What's the most common reason a tire is scrapped?

If tires are well-cared-for, it will probably be wear-out after many miles of use.

If they aren't, it will probably be poor inflation pressure maintenance.

Proper inflation gives tires the right shape under load, minimizing stress for long wear, optimum traction and superior retreadability.

How is that?

Mostly, the problem we see is underinflation.

Underinflated tires (basically the same as "overloaded") flex too much as they turn. That creates heat - the enemy of tires and casings.

What damage does heat cause?

Heat can cause rubber to de-vulcanize, turning it back into the soft, sticky substance it is before curing. Too much heat, and belts can separate from the casing. Most roadside "alligators" probably result from running tires underinflated, not a retreading failure. All that flexing can also fatigue steel cords. Many tire professionals believe this is the principal cause of the dangerous "zipper" rupture.

Many tire experts believe low inflation pressure can overstress steel cords - which eventually break - leading to "zipper" ruptures of sidewalls.

How is that related?

To borrow an analogy from our friends at the Tire Retread Information Bureau, an ordinary paper clip, properly used, would probably last forever. But straighten it out and bend it back and forth a few times, and it's easy to break it. That's what probably happens in a zipper. Cords are flexed excessively for so long that they eventually break, one after another, opening a giant sidewall gash.

Can't inflation pressure also be too high?

Of course. You should never exceed the maximum listed on the tire sidewall, but if you habitually run tires at higher pressures than recommended, you may have more impact breaks, especially on bad roads.

With inflation pressures that are too high, tires can be too stiff to absorb shocks from road hazards - like potholes - resulting in impact breaks.

What's the reason for that?

With higher pressure, tires are stiffer, and cannot "give" or "roll with the punches." That's why tires for off-road service are specially made to flex - so they can absorb shock that could damage the tire.

Can a tire just fail for no reason?

We doubt it. We've seen tires that took an impact, but didn't fail until after many more miles on the road. The shock was not quite enough to destroy the tire, yet more than enough to weaken it. When someone says there was no reason, there might have been a very good reason, just a time delay between it and the result.

This tire was damaged during the mounting process, perhaps by being forced onto too large a rim, inadequate lubrication, or careless use of tire mounting equipment.

Why do new tires sometimes end up in scrap piles?

Sometimes, they are damaged during mounting. Using a good quality, water-free, petroleum-free mounting lubricant is essential.

So is matching rim size to the tire. Improper lubricant, trying to force a tire onto too large a rim - or just plain carelessness - can tear or kink beads, rendering the tire useless. And bead damage is often impossible to repair, so the tire can't be retreaded.

Tires can even turn into scrap - while standing still.

How does that happen?

Exposure to contamination. Let tires stand in a puddle of diesel, and rubber can be damaged beyond recovery. Or, leave stacks of tires in the sun - or store them near an arc welder - allowing UV light to deteriorate sidewalls.

When tires aren't in use, they should be kept in a cool, dry, dark place, away from solvents and fuel.

This tire was probably at the bottom of a stack, sitting in a small puddle of spilled diesel fuel. It has a strong petroleum smell, and the sidewall rubber is spongy and ruined.

Can these tires be repaired?

If cords are exposed for any reason, chances of successful repairs drop, especially if there are signs of rust.

Why is that?

You never know where rust has spread. It's a bit like a leak you notice on your ceiling. Where you see the water dripping is rarely where the water is coming in. Moisture can "wick" along tire cords, causing destruction where you can't see it.

And badly done repairs can cause more damage than they cure.

Incorrect repairs can allow air and water to get at steel cords, damaging them beyond repair.

How can a repair cause trouble?

We'll discuss repairs in a future issue of Real Answers, but here are a couple of examples: A tire that is plugged, but not patched (or patched, but not plugged) can allow air and water to get to cords, causing rust.

Rust is weaker than steel, doesn't adhere well to rubber, and takes up more space, forcing the rubber that surrounds it apart.

A bad repair can also fail to seal properly, creating a leak, resulting in underinflation.

Should we avoid repairs?

"Anything worth doing is worth doing well." Repairing tires is not a job for amateurs. If you don't have properly trained people and properly equipped facilities, you're better off sending them out.

And please, never use a plug or string repair alone, not even on a temporary basis. It might save a few bucks, but think of what you're risking.

Editor's Note: This time, we looked at "mis-maintenance." Next time, we'll see some results of "misfortune."