Good question - but I don't know that anyone knows a definitive answer - otherwise there would be a consensus that does not seem to exist. I WOULD BE INTERESTED IN OTHER OPINIONS, but here's my 2 cents worth.
Some small relatively uniformly distributed checking MAY be a normal sign of age and not to worry - thats what I was told by a dealer about my then 4 yr old Michelin Hydroedge tires about 10 years ago. I ran them another year and replaced them when the edges of the tread started showing little stress marks. As to tread, they would have easily gone another 40k miles but I felt tread compound had aged / hardened to the point I was uncomfortable.
Second experience - As to the then 4 yr old OE Continentals on my 2005 F250 that started throwing small chunks of rubber, the rubber tread compound was distinctly hard. (Maybe that is normal for truck tires, but the Goodyears, Michelins, and even Continentals on new vehicles seemed much "softer" to my fingernail.)
I think the hardness of the rubber was the real problem. Once upon a time I did race tires and we had a durometer - an instrument with a small moveable point that measures rubber hardness. I got used to doing a crude fingernail test on my street tires just as an exercise - if you can leave an imprint or dent on the edge of a tread block, however brief with a strong fingernail push on corner of tread blocks, I considered them "good" - the longer the dent stays visible, the softer the tread compound.
In the case of my Continentals - they were so hard that I could not dent the rubber. Add to that the fronts were chunking more severely and I assume (possibly incorrectly) that the squirm of steering maneuvering a trailer combined with the hard rubber contributed to the problem. The Continental truck tires on front of the trailer eventually did the same thing but that took about 8 years - ditto tread hardness.
One of the sure signs of a problem is a set of well defined and local cracks - typically in the sidewall at the bottom of a parked tire - the result of sitting in one place for a long time, aggravated by low air pressure and sunlight / heat. Personally I would not trust that tire at speed.
Also I would look for more defined individual cracks sometimes seen in the bottom of the tread grooves. Or I would look for any set of more distinct cracks compared to a background of relatively uniform small checking pattern. To me those seem to be obvious signs of age - BUT does that indicate they are unsafe??????
Any engineers currently working in the tire industry - please respond.
Ed
On 3/20/2014 12:12 PM, Ronald Kurtz wrote:
I enjoyed the recent correspondence on tires for ours cars. After reading through this, a question came to mind since these tires could be on some of these cars for years (mine included). What are the signs one needs to look for the to determine if a tire has outlived its useful life and could fail? I was told some time ago small cracks in the sidewall was something to look for.
