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Friday, June 24, 2022

Do you run at or above engine Redline? Why Not?

 OK As a tire design engineer and someone with RV tire experience let's see if I can clear up some of the partially correct and partially misunderstood information we see posted almost daily on one or more of the Internet RV forums.

If you have a Load / Inflation chart for "ST" type tires you will find that ALL ST type tires follow the same information. A chart published by company X may not have all the same sizes and Load Range tires as company Q but when both X and Q make the same size and the same Load Range the load capacity and the required inflation for the tires is identical.
The same applies to "P" and "LT" type. Now we need to pay attention to some other details on tire "type". Some tires made outside the US are not marked with a P, LT or ST before the size numbers. This means that they were made to some different standards. They may have been designed to comply with European or Asian standards to their numbers will be close but not identical to the US standards.  
The inflation number in the chart is the MINIMUM inflation required to support the stated load or to put it another way the LOAD in the chart for your tire is the MAXIMUM load the tire can support when inflated to the specific inflation found in the chart.
Some larger sizes with 19.5 or 22.5 size wheels make things a bit more difficult as some follow the US Standards while others follow European standards but there is no leading letter to help you know which so if you run tires of these sizes it is more important that you learn the Load capacity for the different inflation for these tires directly from the company that made those tires. The differences are not great but they are not identical so you can be off by 5 to 10 psi or maybe a couple hundred pounds load.
RV companies are required to select tires that can support the GAWR but trailers have different "assumptions" than do Motorhomes. For trailers the US DOT assumes a perfect 50/50 split of the load between two axles or 33% each when there are three axles on a trailer and also a perfect 50/50 side to side load split on any one axle. Neither of these load splits are realistic as those of you that have had each tire positions measured know that many axles have end to end load splits of 48/52 to 40/60%. Individual axles on multi-axle trailers can also have significant variation away from a uniform load split.
Motorhomes also have similar problems with the unbalance of each end of each individual axle. Engine placement and dual tire position on the rear axle require paying attention to other details when consulting Load &Inflation tables as "Single" load capacity is always different than when tires are is a "Dual" or side by side mounting as seen on most motorhomes.
Size and placement of water, fuel, propane, and holding tanks can result is 500# to 1,000# being in different corners of different model or different year motorhome so that is why you can't simply ask the RV owner parked next to you about proper tire inflation numbers.

The inflation number on most Certification labels is the MINIMUM inflation needed to meet the above requirement.

A complicating issue is that in 2017 RVIA (Gold oval sticker on the side of most RV trailers) added a requirement that the tire inflation be sufficient to support 110% of the GAWR. This partially addressed then known in-balance found on almost all RVs.

All the above ignores the advantage of having some "Reserve Load" capacity to allow for variation in loading of the RV that can occur if you carry more water than normal or load more "stuff" than normal etc. It also ignores normal engineering practice of not designing components to have some level of margin of load capacity. or for the occasions when trailers are towed at speeds above the tire design limit of 65 mph operating speed.
You can think of running at the max load capacity like running your TV right at the engine Redline. If you think that is a good practice to run with no reserve load then I guess you also must think it OK to run at engine redline for hours on end.


Friday, June 17, 2022

Tire Recall in the news. Why aren't there more recalls?


I have seen this question on an RV forum. It was asked by someone complaining that there were no recalls of what he considered "crappy" RV tires. Other posts in the thread went on to say that complaints to the BBB or the tire importer won't accomplish much. I posted a reply pointing out that expecting the National Highway Traffic Safety Administration (NHTSA), that is part of DOT to recall tires when there had not been sufficient, usable complaints filed, was simply unrealistic.
NHTSA is the government agency charged with the responsibility of writing and enforcing the regulations necessary to achieve improved safety of vehicle systems and equipment. However they cannot order or even suggest that a vehicle or component be recalled without facts and data being collected and analyzed.

 A while ago I worked with a reader of my RV Tire blog, John B., who understood the necessity of providing the information NHTSA needs. He had suffered three tire failures. Luckily he discovered the failures before the tires had complete detachments. In his case there was no loss of air and no flailing of tire pieces. What he did have was a tire that was no longer round or having a uniform tread contour.

Now lets be sure we all have the same understanding of the terms. In this case a "Detachment" would be when a part of the tread or tread & belt package came apart from the rest of the tire. This type of failure can result in damage to the RV as pieces flail around hitting fenders and the side and undercarriage of the RV.

    John wanted to file a complaint with NHTSA and he wanted to be sure his complaints would be useful to the engineers. He understood that partial or incorrect information would result in no investigation and with no investigation there was no possibility of any action being taken to remove "crappy" tires from use. So John contact me and I walked him through the process of collecting all the details needed. He also wanted to  dissect his tire so he could ship the important parts to me for further examination.

When I received the sample I first cut the tread in the locations John had identified but found no serious issues.

I then called upon my 40 years of experience and took the time needed to closely examine and take measurements with special tools to identify a location that was more probably of interest. After cutting the section at the location of interest and found the separation between the belts that was almost all the way across. This separation allowed the tread area to bulge out to the shape seen in the picture of the tire at the top of this post.
For those interested these tires were not made in China as we decoded the serial and learned they were made in Mexico.

With the physical examination complete, John was able to file the three complaints with NHTSA. Now it is important to remember that NHTSA has budget constraints so investigations need to be prioritized. Obvious defects that result in physical injury would receive top priority. Also a single or small number of complaints will be of lower priority than a large number so if the only complaints NHTSA receives on these tires are the three from John there may not be any action. The same situation would apply to any complaint you might file BUT it is important to remember that if the majority of people with tire problems only post to RV forums or grouse to others around the campfire nothing will ever happen or result in the quality of tires improving.

Here are Links to John's information. Link 1     Link 2

A quick review of the complaint on file with NHTSA will show that the majority are of little or no value to NHTSA as the owner didn't provide the crucial information of a correct and complete DOT serial. Many complaints don't even provide the tire size or even the correct tire brand. I believe that if people spent half the time they do on RV forums but provided complete and accurate information to NHTSA we might all end up with better quality tires on our RVs.

If you have a tire problem you need to collect the facts - Size, Brand, DOT serial and collect some good sharp pictures in case NHTSA needs them. Then make the effort to file a complaint. Who knows, you might just be able to grab the interest of the engineers and have an investigation started.



Friday, June 10, 2022

What margin should be used for Low Pressure warning on TPMS?

 In response to a reader from Australia with a question on TPMS Low Pressure Warning level, I responded...

We need to separate the settings specified by US DOT for regular passenger car applications and the more specialized world of Caravans (RVs for us Americans) (both motorized and pull behind).

DOT is trying to protect the worst case which is operating a tire in overload. Most cars have their tire inflation set based on the goals of the manufacturer to meet fuel economy goals while delivering acceptable ride & handling the expectations of the consumer. With our current Federal fuel economy standards, this results in fuel economy being #1 or very near the top of the priority list. This means that almost all tires have sufficient pressure to significantly exceed the load capacity requirements. I have seen some data that shows +30% is not an extreme level. So that means a loss of 25% of air pressure still keeps the tire out of the "overload" situation.

Now when we move to the Caravan market, there are no regulations for fuel economy. So marketing pressure of low cost takes priority. This means the RV manufacturer selects the lowest cost tires possible. While in many cases this means tires that just barely meet the DOT safety requirement tests, it also means the smallest tire size sufficient to support the load as this translated to smaller (lower cost) wheel well space and smaller (lower cost) wheels. This results in most tires needing to specify higher inflation (more load capacity) than if the tire were applied to a car. The bottom line is lower levels of actual reserve load capacity with data showing that the average user ends up overloading the tires or axle or both.

I am a strong supporter of tires & inflation that provides at least a 10% reserve load for motorhomes with towables goal of 15% reserve load being needed to address the Interply Shear forces seen in multi-axle towables. These are what I would consider MINIMUM reserves.

To achieve these smaller margins we need to recommend the Low Pressure setting of TPMS is closer to 0% air loss from the cold "set pressure". If we were to allow a 25% air loss before the TPMS Low Pressure sounded, the owner could have been driving hundreds of miles in an overloaded tire condition.
Here is a post on how I set the TPM system on my Class-C motorhome. 


Friday, June 3, 2022

Why do ST type tires have more pressure gain than LT type tires?

 The main reason for ST type trailer tires to gain more pressure than the LT tires, is that they are forced to support more load relative to their size & inflation than LT tires are.

If you ever look at the Load & Inflation tables and find an LT and an ST type tire of identical physical dimensions you will see that the ST tire is “rated” to carry more load than the LT type tire is. The basic theory behind that increased capacity is that ST tires will be traveling slower because people should not be traveling as fast when towing as when just driving the car or truck. When ST tires were introduced they were limited to 65 MPH MAX in an effort to offset the damaging effects of higher loading than seen in LT tires.

Tire load capacity is basically a function of volume and pressure as seen here Load = K x (Air Volume x Air Pressure) with different type tires having different “K” factor. Tires in LT applications are required to support lower load as a percent of their volume and pressure so they do not have to “work”as hard so they do not generate as much heat.

More heat means a greater increase in pressure. (Approx 2% pressure increase for each increase in temperature of 10F)

Please note that the actual load calculation is much more complicated as the response to air pressure is not linear and different aspect tires i.e. 75 series vs 85 series etc have some different factors that are applied to the actual calculation.


Please note that the above is just one page of many used to calculate load capacity for tires not already published in the TRA Yearbook, so do not go and try to check every size. I show this to demonstrate that the calculations are complex.