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Friday, January 21, 2022

Question about LTX vs. Agilis line of Michelin tires

 Read a question on an Airstream forum:  

 I am replacing Michelin LT225/75R/16/MS2 tire on my 28 Serenity. I am confused ? I am told by the tire dealer that the Agilis CrossClimate has a rough ride. I also read that the LTX tire in not enough for towing. Can anybody help staighten me out ?? Thanks


Load capacity is not a function of the tire "line" but of the type (P or ST vs LT) and Load Range LR-C  LR-D vs LR-E or just XL.


Do not get confused by the "LTX" line as there are both P and LT "types" and there are different Load Range tires in the LTX Line but no ST type which might be what came OE on your RV. Check your placard / certification label.

You should know your actual scale weight of the RV when fully loaded from getting the numbers on a truck scale. Your RV also has a Certification Label (Out side on Driver side, toward the front) that tells you the GAWR.

What ever tire you are looking at it should be capable of supporting AT LEAST 110% of the GAWR with 115% being better, and of course your scale weights should be below the GAWR.

Reports of "Rough Ride" on the Agilis might be from SUV owners with sizes you are not going to run. I would be surprised is you can feel any "rough ride" with a properly balanced set of Michelin tires.

You might want to read my post for a bit more information.

https://www.rvtiresafety.net/2021/08/my-first-recommendation-for-brand-size.html 

 

##RVT1036

Friday, January 14, 2022

How old is too old

 I read a comment on an RV motorhome forum:

"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 guide lines. 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 gal 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. Do you think that all 27 basic components age at identical rates?

Most tires fail for one of a couple reasons. You should have read THIS post if you searched on "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, AZ will fail earlier than tires that spend most of their time in ND, MI, NH, ID, OR. But a tire that spends it life in Phoenix will "die" in maybe 4 years an identical tire that spends it's 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 LT and another 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 TT 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 re-applied 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, even not visible on the internal air chamber surface.

##RVT1035