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Stress Cracks in Tires (dry rot)

My 1968 corvette sits in a closed garage all year (driven 3-days a year) The 10 year old Big-O tires have 500 miles on them but have big stress cracks in the sidewalls. BUT, the 42 year old original spare (Firestone) looks like new with no stress cracks. What’s the difference?

Driving on 10-year-old tires is risky business, regardless of the mileage.

Driving on a 42-year-old spare is insane. I don’t care how good it looks.

I’m guessing the difference is the chemical compound of the tire itself. They just don’t make them like they used to. Thank goodness.

You need to replace tires at 4-5 years as they age from oxidation and such. I’d guess that the chemical formulation of the tires is different. You start with synthetic rubber or natural rubber latex or some blend of the two, and add carbon, strengtheners, plasticizers, anti-oxidant additives, anti-UV additives, and who knows what else. Maybe there was a change in the formula that extends tread life or improves traction but causes the rubber to crack in 10 years (which is twice as long as you should be driving on them anyway).

The difference?

Cracks are caused by flexure. No flexure = no cracks.

Yup. Plus, the spare has probably never seen sunlight, so it’s been saved from some of the damaging effects the street tires faced.

But I bet the minute you try to use the spare, you’ll have another flat on your hands.

That said, keep it and the rim, and get a separate donut/tire for actual use - - It might be of interest to a collector when/if you sell it, if you can say it has the original spare tire.

[b] the 42 year old original spare (Firestone) looks like new with no stress cracks.[/b]

"Looks" is correct.  What you don't see are the micro cracks that have not been stressed and opened up enough to see, but they are there or will be in a few miles on the road.  I would not use that tyre even if I had a flat.  Hang it on the wall as a souvenir. 

 I sure would not take it out for much of a drive with 10 year old tyres.  Maybe a parade at walking speed, but not on the drive to the starting point.

The tire industry has no “shelf life” standard for tires. If stored properly it would seem the life of a tire is indefinite and determined by the apparent deterioration of each tire. Military tires have more additives to protect then from ozone and were individually wrapped when I was involved with their manufacturing many years ago. If the rubber cracks to the depth of the ply stock or tread segments become separated the tire is considered unusable. Surface cracks are acceptable, even on aircraft tires.

We are talking about a 42 year old tire. That tire was manufactured before the moon landing. The tire industry would be amazed to learn that a 42 year old tire is even being discussed anymore.

I think that you should drive the car more often. It’s a great car and deserves to be driven. Try once a month; maybe twice.

I’m gonna be blunt and say that that’s a nice life-sized Hot Wheels car you have there. I had a 65 Malibu a few years ago that I’d have driven everyday if it weren’t for snow or rain(had 1 window that didn’t quite go all the way up on the driver’s side).
But, as others have said, tires wear with age as well as miles. Replace them now and call it a day.