How many "Ply" in your tires?

 Here is a picture and comment I found on an RV Forum. I suggested that the owner review his understanding of tire construction and that he read the material list that is molded on the sidewall of all tires. The failed tire does not have 12 or 14 "ply". I also pointed out that it does not appear to have a "ply" of Nylon over the steel belts.

  Do you know how many "ply" or layers of which materials are in your tires?

The owner said "After many problems with ST 12 ply I have moved to "xxx" brand 14 ply and no more problems. I drive to Mexico and back every year 9,000 mi."

I pointed out the problem with thinking that there were actual 12 or 14 "Ply" in the tires as that term lost its meaning when tires were switched from "Bias" or "Diagonal" to "Radial". I strongly recommend that ST type tires include Nylon or similar material as "Cap Ply" over the steel belts. This can help lower the Interply Shear stress forces found in tandem axle trailer applications.

If you do not know what Interply Shear is then I suggest you review this post. 

The tire failure probably had a contributory cause of impact damage from the poor roads in Mexico. 

Here is my post on the scientific study of impacts and tire failure showing a 100% correlation.

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Interply Shear and am I spreading Fear?

On an RV Forum I saw some posts about tire failures:

There were some posts that mentioned the RV total weight capacity or Gross Vehicle Weight Rating (GVWR). Others were mentioning the total of the Tow vehicle plus the RV Trailer or Gross Combined Weight Rating (GCWR). While others were talking about tire load capacity.

One person responded with: "You need to pay more attention to just what acronyms are telling you. GCWR has nothing to do with tire inflation pressures. The tow vehicle and towed vehicle are individual vehicles and answer to their own standards and regulations.

Maybe you should ask Roger why, during his working years with tires, the interplay sheer problem was never corrected. (Interjecting a fear factor for attention purposes might be Roger's goal).

Tire tread separations are a fact that trailer haulers must respect. Fooling around with RV trailer tire inflation pressures by using less than what the vehicle manufacturer has recommended is, IMO, counter productive".

 

Since I was mentioned if the forum post along with the suggestion I might be spreading fear, I felt the need to reply.

Sorry, but Fear is not and never has been my intent. I covered interply shear and how I discovered that it was different for multi-axle trailers than it is in motor vehicles in my blog post of Nov 20, 2013 when I did the first of multiple posts in Interply Shear as it pertains to multi-axle trailers. The fact is that Interply Shear is well known in the tire industry but not the unique forces imparted on multi axle trailers. It was pure chance that I noticed the irregular path taken by a trailer doing a 180° U turn on freshly smoothed gravel (picture in the Feb 22 2018 post). That was my Ah-Ha moment. 

 

While I was retired by the time I first saw the gravel path, I still had friends that worked with the high power Finite Element and vehicle simulation software and called in a couple favors. I asked them to run a simulation of a truck pulling a tandem axle trailer through a series of "S" curves. All 8 tires were identical with the same load and inflation so we could end up with a comparison of trailer effects on tire belts vs the normal interply forces found in all radial tires in motor vehicles applications. What we discovered after the overnight computer run, was that in  simply S turns (similar to seen in the video) the belts on a multi axle trailer were developing 24% higher internal shear forces than the tires on the truck pulling the trailer.

 

This goes a long way in explaining why tires in multi-axle application have much shorter life and significantly higher failure rate than do tires in car or truck applications.

 

So you see, Science and facts can be used to point out why blindly following 50 year old design decisions and regulations that are the foundation for the current ST tire regulations can be significantly improved upon.

##RVT1054

Another question on "4 corner weights"

 On Tue, Nov 9, 2021 Andy  wrote:

Good afternoon Roger,

 I wanted to double check myself regarding tire position weights.  In reading your blogs you said for double-axle trailers the tire pressure should be the max cold pressure amount as stamped on the tire because of trailer sway and turning forces on those tires is different than on a motorhome.  Is it important to get individual tire position weights for trailers?

 As I just got a new 5th wheel trailer and I am getting ready to install a Tire Tracker tire pressure monitoring system I have a few questions:

1.      Do I need to get 4 corner weights (or in this case 8 tire position weights)? 

2.      Is load on each tire position important for trailers vs. motorhomes, or is axle weight sufficient?  (I can get overall and axle weights easy enough.  And, I just got an appointment for tire position weights with a SmartWeigh club in FL, if you think it is important.)    

3.      When I go to get the weights, should ALL holding tanks be full or only Fresh Water? (The SmartWeigh group literature states that they only want the fresh water tank full.)

 The 4 tires on my 5^th wheel trailer are: (And the spare, too.)

Carslie    CSL16   ST235/85R16

Load index 132/127

Speed Rating: M (81 MPH)

Max Load Single 4400 lbs. at 110 psi. cold

Max Load Dual 3860 lbs. at 110 psi. cold

DOT JETB 1821

 

The 6 tires on my Ford F450 DRW are:

Michelin

225-70R19.5

Max. Load Single 3970 lbs. 110 psi. cold

Max Load dual 3750 lbs. 110 psi. cold

Then they have LRG in an oval followed by 128/126N in another oval and then DOT B6 YB NFL X 0621 in various ovals

 

I don’t have weights on the trailer yet but the GAWR is 7,000 lbs.   GVWR is 17,000 lbs.

 Thank you very much,  Andy

I wrote:

The "4 corner weight" is usually talking about Class-A motorhomes and the data shows that some number of those units can be significantly (1,000# or more) unbalanced side to side on an axle,

Smaller RVs can probably get away with just learning the weight on each axle with the RV and TV loaded to its heaviest.

Then calculating 51% or 52% for each axle and then using that weight number to confirm you are not exceeding the load capacity of the tires.

For towables, including 5vers, your calculated heavy weight should be no more than 90% of the tire capacity when inflated per the Load & Inflation tables. I covered the reason for this 90% limit in  my blog post on Interply Shear and the RVIA (see that Gold sticker near the door on the trailer) has a requirement that tire capacity should be = 110% of GAWR.

The max load capacity for a tire is load in pounds shown on the tire.

Your Dry weight is not important when we are talking about tire loading. The GAWR is just a number for all the tires on any one axle. The tire on the Passenger side has no idea what the load on the driver side is so you can't average the axle weight to learn the max weight on the heavier loaded tire.

"Dual" loading is when 2 tires are mounted side by side on one end of an axle as we see on the rear of most Class-C motorhomes and you have on "Dually" Pick-up trucks like your F450. Dual does not apply to RV Trailers. LT tires do have different load capacities if mounted on the front (single) or mounted on the rear (dual).

Yes, you do not need both freshwater and holding tanks full. Think of your loading when you start a trip. Fresh water is normally full, so is propane, gas or diesel and food pantry but holding tanks are empty, don't forget your tool box. That would be what I would call "Heaviest expected weight".

This is when you get on a truck scale and get readings with just one axle on a scale pad. You may need a couple of readings depending on pad spacing and your axle locations. Get both trailer and truck weights as you should also confirm you are not overloading your TV tires either.

Example:  Suppose you have tires that say 3,900# @ 80 psi. Your GAWR on the certification label is 7,000#   RVIA requires the tires have a capacity of 110% of the  7,000 or 7,700 total or 3,850# each so you might think you are OK but if your RV axle is "out of balance" side to side by 1% or 3,885# on the heavy end you would have a tire in overload. Not a great deal but in overload assuming the scale is accurate +/- 1% and you have a 100% accurate pressure gauge.  What if your axle is unbalanced by 200# or 300#?  TV are normally more balanced so just take the axle load and divide by the number of tires on that axle.

BUT the RVIA 110% requirement went into effect in Nov 2018 so there are many RVs out there that only require that tires be capable of supporting 100% of GAWR so that 10% "cushion" is gone.

I strongly support the RVIA's 110% load capacity. In fact my Interply Shear data suggests that something closer to 125% would be desirable but the RV companies simply have not designed their vehicles with large enough wheel wells to allow that large of a tire even if the cost penalty was less than $200 an RV.

Back to your original question: You do not need to learn the individual loads on each tire position. BUT I do suggest you assume at least a 1% out of balance and apply that to the weight reading you can get at truck scales (just need to pay attention to where each axle is on which scale pad)

##RVT1027

Why inflate your tires to their max, when parking for long time?

I had a question about what inflation to run when parking your RV or vehicles for a long time. I initially said that would lower the Interply Shear.

Then, I was asked
"Could you explain interply shear on a parked R/V?"

Any time a tire is deformed (loaded) the cured rubber in the belt area moves away from the "as cured" shape. Even if not rolling, the area that is now flat on the road has been "bent" from the as-cured original curved state.

This bending causes shear (tearing) forces between the belts, which, at the molecular level, can result in bonds breaking between the carbon, hydrogen and sulfur atoms. If the bending is sufficiently large, the tear gets larger.

If you have higher inflation in the tire, the bending is less than when you have lower inflation which takes us back to decreasing the molecular tearing.

Other things happen too. "Cold Flat spotting" where a portion of the tread ends up flatter than the portion not loaded. This difference can result in vibration and shaking once you start driving. The "flat spot" also has to "work itself out" when you start driving. Again the change in shape when driving is rapid which can result in those broken molecular bonds. Slower changes in shape as when you inflate a tire allow the rubber to move or even "flow" a bit so the atoms have time to re-arrange.

If you ever played with "Silly Putty" as a kid you have experienced this effect. When you slowly pulled the putty it would stretch but if you yanked it quickly it would break. Silly Putty is a form of synthetic butadiene rubber which is very similar to the synthetic rubber used in tires

The whole concept of getting a longer tire life and better belt durability is to decreases these shear (tearing) forces that come about because of the changes in the shape of the belt package.

Now I don't expect people to inflate your tires every night or even every time you go camping for a few days. But if you are parking for a month or more over the Winter, it might be worth the effort. Only you know how much effort it would take to adjust your tire pressure and if you think a few extra weeks or months of tire life is worth the effort.

##RVT936

It appears Goodyear agrees with me

I have written a number of times on the advisability of running trailer tires at the inflation molded on the tire sidewall.

Some posts dove deep into the Science behind the recommendation. I know this can make your eyes glaze over so how about just following what Goodyear says in their RV Tires information web page:

"Unless trying to resolve poor ride quality problems with an RV trailer, it is recommended that trailer tires be inflated to the pressure indicated on the sidewall of the tire. Trailer tires experience significant lateral (side-to-side) loads due to vehicle sway from uneven roads or passing vehicles. Using the inflation pressure engraved on the sidewall will provide optimum load carrying capacity and minimize heat build-up."

##RVT890

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.

Can I run my LR-E at 65 psi? or is this overloading the tire?

As trailer owners start applying the new Goodyear Endurance ST tire, many are discovering that for some sizes the Endurance tire is only available in a Load Range that is higher than their OE tires. Some are concerned about what inflation to run. I have even seen some claim that running a LR-E at LR-D inflation i.e not 80 but at 65 that the "tire will be overloaded, heat up and fail".

While I understand some of the confusion I do not agree with some of the concern or replies.

Tire load capacity is a function of the tire size and inflation level as long as you stay in the same "type" tire.  By "type" I mean P type or LT type of ST type or for large RVs "truck" type.

If you stay with the same type and use the same numeric "size" then the only thing left to change is the Load Range or "Ply Rating". While I do not like using Ply Rating as it is an old and discontinued nomenclature it may help for better understanding in this post for you to think of the old term.

Important Point. "It is the air pressure that supports the load NOT the Ply Rating." This statement is supported for every tire made by every tire company in the world through the use of Load & Inflation tables. These tables show a size and then for different levels of inflation the load capacity of that tire when inflated to that level. You will never see a tire shown where a LR-D at say 65psi can support 1,500# and for the same size the same tire when having a LR-E rating shown a higher load capacity at 65 psi. Not even just 1 pound more.

So a LR-E can support the same load at 50 psi as a LR-C or the same load at 65 psi as a LR-D at 65.

You will not be overloading the LR-E if you load it to the 65 psi rating shown for thet type & size tire and inflate it to 65 psi as you would for a LR-D. Since you are not overloading the LR-E tire it is not going to overheat at 65psi with the 65 psi load so the LR-E tire is not going to "overheat" at 65 psi any more than the LR-D will "overheat" if it is loaded to the 50 psi load rating and inflated to 50 psi.

When going to a higher "Ply Rating" you can then increase the CIP which increases the tire Load Capacity which means it will actually be running cooler because of the greater "Margin". The higher inflation will also lower the Interply Shear which may lead to longer tire life.

When making the change you do need to confirm the upper inflation level for the rim. The wheel manufacturer should provide that information. As an alternative the wheel will have a max load capacity stated. Looking at the OE tire size that comes on that wheel look for the inflation that corresponded to that load and I would consider that to be the wheel inflation rating.

Tire inflation not the same for all trailers

I have written a number of times on the topic of Interply Shear. This is the force that is tring to tear the belts of radial tires apart.
There are some highly technical papers on the topic and you can review tham after a simple Google search on the term. You can also look here on my blog for the posts where the term is tied to a post by simply checking the list of topics on the left side of my blog. Basically this force comes about when the belts in a radial tire are forced to change shape. This means either when a tire rolls and the footprint changes from curved to flat as the footprint rolls into contact with the ground. This force increases when external forces from cornering are also applied to tires.
So why do trailers seem to have such high Interply Shear forces? Well it's not all trailers as the cornering forces of single axle trailers are much lower than the forces of tandem or triple axle trailers.

What tipped me off to this was an observation at a campground that happened to have a freshly smother gravel driveway and a multi-axle trailer happened to make a 180° turn as I was walking by. I noticed that the gravel marks were not a smoth curve but there was a series of turns interupted by discontinuities.

 

A short time later I saw a video from Keystone RV Company of wheel lug nut torque (you should watch the full video some time)
 While watching their video I recall some special high side load tests we ran in our tire test lab.

Here is a short out-take from the longer Keystone video that shows what happens to multi-axle trailer tires.

In the video you can see how the two tires are fighting each other with one bending in as the other bends out. You can imagine that if the turn is made on gravel at some point the high sideload would result is a sudden breakaway or slip. That's what I observed on the gravel turn.

Back at work I had some high powered Finite Element computer programs run to simulate the side load of a multi axle trailer and the results showed that the side loads on a trailer could be 24% higher than on a standard vehicle even with identical radius and loads on the tires.

This post shows why tires get different side loading.

Further analysis showed that increasing the tire inflation could lower the extra shear, but sadly not eliminate it.

Bottom line
In a number of posts I have recommended that Motorhomes set their inflation based on measured static load plus a margin of at least 10% additional PSI. This would also apply to single axle trailers. BUT for Tandem and triple axle trailers I strongly recommend that the tire cold inflation be set to the inflation molded on the tire sidewall associated with the tire maximum load capacity. I also recommend that the measured static tire loads on these trailers be no greater than 85% of the tire maximum with a 20% margin being better.

Another question on why Trailer tires fail more often than tires on motorized vehicles.

On a thread on an RV trailer forum Peter made this statement

"At highway speeds interply shear is not as much of an issue, as I understand things."

I posted this reply:
Speed itself is not the issue that causes Interply Shear in Radial tires. Two things happen to radials as they are driven.
One is the tread (and belts) must flatten out when the tire contacts the road. This results in the steel cords moving relative to each other. This is a shear force.
Second is that when the contact patch or "footprint" is forced to turn a corner there is some slipage between the direction of travel that aligns with the center of rotation and the actual direction of travel.

On motorized vehicles the front tires have a slip angle and the side forces are what actually results in the vehicle turning. But of you were to project the center of rotation toward the center of the driving radius you will find that those centers are close together This is due to the "Ackerman" designed into the front end alignment.This graphic from Wikipedia shows how Ackerman works.

Multi axle trailers however have two axles and 4 tires, with no tire rotating around a centerline that points to the center of the turn radius.

Here is a rough sketch of what happens to trailer tires.

As you can see the center of the turn for the tires on the motor vehicle is the same fo all 4 tires while all 4 of the trailer tires are rotating around different centers. This is why the side loading as seen in the video is significantly higher than it is for tires in motorvehicle application.

These tires are forced to higher than normal "Slip Angle" through any turn, Not just the extreme tight turns when backing into a parking space.
Damage to tire structure is cumulative and while a small turn imparts less shear than t tight ture even small forces can do damage on a molecular level.
Here are some links to various articles on Interply Shear.

Duals on a large truck slip on the pavement when forced through tight turns and with inflations over 100 psi they do not deforn as much as trailer tires with lower levels of inflation.

Increased inflation will lower the interply shear. It will never lower it to the level seen on a motorized vehicle Only Passive Steer axles on trailers similar to wat is seen on the back end of cement trucks can lower the shear too but I don't see any RV trailer company offering that expensive alternative. Afterall they know you can't make a warranty claim on failed tires on the RV company as belt failures from Interply Shear is a long term proposition.

"Dry Rot" is a misnomer regarding tires

Read this on an RV trailer forum, but most of the info applies to all types of tires.

"My understanding is that there seems to be a belief or a known orthodoxy that despite external appearances, the inside could rot out and that tires that appear all fine and dandy on the outside, after that long are not on the inside?

I believe the only "proof of concept" is common anecdotal experience:

People have owned tires that appear fine that are "old yet all seems fine", and they experience unanticipated tread separation and such....several anecdotes to the point of it being a "common wisdom" best I can tell.

Beyond that, I have not seen a "more objective" verification of this...in my mind, after the death of such an "old yet otherwise in good shape and used properly" tire, an autopsy of that tire could show evidence of this "rotting from the inside"....I am not sure anyone has posted details about what that would look like (beside unexplained otherwise tread separation)?

Absence of such "more objective review", I am very very inclined to accept the common wisdom or orthodoxy on this matter until clearly proven otherwise as the cost of being wrong is potentially massive comparatively!!!!!!!!! Many many posts on this site have expressed huge regret about pushing past such "widely accepted advice"

My response to the above:
I did "tire autopsies" for decades before retiring. I have even posted pictures on my blog and have over 30 posts that have "Failure" as a label. Best advice I can give is to read and review the information on my blog. Listen to the only two (to my knowledge) actual tire engineers on RV forums: myself and CapriRacer.

"Dry Rot" is a misnomer. Rubber is a long chain polymer. The chemistry is such that the polymeric chains break down over time. The rate the chains break is related to heat and other energy (UV) input. Nothing is actually "drying out" or "rotting" in the common understanding of the word. Sidewall cracking (dry rot) is just a symptom that suggests the internal rubber compounds have probably lost some of their elasticity, which increases the potential for cracking, which may lead to separation.

There is no single answer to why some people have longer tire life than others except for the fact that some operate their tires at higher temperatures (load, speed and inflation plus ambient temperature) than other people.

Any tire can fail with a Low Inflation sidewall flex failure or "Blowout". Radial tires in trailer application are exposed to significantly higher Interply Shear forces due to suspension design that the tires on the tow vehicle - See my post on Interply Shear.

While operating a tire can help the "Anti-Oxidants" or AO's migrate to the surface, simply driving the tire is not IMO an efficient or effective thing to do, especially when we consider that cleaning of the tire sidewall, which will remove the AO's, can result in more harm that any driving around can prevent.

How many of you have bothered to make load and inflation adjustments necessary for driving your ST type tires any faster than 65mph?

Do you even know the actual loads on your trailer tires?

How many do an annual "free spin" inspection of your trailer tires?

How many are running TPMS (tire pressure monitoring systems) so you get warned when you drop down to the minimum inflation needed to support the measured tire load?

If you feel that checking your pressure with a hand gauge is sufficient, do you make that check every 10 to 15 minutes of operation? If you have a tire leaking air you can destroy it in just a few miles, so the fact you checked the air 4 hours prior to the failure is of no importance.

Sorry for the rant but the FACTS are out there. It takes a little effort to drastically reduce the potential for premature tire failure. There is no magic snake oil spray that will make your tires last 20 years. There are steps that you can take to get 5+ years of life in trailer application and 7+ in motorhome and tow vehicle application.

##RVT798

Question on tread scrubbing on trailer tires.

Got this question:
"I have a question regarding interply shear or scrubbing common on double or triple axle fifth wheels. I have a double axle and try to avoid sharp turns and U-turns as much as possible, but I see visible signs of scrubbing on the tread of my tires. It looks like flat spots on the edge of the tires. After a turn, I can go back and see the rubber I've scrubbed and left on the road. It's frustrating. I have the axles aligned and tires balanced once a year and have individual wheels weighed occasionally. I try to keep side-to-side and front-to-back weights within a couple hundred pounds. I currently carry 85%-90% of load capacity, or about 3300-3400 lbs per tire.

"My question is: Would going to the next load range up (from G to H) or going to a harder compound tire reduce this problem? I currently use Goodyear G614RST tires, size LT235/85R16. I've heard in the past that Michelin uses a harder compound in their tires, which makes for a little stiffer ride, but might this overcome some of the effects of scrubbing? I feel I could get a lot more miles out of my RV tires if I could reduce the flat spots or sculpting caused by unavoidable scrubbing."

My answer:
Axle alignment or wheel balance isn't the problem. There is a sketch in this post on interply shear that shows why the tire tread scrubs. The center of tire rotation is not pointed to the center of the turn radius so the tires are always being dragged around every turn. It is just worse on tight turns.

Lowering the percentage of max load capacity is a good idea. Don't forget that it is the air pressure that determines the load capacity not the Load Range (G to H). You will gain nothing from a Load Range change if you do not also increase the air pressure. You do need to confirm the wheel max psi capacity which, for some wheels, is not easy to do as some wheel manufacturers do not have high pressure ratings easily available.

Regarding tread compound: Sometimes it isn't just the hardness of the rubber but also the tread pattern that can affect scrubbing wear.

In general, the tread scrub is a function of dragging a trailer around.

Send your questions to me, Roger, at Tireman9 (at) gmail.com


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##RVT783

Question on Interply Shear on trailer tires vs tow vehicle tires

A question from a reader of a post on a trailer forum

"Tireman, your concerns over shear puzzle me. G614s are LT tires as are the tires on my 2500 pickup. Only difference is G rated vs E rated. The truck mfg recommends 60 front 75 rear with 80 psi max on sidewall. Why is there no concern about shear on the front tires of my truck. It seems to me the frequency of shear forces is much greater on the truck than the trailer."


Ya, I understand your confusion
 But the reality is that when computer analysis is used to look at the internal structural loading, the fact that tow vehicle tires are all operating at very low "slip angle" (difference between travel direction and angle the tires are pointed to) is significantly lower than for tires on a trailer.

The reason for this is that the center line of of tire rotation for the tow vehicle tires points to the center of the radius while the trailer tires, especially on tandem axle trailers, does not.


This translates into a higher slip angle which means higher internal structural twisting forces on the belts. The computer model suggests 24% higher on the TT tires than TV tires even if all tires were the same with identical vertical load and inflation.

TV front tires have "Ackermann" alignment designed into the suspension but TT have no allowance other than bending of tires, springs, spring mounts and bushings but the forces to bend the springs etc have to go through the belts of the tires.

This is a MAJOR reason for travel trailer tire life to be much shorter than motorhome or tow vehicle tire life.


Hope this helps folks understand a bit of what makes tire engineering a challenge.

Editor: Here is an earlier article Roger wrote about "interply shear," if you want more information. 




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What is the optimum tire pressure?

Load inflation tables identify the minimum inflation for a number of specific load placed on the tire.

Tire sidewalls tell you the maximum load capacity for a given tire when inflated to the maximum pressure for that load range in that size tire.

Optimum implies we have common agreement on which performance requirements we feel are most important. Optimum also implies that there is agreement in the inherent trade-offs of the numerous performance characteristics.

If the only performance we are concerned about is load capacity and if we want the maximum capacity possible for the size and load range of a specific tire, then we have a situation where the optimum inflation is the maximum for the load range, which is also the minimum inflation specified in the tables.

If, however, we do not need to support a load that corresponds to the tire's maximum load, then it is possible that other performance criteria may be considered and as a result there will be trade-offs to arrive at a new optimum.

If we are talking about motorhomes, there are normally performance characteristics other than just load capacity. Some might be fuel economy, noise, ride comfort, tread wear, steering response, etc. Inflation pressure will affect each of these characteristics — some positively and some negatively. So clearly the "optimum" depends on the clearly understood and agreed on priority of these and other characteristics.

Bottom line
As a tire engineer I suggest that people select an inflation pressure that will provide at least 15 percent extra load capacity over the heaviest loaded tire on an axle. All tires on an axle should run the same cold inflation. The above will still provide acceptable ride and provide improved durability and fuel economy.

For multi-axle trailers I would consider improved durability, i.e., reduced chance of failure, to be of primary importance. So in this application, the "optimum" inflation pressure would be the pressure on the tire sidewall associated with the maximum load capacity. Even if you are not loaded to the max load you want to lower the "interply shear" forces as much as possible, as trailers induce much higher shear forces than seen in similarly loaded tires would if on a motorhome.


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"Interply Shear" and other Techno Babble

I have been putting this topic off for quite some time as I was worried about overloading you with too much "Techno-babble" but I find myself having to constantly repeat this information in individual posts on various RV forums I monitor so figured it would be easier to do a complete post that I could direct people that want to understand why the loading of some trailer tires is much more complex than the average person thinks.

To make this less painful I will give the Bottom Line info first, so those not interested, can stop reading before I put them to sleep.

BOTTOM LINE
When a radial tire is loaded, the belts and body have to bend from a round shape to a flat shape in the area that contacts the road. In addition when you turn a corner the forces generated to move the RV sideways have to be transferred through the tire structure.

This causes additional bending of the belt and body structure. The more the bending the higher the stretching of the rubber. With enough stretch, microscopic cracks form and existing cracks get bigger. Eventually with enough cycles and enough force the cracks may grow and join up with the possibility of tire components separating which could lead to a tire failure. You can lower the stretching if you lower the bending and you can lower the bending if you increase the inflation.


So now on to the Engineer Speak and Techno Babble

If you own a multi-axle trailer these forces can be much higher than those seen on a tow vehicle, motorhome or car, where the tires are not close together but at the corners of the vehicle.
I found an excellent video that shows the results of these forces at Keystone RV. Watch the section from time 0:46 to 1:07 and note that the tires on one axle bend inboard while the others are forced outward.

Special consideration for multi-axle trailers. Warning, this gets technical.
When not driving in a straight line there are special side loads on multi-axle trailers because the tires are fighting each other because they are not "pointed" to the center of the radius of the turn. These loads cause interior structural tearing. Sometimes 24% higher loads than those seen in tires on non-trailer application. Initially tearing is at the microscopic level but with time and repeated cycles these forces grow which can lead to small cracks at the belt edges as seen here at the arrows.

 If not spotted these cracks continue to grow to almost the full width of the tread as seen below.

 If you are lucky you will see the bulge in the tread as seen here and now you know this tire has failed and MUST be removed AT ONCE as the separation can grow and  can cause a belt to come off the body of a tire.
You can lower these forces by either decreasing the load 24% on the tire (probably not something you want to do or may not be able to do) or you can increase the inflation to stiffen the structure and decrease the slip-angle. In this case you could increase the tire inflation from the minimum inflation needed for the static load to the inflation associated with the max tire load as molded on the tire sidewall. BUT you need to be sure you are not exceeding the max rating of the wheel.

So the best recommendation I can give to trailer owners is to run the inflation molded on the tire sidewall. For owners of a TV or motorhomes, I recommend you run the inflation needed to carry the actual measured tire load plus at least a 10% margin.