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Friday, July 27, 2018

Question on radial tire belt "Interply Shear" or IPS

Originally posted on Airstream forum, but the answer applies to anyone running radial tires.
"From my reading, if you see a post where the tire failed with the tread coming off AND the tire carcass still held air pressure, that is likely to be an inter-ply shear failure. It is my understanding of the issue, that inter-ply shear (twisting of the tire) breaks the bond of the tread from the tire carcass. This may start as a small section that can be seen as a bubble under the tread. Then rolling the tire expands the failed area until in all comes apart.

For what it's worth, I've seen plenty of these in various posts. I suspect it is also possible for the tread to separate from the carcass and in the process of self-destructing, punctures the tire carcass resulting in tire deflation.
Do I understand the consequences of a high level of  IPS?"

My reply:

The short answer is YES.

OK, now to the questions of IPS (nice acronym BTW).

If you have reviewed my numerous posts that mention IPS you can learn the background and the steps suggested to lower this destructive force.

All radial tires exhibit this force. It is a function of having belts under the tread that are at a high angle relative to the low angle body ply. Here low angle is about zero with the body ply running radially from bead to bead. Belts are generally in the range of 60 to 70 degrees relative to the body ply. The two belts ply or layers run in opposite direction and for the width of the tread. NOTE:  Different tire companies use a different reference for the "radial." Some call that 90 degrees and they say their belts run in the 20- to 30-degree range, but the result is the same as only your reference changes.

Do tires ever fail due to IPS? Yes, it is these forces that initiate microscopic cracks which grow over time and use. Air loss or not is not a controlling factor as air loss can occur because the belts have separated from the body, which allows tearing of the rubber between the body ply cords, which then leads to air loss. This can occur in fractions of a second so the air loss is indistinguishable from the belts and tread detaching from the body. The rapid loss of air can sound "explosive," which leads many to use the catch-all term "Blowout".


In THIS post the two PRIMARY reasons for tires to fail are covered. We are not talking about air leak here.

It is the air pressure that supports the load, not the tire construction. (Yeah, the tire does support some of the load but maybe only 5% at best, so we are discounting that.)

In general, a stiffer tire can generate higher cornering force than a tire with low inflation. Cornering force is not just from the contact area. This is well known in the racing community as our tires generally run higher pressure than we would run on the street. I know this from first-hand experience running and winning numerous road course events in my Camaro.





(6-time winner of the 24-hour race at Nelson Ledges, Lap records at 6 different tracks including Lime Rock, Mid-Ohio, Watkins Glen and others.) I ran real "DOT street tires" as required in my class, not special-purpose-built racing tires that wouldn't last 15,000 miles of street use. Those other tires were only available from race tire dealers. Most of the time I ran 34 to 36 psi cold vs. an estimated 20 to 22 psi, which is what I would have needed to simply support the actual load -- so clearly more contact area from lower inflation did not provide race winning results.


We don't need to get into the sales (price) and marketing decisions of RV companies on what size, type or brand tires they supply. We as RV owners are trying to get the best durability and overall performance from the tires we run on our RVs.

Tire durability (not coming apart) is our number one goal. You can choose to follow our recommendations or not. All I ask is that you not complain if or when you have a "Blowout" that has the root cause of the failure traced to a failure to follow my recommendations. Lowering the IPS force can be accomplished by increasing the margin between the tire load capacity at a given inflation and the actual load on your tires.

You can accomplish this with larger tires or by unloading your RV, but not everyone can do those things. This leaves increasing the tire pressure. Especially on multi-axle trailers, you need to do all you can to increase the margin, and running the inflation molded on the tire sidewall can be done by, and is recommended for, trailer owners.

Friday, July 20, 2018

Internal vs External TPMS test results

Previously I posted the test results for my comparison of internal vs external sensor TPM systems. The weather was cool to cold back in March. Now I can report the results with the ambient in the upper 80's.

All temperatures are in °F
Internal System.
RF 102   LF  104    RRO  104   RRI  111   LRI   111   LRO  102

External System
RF  73   LF  75   RRO  71   RRI   71   LRI  82   LRO   80

I was driving 65 mph with cruse on a level stretch of Interstate. The Sun was fully on the left side of the RV.

Difference  Internal - External
 RF  29   LF  29   RRO  33   RRI   40   LRI  29   LRO   22

These results tend to match the previous runs with the internal reporting about 30F hotter than the external sensors.

Again I do not consider the results of a comparison between the temperature readings to be "meaningful" in the sense that the internal sensor numbers are useless.

Suggestion:  If you have an external sensor TPMS I would continue to use that system. Just be aware that the temperature readings are probably in the range of 27 to 40 F cooler than the temperature readings your friend would be getting with their internal sensor system. If you are concerned you could change your High-Temperature warning level with your external sensor system from the factory 158°F to about 145°F.  Just be aware that if in Phoenix or Death Valley or other location where the Ambient exceeds 120°F  you may get a high temp warning. If you do just pay attention to the pressure readings to be sure you are not losing any air pressure.

Since tire temperature is also a function of load and speed you might bump up the High-Pressure warning level 5°F till you are not getting a warning simply because it is hot outside.  Of course, it might also help to slow down a bit as that will also result in lower tire temperature.

Bottom Line:
There is a difference between temperature readings based on sensor location. BUT this does not automatically make one type of system better or worse than another. As I said in THIS post I am not a fan of TPMS temperature readings no matter which type sensor you run.

Friday, July 13, 2018

Selecting alternate or replacement tires for large / heavy trailers

Found a thread on a forum for folks who own large heavy 5th wheel trailers. This info would apply to non-5'ers too.

While I understand the concern for the tire dimensions, that is NOT the most important specification.
Number one is to ensure any replacement tire is capable of supporting the load you are placing on your tires plus a margin.

The best thing to do is to first confirm your actual tire loading.
Ideally, you would get on a scale, with the RV loaded to the heaviest you ever expect to travel with, and learn the actual load on each tire as there are very few RVs with the load split evenly axle to axle or side to side.
HERE is a worksheet you can use.  You will have to do some hunting around as you can't get individual loading on most truck stop or CAT scales. You will need to find a local building supply or feed or grain dealer or gravel pit or possibly cement delivery company.

Lacking that you could use a truck scale but to be safe you need to apply some math to estimate the load unbalances.
First, assume a split of 52/48 between axles or with a three axle trailer assume one axle is supporting 35% of the total. Then assume a 53/47% split on the heavier axle for side to side loading.  Yes, some RVs have been measured with individual position scales and found 1,000# un-balance.

So with the measured or calculated heaviest loaded tire, and the dimensions checked, you are ready to shop for tires.

You need to realize that ST tires have a higher load capacity than LT type tires. This is because the load formula for ST type tires is based on a max speed of 65 mph even if the "Handling rating" speed symbol suggests differently. So you can't just use the numbers when comparing tire sizes as an ST235/75R16 carries significantly different load than an LT235/75R16  even with the same Load Range ( D or E or F etc)

You can then consult the Load & Inflation tables for the tires under consideration. The good news is that with the exception of Michelin 99+% of the tires out there follow the same table info so you can use Bridgestone or Goodyear etc for LT and Maxxis or Goodyear for ST type tires.. You can look at different tables HERE if you want.

When selecting a tire you need to get the tire capacity at least 15% greater than for your measured or calculated tire load. This allows for sway, load shift due to road crown and wind side load to the tires you are buying.

After you do the above THEN you can confirm tire dimensions knowing the load capacity needed.

On my blog, I cover why you should run the inflation number molded on the tire sidewall (lower the Interply Shear) and why you should always run a TPMS along with other info on Interply Shear and the effect of temperature on tire pressure.  You might even subscribe.

Hope this helps.

Friday, July 6, 2018

Are TPMS of much value ?

Some people still question the need or value of using a TPMS. Some claim that checking tire temperature with an IR gun or touch with the back of their hand every two to three hours is sufficient. They seem to forget that it is possible to destroy a tire in as little as 10 minutes.
Others still think they know what an underinflated tire "looks" like but I have repeatedly demonstrated with hundreds of participants at my RV Tire Knowledge Seminars that no one is able to identify the difference between 35 and 14 psi in a P type tire or the difference between 46 and 66 in a LT tire. Some folks just don't feel the cost justifies the value of an advanced warning of an impending tire failure.
Well, I ran across the following post on an RV forum thread on tire failure on an RV.

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With permission of the author:
 FYI....Be maniacal about checking tire pressure and invest in a TPMS system if you have not already.
This past week, two incidents proved that a TPMS (tire pressure monitoring system) investment is worth every penny.

Coming back from a long trip on Memorial day, my caravan friends, who did not have a TPMS on their 5th wheel toy hauler, lost pressure in a rear tire which became overheated and eventually blew on the interstate. The damage the blown tire caused to their meticulously maintained RV added up to $3000 in body work. I hear this is typical...and often at the low end of cost for damage when a tire blows. My buddy has talked about investing in TPMS...but chose not to spend the $. The insurance deductible alone for the damage was $500. Lesson learned.

A few days later, I was back on the interstate headed for a camping trip to the lake with my 5th wheel toy hauler. I have invested in a TPMS....and am so glad I did. At 65mph....all of a sudden the display on my TPM system flashed red and displayed that my right rear tire on the trailer was "leaking". I was impressed that it alerted me when the tire pressure was down only by 2 pounds...and I could see on the display the rate at which it was deflating. This gave me sufficient time to exit the highway, park in a safe place...and not suffer any damage to my RV....let alone save the tire which picked up a screw.

This device is worth its weight in gold....and I highly recommend it to anyone pulling a trailer of any kind. You will never know you a have a problem until it is too late without it. 


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There you have it -- a direct comparison of the value of TPMS vs no TPMS.