Heat, High Speed and the "Magic" in ST tires.

Heat generation is primarily the result of a combination of High Speed, High Load, and Low Inflation. Factors that can counter some of the negative effects of heat on tires can include the use of more expensive rubber compounds, reduced thickness tread and other construction features that are out of our control as tire owners. Without access to secret formulas and other proprietary information, I know of no way to learn which of these features is in which tires if any. There is one exception, of the inclusion of full Nylon Cap Ply. This would be identified in the material list molded on the tire sidewall along with the number and material in both the sidewall and under the center of the tire tread.

However, the driver] has complete control over the Speed/Load/Inflation factors.

IMO it would be better if all tires had at least a 15% margin on load capacity, with 20 to 25% being better margin. We need to remember that road slant & crown and curves along with significant sideload from wind can easily result in overloading tires that were measured on a level scale. Multi-axle trailers should shoot for a 25% load margin. (read about Interply Shear to understand why )

Ever wonder why RV trailers seem to have a lot of tire problems but your daily driver (car or SUV) doesn't?
Let's look at Reserve Load Capacity of your car:
Many / most have 30% to 40% Reserve Capacity on the tires.

  


Now when we look at an RV trailer we see only 6% with many having 0% margin.
 




 
 If we assume we want to have 25% Reserve load capacity on the above RV, then 13,290/(Tire max load X 4)= 0.75
and each tire max load shall be 4,430 lb.    So the right choice would be an all-steel
ST235/85R16 132/127 (14 with a max load of 4,410 lb.)


Previously I indicated that Inter Ply Shear can be the equivalent of adding 24% more load on the tire while in operation. Using the above tire the IPS effect is reduced considerably.

NOTE in 2017 RVIA and the National Fire Protection Association, guidelines for tire selection for RVs was updated to require a MINIMUM of a 10% margin in tire load capacity over the GAWR. Some RV companies achieved this margin by upgrading their tire load capacity while others simply lowered the GAWR number on the vehicle certification label. Do you know what your RV company did?

   

Speed rating. We should think of this like the engine red-line. Everyone seems to understand that running faster than red line will shorten engine life but that doesn't mean that 5,900 on a 6,000 redline is good to run for hours on end and may contribute to shorter engine life just as running 62 mph on tires rated, based on their original load calculations, for a max of 65 mph.
The high-speed test is a 30-minute step speed test of a brand new tire on a smooth test wheel. No potholes or curb damage. All that is needed to "pass" the test is to meet the target speed and not come apart. The tire is considered scrap after the test.  Would you consider your tires to be "scrap" after you run 70 or 80 mph for an hour or so?
Remember damage (internal cracks) done, even at the molecular level, in tires never repairs itself but only increases in size.

While this post is primarily about Trailer application the limit on high speed also applies to 16 through 22.5" tires that have a max rating of 75 in RV application which would be their "Red Line" but I hear many Class-A owners talking about running over 70 for miles on end but being surprised when they suffer a tire failure.

LT tires would be my first choice in any trailer application where heavy loading was required. I do not understand why so many think that there is some magic engineering used on ST tires that allow them to support 10% to 20% more load than an LT tire of identically dimensions. The load formulas used today for ST type are the same ones used in 1969 when they were limited to 65 MAX (red-line).

I would be very interested to hear an answer from the engineers at the companies that make ST type tires why they were able to suddenly, almost overnight, increase the speed rating from 65 to 75, 87 or even 99 on their ST type tires. Do they have some new materials? Why don't they put those same super materials in LT tires and increase the load capacity of LT tires to be the same as or near to the loads seen claimed for ST type tires?

Sorry, I just don't buy the new claimed high-speed capabilities of ST type tires that exceed the load and speed capabilities of equal size LT tires.

##RVT935

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.

##RVT862

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.

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.

How do I set inflation on my TV and RV trailer?

Got this PM from someone who reads RV forums.

"Good evening, I'm looking for some professional advice. I have a 2500HD pulling a fifth wheel, my steer axle is 4,000 lbs, drive axle is 5,380 lbs, and our trailer is 7,700 lbs. Tires on the TV are rated for 3,100 lbs a piece at 80 psi. The RV tires are rated for 2,800 lbs at 80 psi. Do you think I am ok to run 65psi in the steer axle, 70psi in the drive axle? What I've been doing is 75psi steer, 75-80psi drive, and 80psi trailer. Our TV tires are Michelin MS2 LT265/70-17 the load range chart says I can run much less psi. Also, our trailer tire's are Maxxis 8008 225/75-15 LR E, also the load range chart shows I can run less psi. Any thoughts on what you would do would greatly be appreciated, thank you. New Crusader owner"


To start with it is important to remember that we should never operate tires with inflation lower than what is needed to support the actual load on the tires.
As I have pointed out in my blog we first need to confirm the actual load on our tires. For TV we will find the side to side loading is usually 49/51 to 50/50 split so we can take the measured axle loads divide by 2 and then use the resulting figure when consulting the Load Inflation tables for our size tire.
I have links to many load/inflation tables and related info HERE. Once you find the MINIMUM inflation, I suggest you add at least 10% to that number to allow for day to day variations. Having said this, it is important to remember that the car company spent a lot of time and money evaluating many different tires at different pressures to arrive at the recommendation that is on the Tire Placard on the driver door jam. You should note that the Placard inflation may be higher than my above-suggested calculation as my calculation is only addressing load and not handling, braking or the fuel economy.
The scale weight for the TV for the above needs to be the reading with the RV & TV fully loaded as heavy as you ever expect to tow.

The RV tire inflation is different. Due to SIDE LOADING in trailer application, I strongly recommend running the tire sidewall inflation as your CIP. I think that if you review the RV Placard info from the RV company you will also find that they say to use the inflation number on the tire sidewall.

I know you were hoping I would just tell you to run xx psi here and yy there and zz on the trailer but without knowing tire sizes and Load Range for all your tires, I don't have enough info to provide an informed answer BUT I feel you now have enough info that after reading the sidewalls of your tires - Be sure to check each as they might be different on TV and RV and possibly even different F & R on the TV - You now know how to establish the correct inflation for your current vehicles and for any vehicles you own in the future.

I hope this information and instructions help. If not you can always contact me directly using the email under my video picture to the right.

How would I set inflation on a smaller single axle trailer?

Got this question from a reader. 

Our Jayco Hummingbird came from the factory with P235/75R15SL tires. The TT GVWR is 3,750 lbs and the GAWR for the axle is 3,500 lbs. This is a single axle TT. The P-rated tires were like a pogo stick at max inflation.

We changed from the factory tires to Maxxis 8008's in ST225/75R15 size. The factory aluminum wheels are good for a max of 80 psig according to the stamp inside.

Also converted to metal valves stems for running our TPMS....because I'm an engineer who tends to overdo everything I touch .

The heaviest CAT scale weight has been 3,320 lbs on the axle and 3,780 lb GVWR. We've since removed a few items to stay within the 3,750 lb. GVWR.

I've always kept the tire pressure at the minimum sidewall stamp of 65 psig (Max load of 2,540 lbs at 65 psi cold). After reading some of your blogs and looking at the Maxxis load chart, if I assume an equal split weight on each wheel we would have a worse case of 1,660 lb load. Of course a perfectly balanced load isn't likely to ever happen. But even with adding 10% it would put us at 1,826 lbs per wheel. Maxxis says that for our particular tire 40 psig would give us 1,880 lbs capacity.

I can't say that I'm comfortable going all the way down to 40 psig, but I feel ok with 50 psig, even though this is grossly over-pressurized for the given load. I know that at 65 psig the TT rides like a log wagon and we recently bent a spindle on the axle without even knowing it, I wonder it the limited travel of the torsion axle combined with the tire pressure came into play because we were under the GAWR of the axle and never even felt anything out of the ordinary during the trip, of course we are pulling a 3,750 lb trailer with a '17 GMC 2500HD w/ Duramax so we don't feel much anyways.

So if it were yours what pressure would you choose? I've been running at 65 psig and I think that's too much, 50 psig sounds good to me, but it's still too much pressure according to the weight charts.....

http://www.maxxis.com/trailer/trailer-tire-loadinflation-chart

Thanks!"
Here is the answer I gave him.

My approach
OE tires P235/75R15 are  rated for 2,280#@35 psi but on a trailer we need to De-Rate the load capacity so 2028/1.1 = 1844# load capacity.

Your measured axle load was 3,320#
If we assums a nominal 53/47 side to side split we get 1,760# for heavy end  and a 60/40 split gives 1,982# for the possible heavy end of the axle.

An ST225/75R15 LR-C is rated 2,150# at 50 psi. Since we are looking at a single axle trailer we can check the tables and find 40 psi is rated to support 1,880# and 45  psi can support 2,020#.

Since we always select the pressure needed that can support the heaviest end of an axle and we always inflate all tires on any one axle to the same inflation we could select  40  to 45 psi for our CIP.
I would set my TPMS Low Pressure warning level to 40 psi and my CIP to 45psi.


If this was a multi-axle trailer we would want to lower the special belt shear forces and run a higher inflation. Maybe 50 psi minimum