Why a safety margin on tire inflation is important

 If you have read any of my posts here at RVtravel.com or on my blog RVTireSafety.net or on any individual post on the various RV forums I follow, you have probably seen me suggest you add a “margin” (safety factor aka reserve load to some folks) to either the load capacity or the minimum inflation recommendation, or even to both. You may wonder what are my reasons. I have seen some people try to chip away at my load margin or the minimum inflation I suggest.

Reason for adding extra margins

I do have a reason for adding these extra margins. It has to do with the basic nature of the rubber and other materials used in tire manufacturing. Adding margins to other items such as bridges, buildings, or even the cars and trucks used to haul a trailer or load in the bed, is also fundamental to the nature of Engineering Design.

Engineers and others in scientific fields such as chemistry, medicine or physics all know that the properties of the materials we specify have some level of variation in their properties. For tires, I know and have seen firsthand that you cannot have an absolute number for the strength of a material. Each component of a tire, as seen above, be it rubber, steel, polyester or others, has a set of properties depending on which specific material I select. If we take a sample of material, be it steel or natural rubber (NR) and ethylene–propylene diene monomer (EPDM), we are presented with stated strengths. Even what we call “steel” is available in more than 3,500 different “grades”, each with different properties and levels of strength ranging from 30,000 psi to 72,000 psi.

When designing a tire we can basically choose from one type for the wire used to make the “bead” (the part that keeps the tire on the wheel when inflated—see above) and a different type of steel used in the belts. But then we can also select from a variety of wire cables which is a configuration of individual steel strands. Each cable has its own set of properties of strength or flexibility or even the ability of rubber to adhere to the cable.

As you can see, there is an almost infinite variety of combinations of materials a tire design engineer can select from. There are other things that can also be selected such as the temperature and time to “cure” the rubber, which can also affect the strength and durability of a tire.

All tires for U.S. highway use must pass DOT tests

Now all tires sold for highway use in the U.S. must be certified by the tire manufacturer to be able to pass specific DOT tests. So passing those tests sort of established a minimum “strength.” Tire companies can select to exceed those minimums but generally to exceed the minimums we would need to use more expensive materials or constructions. Since the tire companies want to stay in business, they also need to keep the costs of making a tire in mind. It is also well-documented that if you take a sample of steel cord and pull on it till it breaks, you never get a single result but will get a range of numbers. The same variation is observed in the strength of rubber and polyester, as used in tires. Now to ensure we produce tires that are acceptable to the public and pass the DOT tests, tire companies also have their own internal minimum performance standards. BUT I do not think that all companies design or make tires that perform identically for every type of test possible.

It is also important to remember that the DOT requires that ALL tires be capable of passing the tests—not some or an average or even most. But 100% of the tires made must be capable of passing each and every DOT test. To ensure that all new tires are capable of passing the tests, tire companies use statistical analysis of test variation in an effort to be confident that production tires will pass the DOT testing.

Finally, we come to you, the user. We know that a significant percentage of RV users do not set or even maintain tire inflation necessary to meet published minimums. We also know that many have no idea of the actual load they have on their tires. While some may keep driving speed below 65 mph, some will actually boast of towing at well over 75 or even 80 mph.

A tire’s strength decreases with use and time

In previous posts here and on my blog I have covered the fact that after a tire is used and as time moves on, its ultimate strength decreases. In an effort to decrease this variation, I have been advising that people not run at the lowest possible inflation for the load on their tires. The lower the inflation you run, the more heat is generated, which in turn lowers the ultimate strength of the rubber.

Degradation of rubber strength is not an on-off switch but a continuous process. The more you drive at higher heat, the more strength is “consumed.” The more pot-holes you hit, the more damage you do to the tire structure. This, in turn, can result in a decrease in the maximum strength of your tires.

Load & Inflation tables give you a guide for the MINIMUM inflation for a tire loaded to the stated number and operated at a given speed.

More speed means more heat.

Lower inflation means more heat.

More load means more heat.

And it is HEAT that ultimately can result in a tire coming apart.

However, if you run more inflation than what the tables show, that will decrease the heat.

 

 

In a future post, I will cover what I feel is the improper use of the word “defect.”

Tire Load Capacity is like your engine Red Line.

Many times some people point to regulations for tire inflation and point out that RV companies select the tire inflation and that is what we should run because they think that is the "optimum" inflation.

Well the reality is that the "Load & Inflation" tables actually are giving you the MINIMUM inflation needed to support the stated load.

First we need to remember that the the load is not your estimate or the load your Brother-in-law has in his RV but the actual measured load that you can only learn by getting on a truck scale.

Here is a comment on tire load capacity.

"Ya might have a valid point if the discussion were about OEM tires. However this discussion is about new tires on an existing TT. The owner should go through the process that the TT manufacturer did when determining tire pressures. Tire pressures should be set based upon the tires and their actual loads.

Unfortunately, TT manufacturers don’t have actual load information, so they base pressures on max load. The TT owner has (or should get) information that the manufacturer does not have: actual load. That information should be used to determine optimal inflation pressures."

As an actual tire engineer I would be happier if people understood that the inflation given in the Load tables are the MINIMUM needed to support the stated load.

In the non RV world tire inflation is not based on the minimum needed to support the load. You will find that most cars have inflations that provide a Reserve Load of 20% to 25% or more. That is one reason car tires have such a low failure rate that tires in RV application.

Some people want to believe that all is good as long as you meet the minimum standard, then they are surprised when tires fail.  Well they fail due to the cumulative damage done to the structure when used. This damage reduces the load capacity because damage weakens the structure.

It has been shown that a hard hit from a pot hole can effectively "kill" a tire with the only question being how long can you travel before it finally dies. I have also posted that you can hit a pot hole hard enough to "fatally" damage a tire yet have no recollection of the hit.

Your car or truck has an engine "red line" think for a moment about how long your engine would live if you ran it at 95 to 98% of red line speeds all the time. Well that red line is like the inflation number in the tables in reverse.

Running at the red line is like running at the maximum load capacity for your tire for the inflation shown in the tables.

I bet most of you run no higher than 75 to 80% of red line on your engine, if that high. That gives you a 20% reserve. Maybe you should consider doing the same for your tires.

##RVT973

10% safety margin? Not Over-Inflation for more load capacity.

Originally Posted by Crasher

Roger. When you refer to a 10% safety factor, is the tire any safer at 10% over it's rated load psi? If it is, why don't the manufacturers recommend a higher psi for the load? Or, is the 10% factor to cover the days when the ambient temp is lower which would lower the CIP eliminating the need to adjust the pressure? Whenever I have run tires above the load charts, the center of the tread will wear more than the outer sides. That tells me that the tire was not making optimum contact with the road for best wear and traction. Admittedly, it's a minor issue, but an inquisitive mind has to ask.

My +10% is on the set inflation and is NOT a "Safety Factor" in the normal sense. We know that tire inflation changes by about 2% for each 10°F change in temperature. The intent of this "Flex Range" of inflation is to avoid the need to mess with inflation on a daily basis.
Assume you needed 70 psi to support your heaviest ever expected load (this is why we say get on the scales when fully loaded to your heaviest). So assume you set your inflation to 70 psi and the Ambient temperature is 80F. What happens the next day if the ambient drops to 70F? Your tire pressure will have dropped by 2% to about 68psi which is below what is needed to support the measured heavy load. So you get out and increase your tire pressure back up to 70psi. A few days later it's 90F so tire pressure is now (90F - 70F = 20F so 2% per 10F = 4% increase of the 70 psi so now your tires are at about 73 psi cold so you drop your tire pressure.

See the problem? You are messing around with your tire pressure. almost every day.

However if you have a +10% "Flex Range" above your needed inflation, or in our example + 7 psi You can ignore the day to day pressure variation unless or until the temperature has dropped 50°F.

Tires can tolerate the increase in pressure with essentially no damage but low pressure can result in increased operating temperature which accelerates the "aging" of the belt rubber which can shorten tire life.

Also if you have to mess with your tires a lot, soon you will tire of the chore and stop monitoring and adjusting tire pressure which can lead to low inflation. This extra work can get old quickly and then you stop checking and setting your pressure. I Do Not Want That to happen.

RE center wear. That was an issue with bias tires but I do wonder what micrometer you are using to measure tire tread wear to 0.001" especially given that tire tread wear is normally in the .001" per 1,000 mile range and I doubt that your pressure remains constant over each thousand miles operation. Road surface (concrete vs asphalt) has a much bigger impact on tread wear. 

##RVT965

Another question on "Cold Inflation" vs your "Set Pressure"

Got this question from a reader of an RV Forum:

Thank you for all of your informed comments regarding proper tire care. I need one clarification. I have always considered the cold psi on the side of my 22.5 RV tires to be the minimum to carry the maximum rated load, but have assumed that psi was also the maximum COLD psi the tire should see. From your recent post, am I to understand that unless the tire states that it is the maximum cold pressure, I can exceed it by 5-10 psi?
Thank you for your time, Doug

My Answer:

The wording on tire sidewall IMO was written by lawyers, not engineers or users. Info on the sidewall is the inflation needed to support the Max load.  The difficulty is that few understand that the pressure changes with temperature and the only meaningful pressure, measurement is when the tie is "cold". This still confuses some because some want to apply Chemistry Lab practice of adjusting to theoretical 72.5°F when what "cold" really means for tires is at "Ambient Temperature" and does not include any pressure build-up"   In real life terms this means "Not warmed by being driven on or in direct sunlight for the previous 2 hours"

Now we need to address what is meant by "Cold Inflation" vs the psi to set your tires to or what I like to call your "Set pressure".

I like to suggest the "Set Pressure" for motorhomes to be the minimum needed to support the maximum load on the tires PLUS 10% inflation. 

RV trailers are different because of their Interply Shear problem.

For RV Trailers,  I would like to see a minimum of +15% load capacity over the measured heaviest loaded tire, with +20% to +25% Reserve Load capacity being better. Sadly most RV trailers come with tires that provide +0% to +10% load capacity vs GAWR
NOTE: I am not even addressing the tendency for most RV owners to overload their tires.
So for trailers I try and simplify:
- To lower, but not eliminate the Interply Shear problem I suggest the "Set Pressure" when the tires are "Cold" to be the pressure on the tire sidewall. BUT I still want trailer owners to confirm they have at least 15% "Reserve Load" over their measured scale reading.

##RVT962

"Reserve Load" or Load Capacity Margin

Ran across a post on Reserve Load or Reserve capacity that suggested the RV owner had been given incorrect information. Here is the post and my reply.

Personally, I'd run LTs, simply because of their higher "reserve" capacity; upwards of 30% over the stated load. Given that STs have, at best, 10% (used to be basically 0%), you're still in ST load territory, with a much better tire. Hell, we used to run our old 1/2t trucks with massive loads and just air up to 60-65 psi and go. Yes, it wasn't very far, or very fast, but those tires still lasted 50-60k miles, usually with steel cord showing around the edges. :-) We'd then take them off and put them on a disk or trailer and use them until they sun-rotted.

I think someone miss-informed you about "Reserve Load".

All tires have a stated load capacity for example. "2,340# Max Load" molded on the tire sidewall at a stated inflation level such as  "50" psi.

"Reserve Load" is the difference between the actual applied load and the stated load capacity and is many times stated as a percentage

Example: A vehicle is on weight scales and we learn that a tire has 2200# load on the tire. The tire has a load capacity of 2,750#.   2,750 minus 2,200 = 550#   which is 20% of 2,750. It doesn't make any difference what type tire we are talking about as the math is still the same.

Now, it is true that for a given set of dimensions, e.g., 235/75R15,  the stated load capacity is different depending on type tire and inflation level.   P-type and LT-type and ST-type each have different stated load capacities at their stated inflation pressure. For this discussion, let's keep inflation differences out of the picture.

Let's look at a P235/75R15 at 35 psi is rated to support 2,028# ( In a trailer application P-Type must be De-rated by Load/1.1 giving 1,842# capacity.) An LT235/75R15 is rated for 1,530# @ 35 psi and an ST235/75R15 is rated to support 1,870#

BUT the "Reserve Load" calculation is still  (Tire Load Capacity)/Measured scale Load).

The 10% margin for trailers is the difference between the GAWR and the total capacity of the tires on that axle at their max load.  I have posted in my blog some actual margins showing that many cars have load margins of 25% to 35% while some RVs made before Nov 2017, when RVIA changed the "Margin" to 10%, had margins of tire capacity vs GAWR as low as 1%.

Hope this helps.

##RVT941

Response to some of my information and warnings on tire inflation

After posting on one RV Forum some steps that I felt if taken could result in longer tire life by lowering the Interply Shear Forces, I got this reply:
"Sounds like tires should never be used or stored in any configuration other than the 'as cured' state or they simply self destruct.
Gonna have to figure out how to make these R/Vs hovercrafts."

I offered the following:
A bit of an over-reaction. My advice is intended to offer a series of steps that can be taken to extend the life of their tires rather than actions or inaction that may shorten tire life.

One of the biggest problems is the inability to understanding the real "Root Cause" of tire failure. IMO too many simply assume that somehow, the zip code of the factory where the tires are made is a "cause" for a failure.
Have you read and do you understand the difference between the two major and different reasons (root cause) for failure as covered in THESE posts?


TPMS, when properly PROGRAMED and used  can essentially eliminate one of the two primary reasons for tire failure.


Having a good level (15% to 25%) Reserve Load is the second major thing people can do to get a more reasonable tire life. I would be very surprised to learn that tires with at least 15% actual Reserve Load didn't perform much better than the more normal 0% to 5% level.

"Safety Factor" or "Reserve Load"

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

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

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

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

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

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

Next a larger 5th Wheel RV

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

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

What is your actual Reserve Load?

New GY Endurance only available in higher Load rating. What to do?

Hello Roger,
At the end of next season I plan to replace our GY Marathon tires after 5 seasons and about 20K miles and have been thinking of the new Endurance tire. However, the specs. are somewhat different between them and am not sure what to do.

Our Marathons are LR-D ST225/75R15.  I always run them at sidewall max. pressure 65 psi. The max. load rating is 2540 lbs and gives us about 30% reserve load capacity. The required rim width is 6-7”. The standard tires for our make and model of travel trailer are an unknown brand LR-C and the Marathons were an available option so we upgraded to them. No issues whatsoever with them. I do not know what the max. psi rating for our rims is without taking one off. They are 6-lug.

I like the idea of having a 225 wide tire vs 205 because I *think* it may provide better handling from lateral forces in curves. I have our TV & TT set up for better handling including shocks on the TT. It performs very well in twisty mountain roads at speeds up to 65 mph.

If we were to get the new Endurance tires in an ST225/75R15 width, it means going to LR-E tire which have a max. 80 psi rating. The load rating increases to 2830 lbs. Required rim width is still 6-7”. The increased reserve load capacity would be fine. I have no idea if our rims are rated for 80 psi and would have to remove a wheel. I believe in the practice of running the ST tires at the sidewall max. pressure to get max. reserve load capacity.

If we were to go with an Endurance tire in a 65 max. rating, it would mean going to a LR-D 205 wide tire and the max. load rating drops to 2150 lbs. I do not want to reduce the reserve load capacity that much and would rather not have to go to a skinnier tire.

What to do if going to Endurance tires? If our rims are only rated to 65 psi, could I simply reduce the LR-E pressure to 65 psi? Have not looked at load tables to see what this does. Or stay with LR-D and have skinnier tires and substantially less reserve load capacity? Are tires rated for and inflated to 80 psi going to result in better handling or perhaps an undesirable harsher ride for the trailer? Not sure why GY did not make the Endurance a direct one-to-one swap for Marathons. It looks like GY no longer offers the Marathon?

Comments would be greatly appreciated.

+++++++++

My reply:

Your 30% reserve load capacity is good. I wish other trailer owners had as much. You may not see any benefit in increasing the reserve load above the 30%. The reserve is based on actual scale readings, yes?

I would not go down to the ST205 @ 65 psi.

I see no problem with running the Endurance ST225 LR-E @ 65 until or unless you confirm higher rim psi rating. It may not be marked on the wheel and I doubt the wheels are "brand name". When you change tires you can look for anything that might ID the wheel such as part number. If the wheels are only 65, then 65 is OK.

Yes, in my posts on Interply Shear I have suggested increasing inflation, but your CIP should not exceed the rim rating (all ratings are for cold inflation pressure).

LR-E ST225's have same load capacity when inflated to 65 psi as do ST225 LR-D at 65 psi.

Hope this info helps. 

How are RV tires developed?

 By Roger Marble

If a tire is being designed for a specific vehicle manufacturer such as Ford, Chevy, Toyota, or BMW, there will be a number of tires submitted by competing tire companies all trying to deliver the best overall compromise in performance characteristics. Please note that all original equipment ("OE") vehicle manufacturers have slightly different requirements but all make similar requests for performance improvements in many areas. In the future I will use the term "OE" to include these car and pickup manufacturers.

Compromise: Now is a good time to talk about some of the various trade-offs the engineer is faced with when trying to meet conflicting goals and customer wants. I am sure we would all like an RV that has all the interior space and amenities of a 40’ diesel pusher but gets 25 mpg and can be driven down crowded city streets without knocking off our mirrors. Oh yes, it should also cost under $30k. Well, Bunkie, that just ain’t gonna happen in real life.

The same goes for a tire that handles like an Indy tire, is as quiet as the proverbial mouse, has great off-road traction, is good for 100k miles, and costs $25. One thing few people realize is that most if not all performance characteristics are a compromise. For example, if you improve wet traction you probably hurt fuel economy unless you use a special type of rubber that costs double per pound and is more difficult to process. If you improve handling you might hurt ride and noise. When you improve noise you can significantly increase the cost of making the molds used in manufacturing. The cost of a tire mold can be as low as $10,000 and can approach $100,000 each. Depending on the production volume needs, a tire manufacturer could need 30 or more molds. The list of trade-offs goes on and on.

The competition for a tire application might start three or more years before scheduled start of delivery with two to five tire manufacturers competing for the contract, knowing that only one or two will end up being selected to actually provide tires. The costs associated with building and testing special prototype tires can run in the hundreds of thousands of dollars and are absorbed by the tire company. The only way a tire company can afford this type of activity is by landing a contract for a few hundred thousand tires so the costs can be spread out.

Unlike “OE,” an RV manufacturer may only need a couple thousand tires, so a custom tire designed for a specific RV would be cost prohibitive. Since the RV manufacturer won’t be trying to get custom tires, it doesn’t have staff engineers working on developing specifications for such tires. The RV company will in all likelihood either take what comes already on the cut-away chassis or the bare chassis for Class-C or A vehicles, and in the case of trailers, may buy the tire with the lowest cost that can meet tire size requirements and expected delivery schedule.

For RV applications the one thing that is in the control of the manufacturer is “Reserve Load.” This is the difference between the load placed on each tire with the RV normally loaded and the load capability of the tires at specified inflation.

##RVT802

Safety Margin - Not as meaningful as Reserve Load

I and others really don't like the term "Safety Margin" as it sometimes implies something is safe when in reality all that can realistically be done is to lower the potential for problems.

When it comes to tires the term "Reserve Load" is a more meaningful and descriptive term.

We are discussing the difference between your actual load vs the load carrying capacity of a tire at a specific inflation. If you had a measured load of 4,500# on the heavier end of an axle and your tires was capable of supporting 4,950# at the cold inflation pressure you set the tire to, you would have a "reserve load" of 450# or 10%. To put this figure in perspective most cars have a reserve load capacity of 12% to over 20%.

We have covered proper weighing methods and the importance of having a margin of inflation pressure above the minimum needed to carry the actual measured load you are placing on your tires. So I would ask. Do you know what your reserve load is for the heavier end of each axle?

Despite the above, I occasionally get asked about a tire's "Safety Margin". When asked this question it sounds as if the person is intending to load the tire up to 100% of its rated load plus the additional percent that might be provided by the "safety margin". This is definitely not a good or safe practice.

Yes, some tires may be able to carry more than the max load marked on the tire sidewall (at least for a short time) but I think it is better to think of the average margin rather than the minimum margin.

Federal regulations specify that "all" tires must be able to pass certain tests. So that means 100% of the tires sold are suppose to be able to pass the required tests. Some also want to believe that passing a test that is run on a smooth drum under controlled conditions can always be used to accurately predict trouble free use in real life.

Everyone should understand the concept of product variation. This results from variation in raw materials as well as in variation in the manufacturing process and even variation in testing. 1st tier tire companies have statisticians whose job it is is to target towards minimizing variations mentioned as well as the probability of stacked tolerances to aim for 99.99966% good product hens the term Six-Sigma.  Now not all companies use the concept of six-sigma as this requires a real commitment to ever improving quality from top management all they way down to the night janitor. Such a commitment takes real work, can be expensive and require very involved quality control. Some companies are probably satisfied with 99.9% or maybe 97% or 80% or even lower as they are willing to take a chance that they will not be caught in violation of regulations or that the warranty will expire early enough that there will be no cost to the company for their poor quality. IMO there are companies that no only do not apply statistical quality control such as practiced by 1st tier tire companies and the major car companies of the world, but some companies may not even understand the concept of real quality control.

A company can improve the quality i.e. get closer to 100% good, by over designing the product but if you have large variation you will be building a large number of products that are capable of handling 110% or maybe even 130% of the performance that is needed and this raises cost. Sometimes to the point of pricing yourself out of the market.

Lets assume there was an easy way to measure the quality of tires (remember there is no such test) and we could come up with some numbers. If our target was 7,000 and we were to test a hundred tires from company Q and found that some failed as early as 5000 while others achieved 9000 we could calculate the mean or average and estimate say7500 so some might claim a "safety margin" of 500 (Mean performance minus the goal) when in reality some people would end up with tires that were only capable of 5000. Now if there is a second company Z and with the same test we see that the range was 7000 to a high of 7800 for an average of  7400, would you say the quality or safety margin of these tires was not as good as those from company Q?  After all company Q achieved 7500 while company Z only achieved 7400.
How would you feel if you ended up buying a tire from company Q that only could achieve 5600?


I hope you can see that measuring tire quality can be a very complex issue.  IMO one reasonable way for consumers to estimate the real quality of a product is to look at the basic warranty. Warranty costs are many times greater than the basic cost associated with replacing a product and in the case of tires could amount to as much as three to five times the cost to the corporation of producing a tire in the first place so companies really want to avoid warranty costs.

There are a couple ways to avoid warranty costs. One is to make a good quality product and another is to cut back on the coverage provided by a warranty. If company Z offers say a 3 year warranty while company Q only offers 12 months or maybe as low as 30 day I think it's a safe bet that tires made by company Z have a much better "Safety Margin"  and probably a lower potential for failure.

Rather than worrying about the "margin" of tire brand Q vs brand Z on a theoretical test, I suggest it is better to ensure that your personal "Reserve" is reasonable. To learn what your reserve is you of course need to know your real tire load and that your pressure gauge is accurate and most importantly that at no time you operate your vehicle at a lower than specified pressure and if possible always run a bit more air pressure than the minimum needed to carry the load.


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Can I change from ST to LT tires on my Trailer or 5ver?

This question is a real "Hot Button" on a number of RV forums and blogs. People ask this question because they want a better or alternate selection of brands or they want to improve the durability of their tires.

The answers given seem to range from "Sure, Why not" to "Absolutely Not, Never do it" and some will even offer that they think you are breaking some law if you make any change from the type, size, Load Range or cold inflation from the OE tire information shown on the tire Placard.

As we all should know by now answers about tires are never simple and straight forward and changing tires is definitely one of the more involved answers.

First off, I am not a lawyer but an Engineer and as such I form opinions based on data and facts. So here is the answer based on my 40 years experience as a Tire Design Engineer.

Yes you may be able to change tires  

    

 there are some things you MUST do to ensure that any change you make will actually improve your probability of having better tire durability.

My plan is to provide an outline of the steps you need to take before you make any change. I will try and include each step and each of the points of data you need to collect and evaluate. If you skip a step you may end up with a less durable tire selection which could lead to tire failure, RV damage and even an accident.

Before we start you need to consider that the most conservative approach is to make no change and to simply use the tire Construction (Bias or Radial), Type (ST or LT) and size and Load Range and inflation as specified on the placard and specification documents. This represents the RV manufacturers recommendation based on a number of assumptions as well as some legal regulations the RV MFG must follow plus in many or some cases a desire on the Mfg part to keep their costs as low as possible.

So if you still want to move forward here are the steps you need to take:
1. You need to know the actual load on each tire. This is important because A. we will be basing some decisions on the tire loading    and     B. It is possible that there is a significant unbalance in the tire loading which may be the cause of poor tire durability. With sufficient unbalance it may be impossible to provide a tire selection that would lower the probability of having problems.
To learn the actual individual tire loads you need to either find a company such as RVSEF or agency that has individual scales or to follow the steps outlined on worksheets such as This one  or This one. I have hears some people say that they have been able to get the individual tire loads from their state police or state DOT.

2. Knowing the ACTUAL LOAD on each tire you need to confirm that no single tire is loaded more than the max load molded on the tire sidewall. This is an absolute rule. If any tire is overloaded you should not move the trailer until you either change the load or change the tire.

3. Assuming no tire is overloaded we want to make sure that all tires on en axle are inflated to the same inflation. For multi-axle trailers you can lower the internal tire structural shear forces (the forces trying to tear the tire apart) by running the inflation molded on the tire sidewall. Sometimes this is stated as the Max PSI and other times it is stated as the PSI for the max load. For our purposes we will consider this the proper cold inflation you should always run. 

4. We should have "Headroom" or "Reserve Load" or "Safety Margin" on the tire loading. I suggest 15%. However I know that for many trailers 15% extra capacity above the actual load is very hard to do. Especially since some RV MFG manage to so under-size the tires that even when the trailer is empty they may not have 15% margin.

5. If you don't have at least a 10% margin I would strongly suggest you need to consider changing tires to something with more load capacity when inflated tot he sidewall PSI.

OK so you have some homework to do. We will continue this in the next post.


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Will you increase load capacity with higher Load Range?

The question of load capacity and Load Range gets asked a lot, so it seems to be a bit of a stumbling block for a number of RV owners. Here is an example:
"I need to replace the ST235/75R15 tires on my trailer. Currently have load range C tires which is sufficient for the maximum trailer weight when inflated to 50psi but allows almost no safety margin. I want to change to either load range D or E tires. My rims are only rated for 65 psi so if I went to the load range E tires I would only be able to inflate them to 65 psi. My question is this. Is there any advantage to a load range E tire used at 65 psi vs a load range D tire at 65 psi?"

Now just because he is asking about a 15" trailer application and considering a change from LR-C to LR-D it does not mean the general answer does not apply to others, even if they are considering a change from LR-G to LR-H on a Goodyear 295/75R22.5 Class-A tire.

Let me explain.

As some of you remember, I previously covered the fact that it is the air not the tire that carries the load. For those that still have doubts, here is a picture from a special test machine that measures force and delivers the answer in color. In this case, the lowest force is Blue which increases to Green, then Yellow and finally Red at the highest force. It is clear that the tire wants to be round because of the high inflation pressure but when you press the tire against a surface it becomes flat with  the highest force in the center of the tire. If it was the sidewall that carried the load, as some want to believe, then the Red (high force) should be at the outside shoulders.

Now when you go up in Load Range you are actually looking at the capability of the tire to carry more inflation pressure. Remember to gain in load capacity you need to increase the inflation. Years ago when tires used cotton for body cord we might have 4, 6, 8 or more layers or ply of reinforcement to retain the increased air pressure. Those increments provided steps in the Load/Inflation tables. Since the late 60's and early 70's when materials improved the actual number of ply decreased and the term Load Range was applied to those steps.

So going back to the original question, when increasing the Load Range you gain the possibility of increased load carrying ability but if you don't increase the actual cold inflation, the change in Load Range will not give you any increase in load carrying capacity.

BOTTOM LINE
The answer to the question is;
 He will gain a safety margin if he increases his inflation to 65psi with LR-D but if he stays at 65psi there is nothing further gained by going to LR-E as there is no difference in the capacity at 65psi.
The same thing would apply to the 22.5 example. When we look at the Goodyear tables we see that at 110psi both the LR-G and the LR-H are rated for 6,175# single or in dual application at 100psi they both are rated as 5,675#.

Special note: It is important that you look at the table on the web site of the manufacturer of your tires as not all companies give the exact same capacity number at every inflation level, even for the same size.

Why should I weigh my Trailer tires individually?

Many times when I give a seminar or class to people from the RV community on proper application of tires, I get asked the above question by trailer owners.


Motorhome users seem to understand that knowing the actual load on each corner of their RV will allow them to select the tire pressure they want to run as they balance Ride, Tire Durability, Fuel Economy and Steering Response.
Owners of trailers say they have been told that all they need to do is to run the max inflation on the sidewall of their tires. Almost all the time, that is the information on their placard per the trailer manufacturer’s guideline so that recommendation seems reasonable.

Recently I have been having a discussion via email with an owner of a large 5th wheel trailer.
He even believes that because the load changes on every trip and almost every day it could be “dangerous” to get the trailer weighed as that could mislead the trailer owner to think it OK to lower the pressure to just match the minimum needed to carry the measured load. I would like to offer a different view on why it is important to get the trailer weights on each tire at least once.

More than half of the trailers that have been weighed over the years have been found to have a tire or other suspension component exceeding its maximum load capacity. We all know what happens to people who “Assume” something. When you get the trailer weighed, it does need to be with all the stuff you normally carry on your travels. Knowing this load you will have the facts to see how critical it is to re-distribute the load side to side or front to rear. You will also know if your unbalance has overloaded one or more tires even when the total load seems OK. You will also know if you have proper loading on your pin. I have heard that some axle manufacturers consider having one end of an axle loaded to more than 50% of the total axle rating as overloading the axle. This means that even if you have changed wheels & tires to not overload those components you might still be overloading the axle if you are not careful.

Another thing to consider is the “Reserve Load”. That is the “safety factor” between the actual load and the calculated maximum for a component. Most cars have a 12 to 18% reserve load. One of the other benefits of having a good sized Reserve Load is that it will allow you to occasionally bring home that big load of stuff you just had to buy at the flea market and not overload a component. One way to identify a minimum cushion would be to look at the load capacity of your tires when inflated to a pressure of at least 15 psi below the max inflation and to be sure your load does not exceed that level. A 20 psi cushion would be even better. Reserve load helps compensate for the unbalance that occurs due to side loading because of road crown and wind side loading.

You are probably asking yourself why trailers can’t lower their tire pressure the way motorized units can. The reason has to do with the mechanics of axle spacing and tire side loading whenever you turn a corner. Trailers with two or three axles put enormous side loading on the tires, wheels, and axles whenever they turn corners. A quick look at the tire distortion when you see a big twin axle trailer park will confirm that the tire needs to be as stiff as possible when side loaded. Since we all know it is the air that carries the load we also now realize that it is the air that stiffens the tire and as a result will lower the side deflection and loading.

BOTTOM LINE: Next time you are fully loaded please go to the effort of weighing your trailer and go the extra mile to do the multiple weighing and the math needed to calculate the actual load on the individual tires. Remember a blown trailer tire can lead to substantial damage and financial loss.

How RV tires are developed and why some are a compromise

If a tire is being designed for a specific vehicle manufacturer such as Ford, Chevy, Toyota, or BMW, there will be a number of tires submitted by competing tire companies all trying to deliver the best overall compromise in performance characteristics. Please note than all original equipment vehicle manufacturers have slightly different requirements but all make similar requests for performance improvements in many areas. In the future I will use the term "OE" to include these car and pickup manufacturers.

Compromise: Now is a good time to talk about some of the various trade-offs the engineer is faced with when trying to meet conflicting goals and customer wants. I am sure we would all like an RV that has all the interior space and amenities of a 40’ diesel pusher but gets 25 mpg and can be driven down crowded city streets without knocking off our mirrors. Oh yes, it should also cost under $30k. Well Bunkie, that just ain’t gonna happen in real life.

The same goes for a tire that handles like an Indy tire, is as quiet as the proverbial mouse, has great off-road traction, is good for 100k miles, and costs $25. One thing few people realize is that most if not all performance characteristics are a compromise. For example: if you improve wet traction you probably hurt fuel economy unless you use a special type of rubber that costs double per pound and is more difficult to process. If you improve handling you might hurt ride and noise. When you improve noise you can significantly increase the cost of making the molds used in manufacturing. The cost of a tire mold can be as low as $10,000 and can approach $100,000 each. Depending on the production volume needs, a tire manufacturer could need 30 or more molds. The list of trade-offs goes on and on.

The competition for a tire application might start three or more years before scheduled start of delivery with two to five tire manufacturers competing for the contract, knowing that only one or two will end up being selected to actually provide tires. The costs associated with building and testing special prototype tires can run in the hundreds of thousands of dollars and are absorbed by the tire company. The only way a tire company can afford this type of activity is by landing a contract for a few hundred thousand tires so the costs can be spread out.

Unlike “OE”, an RV manufacturer may only need a couple thousand tires so a custom tire, designed for a specific RV would be cost prohibitive. Since the RV manufacturer won’t be trying to get custom tires, it doesn’t have staff engineers working on developing specifications for such tires. The RV company will in all likelihood either take what comes already on the cut-away chassis or the bare chassis for Class-C or A vehicles and in the case of trailers, may buy the tire with the lowest cost that can meet tire size requirements and expected delivery schedule.

For RV applications the one thing that is in the control of the manufacturer is “Reserve Load”. This is the difference between the load placed on each tire with the RV normally loaded and the load capability of the tires at specified inflation.

Next time:
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Before we cover tire manufacturing -- perhaps dull to you but interesting to me -- we will do our first post on tire loading. I say first, as this topic is the single most important performance characteristic for we RV owners. But few of us truly appreciate the potential safety issues involved.

Updated/reposted for RVT 802