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Friday, July 23, 2021

Is better tire wear AND better fuel Economy possible?

 We all see posts from people wanting better tire wear and better fuel economy. Here is some information from a friend "Dr. Joe"

Wear, traction and rolling resistance are interrelated (but adjustable) factors determined mostly by tread material properties fixed during the design stage of tire development. The rubber chemist's enduring dilemma has been that enhancing one tread attribute influences the other two – usually adversely. Marketing professionals tend to use the term “balance” rather than “tradeoffs” to describe these conflicting variables. Based on my reading of recent industry press releases, tire wear and traction seem to be playing “second fiddle” to rolling resistance in today’s push for “greener” tires – even though fuel efficiency has not been a top priority for US consumers. It is, however, well known that good tread wear resistance is important to motorists purchasing replacement tires. Contrarily, because of CAFE mandates, OEMs consider tread life subordinate to rolling friction in their procurement of new car tires. Unarguably, any decrease in wear out mileage increases the frequency of tire replacement and unwelcome scrap generation.
Tire traction, nonetheless, is important to safety conscious motorists; it is constrained by road conditions which control traction limits encountered by given tread compounds and patterns. These can be optimized for wet, dry or snow performance. While over 80% of tires in service in the US are rated at the UTQG traction level “A”, only 15% are rated at the highest level “AA”. Surprisingly, no real-world data exists indicating that tires with higher rated traction grades are less involved in highway accidents than lower rated ones – with snow tires an exception as they are not UTQG rated, For improved all-weather traction, so-called “rain tires” have been touted on-and- off over the decades, and promoted extensively by U. S. Rubber (now Uniroyal) beginning in 1965. Wet grip was achieved, then and now, by sacrificing tread wear. While this is an acceptable trade-off in Europe, US consumers have consistently rated long tire life more important than tire grip; and with relatively cheap fuel, rolling resistance tends to be a non-issue for individual tire buyers. The most common reasons reported by the influential Consumer Reports (2019) for selecting a particular replacement tire brand are price (44%), tread life (40%) and brand trust (31%). Similarly, the respected J. D. Power new car owner survey (2021) rates the top three tire satisfaction metrics as wear, ride and traction (in order of importance).

Regarding tread life, surveys of large numbers of scrapped consumer tires usually show that the majority are three to four years old when replaced. Since Americans drive between 11,000-12,000 miles per year on average (according to FHWA), this means that most passenger car tire wearout mileages occur between 33,000-48,000 miles. If light vehicle tire lifespans could be increased an additional year, demand for new tires would be reduced with lessened environmental impact. Also, tires become more fuel efficient as they wear. I wonder what an environmental and economic cost-benefit analysis of tire life extension might yield? Mainly due to existing and proposed government mandates, US consumers have been, or will be, forced to unwittingly accept reductions in tire life with increased costs in concert with lower rolling resistance. Then we encounter further demands placed on EV tires – which need improvements in tread wear, traction, and rolling resistance compared to their ICE powered counterparts. Additionally, the
California Energy Commission once again announced plans to implement a program to ensure that replacement tires sold in that state are as energy efficient as OE tires. This will only exacerbate the confounding situation facing consumers valuing tire life.
Ponder Michelin’s
transitional development of the radial tire after WWII. Wearout mileages were doubled while rolling friction was reduced by 25% compared to existing cross-ply tires. Could the industry now develop tread compounds and/or tire constructions that at least make desirable improvements in both wear and rolling resistance without comprising grip? Unexpectedly, Bridgestone, Arlanxeo and Solvay have just announced material technology that claims to achieve just that; up to 30 percent better wear efficiency and six percent reductions in rolling
friction. Hopefully, this technology will be licensed to others if the claims are justified

Joe Walter enjoys teaching Vehicle Dynamics and subjects related to tire materials and mechanics at The University of Akron. He previously served as Vice President and Director of Bridgestone Americas Research Center in Akron and President of its European Technical Center in Rome. Joe obtained graduate and undergraduate degrees in engineering from Virginia Tech a long time ago.

While Dr. Joe's article is aimed at the passenger and light truck vehicle market you can be assured that as new technologies are developed in one sector of tire design the features and advantages soon spread to other product lines. 


Friday, July 16, 2021

Airless tires? Reality? or just a pipe dream for RV owners

 A friend recently sent me a copy of an article from the Akron, Ohio daily paper The Beacon Journal.

 I am in Colorado, ending a 3 week working (giving tire seminars) trip, and celebrating with my wife, our 37th Anniversary sightseeing vacation, so missed the article on "Airless" tires. 




 What do they mean "Airless"? Could we soon be absolved of needing TPMS in our wheels? What will I write about if I can't keep complaining about people not using the proper inflation?

This sounds too good to be true. Well it is true...Sort of. It appears that the latest effort from Goodyear to develop a reliable airless tire is aimed at some "self-driving" vehicles that can transport goods and maybe even people around a small ares of a town or city.

While air filled "Pneumatic" tires have been around since 1847, the background on "Airless tires" is many decades old with many tire companies all trying to be the first to crack to code and present a reliable tire that can function like a regular tire when inflated with air. So far everyone seems to run into the same wall of limited speed or limited durability. While Goodyear hasn't released any specifics, visually their latest effort "resembles similar products being tested by other companies in its open-spoke design and flat tread" according to the picture (above) from the press release and an article in Rubber & Plastics News.

There are many "Airless" tires already in use today. In fact, in 1972, I worked on the Flat-Proof tires for the then brand new Dallas-Ft Worth airport for their passenger transfer system of driver-less vehicles that circulated the airport.  The tires had no air inflation but were filled with a heavy duty "foam-rubber". Other applications of airless tires include lawn mowers and numerous construction vehicles.

 If you wanted, you could order some Airless tires today for your forklift truck and would never need to add air or worry about the tires being punctured and going flat.

One thing that is almost universal with these applications is that they all have significant restrictions on operation speed, This limit is usually below 20 mph and if operation at this speed is required, the distance traveled or even the roadway used  must be strictly controlled.

So, it looks like we RV folks need to continue to wait for the scientific breakthrough that is needed to put tires on our cars, truck , trailers and RVs that no longer need air inflation to carry the load.


Friday, July 9, 2021

Should you use the inflation that is listed on the tire sidewall?

 Many RV owners do not understand the inflation and load information that is molded on the sidewall of the tires they own.  As a tire design engineer, part of my responsibility was to specify all the words, symbols and numbers on the tire sidewalls. Some of the words such as brand name like as Goodyear or Michelin and the "line"  like  "Eagle" or "Transforce" are obviously selected by the sales and marketing departments. 

The tire size or sizes had been selected and it was my job to design and request molds appropriate for the sizes that were to be part of the new line of tires. For each size, I needed to consult the US Tire & Rim Association data book and to specify the appropriate load and inflation numbers for the maximum allowable for the Load Range of each tire. With LT lines we might have LR-C, LR-D and LR-E tires and if incorrect numbers ended up on the tires they would need to be recalled and replaced for free so it was obvious that this information had to be correct.

With the exception of a few tires that are designed for a new model of a specific car or Light Truck from Ford, GM, Toyota, M-B etc, I would never know what vehicle the tires would end up being mounted on so there would be no way for me to specify a specific load or inflation for any given tire that was vehicle specific. Selecting the tire and Load Range and specifying the inflation number to be posted on the vehicle Certification label is, by regulation, the responsibility of the vehicle manufacturer. It was however my responsibility to ensure the tire was capable of passing the DOT tests as well as company minimums based on the use of industry standard loads and inflation numbers.

All tires have a Maximum Load capacity molded on the tire sidewall along with the minimum inflation needed to support that load.

With this background information I hope you now understand that the inflation number molded on the tire sidewall is not selected or made up by the tire engineer as the inflation number comes from the published Industry standards. The inflation on the Certification Label aka Tire Placard is specified by the RV manufacturer and by federal Regulation must be sufficient to support the GAWR (Gross Axle Weight Rating) which is part of the RV company design process.

The placard inflation may be equal or less than the number on the tire sidewall but it should never be higher than the tire sidewall pressure number.

Ideally you would have actual scale loads with the RV fully loaded at its expected heaviest. With that load known you can consult the tables to learn the minimum inflation needed to support your load. However if you don't have scale readings the best thing to do is inflate your tires to the placard information. This is OK as long as you have not overloaded your tires. Note that the tables for tires made by Goodyear are the same as for tires made by General, or Firestone, or Bridgestone, or Cooper. Also most Michelin tires have the same numbers as the other tire companies with the exception of a few tires that were originally designed for European country application where metric units are standard. When converting to the US "Inch-Pound" units the numbers do not always match up so if you have Michelin tires you probably need to consult their tables.  You can also check the tables listed on this blog post.


If you review the above you will see that I am not advising that you use the inflation on the tire sidewall, unless that is the inflation on your Placard.

A side comment on the inflation number of the tire. The words may be "Max Inflation" on some tires, but this is related to the way loads are established for tires based on the inflation. The LIMIT is the stated maximum load. The intent in the wording is to stress that the tire load can not be increased even if the inflation is increased above the number associated with the tire's "Maximum Load".

Regarding hot tire inflation numbers higher than the number on the tire sidewall, that is OK as long as the set "cold inflation" is not set higher than the number on the tire. Tire inflation will increase when the tire temperature increases. The rate  of change is about 2% for a change of 10F. Seeing your "Hot" inflation higher than the tire sidewall is not a big deal. With the advent of TPMS many people are seeing tire temperature or pressure increase for the first time as a driver. Most TPMS have the high temperature set to 158F.  I have found that setting the high pressure to 125% of your cold set pressure should normally avoid having the alarm sound as long you are not overloading your tire or pushing your driving speed. If you get close you might just drop the speed back a few mph. Personally I set my cruse to 62 or 63 mph and stay in the right lane. I do not get people honking and I get real good fuel economy. (10.2 mpg in my last 1,200 mile trip from Ohio to WY).



Friday, July 2, 2021

Tire Life. Why can't I get a straight and consistent answer?

 Have seen a number of posts on RV forums and Facebook asking why there are different answers to the question of when to replace tires in RV application. Some are told "Replace at 5 years" and hear " you will die if you drive on a 6year old tire.

Discount tire has presented this diagram.

You will note that they do not show some cliff that you fall off.  I responded to a question on tire life where the person thought that the tire companies were simply pushing tire sales.

Tire "Life" is not an on-off switch. Rubber begins to lose its strength and flexibility the day it is put in the tire warehouse. Temperature and time are the primary drivers of the loss of strength and flexibility. Tires can fail for a variety of reasons. Hitting potholes creates cracks in the internal tire structure. Most are microscopic but all cracks grow and none repair themselves so the number and size of cracks simply grow till one day the rubber will not be strong enough to tolerate hitting a pothole or piece of road debris and the heat generated with a long run at high speed on a hot day simply lowers the remaining strength of a tire.

The more a tire is driven the more flexing it experiences. Older tires, having lost some of their flexibility, will experience more actual tearing rather than stretching. Driving faster increases the rubber temperature. The higher the rubber temperature the faster the rubber loses its ability to stretch and recover. 

My post on tire covers pointed out the "aging rate' of tires doubles with each increase in operating temperature of 18 degrees F.

Part of Organic Chemistry is chemical reaction rate.

For every 18F increase in temperature the rate of aging doubles. Heat also comes from being in the sun when parked. So if the RV is parked with tires in direct sunlight you can see the tire achieve 36F increase or more which means it is aging at more than four times the rate it would have if in full shade.
If you want to understand the technology behind this accelerated aging due to heat I suggest you can read some of these sources if you have a few hours.

Here are some specific references on tires


The idea that tires be replaced after 5 years of use is based on probability. Some tires fail at 3 years of use and some are still running after 9 but it's the odds that can get you. If you have an RV trailer I can assure you that the science shows that backing into an RV site is much harder on the tires than pulling through. This is because the Interply Shear is much higher backing in because the side forces are much higher. I wrote about that force in this blog post.



Friday, June 25, 2021

UV protection for tires. Still important, and the facts haven't changed in 7 years

 This is a reprint of a post I did in 2014. The facts and data have not changed.

For some time I have been reading posts and advertisements about tire covers and UV protection. As an engineer I prefer FACTS over sales PR.

This investigation has taken more time than I originally wanted as I needed a reasonable way to measure UV and a day with full Sun.
 - Not something easy to find in NE Ohio-

As they say it all came together one day in April. While it was cold 24°F last night and we had an inch of snow yesterday, it is bright and sunny today with only a little haze in the sky.

The test uses a Hawk2 UV meter. This unit is intended to help you judge how much sun you are getting while at the beach but I felt it would serve my purposes as we are not trying to measure an absolute value in milliwatts per square centimeter but a gross relative level of shielding of different materials used to cover tires.
If interested you can learn more about UV HERE and more about the UV Index HERE

I set up a test using my RV.
As you can see the UV of 6 years here in Ohio,  has pretty much destroyed the cheap vinyl used by Coachmen for the side decoration. Anyway the front tire has my normal white vinyl tire cover and there is a standard blue tarp, a roll of window screen and some black cloth backed vinyl similar to what is used in black tire covers.

I will show the meter readings for each "shield".
Full Sun gives a reading of 9 which is considered "HIGH"

 while in full shade the reading in zero.

Under the white cover the reading is zero

 and even under the black cover the reading is zero


  but the screen only reduced the UVI to level 5

I interpret these results to indicate that anything that is not in direct sun or that shields all direct sunlight will provide adequate protection from UV damage for tires.

I would not be worried about reflected light going under the RV to the back side of the tires as this is full shade. After all, tires are designed to be outdoors and we are not trying to protect tires for 20 years but only to get past a normal vehicle usage of 4 to 5 years to the 8 to 10 year range for many RVs. I would not consider open mesh as used in some "tire covers" complete protection but it is probably better than nothing.

NOTE I did not address the effects of heat on tires in this post. I did cover in THIS post and that clearly shows that white covers are the ones to use if you want to keep your tires cooler so they age more slowly.

If you want to protect your tires to give you the longest life possible you need to cover them with white solid covers such as cloth backed vinyl being a most reasonable option.


Friday, June 18, 2021

Are tire pressure recommendations changing?

A question from an Airstream owner.

Hello All,
I have a 2020 Ford F250 and an Airstream 33' Travel Trailer. I've read a lot of discussion about tire pressure, but nothing real current. What I mean is, the newer F250 door column states for these Michelin tires now, 60psi front and 65psi rear. The tires max at 80psi on the sidewall which was my experience with prior year F250's on the door column too, but the tech at the dealership said they've changed and that with normal driving you can run 80psi, but harsher ride and you'll run the "middles out of them."

On to the Airstream. All four tires are Michelin LT225/75R16, Load Range E. The Airstream sticker says 80psi for these tires. I went up to my local Dobbs Tire Dealer and he pulled up the Michelin chart for these tires, and said with a trailer weight on dual axles of between 7980lbs and 8600lbs, you could run the pressure between 60 -- 65psi. I asked if I would have a problem with tire heat or wear and he said no. I did put a 3" lift kit on the Airstream, so as to have better clearance at gas stations and various road conditions. The dealer puts in 80psi, but from time to time I get popped rivets, (if ride is the cause).

My question is this: With an approximately 8200lb trailer, 1300lb hitch weight, and just me and some luggage, (300lbs of luggage), what would you recommend on tire pressure on both vehicles for mostly interstate driving, despite the door stickers, (if recommended)?

My reply:
Not sure why you think the information on appropriate tire pressure is different today than it has been for the last 10 years.

TV Car companies have teams of engineers that work closely with the team of tire engineers to select tires and inflation numbers that will deliver the best overall tire and vehicle performance. The consider and test for ride, handling, steering response, fuel economy and safety in emergency situations. The inflation on the sticker along with tire inflation information in the owner's manual should be followed. Mechanics and service people at the car dealership are not involved with the selection or testing and evaluation of the new tires that were selected for the new vehicle. While they have some knowledge I see no way they can know the performance  features of the tread rubber or the construction features of the body of the tire that was designed, selected and manufactured for that specific vehicle.

TT While I am not aware of any similar testing and in vehicle evaluation on tires for trailers in the RV world, there are still some requirements the RV Certification Label aka Tire Placard sticker has to meet BY LAW. Tires selection is the responsibility of the RV mfg. The inflation specified on the sticker along with the tire type and size must be capable of supporting at least 50% of the stated GAWR per DOT regulations for each tire. A few years ago RVIA decided that having at least a 10% load capacity margin would be a good thing and that having such a margin should improve tire reliability so the sticker inflation for the specified tires must provide AT LEAST 110% of the GAWR and assumes a perfect 50/50 end to end of each axle oad split. So if your RV is RVIA certified the sticker as applied by the manufacturer must meet these standards.

I think a point of confusion is the wording on some tires concerning the Max Load capacity of a tire and the appropriate inflation needed to provide that load capacity. Each tire has an absolute Maximum Load capacity number and that is the number stated on the tire sidewall. The confusion comes in when people think that they can obtain more load capacity if they increase the inflation but that idea is incorrect once the maximum load capacity of the tire has been reached. While it is possible to increase the inflation doing so will not increase the laod capacity and that is why some tires say "Max" inflation but everyone should know or realize that tires warm up when in the Sun or when driven and that pressure increases with temperature (about 2% per each 10F) BUT when discussing inflation numbers or when setting tire pressure we are ALWAYS talking about the pressure when a tire is at the prevailing Ambient temperature i.e. temperature in the shade and the tire has not been warmed by either being in the Sun or being driven on for the previous 2 hours. i.e. "Cold Pressure".

As an experienced tire design engineer I can only advise that people follow the car company and RV company recommendations for tire inflation unless they follow the specific guidelines provided by actual tire engineers if they want to fine tune tire pressure based on actual scale measurement and include the suggested tolerances and margins on load and inflation.


Friday, June 11, 2021

Are ST trailer tires made with "Magic" pixie dust?

 Lets see if I can bring some Science and Engineering facts and history to this issue of speed limits on ST tires.

In '60's & '70 when ST type tires were "invented" and started to be applied to Travel Trailers, the national speed limit was 55 mph and tires were bias.  Trailers were considered "big" if they were 24' long and I doubt there were many if any 5th wheel tri axle trailers on the road. 
Today we see speeds across the country of 70+ and there are many locations where you could set the cruse at 70 and never slow down for an 8 hour drive here in the US. Trailers over 30' are normal with some pushing 40 feet and most have tandem axles with more tripples showing up every day.

The formula for determining the load capacity for all tires follows the basic format 
  Load = K  x  (air pressure)   x  (air volume)
Now the calculation for air volume is the complex part as aspect ratio and a theoretical rim width and other factors such as tread depth come into it but these details do not change the fundamental format of the formula.

The "K" shown above is an important concept as it is really a factor based on the expected service. Trucks are expected to carry heavy loads but not all the time. passenger cars are not expected to be heavily loaded much of the time and while RV are loaded almost all the time, when ST type tires were "invented" we didn't have slide-outs or 35' 5vers or pickups capable of running 80 mph for hours on end.
Standard passenger cars seldom if ever carry their max load. The GVWR and GAWR are not even in every owner's manual or on the Vehicle Certification label AKA "Tire Placard". They are expected to be run at posted speeds but on paved roads for hours on end and driven 10,000 to 20,000 miles a year  i.e. used fairly frequently with many being parked in a garage.

For the sake of this discussion lets assume the K is set to 1.0 for passenger cars.

Now what do you do with Station Wagons and other "multi-purpose" vehicles? These vehicles were expected to carry more load more often so the service is obviously more severe.  When SUV's came along they were places in the "Multi-purpose category" and if a passenger type tire was applied to a trailer that was also considered more severe service. So the load capacity was reduced. many are aware of the "De-rating of P type tires when used on trailers or SUVs etc. So K (multi-purpose) = K (passenger) divided by 1.10 and we end up with lower load capacity. About 90% of passenger.

Lets look at the actual numbers.
P235/75R15 105S  35 psi
  2,028# @ 35 psi 112 mph on a Passenger vehicle
  1,844# @ 35 psi 112 mph on an SUV or P/U or trailer

Moving on to Pick-up service we have LT type tires. The formula is still K x pressure x air volume but with trucks expected to carry even more load most of the time their K factor is different.
Their numbers give us
LT235/75R15 101/104Q  LR-C
  1,985# single 50 psi 99 mph 

This lower load capacity on truck service is clearly because the higher percentage time spent carrying more load.
Before we move on lets look at the ST numbers
ST 235/75R15  LR-C
  2340# @ 50 psi   65 mph
To me the obvious question should be: How does the addition of the letters "ST" on the sidewall allow a 26% increase in load capacity over a P type tire (adjusted for trailer service)
or a 29%  increase over the heavily loaded but occasionally empty truck? The only reason I can see is the significant reduction in speed.

We all know, or should know that more load (more deflection or bending) generates more heat so what could you do to counteract the increase in heat due to the increase in load? Obviously lower the speed would reduce the higher heat and that was part of the original ST tire standard.

Now lets look at the tire type that is of real interest. ST type as used on many RV trailers. 
In 2014 new duties were imposed on imported tires but ST type were exempt, sort of. There were various requirements some of which were requested to be changed or eliminated. The speed symbol was one of these requirements.
Starting in 2017 (possibly earlier in small quantities) many ST type tires started showing up with a Speed Symbol selected from the table as published by US Tire & Rim Association in the LT section.
The problem is that Speed Symbol does not have any standard DOT test or requirements as in the US Speed Rating is really a marketing tool and not a strict performance requirement. A review of various ST tires shows a range of speed symbols from L (75 mph) to R (106 mph) and possibly higher.
Further compounding the confusion is that the speed symbols are from the SAE - Society of Automotive Engineers and according to SAE their test criteria J1561 apply to ""standard load," "extra load," and "T-type high-pressure temporary-use spare" passenger tires." This raises the obvious question of what test procedure, if any, are various tire companies following when they assign the Speed Symbol? While we are talking about SAE symbols we need to remember that DOT does not recognize or test for these ratings.

Let me close with a question I have asked a number of times but as of now have never received an answer for.
What "magic" pixie dust are tire companies putting in their ST tires that allows them to run 75 or 81 or even 106 mph without making any adjustments in load or inflation? and If they have this "magic" engineering available, why aren't they using it in their LT tires?

NOTE Goodyear Tire Care Guide ( clearly shows a blanket 75 mph max speed for 17.5 rim diameter and larger tires.
Some may want to argue that tire technology has improved since 1970 and that is certainly true but I would ask why haven't load capacities for Passenger or LT or heavy truck tires been increased over the past 50 years? 


Friday, June 4, 2021

Are "Blowouts" the result of running a tire inflated to more than the tire sidewall number?

I think people are over-thinking tire over-inflation.

1. Any mention of tire inflation is about "cold" inflation unless there is a specific mention of "hot inflation"

2. "Cold" inflation does not mean you need to refrigerate your tires or to get them to some artificial chemical laboratory "standard" of 68 or 70F. Cold simply means at the prevailing Ambient temperature. i.e the air temperature in the shade. Tires generally get to "cold" after being parked for 2 hours or more, in the shade.

3. All tires warm up when running. Sometimes when driving the Interstate, we even see the tires on one side are 10-20F hotter than the other when constantly in direct sunlight. Think about traveling due East or West in Kansas or Iowa. I know I have seen this happen in my Class-C more than once.

4. If you have your actual tire loads (scale weights when fully loaded) and use that number to learn the minimum inflation from the tire Load & Inflation tables you can add a margin and then be in good shape and not worry about tire temperature.

5. What "Margin" of inflation? If using the tables and you identified the inflation that can support no less than the "heavy end" load. (Always go up to next 5 psi step in the table.) Do not round or try and calculate an inflation that exactly matches your scale number. Just use the table numbers. 

6 Now that you know the MINIMUM inflation required, it is desirable to have a margin so you do not have to get out the compressor every day because the Ambient temperature has changed. Given that tire temperature will change about 2% for each change of 10F in Ambient and that a 20F to 40F change in morning Ambient is in the "normal" range, that would suggest that having a margin of +  8% to 10% will make life easier. If you are running 10% more inflation than the minimum required for the load you will find that the "hot inflation" will not increase as much than if you are starting at the minimum + 2psi.   EXAMPLE Suppose you need to run 70 psi MINIMUM based on the measured load and the information in the tables. I think that if you start out inflating to 77 psi you will see less of an increase in pressure  than if you start out at 72 psi.

7 Maximum Inflation? This causes some unwarranted concern for many. They read the tire sidewall and see something like "Max inflation 80 psi, Max Load 2780 Lbs" IMO it would be better if the tire said "Max Load 2780 at 80 psi" This is because there is an absolute maximum load a tire is rated for and the inflation pressure needed to support, when cold, is 80 psi. This means in reality that 80 psi is the MINIMUM inflation required to support the MAXIMUM load. While I can not speak for all tire companies I do know, from my experience as a tire design engineer where testing for the maximum inflation a tire can tolerate was part of the standard process I saw that for normal street tires. P, LT or Truck type the new, undamaged tires could tolerate anywhere from 100% to 250% over-inflation and not fail. In a few cases some tires were able to tolerate as much as 400% for a short time before failure. Now I need to make it clear that running tires at 100% over the number molded on the tire sidewall is not safe. The tests were conducted in an explosion chamber and not while tires were running. Tire engineers test new designs to confirm they can tolerate significant over-inflation. We run tires in overload on test wheels continuously longer than possible for anyone to run their tires on the highway so we know thy can tolerate significant increase in both temperature and pressure.

8 "Blowouts" do not happen because a properly loaded, undamaged tire was run at highway speeds. "Blowout" simply is a word that is used to explain that there was a sudden loss of air, that made a loud noise, and the driver was surprised. Running a tire at hot inflation of 110 to 130% of the tire sidewall pressure is not, in itself going to cause a "blowout".


Friday, May 28, 2021

Does your TPMS "control" you? or can you control it?

I saw a post on an RV Forum about a tire "Blowout" on an RV that had a TPMS.

The RV owner said they had TPMS but still had a sidewall failure. He also said, he "set his  "baseline" PSI which is the threshold for the low end pressure reading and then the upper limit alarm will tell you if you''re exceeding normal pressures and headed for a blowout. Same thing for the temperatures as well....set the baseline and then it alerts you if the tire temperature is above normal and too high. I used my unit last year on 2 long haul trips to MI and it performed well. Gave me the piece of mind knowing I could keep an eye on the tire pressures and temps. I do recommend the unit. I purchased the unit which can support up to 22 sensors, but got the TPMS system with 6 sensors on Amazon, then bought 2 additional sensors for a total of 8 - 4 for my TV and another 4 for my TT. My trailer has dual axles. I mounted the wifi extender on the front of my TT just behind the battery box and use the clip on leads from the battery to power it. The wifi unit is in my truck and I tuck it in the center pocket in my cab. Works well."

I pointed out that not all TPMS let you set that actual low pressure warning level. Some use a % or number of PSI below the "set pressure" AKA Baseline. Others let you set the low pressure warning level.

I recommend that the low pressure warning level be no lower than 5 psi below the minimum pressure needed to support the measured load on the inflation for the tires on that axle. With some TPMS, this will require some extra calculation and work as you will need to set the Baseline higher than your actual "cold" inflation.

If Tire minimum inflation needed to support the measured load was 70  your low warning level should be no lower than 65.
If your system has the warning at -25% from "baseline" then you need to set your Baseline to 87 psi  (75% of 87 = 65). Some systems use the pressure in the tire when you first connect the TPM sensor as the "baseline" so you would need to pump up your tires to 87 before first connecting the TPM sensor. Once the TPM system is fully connected you could then lower your 87 to  80 (70x 1.15).
The 1.15 is a suggested margin of inflation so you are not pumping up your tires if the Ambient temperature drops. Remember that a drop of 10°F will result in about a 2% drop in the "cold" tire pressure.
Yes this is complex but that is a problem with the TPMS MFG not giving the user full control of your warning and baseline inflation levels. Please learn how your TPMS is programed. Your instructions should tell you how to set the "Low Pressure Warning Level" but if it doesn't you will need to do the above math.

Friday, May 21, 2021

Another post on ST type tires, Speed and tire life

 Post on an RV Advice forum:

"We have ST rated tires. Do we have to run less than 65 mph? We see a lot of RVs passing us."


"Speed Symbol" is actually a relative measure of heat resistance. 

The speed number is based on a brand new tire being able to run for 30 minutes on a perfectly smooth steel drum,  at a constant speed without coming apart. Any tire running the test is considered scrap after the 30 minute run. 


 65 mph is the assumed Max operational speed for ST type tires. 

This is part of the calculations used to establish a tire load capacity. While it is unlikely that your tires will come apart as soon as you run 66 mph, it is true that the faster you run the "faster" you are using up the life of your tires. People running faster than 65 is one of the contributing reasons for ST tires to have shorter life than tires on your car or Pick-up.


I often compare tire speed rating like your engine "Red Line" speed. If your car indicated 4,500 as the Red Line you can probably run 4,495 RPM for some time without the engine blowing up. BUT I would think you know this will significantly shorten the life of the engine. Same with tires.


Friday, May 14, 2021

Can you change tire size or type on your RV?

 As an actual Tire Design Engineer I can assure you that there is more misinformation or partially correct information out there on the internet, than technically accurate information.

It is true that the original tire selection is the responsibility of the RV Mfg. The issue is that once the RV is sold it seems that most RV Manufacturers have little or no interest in standing behind their choices with any actual warranty service when it comes to tires.

It seems that OE tire selection for most RVs is based on one goal. Find the smallest, lowest cost tire that will meet the requirements.

The only Federal (DOT) requirement is that the tire load capacity, times the number of tires on the axle, be AT LEAST equal to the maximum load rating of the axle. While RVIA now requires 10% Reserve Load capacity, DOT does not. As a point of reference most cars come with a 20% to 30% or higher Reserve Load capacity

A smaller tire can mean the RV Manufacturer can get away with less costly (smaller) wheel and maybe a smaller wheel well so this is extra pressure on purchasing Dept to get the minimum possible tire that can meet the requirements.

Given the above, it is up to you, the owner, to decide if you want any, some or more "Reserve Load capacity" for your RV. You may have the option of larger tires or you may be restricted to trying to find tires of the same dimensions but with higher load capacity.

You need to educate yourself about the requirements and limitations of the four "types" of tires that are in the market.  'P" is Passenger type. If used on an RV (trailer or motorhome) the load capacity Must be reduced by dividing by 1.1 but not everyone will know or do that. LT type can be used in RV service but you will soon discover that LT tires with the same dimensions and Load Range (ply rating) have a lower load capacity than the same dimension ST type.
ST type have the highest load capacity rating for a given set of dimensions, but you need to remember that the ST tire Load formula that is used to calculate the tire load capacity is based on an assumption of a 65 MPH Max speed. We all know that there is "No Free Lunch" and the trade-off for increased load capacity is lower speed capability. The "Speed Rating" symbol on many ST type tires is based on a 30 minute test so you need to decide if you want to depend on such a short term test when making a tire selection.
Finally there are actual "Truck / Bus" tires. These have no leading letter and are usually on 17.5" or larger wheels. These tires have higher Load Range, usually F or higher These tires are almost all rated for 75 MPH in RV use on the highway.

Do your homework. Ask questions, but remember there are very few really knowledgeable people out there who have the training or experience in tire engineering. Just having driven on tires for 40 years is not the same as having been held responsible for designing tires for Truck, Passenger, Trailer, or Indianapolis racing application. Also, being able to read Federal Regulations is not the same as having to work within those regulations while meeting the goals and demands from GM, Mazda, Toyota, Honda, Freightliner, MB, Nissan, Ford, or Chrysler.

I am only aware of two actual Tire Design Engineers who regularly post on various RV Forums.


Friday, May 7, 2021

What is the "BEST Tire"?

This question is usually framed as "What tires should I buy for my RV Trailer?"

I usually stay away from brand recommendations. As a certified Forensic Tire Engineer I will say that what ever you get, you should ABSOLUTELY be sure the new tires have a load capacity that is at least equal to the OE tire capacity. Better yet get tires that can support 120% of the load of the OE tires.

If the RV was made before Dec 2017 each tire may have only been rated to support 50% of the GAWR (see your certification label). If mfg after Dec 2017 each tire might have been rated to support 55% of the GAWR.

I strongly recommend you be sure your tires are of sufficient size and Load Range to be able to support 120% of the measured load (scale weight, not printed limits or someone's estimate) of the heaviest loaded tire on the axle. All tires on an axle should be inflated to the same "cold" inflation.

RE TPMS  I recommend the Low Pressure warning be no lower than 95% of the goal Cold pressure. Your High pressure can be set to 125% of your Cold inflation.  Your high temperature is probably set by the TMPS mfg to 157°F (70°C) which is a reasonable level for aftermarket external TPMS..

Finally, I will go so far as saying you will probably be better off if you ensure that what ever tire you select shows that it has Nylon aka polyamide listed as one of the materials under the tread in addition to the steel ply. This will improve heat resistance and high speed durability.


Friday, April 30, 2021

Are ST type tires better because they have a higher "Speed rating"? Maybe not.

Saw an RV forum post about better durability of trailer tires. 

The poster said: " Old trailer tires were rated to 65 mph. Newer tires are rated to 81 or 88 mph. That is a big difference, and since i upgraded to Load Range D and Speed Rating M (81 mph), my trailer blowouts have disappeared. "

My response:
Glad you are getting better durability from your trailer tires. I would you not assign all the better durability on the higher speed rating as we are seeing that many/most ST tires manufactured since 2018 to be more durable than the tires of 2000. The increase in Load Range probably gave your tire durability a good boost. One thing you can look at is the material list molded on the tire sidewall. I think you will see that older tires did not list a layer of Nylon or other material on top of the two steel belts in the tire tread, while many of today's ST tires list the Nylon.

We need to remember that "Speed Rating" is a bit misleading. ST type tires have their load capacity formula based on a max operating speed of 65 MPH since 1970's, otherwise their load capacity would be similar to LT type tires. The test used to establish a "Speed Rating" is a 30 minute step speed test designed for passenger type tires. To pass a "speed level" a tire only has to be able to run 30 minutes without failing, after which, the tire is scrap. So clearly a tire with a 81 mph "rating" should not be considered acceptable for running many cumulative hours at 80 mph.

We really need to only use the "rating" as a measure of RELATIVE heat resistance. ALSO it can be misleading to try and compare the rating on tires from company "A" with tires from company "B" as each company will use test results from a small number of tires that are actually tested, to establish the "rating" symbol for that group of tires. This is done statistically and the statistical prediction used by company "A" is not going to be the same as used by company "B".

Also there is no DOT test for this rating so I doubt that you can depend on all tires of a specific speed rating to perform the same. With no federal regulation for speed performance, I do not see a reason to expect any ST type tire to be capable of running at the stated speed for more than 30 minutes when brand new on a perfectly smooth surface as used in the test laboratory.

Bottom Line:

When shopping for new tires for your trailer I suggest you "read the fine print". Look at the material list for the tread area molded on the tire sidewall. An ST tire with more than just steel and polyester listed i.e.  Polyester + 2 layer Steel + 1 layer Nylon, will probability be more durable than one without the Nylon when operated at speeds above 65 MPH. IMO a "Speed rating" of L ( 75 mph) should be more than sufficient for RV trailer use.


Friday, April 23, 2021

Internet "Experts" have Dennis confused. Maybe I can help

 Dennis said:

Okay ... I've been RV'ng in Class-A's for 40 plus years and thought I had this figured out, but the "experts" have emerged to confuse me, once again. So, what is the definitive answer to the question of tire pressure ... should it be the coach manufacturer's recommendation on the placard in most every RV, or the tire manufacturer's inflation recommendation? I know for sure, the "cold pressure" stamped on the tire is NOT the recommended pressure. Please advise.
Dennis, I completely understand your frustration.
You need to remember that there are three entities that are trying to answer three different questions when it comes to tire inflation.
First the tire company.    They make tires that must meet various tire industry standards for tire dimensions and load capacity. The tire company must also certify to DOT that the tires they make are capable of passing a number of different strength and durability standards if they want to sell tires for use on US roads. These requirements are why you end up with the numbers molded on the tire sidewall. These include the Maximum load capacity for both Single and Dual-position. They must also identify the minimum inflation needed to support both the single and dual-position loads. We need to remember that the tire company does not know which RV or truck the tire will be mounted on. The tire company also doesn't know if the tire will be on the Front, in Dual on the drive, or even if it will be on a TAG axle so they can not give a single inflation number that meets all these requirements

Second, the RV Mfg has to meet some different DOT regulations. The primary one is that the Certification Label aka tire placard must specify inflation for the tire that would be sufficient to support the stated GAWR. Example: If the GAWR for a front axle was 6,000# then the tire they select must be capable of supporting 3,000# and the RV company must tell you, the owner, the MINIMUM inflation needed for the tire to be able to carry 3,000#. Since each axle probably has a different GAWR that is why you may have different inflations on the Certification label for each axle.

Third If the RV company wants to be able to meet RVIA standards the tire must be capable of supporting 110% of the axle load so in our example that means 3,300#. Here things can start to get messy. The inflation needed to support 3,300# is almost certainly different so the Tire placard would need to state the inflation for 3,300#. It is OK for the inflation on the placard to be higher than what DOT requires but it can not be any lower.

Finally, we get to you, the owner.
Option A. is to simply inflate to the number on the tire sidewall. This, in some cases, is significantly higher than the tire needs so you may get a hard ride and in extreme cases more rapid center wear.
Option B is to follow the placard inflation which on many Motorhomes is lower than the inflation number on the tire. So the owner gets confused.
Option C is what I and other tire engineers recommend. We have solid reasons for saying that the Rv owner needs to get on a truck scale, learn the actual load on the tires, and then learn the minimum inflation needed to support the actual load on the tires. We know this is the best for the tire because the data shows that a majority of RV have a tire or axle in overload and this is a major reason for tire problems in the RV world.
Option D This is a better version of Option C. This is even better but is not as easy as Option C. The data shows that almost no RV has the axle load split 50/50 side to side so simply taking the truck stop scale reading and dividing by two can be misleading. Some RVs have been found to be as much as 1,000# out of balance side to side. Option D means you need to learn the individual load on each tire position when the RV is loaded to its heaviest. This is almost impossible at regular truck stop scales. To learn the individual tire position load you need to get a company like RVSEF or Escapees to use their scales to get the individual weights. It is also possible, with a little work to find a platform scale at a gravel yard or some lumber or cement block companies have large scales. Here ere is a worksheet to help with the numbers. Once you learn the actual load on the heavy end of each axle you would use that weight to consult the tire Load and inflation tables to learn the MINIMUM inflation for all the tires on that axle. We also recommend that if possible you add 10% to the table inflation to give you a nice margin. Just do not exceed the max inflation rating for your wheels.

I hope this helps clear up your confusion.

Friday, April 16, 2021

It's rubber and needs some protection but I'm not talking about tires

 Had a windshield wiper fail when I pulled my RV out of storage before I headed South to the FMCA Big convention in Perry, GA a few weeks ago.





 Why did it fail?  It got old, just like tires do. Also I failed to protect the rubber blades. 

So yes this is a situation of do what I say not what I do. So how do I protect my rubber wiper blades? I got some white vinyl table cloth material, cut it and hot melt glued one edge.

This should give an extra year or two to the blades.

Friday, April 9, 2021

What's the "Best" TPMS

 No I don't want to start a fight. Probably everyone that already has a TPMS will say that what they have is the best.

I would prefer to identify the features that I think are important and then let the customer do the shopping because there is no way to predict what the sale price will be next week or what new brand or model TPM will hit the market next month.

Here is my list of Features:

Key features for me would be (sort of in order. but as they say Your mileage may vary)
1. Ability to set the low pressure alarm level and not have to change other pressure levels.
2. Lifetime warranty
3. Ability to set the high pressure level and not have to change other pressure levels.
4 Ability to set the high temperature level to 158F (70C) if it is not already set to that level.
5. Include a signal "repeater" or booster
6 Ability to add 1 to 9 more sensors in the future if needed (+1 more for a spare, + 2 or 4 more if you move the system to one with more tires, +4 if you want to monitor your toad.) This avoids the need to buy a new system in the future when you only need additional sensors.



Friday, April 2, 2021

Got a flat. Is "Sealant" a good "fix"?

 Improper Repairs: NHTSA (DOT)

“A plug by itself is not an acceptable repair.”


 “The proper repair of a punctured tire requires a plug 

 for the hole and a patch for the area inside the tire

 that surrounds the puncture hole.”


 “Punctures through the tread can be repaired if they 

are not too large, but punctures to the sidewall should 

not be repaired.


 Tires must be removed from the rim to be properly 

inspected before being plugged and patched.”


NOTE: Michelin, Goodyear and Bridgestone say the same thing.



The use of sealant or Slime or "Fix-A-Flat" or similar is in some cases worse than using a plug. Not only can't the inside of the tire be inspected but the use of such product may void any tire warranty and make if difficult or impossible to make a proper repair.

Here are some pictures from one of my seminars on RV tires.

In the first picture you can see all the crack damage in the tire. These can lead to unexpected tire failure.