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Thursday, August 8, 2019

How old is too old ?

For RV application this is a tough question, because "It Depends".

Primary factors that affect the answer include 1. Type of RV -Motorhome, tandem axle trailer or single axle trailer.   2.Cumulative temperature i.e. How much time with tires below 60F  Below 70F,  below 80F  below 90F  Above 90F, Above 100F, etc up to Above 140F     3. Actual Reserve Load   4  If a trailer, how many times you backed into a parking spot  5. Do you use white tire covers or is the RV parked in full shade?

Ya, a lot to consider but each of the above items can shorten tire "life".  Would it be possible to construct a formula or spreadsheet with answers to the above to calculate the answer?  Yes but I doubt it would be worth the effort as the life of one brand or size would probably end up with different detailed factors. So let's just consider some of the items I listed so you may gain a better understanding of "tire life"

#1 Type of RV. This is where "Interply Shear" (a force that is trying to tear the belts off the body of a radial tire) Based on computer simulation this shear or tearing force is 20% or greater in tires applied to multi-axle trailers than if the same tire was applied to a motorhome, even with identical weight.  For those that care there are a number of posts on IPS on my blog if you really want to understand the background and how this force affects tires.

#2 Heat. This is the number one "killer" of tires if we set aside the obvious failures due to loss of air pressure. In fact, even the loss of air pressure results in excess heat that can lead to rubber reversion or even melting of body cord which leads to many "Blowouts".  Heat damage is cumulative and has a MAJOR effect on tire life. The reason for this is that rubber is always "curing". In technical terms, this means the molecules of Carbon and Sulfer are continuously linking, although at a slower rate when relatively cool 70F and below and the rate of chemical reaction doubles with each increase in temperature of 18F. I have a number of links to Scientific papers on the chemistry of tire curing in THIS post. It is also important to understand that the hottest area of a tire can not be measured with a heat "gun" or even with TPMS. Here is a graphic showing the relative temperature in different areas of a tire. The internal red area can be 20°F hotter than the rubber just 3/8" away and the range of temperatures in this example is 80°F so measuring at the correct location is critical as well as very difficult as it takes specialized needle probes.
 Also, this is the location of the highest level of Interply Shear.




#3 Reserve Load  This is the difference between a tire's load capacity at the cold inflation number in the tables vs the actual measured load on a tire.  Greater reserve load means cooler running temperature. See #2 above

#4 Backing into a parking space. This is something few think about but in extreme cases, the interply shear might get to 100% higher in a tandem axle trailer when doing a tight turn into a parking space. This high tearing force can initiate cracks in the rubber which can just grow and they never "heal) which can lead to belt separation many hundreds or thousands of miles later.

#5 Tire Covers.  See my post on using white tire covers.  Yes, this simply goes back to cumulative heat. See #2 above.

Long term readers of my may remember my post on how to maximize tire life.

One thing that is true is that Michelin and some other tire companies have set 10 years at the maximum life of a tire no matter what it looks like. This does not mean that no tire will fail due to "age" or the cumulative effects of the conditions I have outlined above. All of these conditions simply are shortening tire life from 10 years down to maybe as little as one or two years in extreme cases.

##RVT909

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