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. 




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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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#rvt743 

What do do with your old tires?

Most informed folks in the RV community know that our tires will normally "age-out" before they "wear-out" so they are confronted with spending a bunch of money on new tires and sometimes even being charged by the dealer to take the old tires.

The other day we had some service done at home and I noticed the truck had 14 tires on it and they were all 22.5 size but they were not all the same size ( same on each axle but different between axles)

Now obviously this truck is used for local short haul service with probably a lot of relatively sharp turns in and out of driveways that result in rapid tread wear and a good portion of the time at less than max load.

Just something to consider when confronted with the question of what to do with your old tires. IMO there is a good chance companies like this might be interested for the right price.

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Which brand tire is better ? I can set up a test if you really want the answer.

Many people ask the question in various RV forums "I need new tires. What is the best brand tire I can get?" or "Why do tires made by company Q seem to have a high a failure rate?"

Invariably the thread quickly devolves into people providing their opinion but I do not recall ever seen the suggestion that one brand is better than another supported with any data based on direct apples to apples comparison of two brands of tires.

I believe there are two reasons for not getting a clear picture of which tires provide better service in RV application: One reason is personal pride and the other is a lack of direct comparison based on equivalent exposure to service problems.

Personal Pride
I think you will find that many posts IMO are based on the concept that people do not want to consider the possibility that the product they bought isn't the best. To think so could be an indication that someone might not be making the best purchasing decisions. For example, if someone asks about Acme brand anvils, and someone just happened to have purchased an Acme anvil they are more than willing to "badmouth" anvils made by almost any competitor even when they lack either meaningful data or solid experience to justify their position. After all why would they admit to just having purchased a good quality anvil?

Direct comparison of tire performance.

You may not be interested in ST type tire but I would hope that if you review this information it may help you make a more informed decision when asking what brand to buy.

If you consider that the OE market in ST type tires in RV trailer application is dominated by Goodyear and many dozen 3rd or 4th tier tire companies who make tires in small lots and don't even apply the name of the tire company to the tires they are making. With GY having a majority of the market and no other brand having much more than 5% I think you can see how it easy to end up with skewed impression that GY tires are bad, based simply on the observation that the greatest number of complaints are about Goodyear ST type tires. This is much like the concept that tires made in China must be poor quality because so many trailer owners that have problems with their tires discover they are made in China. I have to wonder how people expect to be able to make a reasonable comparison of tire quality based on country of origin when probably 90+% of RV trailers with ST type tires come OE with tires made in China. Wouldn't it seem reasonable then that 90+% of problems experienced on RV trailers with ST type tires would have tires that were made in China?

Now in 22.5 rim diameter tires found on Class-A RVs,  I think we see Michelins with sidewall cracking complaints and Goodyears with irregular wear complaints and since those two companies probably are on 70+% of coaches of course those are the ones that would have the most and therefore most noticeable complaints. Even if another company with maybe 5% of the market had tire failures  at twice the rate of GY or MI cosmetic issues, it would still appear that GY and MI are the only tires with problems.

Check out my post on Causation vs Correlation for more on the topic.

Thorough tire testing costs in the neighborhood of $30k to over $100k per brand for just basic outdoor wear and general durability is it any wonder that no independent agency has stepped up and run such a comparison?

Now I have the contacts in the tire industry to an independent testing company that could run such a comparison. All I need is for people to send me contributions - maybe in $500 increments and I will develop the specifications for a direct comparison between Goodyear ST type tires and Maxxis brand ST type tires plus a set of TowMax or similar ST type tires. I would use a popular size and load range made by all three companies. To keep costs down we would limit the test duration to about 10,000 miles or till there are enough failures to discontinue running that brand. I figure two failures would be enough to stop running a given brand.

So do you really want to see actual data? All I need is $35,000 in the bank to start the evaluation. Test results would be sent only to those making the minimum $350 contribution or more.
If interested send me an email and I can send you specific details of this offer. We only need 100 individual contributions to get started.


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Tire Temperature always seems to be a hot topic

From a couple of RV forum threads.

Hot tire temperature is important but the problem is in learning what the actual temperature is.
The average temperature is not a good indicator and that is about all you can get by reading the Contained Air Temperature. While many seem to believe that their external TPM is giving them a reading of the CAT, I can tell you that the TPM will always read significantly cooler than the CAT and that the CAT is always cooler than the hottest and most critical location in the tire.
Rubber is a good insulator and does not transfer heat from the hot spots to the cooler locations very well. As a result you can have a tire failure from extreme heat in one location while another location only inches away can be perfectly OK.

I read that many TPMS have set the high temp warning point at about 158F. That is all well and good if you are measuring the actual CAT but I do not know how an external TPM will sense that temperature especially if it is out in the cooling air stream on a metal stem or at the end of an extension hose

I would always be more concerned about the pressure measurement as that is not going to be affected by cooling of the valve stem or extension hose. We know that for our purposes pressure does follow the "Ideal Gas Law" and it doesn't matter if you use air or Nitrogen the results are essentially the same unless you are measuring your pressure to the 0.1 psi.

So knowing that tire pressure will change by about 2% for each change of 10F in temperature of CAT we can get a reasonable estimate of the CAT and will note that out TPM is showing a lower temperature change than indicated by the pressure change.
I would not use the temperature reading from a TPM to set tire pressure. If I got a high temperature reading but the pressure reading was reasonable I would suspect a mechanical problem such as bearing or brake issue.

BOTTOM LINE
A motorhome properly loaded with the inflation set based on actual measured load on the tire and consulting the load Inflation tables will not result in an over-heated tire. NOTE Trailers should use different method to set pressure due to the Interply Shear issue of trailer axle alignment and should set the cold inflation to the tire sidewall pressure.

If the above isn't enough detail for you you can review the other posts that have Temperature as part of the topic.

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