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Friday, December 28, 2018

"Safety Factor" or "Reserve Load"

"Safety Factor"   The dictionary offers this "the ratio of the maximum stress that a structural part or a piece of material can withstand to the maximum stress estimated for it in the use for which it is designed."
While that sounds reasonable it really only works when talking about pieced that fail from simply increasing the load placed of the component.

Items like tires do not really have a "Safety Factor" as tires generally do not fail from simply increasing the load too much. In a non-rolling situation, I would not be surprised if we could load tires to 200% or maybe even more than 300%  of the load marked on the tire sidewall. However as soon as you introduce rolling or time or operating temperature the maximum load before failure is much closer to the max load molded on the tire sidewall. The exception to Max speed is affected by temperature time and load. With zero load many tires can probably handle 200+ mph but again for how long and at what temperature?

Since tires are basically a structure made of "organic" components tire and temperature can have a significant impact on the maximum load capabilities of the tire.

If we think of non-organic items like a steel girder or maybe even a stone block as used in the pyramid we can see that time and normal atmospheric temperatures have essentially no impact on the long-term maximum strength. The exception would be if we were to allow steel to rust or stone to be exposed to water and freeze/thaw cycles.

Tire Engineers prefer to use the term "Reserve Load" when talking about the load capacity of a tire. Here we would find Tire Engineer definition as the difference between the tire's maximum capacity when inflated to the stated level for the specific application (the inflation on the tire Placard) and the actual load to be placed on the tire.

Here are a few comparisons: First some normal car and truck applications.

Next a larger 5th Wheel RV

When you compare the reserve load percentage of the different groups you can easily see the different level of reserve load.

What should the reserve load be for your RV?  Currently, RVIA considered 10% to be the minimum Reserve Load. However, the few actual Tire Engineers that are posting on RV forums are suggesting a MINIMUM Reserve Load of 15%  with more being desirable.
I know that on my Class-C I am running closer to 20% Reserve load based on actual "4 corner" weights i.e. individual tire position scale readings.

What is your actual Reserve Load?

Friday, December 21, 2018

Video showing why Interply Shear happens

As we head into the Christmas Holiday, most of us are not thinking about our tires.

Here is a quick post that might help people with a better understanding of how and why tires can develop tread/belt separations. The video is focused on multi-axle RV trailers but ALL radial tires have this force that is called "Interply Shear." It's just more obvious on these trailers.

Now go finish your shopping. One gift suggestion I can make is a good digital pressure gauge. The gauge I use as my "Master" is the Accutire MS-4021-B digital tire pressure gauge. Learn more or order on


Friday, December 14, 2018

Better Fuel Economy for Class-A

Smartway is a program from the EPA that is supposed to help truckers select tires that have better Rolling Resistance (the way tires can be measured) for better fuel economy.

Review the information provided and you will see that most truck/bus tire companies have a line of tires available that meet the Smartway standards.

Will selecting one of these tires get you 20 mpg on your 40' DP  No it won't but it might get you a 5% improvement which could be like getting $ 0.15 off a gallon of fuel which
I bet anyone would like.

While we are on the topic of fuel economy here are some tips from RVTechLibrary

ß Every 2% reduction in aerodynamic drag results in approximately 1% improvement
in fuel economy.
ß Above 55 mph, each 1 mph increase in vehicle speed decreases fuel economy by
0.1 mpg.
ß Worn tires provide better fuel economy than new tires, up to 7% better fuel economy.

ß Ribbed tires on the drive axles provide 2–4% better fuel economy than lugged tires.
ß Every 10 psi that a tire is underinflated reduces fuel economy by 1%.

ß Tires make biggest difference in mpg below around 50 mph; aerodynamics is the
most important factor over around 50 mph.
ß The most efficient drivers get about 30% better fuel economy than the least efficient
ß Idle time is costly. Every hour of idle time in a long-haul operation can decrease fuel
efficiency by 1%.

Friday, November 30, 2018

Can I change from LR-F to LR-G or H? (ply range change)

A motorhome owner needs to change his 245/70R19.5 LR-F  tires and asked some questions:

My questions are:
  1. Will the truck "H" rated tire give a much different ride than the recommended "F", or "G" rated tire?
  2. Some manufacturers claim UV protection. Does it make a difference?
  3. Are there any particular brands to steer clear of for RV tires?
  4. I qualify for veterans/AARP discounts. Are there other good discounts?
  5. What else should I be looking for/be concerned about?
My answer:

LR-F  vs LR-G  vs LR-H
You should never go lower. If you go up the only way you will get increased load capacity is by increasing the inflation. It is unlikely that you will get a different ride if all you change in the marking on the tire sidewall for what the Load Range is. Now if you also change brands then ride can change and if you change from a ribbed design to traction design then I would expect ride and noise to change. When you check the load/inflation tables you will note that the capacities for LR-H match the capacity for the LR-G at the LR-G inflations so it is OK to run an LR-H at less than the max inflation on the side of the tire as long as you know the actual load on each tire position and NEVER run lower inflation than what is needed to support the actual measured load.
Remember it is the air pressure, not the tire construction that actually supports the load.
I have a number of posts on my RV tire blog on Inflation as well as other tire topics so you can learn quite a bit there.
RE Brand. Yes, many to choose from. One thing to consider is how many and location of dealers."Billy-Jo-Bob cheap tire emporium and bait shop" may have the best price but with only one store how are you gonna get service if you travel?
Many of the "majors" also have alternate lines at lower cost, sometimes even made at the same plant with most of the same materials. For example in Warren Co TN Bridgestone truck tire plant also makes Firestone and Dayton brands which are less expensive. Do some research to see what other brands have.
Does the country or origin make a difference? If "Made-in-Canada" or where ever is important then be sure to read the tire sidewall as the country of origin is always listed. You can also look up the actual individual plant location by reading the first 2 characters of the DOT serial and checking HERE or HERE for actual plant location.  

UV protection:  All tires have some level of protection. There is no industry standard so it's impossible to get a meaningful comparison. Some will claim "more" or "better" but more than what isn't answered. Covering your tires will do a better job of extending tire life than simply depending on advertising.

Discounts. Yes, there are a number out there. Some might even vary depending on location as some large dealers can offer better discounts simply due to their volume.

If the price is a major concern you may be able to do a rolling change over a three year period as I suggested in THIS post.

Friday, November 16, 2018

Setting tire pressures on cold days?

Here was his post...
"I am looking for a formula for adjusting "cold inflation" pressures for large tires. I have read that for smaller (car) tires the recommendation is 1 psi per 10 degrees F.

What is also not clear is what the assumed "cold" temperature is. Is assumed to be 65 F, for example. So if we are sitting at 25 F, what is the adjustment?"

My reply
Here is my blog post that explains "Cold Inflation"
We are not in High school chem class so there is no adjustment need to get to "standard " temperature.

If you are running a TPMS, which you should be, you will soon recognize the normal range of temperatures and pressure variation in your tires.

I would not worry about adjusting inflation pressure is today's expected temperature is +/- 10F from yesterday. After all, you should be running +10% over the inflation you need to support the measured load.

Let's say your minimum needed is 90 psi. Adding 10% that means your cold inflation (cold meaning the tire was not driven on or in direct sunlight for the previous two hours)  should be 99 psi so you round to 100 psi. All is good.

The next day's weather is 20F colder. That would mean the tire inflation will drop 4%  ( 2% per 10F change)

So now your tires would be 96 psi but since you have to stay above 90 I would say there is no reason to worry about adding air. Just enjoy life and head out.

If the next day your weather went up 10F instead of down that means tour "cold" inflation would go up 2% from the 96 or to about 98psi. Again you don't need to do anything.

I would not worry about lowering the pressure until I saw pressures 10 psi above the pressure molded on the tire sidewall. When you get to that point you could bleed off a few psi but remember if the weather gets cold over the next couple of days you will be needing to add air again.

Don't get your shorts in a bunch about inflation. Just watch your TPMS and you will learn what is normal for your coach.

Friday, November 9, 2018

Remember: A tire is just a "tool" you use to get a job done

This in response to a report of tread cracking of some traction design tires placed on a custom heavy car hauler trailer. The owner was blaming the 4-year-old tires.

I do note that when there are tire problems, including actual "failure," it's common for some to say "My xxx brand tires failed, I will never buy xxx tires again".

Well, sorry to tell you but there is no such thing as "Fail-Proof" tire. This was even part of the testimony mentioned by DOT spokesperson during the Ford Explorer rollover fiasco of 2000.

Today's tires are amazingly robust. Even when they are made in Japan or Timbuktu, and we all remember how bad "Made In Japan" was when were growing up. As I pointed out in the thread, the subject tire an All season traction design was not at all appropriate for heavy trailer application. Both the tire type/size was wrong and the tread pattern was wrong.

Why is this the tire's fault?

If you put a truck tire with a heavy off-road mud traction tread design on the front of your 40' DP and had loud noise and vibration and harsh ride would that be the tire's fault? Would simply changing tire brands from say Bridgestone to Michelin solve the problems if you selected just another heavy off-road mud traction tread design? No of course not.
From my experiences as a tire engineer, I can tell you that I can probably "fail" any tire in under an hour and under 50 miles if you let me set the conditions.

A tire is just a tool you use to get a job done. If you don't select the correct tool that is appropriate for the job you want to be done why is it the fault of the tool manufacturer? Think of the absolute best tool company. Now select one of their flat blade screwdrivers.
 OK now start using it as a chisel and pound on it as you try and cut through some rusty bolts. After cutting through a few bolts would you blame SK or MAC or  Snap-On or ???? if the point of the screwdriver is dented and chipped?

Different tires are just tools with different intended usage.

Basically I would suggest "Rib" or "All position" designs for the front position and if you intend on occasional off-highway travel (a gravel road at a campground doesn't count) then you could select something with a bit more traction capability but I would consider any "Drive" position as not a great choice for Motorhomes or trailers.

Friday, November 2, 2018

Do you need your own air compressor? & Certified tire inspectors

Saw an RV forum post on this topic.  Here was my answer.

If you are running a TPMS (which you, of course, should be), you should have received plenty advance notice of needing to add 3 to 5 psi. This slight loss of pressure is due to normal air loss and pressure change due to change in ambient temperature. You can easily top off your tires at your next fuel stop.

If you don't have TPMS and discover you have been driving on a tire that needs more than 20% of it's required inflation, you should be calling road service and have the tire changed, as there is a good chance you may have done permanent internal structural damage. I consider this operation on the under-inflated tire made the tire unsafe to re-inflate until the tire has had a complete internal and external inspection by a trained tire service person, not just the guy that mounts tires who probably has not received the training.

AFTER the inspection, the tire should only be inflated in an approved safety cage as doing otherwise can lead to serious personal injury.

 Regarding how to find trained, certified tire inspectors...  Use THIS link    from Tire Industry Association.

There is a directory that you can search by zip code. Those listed are TIA Members, and those with the Certified Patch next to them have been TIA Certified.

I will suggest that folks with 19.5 or larger tires or with Load Range E, F, G or higher or with any steel body ply of any Load Range go to Certified,  Commercial inspection

People with Passenger, LT or ST type tires of lower load range, can use the "Automotive" link but a certified Commercial person should be able to inspect smaller tires too.

Remember as Sgt. Esterhaus said 'Let's be careful out there'

Friday, October 26, 2018

Another thread on how fast but from the drivers view. Have a drink.

Some folks insist that the "have" to drive at 70 to 80 while pulling 15,000 pounds of trailer on old, too small tires. I had to jump in with this reply.

My reply on forums like this one is from the viewpoint of trying to give the best answer from the point of extending the life of your tires.
Personally, as a retired professional race car driver, I don't buy the "Speed Kills" mantra. If that was true I would have died in the 70's when I started racing.
IMO it is inattention and the false belief that "I am the best driver around" that most people seem to believe. If you are listening to the radio you are not paying attention. Worse, if you drive and are having an extended conversation or listening to an audio book or the news, you are not paying full attention to the task at hand. Do you keep both hands on the steering or just a couple fingers from one hand? There are more but you should get the question.
Having only driven cross country 4 times I have a little experience but do not understand those who claim you have to drive the posted speed or "get run over". I set my cruse at 62 to 63 and do not recall anyone passing and honking their frustration. But then I keep to the right lane and keep an eye on the vehicles behind me so the occasions when I pass a slower vehicle I do not pull out in front of someone who feels they own the highway.

As a Tire Design Engineer, I know that tires have a finite life of cycles and tread wear is not the only limiting factor. The higher the load or lower the inflation or faster the speed or hotter the temperature or longer you park the tires in full sunlight, the sooner you will "consume" the finite life in any tire, no matter the brand. There are just too many variables to predict the moment when a tire will "give up the ghost". Also, tires do not always fail the instant you have done "fatal" damage to them. Sometimes it might take many miles or days or weeks to come apart and surprise you while you are drinking a coffee with one hand and steering with your knees.

This blog has over 300 posts on the numerous ways you can damage tires and shorten their life. I still get replies, almost every day, from those who think that because they haven't had a tire "Blow Out" or been surprised with a tire failure they know all there is about tire life and proper usage.

There is a saying a friend uses frequently. "You can lead a horse to water but you can't make them drink".  IMO this blog is  a large body of cool clean refreshing water, and it's free. Drink up.


Wednesday, October 17, 2018

Do RV owners know about tire age?

There are many posts on how to learn the calendar age of tires by reading the DOT serial number and "de-coding" the last four digits to learn the week and year a tire was made.

But just knowing the calendar age doesn't tell you the useful life of a tire.

Some folks think ten years or longer is OK; others think it is a good practice to change every two years.

We each have different operating conditions which affects the "functional age" of our tires.

I arranged for a poll to be taken on It asked the question: "Have you ever heard the phrase 'Tires age out before they wear out?'"

I was pretty sure that many of you had heard the phrase, but now I have the numbers.

If you participated, Thanks!

Friday, October 12, 2018

Do you need a spare tire?

Just read a tale from a Class-A owner who suffered a sidewall puncture on a weekend and had serious difficulties getting service or replacement tire even though he had been paying for RV Roadside Assistance.

I have posted about spare tires on this blog and on various RV forums.

One thing almost no one considers when shopping for a new RV is the potential need for a spare tire. After all, lights in the ceiling seem to be more important to many as is having 3 televisions

The assumption that having "Roadside Assistance"policy is good enough, many times does not work as well as the advertisement.
Competent truck tire service centers have the ability to change tires at the side of the road but it appears that the insurance companies (ones selling the idea of assistance) are really only knowledgable about cars.

I have suggested that if you don't have a spare you should be able to get a used tire, in your brand, type and size and find some way to carry it around. Used tires should cost closer to $50 for 22.5 sizes and if you know a fellow RV owner who is getting new tires I bet you can make a deal.
Having a tire only, would allow a regular truck tire service truck to come out, mount, inflate and change the tire. You still will need a new tire but you should be able to get a better price on a new tire somewhere in the next couple days when you have time to do some shopping around.
Your "old" spare tire should be thoroughly inspected but should be OK for a few hundred miles at 55 mph max on a rear dual position.

Friday, October 5, 2018

Is it OK to run 70 or 80 or faster with ST type tires/ Yes, If you believe in MAGIC

More posts on an RV trailer forum about driving at 70 to 80+.  My reply here....

The load formula that resulted in the numbers in the Load & inflation charts has not changed since the late 60's. That formula was based on a 65 mph MAXIMUM operating speed. The lower speed was the trade-off you get when you increase the load capacity. "THERE IS NO FREE LUNCH"
Look at the load capacity of a similar sized LT tire. If ST tire companies suddenly discovered some magic rubber or special tire design feature why don't they put that magic stuff in the LT and Passenger tires?

The "Speed Symbol" is simply an indication of temperature resistance of a tire on a short-term(30 min) step speed test.   The symbol is based on an SAE test that specifically is for Passenger type tires. ST type is not covered by SAE.

Those who choose to run at 65+ need to remember that you are "consuming" the finite number of cycles that make up a tire life. Faster speed = higher temperature. Higher Temperature = shorter life.

This a Science folks, not magic. So please when you have a belt separation failure before you wear out your tires please do not come back here with some complaint about "Defective" tires of "China Bomb" tires. You were warned but decided to not heed the warning and advice.

The high load capacity in ST tires was developed based on a stated 65 mph Max speed.
IMO The addition of higher speed ratings was not accomplished with improved engineering but because of Import Tariffs being imposed on non-speed rated tires by US Dept of Commerce in 2017 and as if by magic each and every tire company making ST type tires almost overlight added high-speed capability to the tires. There were no years of incremental improvements in speed capability. If that were the case we should have seen so 70 mph tires then 75 etc but we didn't.
Some have suggested the increased speed capabilities are the result of years of incremental improvements and that tire companies have seen numerous slight improvements. I understand this concept. The problem is I don't know how to exceed a "Step Speed" test and translate a non-specified step into a speed capability that is not part of the test.

Here are the actual test requirements
FMVSS High-Speed test says
"S7.4   High speed performance. (a) Perform steps (a) through (c) of S7.2.
(b) Apply a force of 88 percent of the maximum load rating marked on the tire (use single maximum load value when the tire is marked with both single and dual maximum loads), and rotate the test wheel at 250 rpm for 2 hours.
(c) Remove the load, allow the tire to cool to 35 °C (95 °F), and then adjust the pressure to that marked on the tire for single tire use.
(d) Reapply the same load, and without interruption or readjustment of inflation pressure, rotate the test wheel at 375 rpm for 30 minutes, then at 400 rpm for 30 minutes, and then at 425 rpm for 30 minutes.
(e) Immediately after running the tire the required time, measure the tire inflation pressure. Remove the tire from the model rim assembly, and inspect the tire."

The RPM converts to 50mph, 75 mph, 80 mph & 85 mph so the steps are not linear.  What would you consider exceeding the specified test by 15% look like?  run 34.5 minutes at 85  or run another 30-minute step at 97.75 mph?

The Speed test above is required for a variety of tires with ST being one type. LT and Pass tires have different standards and different test requirements.

None of the DOT tests support any "Speed Symbol" claim as DOT doesn't recognize the speed symbol.
SAE high-speed test is not a regulatory test.

"This SAE Recommended Practice provides a method for testing the speed performance of passenger car tires under controlled conditions in the laboratory on a test wheel. This procedure applies to “standard load,” “extra load,” and “T-type high-pressure temporary-use spare” passenger tires."

Maybe someone can explain how a P235/75R15 would be rated for 1,874# @ 32 psi and can carry an "S" speed symbol (112) but in trailer or pick-up service is only rated to carry 1,703#
An LT225/75R15 is rated for 1,445# @ 35 psi and can carry an "S" speed symbol
Then we see an ST225/75R15 is rated for 1,760# at 35 psi with a Speed Symbol of "N" (87)

If you only want to look at dimensions, load & inflation I don't know why all three wouldn't be rated for 1,703 to 1,760# load capacity.
 Why haven't the tire companies put the same better construction features they apparently are putting in the ST tires in the LT tires? where load capacity is an important factor in selecting the vehicle?


Friday, September 28, 2018

Are ST type tires "Required" on trailers?

Had a question:
"My 2012 Airstream Classic came with Goodyear Marathon ST225/75R15 LR-D tires. I would like to tell you how many original installed tire I have had blowout using the original and same size tires as replacements. I even replaced the LR-D's with LR-E's and still only get about 3,000 miles before blowouts. It is the damage caused by the blowouts that concerns me most. I am tired of fixing my trailer after blowouts.

As I alluded to in my earlier message, I owned an Alpha 5th wheel that had 16" wheels as the original size. After 5 blowouts in one 6,000 mile trip I replaced the tires with Michelin LT235/75R16 XPS RIB (All Steel) tires. These tires are made with steel sidewalls. Many thousands of miles later I traded the trailer for my present Airstream without any more blowouts on my Alpha.

I have another question that does not appear to have a good answer....If trailer tires are so good and must be installed on a trailer when truck tires wear so much better, what is the rational for requiring the ST's on a trailer? My truck, 2002 GMC Duallie uses LT tires and I can not wear them out not blow them out before I feel it is safe to replace them (7 years). I am running LR-E' LT types on my truck.

I am sorry to say that I do not agree with the idea that ST tires for trailers is the safest and best option.

I have never seen any "requirement" that trailers run ST type tires. However, ST type tires can only be installed on trailers, so that might be the "requirement" you are thinking of.

The selection of tires to use on any vehicle is up to the RV company. As far as I can see they select tires based only on load capacity, physical size and cost. I don't know what the RV company actually pays for ST tires but it is probably less than an LT of equivalent load capacity.

If your only concern was for lower cost when comparing two tires of equal load capacity and the ST tire was easier to fit (it was smaller and did not require you make larger wheel wells), Why would you select the higher cost tire?  If you don't offer a meaningful warranty, you do not face the expense of warranty costs if there is a tire failure so what is the incentive for the RV company to spend more?

Friday, September 21, 2018

How to take good pictures of a tire tread (Not too technical I hope)

I occasionally  see posts from folks complaining about some tread or sidewall condition but either they can't properly describe the condition or if they have pictures the image is too small, poorly lit or from too far away to allow proper inspection of the condition for me or others to help and offer an opinion.
I recently wrote this post:
Some general guidelines and suggestions for taking good pictures of tire conditions.

Full sunlight and NO flash.
Set the camera to maximum pixels or max quality
Take close enough to only include 8" to 10" of the tire surface. Closer if you have a camera that allows "Macro" or close-up.
If all you have is a phone you need a minimum of 10 Megapixles.
Only use "Optical Zoom" not digital zoom if possible.

Here is an example of a good quality tire picture that would allow someone to make a judgement on a tread surface complaint.

The following, while showing an obvious problem really would not be good enough to show a sidewall or tread cracking complaint.

Friday, September 14, 2018

Do you have to replace valve stems when installing TPM sensors?

Got this question on an RV forum:
"I understand from the thread that you should use metal tire stems with TPMS caps. The ends of the tire stems on our trailer are metal with the only visible rubber part right where they go through the wheel. I am guessing these are still considered rubber stems and should be replaced prior to installing TPMS?"

"Metal" stems can be a bit misleading as some newer stems may have some brass showing. I posted pictures on THIS post showing standard passenger rubber valve, a "bolt-in" metal valve, and a new style rubber/brass valve.
When it is recommended that "metal" stems be used, what is actually meant is "bolt-in". You will see the nut that retains the bolt-in stem on the outside of the wheel for most of these stems.

The concern is that over time (months or maybe years) the extra weight of TPM external sensors may cause some vibration and movement of the rubber stem body which might result is a failure of the rubber portion of the stem.

I am not aware of any testing done on the new style rubber/brass stems with a TPM sensor screwed on, so I and others are erring on the side of caution when we recommend "bolt-in" metal stems.

Cracking and failure of the rubber part of valve stems is another time & temperature aging thing, just as it is with your tires. You visually inspect your tires monthly, as outlined in your RV  owner's manual, so just include the rubber part of your valves and just as you would replace your tires when signs of significant signs or aging (cracks) are found do the same with your valves. 

You could consider running your current valves until you get new tires and at that point have the bolt in valves installed. In the meantime, I would take a close look at your stems when doing your monthly visual inspection. You can push sideways a bit on the rubber/brass stem and look for any cracks. If you find any, that would mean you need to replace them sooner rather than later.

Friday, September 7, 2018

WARNING - Super Technical Post Tire failure and Interply Shear

I have had some folks who seem to want to replace Science with what they term "common sense".
In my opinion, this is why some people still think the Sun and rest of the Universe rotate around the Earth or that believe the Earth is flat or the Moon landing is a hoax simply because they don't understand the Physics and Science behind the stated facts.

I mention this because I continue to get people claiming that because they checked their tire pressure last week or yesterday and had a tire failure today, some sort of "magic" must have happened to cause their 65 psi or 80 psi tire to suddenly go sky high in pressure to cause the tire to explode due to high pressure. You don't have to do the technical research yourself just as you don't go to medical school to learn about some ailment you have. You do have a choice. You can trust your Doctor or go to Medical School or in the case of tire failure, you can put your trust in those who have spent years working on and constantly improving tire design, or you can simply believe that tires fail because of some unidentifiable "defect" that was built into the tire.

I have previously posted on how Sidewall Flex Failures can easily mislead the inexperienced into believing they had a "blowout due to high pressure." I also have some who do not want to accept the Science behind the need for tires in trailer application to run lower speeds and higher inflation in an effort to lower (but not eliminate) the probability of Belt Separation.

So I decided it is time to get out the "Big Guns" and cite some actual tire Science.

Here is a question from someone who took issue with my recommendation to increase the inflation in trailer application.

Did you ever notice that the two rear tires on the tow vehicle are putting hundreds of horsepower to the road? Did you ever notice that the two front tires are steering the whole assembly?
Here is my reply:

Yes, I have noticed that. I also know that the internal structural forces are different for torque than for high "slip angle" which is the situation in trailer application. Front tires on cars, motorhome or tow vehicles do go through slip angle but usually in the 1° range while trailer tires are subject to angles in the 10° and greater range. The forces are NOT linear. 10° can generate significantly more shear than 1°.

If you want you can purchase the software package HERE for the vehicle response and handling. The results of this vehicle simulation show the vertical and side loads being applied to tires as you drive around a corner. These forces can then be used as the input into Finite Element software programs to determine the structural loads on tire components.

Here is is a technical paper on "Interply Shear Stresses and Coupled Deformations of a Folded Belt Structure Under Extension"

Personally, I prefer Dr. Song's paper on"FATIGUE OF CORD-RUBBER COMPOSITES FOR TIRES."
Here is the abstract.
Fatigue behaviors of cord-rubber composite materials forming the belt region of radial pneumatic tires have been characterized to assess their dependence on stress, strain and temperature history as well as materials composition and construction. Using actual tires, it was found that interply shear strain is one of the crucial parameters for damage assessment from the result that higher levels of interply shear strain of actual tires reduce the fatigue lifetime. Estimated at various levels of load amplitude were the fatigue life, the extent and rate of resultant strain increase (“dynamic creep”), cyclic strains at failure, and specimen temperature. The interply shear strain of 2-ply ‘tire belt’ composite laminate under circumferential tension was affected by twisting of specimen due to tension-bending coupling. However, a critical level of interply shear strain, which governs the gross failure of composite laminate due to the delamination, appeared to be independent of different lay-up of 2-ply vs. symmetric 4-ply configuration. Reflecting their matrix-dominated failure modes such as cord-matrix debonding and delamination, composite laminates with different cord reinforcements showed the same S-N relationship as long as they were constructed with the same rubber matrix, the same cord angle, similar cord volume, and the same ply lay-up. Because of much lower values of single cycle strength (in terms of gross fracture load per unit width), the composite laminates with larger cord angle and the 2-ply laminates exhibited exponentially shorter fatigue lifetime, at a given stress amplitude, than the composite laminates with smaller cord angle and 4-ply symmetric laminates, respectively. The increase of interply rubber thickness lengthens their fatigue lifetime at an intermediate level of stress amplitude. However, the increase in the fatigue lifetime of the composite laminate becomes less noticeable at very low stress amplitude. Even with small compressive cyclic stresses, the fatigue life of belt composites is predominantly influenced by the magnitude of maximum stress. Maximum cyclic strain of composite laminates at failure, which measures the total strain accumulation for gross failure, was independent of stress amplitude and close to the level of static failure strain. For all composite laminates under study, a linear correlation could be established between the temperature rise rate and dynamic creep rate which was, in turn, inversely proportional to the fatigue lifetime. Using the acoustic emission (AE) initiation stress value, better prediction of fatigue life was available for the fiber-reinforced composites having fatigue limit. The accumulation rate of AE activities during cyclic loading was linearly proportional to the maximum applied load and to the inverse of the fatigue life of cord-rubber composite laminates. Finally, a modified fatigue modulus model based on combination of power-law and logarithmic relation was proposed to predict the fatigue lifetime profile of cord-rubber composite laminates."

Let's see if I can help. Here is a key phrase "the fatigue life of belt composites is predominantly influenced by the magnitude of maximum stress."  Now, think of the tire side bending when you back a trailer into a campground site.

The video in THIS post shows the side loading during relatively low angle turns

I apologize for going so deep into tire engineering but sometimes the facts are needed to demonstrate that "common sense" doesn't always lead to the actual facts.  Before writing my posts on Interply Shear on trailer tires I had both vehicle simulation and tire structural shear forces run. We learned that for tires on multi-axle trailers, like RV trailers, the belt shear forces can be 24% higher than the belt shear forces of identical size, load and inflation tires on a motorized vehicle. This is why I suggest a different approach to tire inflation for trailer application than motorhome applications.

I will try and "lighten up" a bit in the next few posts.


Friday, August 31, 2018

TPMS Comparison TireTraker external vs TST 507 internal. Summary and my opinion

Since March I have been conducting a comparison test of two different TPM systems. When I started in March the Ambients were down near freezing. I completed my comparison at the end of July with the ambient temperature above 90°F.
I started by checking the accuracy of the various sensor pressure readings. The summary of that post indicated there was no meaningful difference between the systems for pressure readings for the 12 sensors I tested.

In THIS post I confirmed the temperature readings were also essentially the same by comparing the morning readings after a night long temperature soak.

The next step was to see what the systems reported for the hot running temperatures. I expected that the numbers from the internal system would be higher than the numbers from the external sensors. The reason for this is that the external sensor is actually reading the temperature of the outer end of the metal valve stem that is wiping around in outside air being cooled. The small collum of hot air inside the valve stem just can't transfer enough heat as fast as the metal (brass) valve stems is being cooled off. Here are the numbers. But I had to wonder if the cold ambients might somehow be skewing the data so, knowing I was planning to travel to Indiana in April, Michigan in June and  Wyoming in July I was hoping that the ambient temperature on one or more of these trips would be significantly warmer than my March trip.

In June I posted my opinion on the value or lack thereof of temperature readings from TPMS. This opinion was not based on any specific results from my testing but just from some serious contemplation to tire temperature recording I had been involved with when working on Indianapolis race car tires and my observations in test laboratories while I worked as a tire design engineer.

Back in May 2012, I posted some actual running temperature images recorded by some high priced laboratory instruments. You can see the results here. Clearly using a handheld IR gun after you come to a stop or depending on the temperature of the air inside the tire, which is obviously an "average" of the hot and cooler areas of a tire, is not going to give you a reading of the hottest part of a running tire. 
If we are concerned about the advanced warning of a tire failure, tire temperature numbers from a TPMS is not going to be sufficiently precise to identify the temperature of the hot spot. While high temperature can lead to a tire failure, the failure will most likely occur at the hottest spot which is not the "average" of the internal surface of a tire. Also, extended periods of time at elevated temperature can contribute to the degradation of rubber which could eventually lead to a failure like a belt separation while never being hot enough to set off the high-temperature alarm.

Finally, in July, I could review the results of my readings with higher ambients of the different readings observed with the internal TST 507 system vs the external Tire Traker system.

In mid-Aug I posted the test results of the external Traker system vs the internal TST system.

OK, so what is the bottom line?

IMO the performance of the two different systems is similar enough to make recommending one over the other impossible.

There is a cost penalty with the internal system of a little over $100 plus any purchase price difference. Looking at the two different web sites I find the 6 sensor Tire Traker system with booster available at $398.  The Truck System Technology 6 sensor internal system with the booster is listed at $599. You will need to figure there will be an extra charge to pay for the dismount, mount, and balance of the internal system. I had the TST system installed locally for $109.07 which would bring my total to $708. 
To answer the question some of you may have. I purchased and use a Tire Traker TPMS in 2009 befroe I started this blog and plan on continuing with that system for the foreseeable future.

The advertisement you see on this blog does not involve me as it is between any advertiser and

Friday, August 24, 2018

When should you replace your tires? HOW OLD is too old?

A lot of folks talk about tire age and when they "age out."

Here is what a few tire companies have to say:

Passenger & Light Truck Service Life

Japan Automotive Tyre Manufacturers Association (JATMA) recommendation: At 5 years have tires inspected by "Qualified tire service person" and no matter the condition replace any tire at 10 years.

Bridgestone & Firestone issued a Tech Bulletin that supports the 10-year maximum but advises that "some tires will need to be replaced before 10 years due to operational conditions.

Michelin, Falkin, Yokohama and Vredestein suggest tires be inspected by specialist annually starting at 5 years and replaced at 10 years as a precaution "even if such tires appear serviceable and even if they have not reached the legal wear limit"

GM recommends tires be replaced after 6 years

Hankook says to replace tires 10 years after manufacture "even if the tire was never used"

Goodyear, Continental and Cooper do not give a maximum calendar age but stresses "Monthly Inspection" along with proper storage and monitoring of inflation pressure.

Kumho says to have tires inspected annually starting at 6 years and to replace the tires at 10 years, even if the tires appear serviceable.

Vredestein suggests tires be inspected by specialist annually starting at 5 years and replaced at 10 years as a precaution even if such tires appear serviceable and even if they have not reached the legal wear limit. They also say that tires on trailers, campers, horse trailers and caravans (motorhomes) be replaced at 8 years.

They are talking about the annual inspections after 5 years to be done by a "Tire Specialist", not just anyone that sells tires. I would suggest that when having this inspection done you ask for a written assessment as a minimum. You can ask if the dealer has an ATS Certified Technician on staff.

Friday, August 17, 2018

Internal vs External TPMS comparison Hot ambient

Since March I have been conducting a comparison of a couple different TPM systems. One from TST has internal sensors, as seen here.

Please note this shot shows the TPM sensor and the steel band retainer. I was in the process of removing the extra band material as this is a 16" wheel. A larger wheel would need a longer band.

The other from TireTraker has external sensors that screw onto the end of your metal valve stems. Or as I did on the end of my dual tire extension hoses seen here.

I also had some TST external sensors that mount on valve stems and look very similar to the TireTraker brand sensors.

This is the final post on the direct comparison of different sensor location. I previously posted results HERE but there was a question raised as the internal system was one brand from TST and the External sensors were from TireTraker. We needed to learn if the minor differences were due to the different brand or the different sensor location.

To accomplish this I mixed the external sensor brands and used three external sensors from TST vs three from TireTraker. Here are the numbers.

Ambient was in the upper 80s to lower 90s for this part of the test.

Internal vs External TPMS test results

All temperatures are in °F and pressure in Psi
Internal System.            
Temp.  RF 102   LF  102    RRO  104   RRI  109   LRI   113   LRO  106
Press          69           70                 82              84              84              86

TST External System
TEMP    RF  86                                      RRI   84                         LRO   86
Press           70                                              84                                  86

Second run
Internal System.            
Temp.  RF 108   LF  111    RRO  115   RRI  124  LRI   117   LRO  109
Press         70           71                 84              87              84              87

TST External System
TEMP    RF  79                                      RRI   90                         LRO   82
Press           71                                               86                                    87

Tire Traker External System
TEMP                    LF  78     RRO   100               LRI   91
Press                          70                  82                         83

Now I realize that reading the above doesn't make the differences and similarities obvious or even easy to see so here is a summary of the findings.
TST system  External vs Internal pressure readings are essentially the same
TST system  External temperature readings run 20F to 30F cooler than TST Internal temperatures.

TST system internal sensor pressure vs external sensor pressure readings Tire Traker are essentially the same
TST internal temperatures readings run 15 to 33 (average 25F) hotter than Tire Traker external temps.

Reviewing the above results and the results posted in the July 20 report and the two March reports from tests with Ambient in the 30s and 40s, it is my considered opinion that both TST and TireTraker systems provide similar numbers for inflation pressure and that external sensors report temperatures that are 20F to 30F cooler than internal sensors for both brands.

Given the above comparison on the data readings, I would consider these two systems a toss-up for reporting the temperature or pressure, as long as you remember that external sensors are being cooled by external ambient so will report 20F to 30F lower temperatures.

There are some differences between the two systems and some of these are subjective. I will cover these differences in a future post.

Friday, August 10, 2018

Do inner duals fail more often? If so, why?

Read this on an RV Forum thread:

"Isn't it weird how it always seems to be the inner tire that goes bad or blows? Seemed to always be my experience (bad luck) when I was driving semi's for a living."

While I have never seen a documented study, I can certainly believe this observation can be made by many. There is Science behind the "Why" this may be true.

For those that have reviewed the post on Tire Covers that explains the effect of temperature on tires, you have learned that it is the higher temperature that accelerates the tire "aging" process. If you look at the temperature readings on sets of dual tires you will see that when the tires are inflated to equivalent pressures the temperature of the inner dual on Motorhomes will usually show as a bit hotter. The difference isn't a lot but the effects of that difference I believe are cumulative.

It is also true that older tires are more likely to fail due to the degradation of the rubber flexibility and strength.

Please do not take this observation and assume you need to start adjusting the inflation in your duals to run more air in the inner tire. Doing this could end up resulting in a shift in loading between the pair of tires to place more load on the inner tire, and we know that increased load results in increased operating temperature.

Tire operating temperature develops from complex actions of flexing of the belts and of the lower sidewall which are the two hottest locations on a tire. You might be able to lower the temperature in one location while increasing the temperature in a different part of the tire.

The best practice I can suggest is that you:
1. Confirm the tires in each pair of duals are a "Matched Set" (See THIS older post).
2. Ensure you know the actual load on each set of duals not just the total axle load.
3. Use the Load and Inflation tables to learn the MINIMUM Cold Inflation Pressure for the heavier loaded axle end.
4. Add a 10% inflation to that minimum number to establish your CIP.
5. Inflate all tires on the axle to the same CIP. (matching the inflation within +/- a couple psi is good enough.
6. Run a TPMS to monitor your pressure whenever driving.

Friday, August 3, 2018

Can you change tire size on your RV?

There seems to be a bit of confusion when it comes to selecting tires to replace the size/type that came as Original Equipment on your RV. What you can do and what you should do are not always the same thing.

Here are some statements collected and posted by a knowledgeable person who frequently responds to questions on some RV Forums.

"Goodyear: Never fit tires to a vehicle that have less load carrying capacity than required by the Original Equipment Manufacturer.

Michelin: Never choose a tire that is smaller in size or has less load-carrying capacity than the tire that came with the vehicle.

Cooper: The new tires must have a load carrying capacity equal to or greater than the maximum load carrying capacity specified on the tire placard on the vehicle.

Toyo: Any replacement tire must be of a size and load range that will offer equal or higher load carrying capacity compared to the original equipment (OE) tire on the vehicle

I am not aware of any "legal" requirement that specifies what the RV owner is required to do. While it is a legal requirement from DOT that the vehicle manufacturer must select and specify tires and the inflation necessary to support the stated Gross Axle Weight Rating, as far as I know, this legal requirement does not apply to individual owners of vehicles.

HOWEVER, I doubt that you will find any company or responsible individual willing to state that it is good practice to select replacement tires that do not have a stated load capacity that is equal to or greater than the load capacity of the original tires.

All tires sold for use on public highways have a stated maximum load capacity at a given inflation pressure molded onto both sidewalls. This fact, along with the information in published Load/Inflation tables, makes it relatively easy to find tires with the needed load capacity.

There are a number of reasons to consider an alternate size tire (availability, cost, brand reputation, etc.) but in every case you should only select new tires that can support at least as much load as the OE tires.

The above is based on an assumption that the load on your tires is split equally 50/50 side to side on each axle. Since this is seldom the case, this is an extra bit of information you should consider when shopping for new tires.

I have numerous posts on this blog where I outline the importance of confirming the actual load on each end of each axle. This is because it is possible to have the load unbalanced to the point that one tire may be overloaded even if the total tire load capacity for all the tires on an axle numerically exceeds the total axle load measured on a truck scale.

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.

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. 


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

Friday, June 29, 2018

TPMS temperature readings. Of little value in my opinion.

A comment on TPMS: As a tire engineer, I see little reason to present the temperature of the sensor. IMO this extra information has some problems.
 1. Tires fail from low inflation primarily due to air leaks (puncture, cut or valve problems) but not from just getting hot. You only get a tire hot from running too fast and/or running to low air pressure. The sensor is not actually reading the temperature of the critical location in the tire as the hottest location is internal to the tire construction and measurement of that location can only be accomplished with a needle probe as done by Race tire Engineers ( I did do that as part of my job once upon a time).
2. The temperature number is distracting and is actually the temperature of the sensor, not the tire and the sensor is cooler than the wheel.
3. The TPMS temperature reading is significantly (25F to maybe 50F ) different i.e. lower than the tire temperature for most applications. More on this temperature difference in a future post. While it is possible for a sensor to report a dragging brake or failing wheel bearing an IR hand gauge is a better tool to use if that is your concern, as the hub will be hotter than the wheel and provide an earlier warning.


Friday, June 22, 2018

How to prepare for and respond to tire failure

There are numerous posts on RV Motorhome forums on how the driver whould respond to a sudden tire failure (Do not stomp on the brakes but apply the gas for a couple seconds to be sure the driver has full control. THEN slow down and pull over.)
If towing a trailer you are less likely to lose control of the tow vehicle so you just need to slow down as soon as possible and pull over.

BUT the reason for this post is not to address vehicle control but to identify the actions you should take concerning your tires.

1. Before you have a tire failure you should be sure you have the full DOT serial for each tire written down somewhere.
2. With ANY failure, other than puncture or valve failure you should file a complaint with NHTSA. They need the FULL DOT tire serial and full vehicle VIN.
3. Take pictures.  In full sun, close enough so only half the tire fills the frame So more than a couple pictures are needed. Snapping a shot in the shade from ten feet away under the RV just isn't useful. Get tread, sidewall failed area, and non-failed area. Make sure the pictures are in FOCUS and at the highest quality possible for your camera.
4. Let NHTSA know you have pictures when you file your complaint.
5. Contact the tire dealer, RV dealer and RV MFG of the failure and the NHTSA complaint number.
6. KEEP THE FAILED TIRE. Tire company may be willing to compensate you but you must have the failed tire. Carry an HD trash bag if you must. A folded trash bag doesn't take space and is only a few ounces weight.  No tire = no possible compensation.

Note I have posts and video on how trailer owners, Class B and Class C owners can inspect your own tires each year for belt/tread separations. Running a TPMS will give you an advanced warning on Run Low Flex Failure.
Having the actual scale load on each tire as covered in the owner's manual will help substantiate your claim. Always run the tire pressure on the tire sidewall for your CIP if you are pulling a trailer. Motorhome owners need scale weight printout and a copy of tire load tables.

IMO if you don't do all of the above I am not interested in hearing your complaints.

Friday, June 15, 2018

What should Airstream and similar type trailer owners do?

One of the Forums I follow is on Airstream trailers. There was a post from an owner who was having problems getting the actual loads on the trailer, hitch, and TV. There were a number of posts on what he needed to do.

However, I saw no mention of tire loading or tire inflation so I added the following post to the thread.

"When you are done making adjustments and moving stuff around and get new scale readings. You need to look at tire loading and inflation.

Ideally, you would get the individual axle load as they are probably not split 50/50

You are also probably not 50/50 side to side loaded either.  The good news is that you don't have slides or residential refrigerator so are probably 51/49 side to side or 48/52%

The best approach is to take the heavier loaded axle and assume a 52/48% split and compare the 52% number with the tire load capacity. You should have at least a 15% load capacity margin.  (52% of the heavier axle is no greater than 85% of tire max capacity.

Finally, with a multi-axles trailer, you should use the tire sidewall pressure number as your "Cold Tire Inflation" to try and lower the Interply Shear forces that are trying to tear the tire belts off the carcass.

You can learn more on my RV tire blog."

Note: While I wrote this in response to a specific post/question on an Airstream forum the information on tire load & inflation would apply to similar trailers that do not have slide-outs or heavy residential refrigerators or other obvious unbalanced equipment.

Friday, June 8, 2018

Setting Motorhome inflation for "Newbie"

Reading a forum post that said:
"We are moving across the United States for our FIRST trip in our 06 Mountain Aire. Don’t think that’s a great idea for a first trip, but that’s what’s gonna happen. LOL. I do not have access to a four corner scale. I do have access to a cat scale. I downloaded the Michelin tire pressure guide. From what I remember here, I take the axle weight and use the Michelin guide? I seem to remember someone saying to add 5 psi to compensate for the lack of a 4 corner scale?"
My reply:

You can read my RV Tire blog for more details for Motorhomes (but not towables)

1. If you have no scale reading follow the Vehicle Certification Label AKA (Tire Placard)

2. As soon as possible, get on a truck scale when you are at your heaviest (full of fuel, food, water, propane, clothes, people etc).

3. Using the axle load numbers: Confirm no axle exceeds GAWR (gross axle weight rating) (on placard) and the RV does not exceed GVWR (gross vehicle weight rating) (on placard). If you exceed either max weight rating you must move or eliminate some "stuff" in the RV.

4. If your weights are lower than GAWR then you MIGHT be able to lower your inflation a bit but there is some effort involved.

5. Since you will not have "4 corner weights" you will not know how much out of balance side to side you actually are. Some RVs have been found to be 1,000# out of balance on an axle.

6. I suggest at a minimum you assume one end of each axle is carrying 53% of the total axle load. You could use the 53% number to consult the Load & Inflation tables.  WARNING Michelin tables assume perfect 50/50 load balance which IMO is not realistic, so you need to calculate the individual axle end load ( divide Michelin number by 2)

7. Using the 53% load number and the calculated Michelin load number you now know the MINIMUM Cold Inflation aka MCI number. It is suggested you add 10% to that inflation number for inflation when setting tire inflation. This is your daily measured Cold Inflation Pressure aka CIP. This 10% covers you for day to day temperature and elevation changes.

8. If or when the measured cold inflation (or inflation reported by your TPMS) drops to your MCI number you need to add some air to get back to your 10% margin number.

9. I see no reason to ever bleed off pressure from your daily measured CIP unless your reading is higher than the inflation number molded on the tire sidewall. If you get that high you can bleed down to the tire sidewall pressure but never below the MCI.

Ya a bit of work but once you think about your goal - Stay above the MCI and below the inflation molded on the tire sidewall you are good to go. A TPMS makes the job of checking inflation on each day of travel much easier as you simply check each morning before you start out. TPMS will also give you a warning when or if you get a puncture od have a leaking valve and are losing air. A good TPMS will have some form of "early warning" when you start to lose a couple PSI while running down the road. This could prevent a serious tire failure.

Friday, June 1, 2018

ST type tire speed ratings. One man's opinion.

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 fundamentsl format of the formula.

The "K" shown above is an important concept as it is really a factor based on the expected service.
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 catigory" 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 DT does not recognise 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" engineering are tire companies puting 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? There are construction features that will improve the test performance of tires, maybe even improve the speed rating a step, but I have serious questions on the ability of tire company to increase load capacity by 20 to 25% which is effectively what some appear to be trying to do.

NOTE Goodyear Tire Care Guide ( clearly shows a blanket 75 mph max speed for 17.5 rim diameter and larger tires.

Wonder why they didn't increase the speed rating of these tires using similar construction features they applied to the GYMarathon to turn it into a GY Endurance.