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Wednesday, May 25, 2016

Question on Interply Shear on trailer tires vs tow vehicle tires

A question from a reader of a post on a trailer forum

"Tireman, your concerns over shear puzzle me. G614s are LT tires as are the tires on my 2500 pickup. Only difference is G rated vs E rated. The truck mfg recommends 60 front 75 rear with 80 psi max on sidewall. Why is there no concern about shear on the front tires of my truck. It seems to me the frequency of shear forces is much greater on the truck than the trailer."

Ya, I understand your confusion
 But the reality is that when computer analysis is used to look at the internal structural loading, the fact that tow vehicle tires are all operating at very low "slip angle" (difference between travel direction and angle the tires are pointed to) is significantly lower than for tires on a trailer.

The reason for this is that the center line of of tire rotation for the tow vehicle tires points to the center of the radius while the trailer tires, especially on tandem axle trailers, does not.

This translates into a higher slip angle which means higher internal structural twisting forces on the belts. The computer model suggests 24% higher on the TT tires than TV tires even if all tires were the same with identical vertical load and inflation.

TV front tires have "Ackermann" alignment designed into the suspension but TT have no allowance other than bending of tires, springs, spring mounts and bushings but the forces to bend the springs etc have to go through the belts of the tires.

This is a MAJOR reason for travel trailer tire life to be much shorter than motorhome or tow vehicle tire life.

Hope this helps folks understand a bit of what makes tire engineering a challenge.

Editor: Here is an earlier article Roger wrote about "interply shear," if you want more information.

Tuesday, May 17, 2016

Tire Dry Rot is a misnomer

I see the term "Dry Rot" used by many in the RV community when describing old tires that have visible external cracks.

Some have attributed this to the dry air in the Southwest part of the US. Some with specific references to Arizona.

Sidewall cracking occurs for a number of different reasons but these reasons all end up at the same place. The elastic properties of the rubber have been degraded over time and when the rubber is flexed it cracks rather than stretches.

Some reasons for the loss of flexibility or "stretchyness" can include exposure to UV or Ozone or simply old age. Each of these items is different but they each attack the bonds that exist between the various  materials such as rubber, sulfur, carbon black, oils waxes and numerous trace materials used in the process of manufacturing rubber. When the chemical bonds break or "crack" the loads in the rubber get transferred to the nearby material which then has to resist the forces trying to stretch it.

These cracks may start out at the molecular level but they do not repair themselves so they can only  continue to grow. Eventually they are large enough to be seen on the outside of the tire and if the tire is exposed to the damaging elements long enough the cracks can grow large enough to allow air to escape or for tire components separate.

I have previously discussed the way increased heat can actually accelerate the aging process of rubber in posts about direct exposure to sunlight and that the use of white tire covers can reduce the accelerated aging process.

For those interested HERE is a report issued by NHTSA on tire aging. You will note that on page 3 of the report titled "Background" they identify that "degradation is accelerated with higher temperatures",  You may also note that there is no mention of UV as a significant contributor to the aging process.

IMO Sidewall cracking is in itself seldom more than a cosmetic issue. However it can be an indicator of possible "old age" and degradation of the internal structure of a tire. Maybe a good analogy is when you run a temperature.

I do not recall ever hearing of someone having a temperature for no reason. Your elevated temperature is almost always an indication or symptom of some other medical problem that needs attention.

Since the consumer has no good, low cost way to learn the condition of the tire structure you are confined to looking at various symptoms.  Spotty tread wear is one symptom. Tread and/or sidewall snaking is another and of course sidewall cracking can be another.

Bottom Line
Tires do not actually suffer from "rot" as one might see in a piece of wood or some old food. They can have signs of surface cracking but as long as the cracks are shallow and a tire dealer has completed a full inspection of a tire and said it was okay to run I would go with the dealer finding.

You might want to review my post on How do I inspect my tires and note that signs other than just cracking can be much more telling than just sidewall cracking.

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