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.”

4 questions on inflation, a long back story and RVs are not the same as HD Trucks

 [QUOTE=yeloduster;6070640]This is a question for Tireman.

A little background first. My older brother was an independent OTR for years. The last 15 years or so of his career he had a contract that took him from his home in Roosevelt, UT to Houston, TX. His gross weight were ~80000 lbs. He typically left early Monday morning and was home by 4 PM on Friday.

In the winter he might leave Roosevelt at 5000 ft elevation and maybe -20°. He would proceed through Colorado and top out on Berthoud Pass at 10,300 feet and proceed to Houston where it might be 85° and elevation 100 ft.

His tire cold inflation pressures would experience a difference of ~5 PSI due to elevation change and ~10 PSI due to temperature change. In addition because he was hauling oil field equipment on a flat bed his load could be heavy on one side because of the odd shape of his load. I know he couldn't measure side to side weights. He relied on axle weight to set his air pressure.

He and I talked about tires a few times. I don't recall talking about specific pressures but I know he checked his pressures each morning before leaving wherever he was but didn't change the pressure unless he found a low tire.

He drove millions of miles and wore out several trucks and a truckload of tires. He says he had very few blow outs because he used good tires and monitored his pressures. The few blowouts he experienced were road hazard type events.

His experience would suggest that tire manufacturers load charts take into account a certain amount of side to side imbalance of the load and account for large changes in elevation for over the road trucks.

Actually I guess I have 4 questions:

• If the engineers who developed the load charts took into account reasonable side to side load imbalance and large elevation changes shouldn't we just follow the chart?
• If the engineers who developed the load charts took into account changes in ambient temperature when developing the chart would it be better to set your cold inflation pressures according to the chart and add or subtract about 1.5 PSI for each 10° the ambient temperature varies from 72°?
• Most RV owners do not have a calibrated tire pressure gauge. I've read that moderately priced tire pressure gauges are accurate ±3%. Knowing all these uncertainties about inflation would a RV owner be wrong if he set the pressures with his gauge according to the weight on his axles and checked them regularly. As long as the pressures were his original set point ±2-3 PSI everything is OK?
• Some of us are OCD and getting 4 corner weights, studying the charts and getting the exact pressure according to the chart and then setting pressures to ±1 PSI according to our gauge satisfies our OCD but does being that precise really matter for safety, driving comfort and tire wear?

I'm thinking if we inflate to the chart value for our axle weights taking into account the actual cold inflation temperature we should be good![/QUOTE]

 

First off it's important to remember that HD Trucks are weight limited not tire limited so many times they do not have to run as high a pressure as some RVs do. HD trucks also may have tandem duals which means 18 tires while Large RVs may only have 8 tires total so don't get hung up with that comparison.

1. The charts have the MINIMUM psi required to support the stated load. There is no stated or implied "safety factor" that would allow overloading.

2. Tires are to be inflated to the appropriate inflation when the tires are at ambient temperature before you start driving. 72°F is not some special temperature. The chart is based on AMBIENT air temperature wherever you are.

3. My hand gauge (Accutire brand) cost $12 from Amazon and is accurate to +/- 0.5 psi at 80.0 psi ( 0.6% accuracy) when checked against an ISO Certified Laboratory gauge. Paying more for a gauge is no guarantee of better accuracy. I prefer digital gauges over dial gauges as they are easier to read. I do not like stick gauges as they can get way off if they get worn from use or dirty. Dial gauges can sometimes be hard to read.

4 I would suggest if possible to get 4 corner weights when the RV is at its heaviest. Consult the tables for the inflation for the heavy end of each axle. That is the MINIMUM pressure to set the low pressure warning on your TPMS. I would add 10% to that MINIMUM for your "cold" tire (ambient) pressure.

5 If you are traveling back and forth between Phoenix and Pikes Peak just be sure that in the morning at Pikes Peak in the snow you are not below the MINIMUM psi shown for your scale weight and the tables advise.

6 It is the under inflation we tire engineers are concerned about. In my Class-C I tend to run +15% to +20% higher inflation than the tables suggested minimum because I have a light weight RV based on my 4 corner weights.

7. Run a TPMS that you have set properly and test at least once a year.

##RVT1040

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

"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

"Safety Margin"

Some people ask "How much Safety Margin should I have with my tires?" While this concept is simple, the reality is quite complex.

If you want to skip over the "Why" safety is complex just jump to the "Bottom Line" below.

In engineering it is more proper to talk about "Safety Factor" and Wikipedia covers the topic quite well. " Essentially, the factor of safety is how much stronger the system is than it usually needs to be for an intended load". For tires this can become difficult to establish for unlike many materials such as steel or aluminum, tires being made of a number of complex organic compounds both natural and synthetic that have properties that can vary from batch to batch. Even how the raw materials are handled and stored can affect the end product. Also the "strength" of the tire rubber varies with both time and temperature history and as I have previously pointed out the temperature history is not established by just considering the ambient temperature as tire load, inflation and operating speed as well as even storage conditions play a part in establishing the temperature of the more critical components of a tire. Some of these factors can be controlled by the vehicle owner while others can not.

Another part of the calculation concerns the consequences of failure. With some products, the consequences are just an inconvenience say as when a pencil breaks or the ink in a pen stops flowing. With tires the failure can range from an inconvenience if the tire wears out faster than expected or property damage may occur or in extreme cases personal injury can result.

Over the past decades the tire industry has developed a series of guidelines as they try to anticipate the variation in service the vehicle operator will subject the tires too, but even here outside factors such as changes in speed limits or legal load limit changes can affect tires made years before these operating conditions were contemplated.

Top line tire companies have staff of engineers, chemists and statisticians who constantly monitor variations in raw materials and in the finished product. Different plants have different requirements as even something as mundane as the water source can have an affect on the end product. Test labs at each plant are constantly monitoring the quality and consistency of the products that plant makes. not every tire plant makes the same type of tires so along with sales volume requirements plant capabilities are taken into consideration.

Ya but you are thinking "So what? I just want to know the Safety factor of my tires."

Basically I and other tire engineers have tried to consider all of these factors and are constantly looking at tires that have been run on both test tracks and by end users such as yourself. We adjust our specifications to allow our tires to meet and exceed a list of special tests that over time have proven very reliable at predicting the potential for tire failure. While we shoot for zero failures we also know that due to factors out of our control that goal is never possible given the constraints of real life tire use.

I have seen some figures that show a failure rate in the range or 0.05% for many tires but I have also heard of some specific tires (brand, size, design) having a rate closer to 5% or even 10%.

The bottom line
The best I can do is to suggest that you obtain and read the product maintenance manuals for the brand tire you have or are considering of buying. You will probably find that the information across brands is pretty constant so I suggest you at least take a look at a couple different documents. Some of the top line tires have RV or truck application specific reference materials such as can be found from Michelin or Goodyear or Bridgestone or Maxxis or you can check some of the links on THIS post.
- If you have a Motorhome or pick-up slide-in camper you need to confirm the load on each tire position and using the highest loaded end of each axle and the Load Inflation tables from your tire company learn the MINIMUM cold inflation pressure.  I suggest you add 10% to the table number and use that for all tires on that axle for your minimum. 
- If you have a towable (trailer or 5th wheel) also confirm that no tire is loaded to more than 85% of the max load molded on the tire sidewall. AND inflate to the inflation molded on the tire sidewall associated with its maximum load capacity.
- Get and use a TPMS. I have written on how I would set the TPMS warning levels HERE.

- Inspect your tires. Motorhomes can have your tire dealer do the inspection. Trailer owners can follow THIS procedure at least once a year or every 5 to 7,000 miles if you travel that much.

- Never exceed 75 mph with any tire in RV application and if you have ST type tires with no speed symbol never exceed 65 mph.











In my opinion if you follow these guidelines I believe you will have a reasonable and realistic safety margin for your tires.