Maximum Operational Tire Speed

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

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

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

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

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

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

 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

 

 

Tires are more than just Round & Black things

A question came up about the Max speed recommendation as published by Goodyear, Michelin, and Bridgestone for tires in RV use.

To help clarify:

Many times the "application" or use of an item results in it additional or different limitations or ratings.

Example

 If I have a tire that is a 225/75R15 and inflate it to 35 psi, How much load can it carry?

If the intended use is on a passenger car 1,874#

If as a single on an LT it is rated for 1,445#

If as a dual on a LT 1,315#

If a Single on a trailer where having passengers in the trailer is actually prohibited 1,760#

and if in a dual application on a trailer 1,570#

Part of the decision process for a tire's capacity in each of these different applications includes such things as:

Will the vehicle carry passengers?  Is the "Reserve Load" of the tire in the application normally close to zero or maybe 10% or normally closer to 25%

An obvious example where the application affects the load capacity is well established in the use of "P" type tires on a truck, trailer or multi-use vehicle such as an SUV. In these applications the load capacity is reduced by Industry standards by dividing by 1.10 so the 1,874# capacity becomes 1,703#

For heavy truck there are published guidelines that allow an increase in load capacity if the max speed is significantly reduced.  As the speed is lowered the load capacity can be increased up to 16% with no increase in inflation.

Similar to above if the tire is made for a specific market a tire company my have more demanding requirements on the tire during the development process. An example might be it the tire was being made for a market where the speeds and Ambient temperature was both very high the tire might require a more robust construction. Another example might be providing a special tread rubber if the tire was to be used at extremely low temperatures where the tread rubber might even crack if a "High Speed" rated tire tread were used.

Tires are much more complex than many people suspect. Ideally, an owner would be more informed and knowledgeable about tires and their limits and capabilities when making a purchase. They are much more than just "Round Black" things that almost no one "wants" to buy.

 

##RVT964

ST tire speed rating

Point of clarification.

The Tire & Rim Association Load formula used by the industry for ST type tires is based on a 65 mph max operating speed. They even identified inflation increase and load capability reductions necessary to operate up to 75 mph.  Neither P or LT tires have stated max speed associated with their load formula or calculation as far as I can find. P-type do have the stated 1.10 "De-Rating" of load capacity when used on a trailer or P/U.

If you look at similar physical sized tires and include the P Derating you will see the de-rated P and LT tires give similar results for load capacity.

BUT the load capacity of the ST type is significantly higher. Why would that be if there were not some other limiting factor such as the 65 mph speed limit?
Some people would have us believe that ST tires all had some magic rubber or body cord added to them in 2017 when speed ratings higher than 65 were added. The reality is that the speed rating was added to avoid import tariffs. Maybe you think that all the different tire companies used this "magic" rubber to give them an overnight increase in speed capability.

Why isn't there a single tire company anywhere in the world using this "pixie-dust" in their LT or heavy truck tires to give them an extra 10% to 25% more load capacity?

I have seen a section of a GY Endurance and it clearly has "better" construction than some non-speed rated tires. Only time will tell if this new construction results in a better life, But I seriously doubt that you can go from 65 mph max to 99 or 106 mph with an ST tire and not suffer some negative consequences.   Yes, many will say they do not drive that fast but we have all been passed by trailers going faster than 75. Some even say they tow at 80+.

Going faster can do structural damage and we have covered how damage is cumulative.

What I have not seen is anyone changing their advice on tire life in RV usage.
What ST tire company now says up to 10 years life or even claims over 5-year life?  I have reviewed a few different owner's manuals and see 3 - 4-year life suggested.
I have previously written about Interply Shear and while the addition of a Nylon cap on top of the belts may extend the life of a tire and it may also improve the tire's ability to pass some speed test designed for passenger tires I still have serious reservations about any trailer being towed at any speed above 65 mph.

I'm not saying "Don't buy tires with higher speed symbol" What I am saying is that you should still stay to 65 mph max or you may end up paying the consiquences with shorter than expected tire life.

But that's just my opinion.
 

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.

##RVT869

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.

J1561_200102
"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?

##RVT866

How you might run ST type tires at 75 to 80

RE Speed Ratings and operation speed.

My comparison to engine redline seems to me to be the easiest for many to understand. It is possible to run an engine right at redline or even above for a short time but I can't find anyone that advocates operating an engine at the rated max speed for any significant time or distance.

High Speed test is not a DOT regulatory test but is a test based on Society of Automotive Engineers testing. To be marked with a speed symbol a new tire needs to run for 30 minutes on a smooth drum at the stated speed.

There is no requirement for a tire to be "conditioned" with a few thousand miles at lower speed over potholes and up on curbs or with 110% of the rated load or for the tire to be able to pass DOT regulatory durability tests after running the SAE High Speed test.

Yes, an increasing number of ST type tires now come with a speed symbol molded on the sidewall. The primary reason for this seems to be to avoid import duties. What many want to ignore is the fundamental truth that the load capacity of ST tires is much higher than an LT type tire based on the premise from 1970 that the ST tire would be on a single axle trailer that was limited to 50 mph operation speed.

Molding the letters "ST" on a tire is not magic. Physics still applies. If people want to drive at 75 or 80 while towing as they would with their LT type tires and not have failures, then I suggest they pay attention to the Physics and limit the actual tire load as if it were an LT type. This is easy to do.

First simply look for an LT type tire with the same dimensions i.e. ST235/75R15 > LT235/75R15 in the Load tables and limit their measured load to the number found in the LT tables.

If you do that you will probably see a significant reduction in tire failures. Of course this also means you are not in the 50+% of RV owners that operate your tires under-inflated.

If you ignore the Facts and Physics of reality you will have to live with the consequences no matter how much you want to believe otherwise.

Bottom Line. If you want durability and life more like LY type tires then treat them as if they were LT type tires.

IMPORTANT NOTE and WARNING
As with the engine in your RV or tow vehicle it may be possible to run faster but it does not mean you will avoid all problems. This post is just about tires and not about safe and reasonable operation of your RV. Personally I think 70 is too fast to drive a "big rig" and 75 is certainly too fast for towing. I have heard comments about an increase in truck tire failures due to increased speed limits in many states. A MAX of 75 is stated in some tire information guides published by major tire companies and as with any maximum, the closer you are to it the more likely you will have some negative consequences.


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##rvt750

When is Minimum inflation the Maximum inflation?

Sometimes the information on inflating tires can lead to some confusion.  OK a lot of the time it can be confusing to many.

As many of you know I try and follow a number of RV forums and offer comments. I try and focus the comments to correcting serious or significant errors or misunderstandings, especially when Safety Related.

Recently there was some confusion about the Maximum allowable inflation for a tire. Some wanted to co-mingle some information from PSR or passenger car tires with information about large TBR, truck bus radials. The discussion then went way off track. Rather than limit my audience to those following that thread, I decided a blog post would be more appropriate. Also I wanted to be sure to have all the information up to date and accurate I contacted an "old friend" from the tire industry and he sent me this nice summary. With his permission I re-print it here.

First some definition of terms may be appropriate as "tire engineer speak" may confuse some.

"Seating pressure" this is the inflation needed for the tire beads to "pop" home against the wheel/

"FMVSS" these are various Federal Motor Vehicle Safety Standards. If you need some reading that will put you to sleep HERE is a link offered by NHTSA.

 "NHTSA" NATIONAL HIGHWAY TRAFFIC SAFETY ADMINISTRATION

"PSR"  Passenger steel belted radials or the normal tire you find on todays cars

"TBR" Truck Bus Radial  as found on most Class-A RVs

"LTR"  Light Truck Radials 

"CIP" Cold Inflation Pressure

OK here is the nitty-gritty

"First, the max pressure to seat beads as a matter regarding the technician’s safety is 40 psi, whether passenger, light truck, or truck-bus.  This bead seating pressure is totally independent of the tire maximum operating pressure.

It is important to clarify the differences in sidewall markings of the tires you bring up:

PSR & LTR Tires (load range E or less):  Since FMVSS 139, these tires have sidewall markings indicating maximum load AND maximum pressure.  Maximum load means max [static] load, and maximum pressure means max [operating] pressure (cold).  With respect to the minimum pressure that carries the maximum load there is a difference:

·         Passenger Tires:  These tires are usually marked with a maximum pressure that exceeds the pressure necessary to carry the maximum load marked on the sidewall.  For example, the tire may be marked with 44 psi max pressure, but only requires 35 psi to carry the max load.

·         Light Truck Tires (load range E or less):  These tires are usually marked with a maximum pressure that is also the pressure required to carry the maximum load.  For example, a load range E tire marked with max 80 psi would need that same pressure to carry the max load.

TBR Tires (and light truck load range F and higher):  On these tires the sidewall markings indicate maximum load AT a certain pressure (the word “maximum” is not used in regards to pressure).  Maximum load means max [static] load, but the pressure is not the maximum operating pressure (cold).  This marking just follows FMVSS 119.  Essentially, the pressure marking is informative, simply telling the reader the pressure that is required to carry the maximum rated load.

However, for most practical purposes, on TBR tires the pressure marking is typically considered the maximum pressure recommended in the tire while in ordinary service.  Certain situations may permit cold inflation pressure higher than the marking, usually in consultation with a tire manufacturer for a specific product, application, and service.

Regarding load-inflation tables:  As long as you are looking at the right table, this is where you find the pressures needed to carry certain loads for a given tire type, size, load rating, etc.  Note that for truck-bus, you might need to make sure the tire is a “T&RA tire” or an “ETRTO tire” since the tables can differ, even though the size codes are the same (such as 295/75R22.5).  Also, a tire manufacturer may have unique load-inflation table(s) associated with certain tire models, sizes, etc.

If an operator is running at max load, and the pressure to do that happens to be max pressure, then yes, they need to be diligent.  But it is manageable, and they owe it to themselves and to others on the road to do so.  Pressure loss through permeation requires minimal adjustment approximately once a month.  For predictable swings in temp, set the pressure when it is likely the coldest, and try to consistently check it during those times, such as early in the morning before setting out.  No one is saying that everyone in all circumstances needs to set pressure to +/- 0.1 psi every morning, noon, and night with three hours of ambient cold-soaking before taking measurements.  I agree with you about reducing load; not just due to the tire influence, but also drivetrain/axle and chassis wear and tear, braking performance, fuel economy, etc."

So we see there are similar but different words on the sidewall of tires. Some have a stated Max cold inflation others do not. This is one reason why it best to have tire service done at a store that has the appropriate equipment and training to handle safe and proper mounting and inflation of the type tires you are working with. This does not mean you can't add 5 or even 10 psi to your tires but IMO if you need more than 20% of the CIP there is something wrong and you really need to consider having a professional inspect and re-inflate your tires. Inflating an improperly mounted, improperly repaired or damaged tire can injure or even result in death if not handled properly.
OK now back to our regular programming.
One other comment I have is that many times some think tire failure "Blowout" is caused by too high a pressure but this is essentially incorrect. Unless you have damaged the body ply cords, be they Nylon, Polyester, Steel or Rayon by over flexing and running significantly under-inflated, tires are designed to tolerate the normal pressure increase seen when running highway speeds at the approved load.

But if you have damaged or run the tires in overload or under-inflated for the actual load or perhaps at a speed higher than the tire rating you may have damaged the cord sufficiently that it has lost a portion of its strength so in that case even normal cold inflation may be too high. This is one reason any tire that has been damaged must be rendered un-usable or if it appears to be OK then inflated in a Safety Cage.


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80+ mph RV tires ?

Recently read a post on a forum where the question was asked about by-passing the speed limiter that was on the poster's Class-A unit as the poster wanted to drive faster than 80!

The general consensus was that this was not a reasonable speed in an RV and almost universally people knew that their tires were rated for 75mph or less.

On my trip from Ohio to Oregon last summer I noted that many interstates now had updated the posted speed limits from the old 55 to 65, 70 and as high as 80mph. While I "puttered" along in the right lane with my cruise set at 60-62 mph, I saw all manner of vehicles whiz past me, including many RVs, some of which were clearly exceeding the speed rating of their tires.

In March  I wrote about tire speed ratings, but this is a real safety issue with potentially both short- and long-term consequences. 

 

 

I wrote again in April on "How fast can I safely drive?"

First off, as a professional race car driver and police driving instructor, I think it would be unsafe and foolish to drive an RV in excess of 70 mph. A quick search of the internet finds numerous charts showing the effect of increased speed on stopping distance, with large RVs needing 40% to maybe 80% more distance to come to a stop as speed increases.

At 55 mph, you will "only" need 100 feet more than the length of a football field to stop; at 80 mph, it would take you the length of three football fields to come to a stop. While you might get lucky as you sit high above the traffic, have you asked yourself how you would feel after driving completely over a passenger car that can stop in maybe half the distance it takes you to stop?

BUT this blog isn't about general driving safety, as hopefully you have a good understanding of your vehicle capabilities, having done a practice emergency stop on a "closed course." I want to address the effect of speed on tire durability.

Tires have a speed rating. This is the speed at which a tire can be driven for a short time without failing immediately. For most ST type tires this is 65 mph. There are some special high load trailer tires that are only rated for 62 mph.  LT tires and TBR (Truck-Bus Radials) are rated for 75 mph in RV service. If you look at the side of passenger car tires you will see a "Speed Symbol" letter.
An example might be LT235/85R16 114/111 Q   with the 114/111 being the "Load Index" single and dual and the Letter "Q" being the speed symbol.

Many LT tires also have such a symbol but if you place a passenger tire or LT tire in RV service the manufacturer has lowered the speed rating to 75 max. If there is no Speed Symbol then the speed rating for LT tires should be considered to be 65 mph.

As I wrote previously, some ST tires may be rated with a speed number or symbol by the manufacturer. This number should be molded on the sidewall. I would not trust information that is just verbal from a tire salesman.

I just updated my "Links" post with information on an importer of ST type tires that has published the speed symbol for their tires as "L" which corresponds to 75 mph.
Here are some of the Speed Symbols and the corresponding max speed capability:
F = 50 mph              G = 55 mph        H = 60 mph        J = 62 mph     K = 68 mph     L = 75 mph

IMPORTANT INFORMATION
It is very important that you understand that all the above is based on a laboratory test done on new tires. If a tire has hit a pot hole or been repaired or been parked in the sun for months, then there has been some degradation to the tire's strength and I would not expect it to pass the same test requirements. In fact, if a tire has been repaired, it's in many pieces of literature that the tire looses its speed rating.

Another bit of information to remember is that running fast will increase the temperature of a tire and as I covered in depth in my post on tire covers: "Increased temperature causes continued and accelerated chemical reaction which "ages" a tire faster than when the tire is cool. A rule of thumb would be that the rate a tire ages doubles with every 18°F increase in temperature. We can see the result of old rubber on the surface. What we don't see is the more brittle rubber of the internal tire structure. As rubber gets more brittle with age it also looses strength." Running a tire at 70 rather than 60 will probably increase the internal temperature by about 18°F so every hour you are driving that fast is costing you two hours of tire life. Damage is cumulative and slowing down does not repair the damage you have already done.

Drive safe.  Slow down and enjoy the journey.

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Understanding Tire Failure

Yes, when a tire fails there is almost always evidence left behind that points to the Root Cause. I have shown this in tire "autopsy" posts on my blog.

Some "causes" are easy to see or understand:
Punctures, cuts and external damage from hitting potholes or other "road trash" can lead to leaking air (fast or slow) or in some cases broken body cord (see my blog post on sidewall bulges).

"Blowouts" with sidewall flex failures are most likely the result of air leak, not just low air pressure due to poor maintenance. The leak could be caused by puncture, cut, leaking valve, cracked wheel or the like.


Other failure modes are harder to detect, such as:

Belt separations come from long-term heating of the tire belt/body interface. This heating comes from a combination of insufficient air pressure for the load and excessive speed.

Now while separations may be harder to detect, many can be found if a little effort is made by the person doing the tire inspection.

I previously published a video showing one way to "measure" a defect.

Sometimes you don't need to "spin" or rotate the tire but just get a good look at it and roll it across the floor for a full revolution as one part may look OK while another portion clearly would only look straight to a drunk.

If any of your tires look like the above, I would bet $$ that they had a belt separation similar to what is seen here.

Driving on a tire in this condition is simply asking for trouble as it could come apart when you least expect it as you drive down the road.

As we move out of Winter and you "DE-Winterize" your RV, especially trailers, Please take the time to do a thorough tire inspection.  Spin or off-the-vehicle inspection is much better than a stationary on-the-vehicle "Look At".




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Quick Summary here

I have been following a number of RV forums and want to do a bit of a summary for those that don't have the time or interest to read a number of posts on this Blog: Remember more than half of the thousands of RVs that have been checked have been found to have one or more tire and/or axle in overload. So nect time you camp you can be pretty sure that one or both of the RVs parked next to you are overloading a component.

- You should have a Digital tire gauge. I have posted the results of my tests of dozens of gauges that RVrs were using and the bottom line is that about 15% were off by more than 5 psi which  IMO makes that tire gauge a good door stop or tent peg. I have provided information on how to do your own gauge "Calibration Check" to confirm the accuracy of the gauge you use daily and how to maintain your "Master Gauge"

- If you follow the advice from the two Tire Engineers on this thread you will be setting the pressure based on the appropriate method here:
     - On your TV inflate to pressure based on the Door sticker and Owner's manual for "Fully Loaded" operation. BUT get on a scale at least once to confirm you are not overloaded.
     - If you have a multi-Axle trailer you should be running the pressure on the tire sidewall. I have written in my Blog and provided the technical justification and need to follow this advice. You should still go across a scale and calculate the actual tire loads to confirm you are not overloading one or more tires
      - If you have Motorhome you need to confirm your unit's "corner weights". Use one of the worksheets I have provided links to. Use Load/Inflation chart from your tire manufacturer, to confirm you are not overloaded and identify the heavier loaded tire on each axle. Using the pressure for that load add 10% (without exceeding the wheel max) and that is your Cold set pressure for all tires on that axle.
      - If you follow the above there should be no reason to be chasing your tire inflation around unless you have a major change in Morning temperature ( More than say 30°F ) as you would only need to "Top Off" the air once a month at most.

- Don't be too anal about inflation. Variations of 1 to 3 psi day to day are normal. You should not need to make adjustments unless the Cold pressure is 4 or 5 psi away from your goal. (Multi-axle trailers might try and get no lower than 3 psi below tire sidewall pressure)

- TPMS are a WARNING device and like Insurance we should all be using them. Also just like Insurance they cannot "Prevent" bad stuff from happening but with luck they may give sufficient advance warning to lessen the impact of the bad stuff that does happen.

- If you are parked for more than a day you need to cover your tires that will be in direct sunlight with WHITE tire covers. This protects against both UV and higher temperature damage which can shorten tire life.

Breaking News - Max Speed on ST type tires

Finally chased down the answer. Getting to the bottom was a challenge because I ran into "customer service" people who could read from a script but had no idea why they were giving me the answer they were.

Even got to an engineer at Tire & Rim Association and based on his answers it was obvious TRA had not thought about ST type tires or the unique nature of most RV trailer applications for many years so I believe he acted a little surprised when I identified the engineering analysis that indicated that the current loads too high for many trailers and that the load capacity should be decreased by 15% to 25% or more if the failure rate was to be decreased to a more tolerable rate rather than the 5 or 10% or higher some are reporting. This is not his or TRA fault as I understand it is the responsibility of tire company representatives to TRA to present and review technical matters.

Anyway here is the bottom line.

For Goodyear Marathon tires only:
There is a Goodyear  Tech bulletin   PSB#2011-13 that uses information from TRA.   
The tech bulletin says that the normal max speed for Marathon ST type tires is 65 mph. This speed can be increased up to 75mph only if the inflation pressure is increased by 10 psi. This new inflation can be 10 psi higher than the pressure molded on the sidewall of the tire that is associated with the tire max load.
The load does not increase beyond the load molded on the tire sidewall.
Goodyear does not support or condone operation above 75 at any time.
The trailer owner is responsible to confirm the rim is rated for the new inflation level.
While TRA indicates a further increase in speed might be achieved Goodyear has decided not to adopt this option.

The person I talked with at Tire Rack said they had already changed their web site to indicate this applied to Goodyear tires only and that they were attempting to learn if any other ST tire manufacturers were willing to make a similar change. As of Saturday Oct 19, 2013 they had not identified any other tire manufacturer willing to make an increase in max speed above 65.

++++++++++++++++++++++++++++
Info added 10/26/13 10:51 AM EDT
Example:
If you have a GY Marathon ST225/75R15 LR-D it is rated for 2540# @ 65psi and a max speed of 65 mph. If you increase the cold inflation to 75psi the tire would now be rated for 2540# @ 75psi and a max speed of 75 mph.
The example currently only is OK for Goodyear Marathon ST tires as I have not seen a published document from any other tire manufacturer. 

The pressure increase must not exceed the max pressure capability of the wheel. You need to find a max pressure rating stamped on the wheel or get something in writing from the wheel MANUFACTURER.
+++++++++++++++++++++++++++

My observation
This Max speed is like your engine red-line. While it might be possible to exceed this speed for short periods it will significantly reduce long tern durability.

I have sent a copy of this post to both TRA and Goodyear. If they ask for corrections or additions I will edit this post accordingly.

Why did my engine "blowout"?

Why is it we almost never hear the above question on various RV forums?

Could it be that we don't hear this question because people know it is not wise to exceed the RPM limit or "Red-Line" specification of their engine so they simply don't run at and above the engine speed limit?

But I do see questions of what has to be done to run 75 mph with a standard ST type tire when the RV manufacture specifies the pressure on the tire which is one of the factors limiting the tire speed capability.


The interaction of tire types, load capacity, inflation and speed "rating" are complex, as are almost all topics about the various performance trade-offs with tire selection

I did a post just on the topic of "How fast is safe to drive on your tires?"   I have also pointed out that if you are going to deviate from US industry guidelines, such as published by Tire & Rim Association, you must follow the published guidelines for your specific size, type and brand. You cannot use Goodyear document to learn the appropriate load & inflation for your Maxis tires. It even means you cannot apply specifications, including speed, load & inflation from a Goodyear Marathon with a Goodyear G614.

Now as we all know, an ST type tire has a higher load capacity rating than a similar sized LT type tire.
ST235/75R15 LR-C is rated 2340# @ 50 psi (min) and 65 mph (max).
LT235/75R15 101/104Q LR-C is rated 1985# @ 50 psi (min) and 75 mph (max).

We have also established that load capacity is nominally a function of tire size (air volume) and its inflation pressure. If however we decrease the tread depth and the lower the maximum operating speed we can increase load capacity slightly. In my post of Oct 12, 2011 I identified a Michelin trailer tire with a higher load capacity but many want to ignore its max speed rating of 62 mph which is even lower than ST type tire rating of 65 mph.

Now to the specific question of how to operate a specific tire at 66 to 75 mph. This is accomplished by increasing the inflation pressure by 10psi according to a document published by the tire manufacturer. This is not as simple as many assume.

If you select the 65 mph max speed for an ST245/75R16 LR-C you will find a load capacity of 2,600# with a minimum of 50psi if you stick to 65 mph max. But if you want to drive at speeds up to 75 you need to increase the inflation pressure in the industry standards as published by Tire & Rim Association. So now to carry 2,600# you need an inflation of 60psi but this exceeds the max inflation rating for this size @ LR-C so you either need to get new LR-D so you can run 60psi or to limit your load to 2,270# and 60psi which is the published load capacity for 50 psi inflation of the subject size tire. This higher inflation may also exceed the rating of the wheel, but again many choose to simply ignore that rating too.

One obvious item many simply ignore is that the max speed for a tire is much like the red-line rev limit for their engine. It clearly is possible to run an engine with a 6,000 red-line at 6,300 RPM and possibly higher but what about long term engine life?

There are hundreds if not thousands of posts on RV forums from people complaining about tire failure but I have to wonder why they seem to understand the effects on engine life of high RPM operation but fail to understand that running a tire faster than its design specification will also contribute to shorter tire life.

How fast is safe to drive on your tires?

I found and responded to a forum thread on tire speed ratings and thought I would share what I learned with you here.

I think it best to think of a tire's "maximum speed" a bit like the engine red line. Can you exceed the red line on your engine? Yes, occasionally, but is it good for the long term durability of your engine? I think you will agree that it's not a good policy unless you like blowing up engines (or tires).

A tire has a finite life. Heat and cycles consume that life. Heat comes internally from under inflation, overloading and fast speed and externally from spending all your time in the South where temperatures are higher than in snow country. Cycles are simply miles.

Heat generation occurs inside the tire structure at the molecular level. It is not linear so one mile at 75 mph “uses up” more of that limited life than two miles at 40 mph. It’s even possible that a mile at 80 mph is worse than two miles at 60mph.

The maximum speed rating is also based on the assumption that the tire has NEVER been operated while 'flat." Note that a loss of 20 percent of the minimum inflation needed to carry the load is considered flat.

Also if the tire has ever been repaired, the manufacturer may no longer support the speed rating marked on the tire. You need to consult the manufacturer's publications to confirm its policy. This would apply to tires with a speed symbol letter as found on most Class-B and Class-C motorhomes.

An example might be LT235/85R16 LR-E 120/116Q. The 120/116Q is the service description with the 120 and 116 being the load index for single and dual position. The “Q” is the speed symbol for 99 mph max. A Class A motorhome might display 255/70R22.5 LR-G 138/134L. The “L” is the speed symbol for 75 mph.

As I cover the topic of speed rating in my tire seminars, there are different requirements for tires in Europe than the USA and Canada. In Europe vehicles are capable of higher speeds (100+ mph). Roadways exist to run them, and it’s legal. The tire description system was created to assure replacement tires were also capable of those higher speeds.

However in North America we have speed limits that do not allow driving at excessive speeds. So the emphasis is placed on handling, not "all out" speed.

The same construction features required to achieve higher speed ratings also improve handling. So, in the Americas a speed rating also tends to be a performance or handling rating.

I know of no highway tire that can run its entire life of 40,000 to 90,000 miles at its maximum speed. The tests to establish the speed rating only require an hour or so to complete and most of the test time is well below the maximum. This test is used to measure the rate of heat energy generated with tires failing due to too high an operating temperature.

Finally.
If you tow a trailer that has ST type tires you need to remember that they are rated at 65 mph maximum. A few trailers come with commercial tires that are only rated to 62 mph. So your maximum speed is based on the lowest rating of all your tires.

Careful when you do research or before you buy

Recently I was asked to clarify some information an RV owner discovered as he did some research. What we see here is an error that is easy to make. Even for the more experienced RV owner.

The RV owner asked about a Michelin XTA
215/75R17.5 LR-J tire that came on a 5th wheel trailer. He had confirmed the load capacity of the tires at their max rated inflation and said they could handle the load on his 8,000# axles when inflated to the placard inflation of 120 psi. His question was about the max speed rating of the tires.



He consulted the Michelin Truck Tire Data Book and found that the subject tires were intended for Low Platform Trailer application.
In fact the brochure for his tires even identified the maximum speed rating for his tires as 62 mph.


He wanted to know if the maximum speed rating had been waved for RV application, and if it had what was the new speed limit. He found a statement from the Tire & Rim Association that specifically stating that when a tire had a speed rating below 65 mph it could not be manipulated (increased) with adjustments in inflation and load. He also found a Michelin truck tire service manual specifically saying that low bed trailer tires like the XTA, cannot have their speed increased.
Despite all this information he wanted to know how to work out the adjustments needed to allow a higher speed limit for the RV and could I please help him understand what he felt was confusing information given that the RV manufacturer had selected these tires for his trailer.

I had to tell him that in my opinion there was no way the speed limit could be adjusted. I pointed out that his error was in looking for information in data books and manuals of different type tires such as Truck Tires, RV tires and Low Platform trailer tires and assuming that since his tires were sort of like truck trailer tires and looked like tires he has seen on other large 5th wheel trailers, he could pick and choose which information applied to his specific tires.

The lessons to learn here are:
1. Not all RV manufacturers pay a lot of attention to proper tire selection for the application.
2. When doing research on tire type and specific operation limitations you need to look only at the literature for your specific tire if such literature is available from your tire manufacturer.
3. You cannot depend on the RV dealership to warn you about the real quality or limits of some components of your RV. You need to take responsibility for what you purchase. i.e. Let the buyer be aware.

Important Note:
Speed ratings are like the “Red Line” for your car, truck or motorhome engine. While it is possible to exceed the max speed rating of a tire just as it is possible to exceed the rev limit and run your engine in the red zone, I think we all understand that if we do run our engine that fast we are pushing the odds. In the same manner if your tires are rated for 65 mph max you might be able to run faster for a short time but you are consuming the finite limit of the tire structure and simply slowing down does not repair the microscopic damage you have done to the internal structure of the tire. You should not be surprised if you blow your tire just as you would not be surprised if you blow your engine after running in the red zone for engine speed.

Tires - Dull or Dynamic? Part two

Following is the second part of an article I was asked to write for a tire industry trade publication in the UK. “Tyres International.” I thought you might find it a bit interesting.

“To the untrained eye it might seem like the tire industry has not done much to improve John Dunlop's original. But looks can be deceiving.

Part two
A cost/performance comparison on the improvements in wet and snow traction, ride comfort, low noise, blowout resistance and handling response is significantly more difficult, as the tire of 1920 was so poor in many of these areas as to be considered useless when compared to today's tire.

When one considers high-speed capability it is a little easier. In 1920 most cars were not capable of a sustained 85mph, yet today that is considered an absolute minimum high speed capability for a tire to be sold for use on the public highway.

Material World
Some would have us believe that today's tire engineer is not willing or even encouraged to look at new ideas or materials. Even the use of rubber, both natural and synthetic is used as an example of the perceived lack of advancement in tire technology.

I know from personal experience that alternate materials for both wheels and tires have been investigated but either lack of performance or poor reception in the market prevented those ideas from making it into production. I have seen non-metallic wheels that were too flexible to hold air at high temperatures and too stiff so they shattered at low temperatures. New tire sizes with improvements in safety and mobility were not accepted because non- standard wheels were required. Even well engineered items such as the mini-spare, still meet with resistance and ridicule despite the fact that they can deliver acceptable performance when needed. They even have an impact on vehicle fuel economy when not needed

Rubber band
The basic material in Mr. Dunlop's tire, natural rubber, comprises less than 10 per cent of today's modem passenger radial. Today's tire, with 25-30 different materials, made up of hundreds of different chemicals, is one of the more complex components in a modem automobile. This is especially surprising when you consider that some materials are considered contaminants and are incompatible with other materials in a tire yet we have managed to make these incompatible materials work together to deliver improved air retention and blow- out resistance.

It is likely the concept that tire materials have not changed is a concept only held by those with little training in the an of tire design. There are few materials that are capable of 300 per cent strain for tens of millions of cycles over an operating range of temperatures from -20°F to +200°F while at the same time having a coefficient of friction of 0.8 or higher. Today's steel belted radial could be improved upon and even have its weight lowered, with increased use of rayon, fiberglass or other materials as belt material. There are however, restrictions on pollution or customer resistance to materials other than steel which have so far proven insurmountable obstacles to broad appeal for the average consumer.

Some of our biggest challenges will come in the next decade as we are asked to change from making a product that will last indefinitely under extreme conditions as the tire industry has been asked to do for more than a hundred years. We are now being asked to design a product that will be almost indestructible until the user wants to change it, then the tire should, as if by magic, become easy and inexpensive to deconstruct into its chemical components. Some OEMs are even starting to suggest that old tires should be able to be recycled into new tires with no loss in any performance characteristic.

I have every confidence that the tire industry will rise to this new challenge and methods will be developed to address the disposal and reuse of materials in a tire. It is unlikely the recycled materials will be used 100 per cent in another tire just as the OEM will not be able to recycle the leather car seat into a good as new' leather car seat, but we will incorporate an ever increasing percentage of recycled materials in tires and we will find acceptable methods of recycling them into some usable material or product at the end of their useful life as a normal tire.

To the uninitiated it is easy to look at John Dunlop's tire of the late 1800s and say that since today's tire is still made of "rubber' it is not really any different. Thus some would consider this sufficient proof to postulate the tire industry is not capable of looking at history, learning from it and moving on.

It is my belief that this thinking ignores the advancements in both the materials and performance delivered at a very low cost to the often uncaring consumer.

Light Truck tires on trailer

Had a question from a reader
Tireman:
Thanks for your expertise. I am, frankly, baffled about what to do about our tire situation. In this instance, our tire that went flat definitely was not overinflated. It had been checked about two days prior and had 63 pounds. We have been running nitrogen since the tires were put on and have not lost a pound in any of them in 3 months.
When we upgraded to 80 pound, E-rated tires three years ago, I asked about the rims' ability to hold that pressure. Stupidly, I took the tire dealer's word for it.

In 2008, we started blowing Denman tires all over Montana: three of four blew, tires with about 8,000 miles on them.
On our way to Maine in May, we blew yet another tire and sought advice from the guy who owns the campground where we were headed. He is a tire dealer, and advised us to go back to the original equipment 65-pounders, which we did. Now, we have a rim problem.
An 80-pound Maxxis, which we kept as a spare, is now on the trailer. The one that went flat will be on a new rim and used as a spare until we get home. Clearly, we had lost pressure in that tire before we got to the CG; how much is the question. It was not flat or we would have noticed it when we put the chock between the wheels. That tire will undoubtedly go bye-bye when we get home.

I will research rims. It sounds like I should replace all rims with ones that are capable of handling E-rated tires if we want to go that route again. I do not think we have room to upgrade to 16-inch wheels, but I now am going to find out for sure. If it's possible, we will do that.

I am adamant about checking tire pressure, so I am confident we have not been running on over or under inflated tires. We are not overweight, and our axles are fine.

It is obvious we are doing something wrong. I'm beginning to feel like we are too stupid to own this 5er!

Hope your eyes don't glaze over reading all this...thanks for your input.

Answer
You didn't mention if you ran 65psi or 80 psi in your "80 pound" tires. I wonder if you are running metal valves, as standard rubber valves are not rated for more than 65 psi. Also the rims have a max load and max inflation rating. Did you check the rim stamping?

You also were not clear if you were running LT or Light Truck tires or had been running ST or Special Trailer tires. These two different type tires have different load ratings even if the rest of the numbers were the same. A LT235/75R16 Load Range E has a different rating than an ST235/75R16 Load Range E.

An "80 pound" Load Range E tire rated for 80psi will carry no more than a Load Range D tire will if they are both inflated to 65psi. It is the air (nitrogen) that does the work and carries the load not the tire. Just because you checked the tire two days earlier is no guaranty you didn't get a nail as you left the CG that day and drove for two days on a leaking tire. This is one of the best arguments in favor of TPMS.

With all the tire failures I have to ask what your real ( not sticker or guess) axle by axle side to side loads are when you are fully loaded going down the road. With those numbers you can consult the manufacturer’s charts.

While you are getting the TT weighed it won't hurt to get the four corner weights of your tow vehicle too.

One thing few realize, including the tire salesman, is that according to Industry Guidelines LT tires require an inflation increase or even a load reduction if you ever drive over 65mph. ST tires also have a speed restriction that needs to be considered. The only way to know if this is not needed is to consult printed literature for your brand and design tire from the tire manufacturer. I would not assume the tire salesperson knows the correct answer.