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Thursday, November 21, 2019

Are sidewall cracks the cause of tire failure?

Read an RV forum post about sidewall cracks. From the picture, it appeared the tire had what I would call Ozone or UV cosmetic cracking. The owner was concerned about the tire durability as he had suffered a couple "Blowouts" previously.  I responded....

"Not being able to inspect the "Blowout" tires or even see some pictures, I have no idea for the reason for the tire failures as "blowout" is not specific enough to suggest a possible cause. They might have been run for a few miles while losing air. They might have suffered impact damage 10 or 100 or 1,000 miles prior to the ultimate failure. They might have been run overloaded or underinflated or over speed recommended by tire manufacturer or tire industry engineering guidelines for thousands of miles.

Yes, tire failures can be expensive if the failure is only discovered by operating the tires at speed and not during the regular detailed close inspection by a "qualified specialist" as outlined in the Michelin Tech bulletin on RV/Motorhome tire inspection.

It is also true that sidewall cracks, in my professional experience, are not in themselves the root cause of tire failure. Just as a person developing a temperature of 101F or 102F is not the cause of an illness but is just an external symptom. Tire sidewall cracks are an indication of extensive tire age with the cracks developing due to time, temperature, flexing and exposure to Ozone and UV. All of which are detrimental to tire life.

"Zipper" sidewall failures of steel body ply are the result of fatigue from operating for miles when significantly underinflated or overloaded or a combination of those. Sidewall crack inspection guidelines published on pg 8 of THIS guide, suggest a maximum depth of about 2/32" on large radial tires (22.5" sizes) which usually have rubber thickness closer or thicker than 0.10".

Tires "Fail" for two basic and different reasons:
1. Sidewall Flex failure from low inflation/High load
2. Belt separation from long term rubber degradation due to excess heat and age

I covered these in two separate posts in 2012  and again with a slightly different focus in 2014.

While I can conceive of someone running tires for many miles with sidewall cracks that get deeper than 2/32", those cracks would need to penetrate deeper and get completely through the sidewall rubber to the depth of the steel and then water would need to be introduced and enough time pass to allow a significant portion and number of steel cords rust and be weakened before a "Zipper" like failure could occur.

The above would, in most cases, take many months of operation and would require that no or improper or incomplete inspection take place before a tire would suffer a catastrophic failure.

So the bottom line is two-fold.

1. Ensure you are not overloading your tires and that they are ALWAYS inflated to what is needed  for your application by running a TPMS

2. Starting at 5 years (3 years in trailer application) from the DOT manufacture date molded on the tire sidewall and annually or every 2,000 miles whichever comes first, have them inspected.  I have covered the topic of inspection previously.


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