Why can't you make a tire that stays inflated forever?

We wish we could. It would certainly make fleet tire maintenance simpler, and would eliminate the necessity for automatic inflation systems (Editor's note: See related story in Technically Speaking within this issue).

How does the air get out?

There are at least five ways. One that should be pretty obvious is through a puncture. And you can also lose air through a defective or dirty valve stem.

If grit or ice gets inside valve stems, it can hold the valve partway open, allowing air to escape.

That's one reason we're so adamant about metal valve caps with internal rubber seals. If the valve stem develops a problem, that valve cap may be the only thing keeping the air inside your tire.

What if the valve stems are OK and properly capped?

If so, that's good. But there are other vulnerable areas, the seal between the tire and wheel and between the valve and wheel.

If your wheels are damaged or corroded, or if your tire beads have been damaged through incorrect mounting procedures, or if you don't have a uniform layer of proper mounting lubricant on the bead seats, you can lose air around the beads.

An old, cracked valve grommet can also allow inflation loss. Most shops try to replace these anytime a tire is mounted and demounted.

Again, what if there are no punctures, no valve problems and proper mounting?

Even then you can still lose 2-3 psi per month. The air actually migrates through the tire, especially the sidewalls where the rubber is thinnest.

How does air get through solid sidewalls?

They aren't so solid. Covering the inside surface of the tire is a thin rubber layer called the "innerliner." It may have two or three layers itself, and it's designed to bond firmly to the tire casing, yet resist tiny molecules of air passing
through it.

But it, like the tire itself, is not "solid." In fact, if you think of the structure of rubber, it's probably better to think of a very dense sponge.

Or, maybe a thick hedge around your backyard.

You mean there are lots of "holes" in it?

Think of the hedge for a moment. It would be tough for you to walk through it, but it might not be so hard for a bumblebee. It's sort of like a maze.

The bee would have to stop and change direction a lot, and it certainly would take a lot longer to fly through the hedge than over the same distance unimpeded, but it could be done.

That's how air escapes through what appears to be a "solid" layer of rubber in a tire. Of course, the spaces in the spongy rubber are a lot smaller, but so are air molecules.

What can be done about these escapes?

Better and better innerliner compounds seem to hold the most promise. That's one of the things Bridgestone engineers have been working on.

How does this new innerliner work?

Bridgestone's new innerliner compound incorporates a mineral additive that is shaped and oriented to create a maze-like path to slow air from passing through it.

A special mineral component is added to the innerliner compound. The tiny particles of this additive have a shape that's rather long and thin, and being mineral-based, they're sort of like stone walls, scattered throughout the rubber.

A special processing method helps to align these long, thin particles more or less parallel to each other. The result is that if you look at a cross-section of this new innerliner compound, you can see that it creates a sort of "maze" the air molecules must pass through in order to escape.

So the new innerliner slows down air loss. How much?

Initial results are very encouraging. Testing indicates this new innerliner can reduce inflation losses by about 40 percent.

That means our old figure of about 2 psi loss per month - simply through air permeation alone - could be cut to as little as 1 psi per month or less.

That's comparable to the kinds of results people are getting from nitrogen inflation. And it's achievable without any need for a nitrogen generator.

And if you combined this with nitrogen inflation?

The results would probably be better still. Not quite the "never goes flat" result many maintenance managers want, but a big step in that direction.

Are there other benefits to this new innerliner compound?

Any time you reduce the amount of air passing through the walls of a tire, you also reduce damage to the steel cords. Rust and corrosion are the direct result of contact with air and moisture, so the less of that the better.

And, rubber itself ages as a result of chemical reactions with the oxygen in air. That's one of the reasons nitrogen helps with tire life - besides helping maintain pressure. So, with less air penetration, rubber aging is reduced as well.

So, both the rubber and its steel cord framework last longer.

How soon will we see this new innerliner in our tires?

There is still a lot of research and development to be done. While these initial experiments are very encouraging, Bridgestone engineers must fully evaluate these new innerliners, consider any possible tradeoffs and devise ways to mass-produce the necessary material.