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Friday, November 29, 2024

Tire Inflation. We all need to pay attention to it

 

It doesn't matter if we drive a Diesel Pusher that is 48' long or pull a 6' "Pop-Up" trailer with our car. It doesn't matter if our vehicle runs on Gas or Hydrogen or Diesel or is an EV, our vehicles currently run on tires that are inflated with "air". Even if that inflation "air" is 78% Nitrogen or 98% Argon we must use some gas in our tires. What is constant for all vehicles on the roads today is that we have tires that require inflation with some sort of "gas".

It is basic Physics that It's the "air" pressure that supports the tire load, not the tire structure, and while we all know that there are folks on the Internet, who will claim to know more about any topic than the people with years of training, I can only hope that the majority of readers of this blog will trust the actual Science of tire design and engineering rather than some random guy spouting his opinion on what level of inflation you should run.

I follow a number of RV forums and find a number of folks offering their interpretation on what the proper inflation is for tires in RV application. Some insisted the only correct inflation pressure was what was shown on the "Tire Sidewall" while others insist the "Vehicle Certification label" is the only correct level and of course there are more than a couple that have developed their own inflation tables and will be happy to tell you what their personal calculations indicate.

Here is my take on the subject:

We have two separate issues to consider when selecting the minimum cold tire inflation pressure in RV application.

A vehicle MFG has DOT requirements they must follow. They are to specify the minimum inflation needed in the tires they provide on the new RV. That inflation is required to be sufficient to support the stated GAWR divided by the number of tires on that axle. Basically this means the axle rating is used to establish tire inflation based on the published load & inflation tables from the tire industry.

The owner of the RV needs to confirm the actual loading for their RV and should confirm they are not exceeding the GVWR or GAWR or the individual tire load capability.

You will immediately note that the RV company is making an assumption that is just not a reflection of reality, that being that axle loads are always split exactly 50/50 side to side. While many RVs may have the load split only a couple percent off of 50/50 some have discovered one end of an axle may carry more than 1,000# more than the other. Others have learned their unbalance is closer to 40/60 and some have discovered that their tires are overloaded as soon as they fill their water and fuel tank before loading anything else into the RV. Obviously a responsible owner needs to do a better job than some RV companies are doing.

While some feel it is sufficient to just read the regulations about the vehicle certification label, I and other tire engineers know that "policy" is not always the best way to get the best combination of ride, handling and tire durability.

Lets see what Goodyear says in their RV tire application web page.
"Correct tire inflation is a key component in tire care. The recommended inflation pressures for your tires are indicated on the certification label or in your owner's manual. Since RVs can be loaded with many different configurations, the load on each tire will vary. For this reason, actual air pressure required should be determined based on the measured load on each individual tire. Inflation pressure should be adjusted to handle the tire carrying the heaviest load, and all tires on the axle should be adjusted to this standard."

Maybe you don't think Goodyear knows more than some RV owners. What does Michelin say in their RV documents Here on pg 4:
"The amount of inflation pressure required in each tire depends on the weight of the fully loaded vehicle, to include passengers, cargo load, fuel, and water."
" To determine proper inflation pressure, Michelin recommends weighing each wheel position of the recreational vehicle individually. Weighing each axle end individually will give a clear indication of how the weight of the recreational vehicle is distributed across the axle.
 "

Also in their tire maintenance document on tire inflation:
"Unlike commercial light truck and truck applications, we recommend weighing motorhomes at each wheel position of the vehicle (right front corner, left front corner, etc.). This is the only way to get a clear indication of exactly how the weight of the motorhome is distributed. Otherwise, one wheel position may be significantly overloaded even though the GAWR has not been exceeded. Tire inflation for a given axle should be set according to the pressure corresponding to the higher of the two end loads."

Others are allowed to have and express their opinions and their interpretation of how the regulations are to be applied, but it was my job as a tire engineer to understand and work within FMVSS regulations for some 40 years.

I will leave it up to you to decide if the only acceptable inflation is what is on the vehicle certification label or that inflation is learned by reviewing what major tire companies say in their  published guides or if some guy pontificating at the campfire is a more reliable source.

Monday, September 2, 2024

Maximum Operational Tire Speed

 There are still a number of folks asking about tire operational speed and specifically with ST type tires. I hope this post will help.

Back in the 70's there was a National Motor Vehicle Speed limit of 55 MPH, and the RV market wasn't as big as it is today. Plus RV's were much smaller vehicles and many used Passenger Vehicle type tires.

Then the RV market started to grow and RVs became larger and heavier so tires offered on RVs needed to increase in capacity.

Based of info from someone in the tire industry at the time, we know that some RV companies did not want to use P type tires because of the loss of 10% of the load capacity and they did not wan to.pay for more expensive LT type tires so the RV companies asked for "Special Trailer" tires.

Not all tire companies agreed to the proposal and even today many companies do not make ST type tires.

The formula to calculate tire load was adjusted and included a statement on 65 MPH max speed. This showed up in the publish Industry standards as seen here.

 

 Goodyear even issues a Bulletin PSB #2011-13  in 2011 stating that "Industry standards dictate that tires with the ST designation are speed rated at 65 MPH (104 km/h) under normal inflation and load conditions."  Adjustments for higher speed up to 75 MPH would be allowed if there were either a decrease in Load capacity or increase in Inflation limit or both. The wording is convoluted but it specifies "Based on these industry standards, if tires with the ST designation are used at speeds between 66 and 75 mph (106 km/h and 121 km/h), it is necessary to increase the cold inflation pressure by 10 psi (69 kPa) above the recommended pressure for the rated maximum load.
- Increasing the inflation pressure by 10 psi (69 kPa) does not provide any additional load carrying capacity.
- Do not exceed the maximum pressure for the wheel.
- If the maximum pressure for the wheel prohibits the increase of air pressure, then the maximum speed must be restricted to 65 mph (104 km/h).
- The cold inflation pressure must not exceed 10 psi (69 kPa) beyond the inflation specified for the maximum load of the tire
."

Please note that the Goodyear bulletin does not completely match the publisher TRA guidelines.

In 2017 the US federal Trade Commission placed a tariff on all "non-speed rated" tires so the tire companies in Asia where the low cost ST tires were  made quickly adopted the use of a "Speed Symbol" based on the use of the SAE Passenger car Speed test to justify the use of higher speed rating.

This "Speed Test" only requires that brand new tires be capable of running 10 minutes at a stated speed so in reality the SAE test is just a high temperature resistance test. After running the test, tires are not subjected to any additional testing and are considered scrap.

Through all of this there has been no adjustment to the original load calculations that specified a 65 MPH Maximum operational speed.

We have all learned or read of the general degradation of tires in Trailer application with Interply Shear being a major physical cause for this degradation of the belt strength which can eventually lead to a "Belt Detachment"  AKA "Blowout".

So what does this all mean?  In my expert opinion:

1. ST type tires should not be run for any length of time above 65 MPH at the published Max Load for the given Load Range of the subject tire unless adjustments to load and inflation are made.

2. If you want to run your ST type tire at a speeds above 65 but no greater than 70 MPH you MUST increase the cold inflation by 10PSI. This inflation increase DOES NOT provide any increase in load capacity

3. If you want to run your ST type tires at speeds above 70 but not greater than 75 MPH you MUST increase the cold inflation pressure by 10 psi PLUS you must reduce the tire load MAX capacity by 10%

4. At no time should you ever increase the cold inflation pressure above the maximum PSI rating of the wheel as identified by the wheel manufacturer.

You should note that the Loads and inflations published in the Industry Standards book AKA TRA manual are based on a maximum operation speed of 65 MPH. The "Speed Symbol" is a sales and marketing tool and I find no information in the TRA manual that recognizes the SAE speed Symbol as superseding the existing TRA standards.

If or when such approval is issued by TRA I will adjust my statements above accordingly.

If you want to understand Interply Shear I suggest you review the following posts.

- https://www.rvtiresafety.net/2013/11/interply-shear-and-other-techno-babble.html

- https://www.rvtiresafety.net/2018/09/warning-super-technical-post-tire.html

- https://www.rvtiresafety.net/2018/07/question-on-radial-tire-belt-interply.html

- This link has a video showing the effects of tire cornering and the resultant Interply Shear

       https://www.rvtiresafety.net/2018/02/tire-inflation-not-same-for-all-trailers.html



I hope this clarifies the question of why ST tires in RV service seem to fail at a higher rate than regular Radials when used on cars and Light Trucks.