By Roger Marble
I read this comment on an RV motorhome forum about tire life:
“I continue to read posts indicating that tires will calendar out in 5 or 7 or even 10 years. After some searching I have yet to find a technical document regarding tire life expectancy. I’m not interested in sales brochures or salesmen guidelines. I’m looking for a document written by a subject matter expert in the tire manufacturing industry. IMO, tire life is more a function of proper storage, inflation, maintenance, environment, and care. I believe a physical inspection by a tire professional is more reliable than an arbitrary date.”
So I offered the following:
As an actual Tire Design Engineer with 40+ years experience, I believe I can provide some information. But if you are looking for an answer such as 62 months, 2 weeks and 5 days you are out of luck. It’s just not that simple.
Maybe you can tell me, in hours, how long a gallon of milk will stay “good”. Since you know you can’t answer that simple question, why do you think the answer for a complex structure such as a tire age should be that simple? Obviously all 27 basic components don’t age at identical rates.
Most tires fail for one of a couple reasons. You may have read THIS post if you searched for “why tires fail.” I have also covered this in detail in a number of posts on my blog “RV Tire Safety“. But we can cover the topic again.
Polymer Cross Link Density is the property that determines how flexible a piece of rubber is. If it is too flexible or elastic it will not hold its shape. If it is not flexible or elastic enough it will develop microscopic cracks. These cracks will grow with every revolution of a tire and also some will grow simply given enough time.
Heat and time will change the cross link density. The rate of change is not linear but doubles with each increase in temperature of 18°F. This means 4 times if it is 36°F hotter or 8 times if the rubber is 54°F hotter. This is why tires that are on RVs that spend most of their life in Southern Tier states like FL, GA, TX, or AZ, will fail earlier than tires that spend most of their time in ND, MI, NH, ID, or OR. But a tire that spends its life in Phoenix will “die” in maybe 4 years, while an identical tire that spends its entire life in Flagstaff may live to 8 years. But there are other factors that can have significant effect on tire life.
If we were to put a set of tires on a light truck and an identical set on a 4-tire trailer, then load all 8 tires to identical load and inflate to identical level, the tires on the trailer may have a life that is 25% to 50% shorter than the tires on the truck. This is due to Interply Shear, which is the force in all radial tires at the belt edges that is trying to tear the tire apart from the inside. (See “Interply Shear” if you want to see my posts on that topic.)
This post is on a Class-C thread, but I can guarantee that information published here will be incorrectly applied to information on a travel trailer thread because people do not understand the significant different forces internal to a tire.
Michelin has published a guide on tire inspection which basically suggests at 5 years the tire be inspected inside and out annually and reapplied if no problems are discovered. BUT they still put a MAXIMUM life on the tire, as unless you have “X-ray” vision, the structure can have cracks that are not visible on the tire surface, and even not visible on the internal air chamber surface.
Have a tire question? Ask Roger on his new RV Tires Forum here. It’s hosted by RVtravel.com and moderated by Roger. He’ll be happy to help you.