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.
Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.
##RVT1035
Well described, thank you Roger for your consistent reporting. I store my Class A (with Michelin tires) indoors and with much of the weight transferred to the deployed leveling jacks. I also “exercise” the MH on a 1hr road trip each month. Replace the front tires every 5-6yrs, and the rear tires every 6-7yrs.
One thing I didn’t see mentioned is that older tires get “hard” and lose some traction ability over time. I’m with the 5 year rule also. I would also suggest going up one rating class when you buy new tires for the first time.
I assume you are talking about Load Range when you mention”rating class”. The Load Range letter system i.e. LR-C, LR-D, LR-E etc replaced the old “Ply Rating” numbers i.e. 6, 8, or 10 ply rating. People need to remember that you get the load capacity from the inflation, not the tire construction so if you go up in LR you will need to go up in your inflation or you will get no benefit of the change.
I have chosen to stick with a 5 year replacement cycle. The difference between a 5 year cycle and a 6 year cycle is one extra set of new tires in 20 years (4 sets in 20 years 3.5 sets in 6 years. For the safety of my wife and me and those on the road around me well worth the extra cost.
In my opinion “inspection by a professional” annually after 5 years is a losing game. Why? Because the inspection needs to be from the inside and the outside, not just the outside while mounted on the vehicle.
Jack it up safely, dismount the tire, break the tire off the wheel (rim), closely inspect the tire, clean the rim, remount the tire on the wheel (rim), and remount on the vehicle. And a balancing if needed. Then repeat for each tire. That’s a lot of labor multiplied by the number of tires being inspected to save on the price of the tires.
And the inspection will have no guarantees by the company that performed it.
And what does “5 years” even mean? 5 years from the date of manufacture? The Michelin motorhome tire manual says after 5 years of being in service. When does a tire go in service? When it’s first mounted and filled with air? When it’s first driven on regularly? I dunno.
But to each their own.
Hey Roger…I just finished reading your article and am now convinced that it is time for me to replace tires on our class C. While we only have 20K miles on our 6 year old Michelin tires and store our RV in a covered area, we do live in the So. Cal. desert where temps are in the 110’s consistently during the summer months. I am now convinced that our many trips across the desert to Tahoe and Mammoth have had an impact on our tire degradation. Better to be safe than sorry!!
It sounds like a reasonable plan given the temperatures in the location where you travel.
Excellent summary Roger. Was really surprised to learn of the logarithmic progression of material density that ambient heat has on tires. Also, it is easy to overlook the effect of hidden shear that occurs when making sharp turns with a 4 or 6 wheel trailer. Thank you for the article,
Nice article!
Love the milk analogy!
I am on year two of my 5th wheel, and for me I drive about 55 mph, do not overload the trailer, tires properly inflated, and keep the tires clean.
I will visually inspect all the time, but at the five year mark I will look to replace if cracks or wear appear. I mix my driving from snow conditions to hot, so who really knows how long they will last? Cost of RV’ing., another first world problem.
Thanks!