to pass at a time is enough for the tire to eventually deflate.
Some molecules (such as hydrogen and helium) are small enough to pass directly (albeit slowly) between molecules which form the container, which makes storing these elements in their gaseous state a challenge. This is why hydrogen and helium are usually stored as cryogenic liquids.
Oxygen molecules are really, really teeny tiny compared to other molecules.
They’re so small that over time, they simply squeeze out from inbetween two rubber molecules.
Rubber isn’t actually air tight. It’s just mostly air tight. When you are talking air that is made up of just atoms and very, very small and simple molecules, the very large molecules of the rubber don’t seal well enough that a few atoms can squeeze through occasionally.
Rubber at the molecular level is somewhat like weaved cloth. Where the rubber doesn’t quite weave together perfectly atoms can pass. The more pressure in the tire, the more the flaws get exposed and the more energy there is to push the tiny molecules in the air through.
Latest Answers