Is there a specific "Safety Margin" on tires?

Read a question on tire "Safety Margin" 

I pointed out that the term "safety" margin really doesn't apply to tires but a "durability margin" might be a better way to think about tires.

Tires seldom, if ever fail as soon as they are run 10% overloaded or run 20 psi low or 10mph over their stated speed rating.
Damage is cumulative and I don't know of any automotive parts that "repair" themselves. In some mechanical parts the damage is in the form of accelerated wear. For a moment think of running your engine low on oil. Will the engine fail if you run it 5% low or 20% low or 80% low on oil? In how many miles? I don't know the specific answer to the number of miles based on how low on oil you are. Does the fact that you might not have an immediate engine failure from running 50% low on oil mean it is OK to do that?

In tires, the damage starts at the molecular level with chemical bonds breaking (cracking) which can grow and may become microscopic in size and eventually might result in cracks that grow large enough for the structure to fail. i.e. a belt separation.

Overload or low inflation or over-speed, lets call these damaging events, can each initiate or contribute to an accelerated growth of these "cracks". In rubber there is both an initiation phase and a growth phase.

It is conceivable for tires to tolerate "X" level of damaging event for many thousands of miles and the tire never "initiates" a crack but also there can be a single damaging event such as hitting a pot-hole of a certain size, depth, length, angle of incidence and speed that is the "initiation" event and this results in a belt separation 5 miles or 20,000 miles later, with the variation be the result of different growth rates. This was covered in detail in my blog in Jan 2020. I examined a Tire Industry technical paper on tire forensics and impact damage that identified a 100% correlation between impact damage (such as you might get hitting a pot hole) and belt separation failure.

Many times I read reports from RV owners saying..."I was driving 55 mph down the Interstate and had check the air just an hour previous and was not overloaded when the tire failed for no reason." Sorry but there is always a reason. That reason might be because of the speed, load, inflation road surface, i.e. damaging event, that the tire was driven on 1,000 miles previous to the actual failure.

Forensic tire analysis is a very complex science and there are very few engineers that have the decades of experience it takes to accurately identify the Root Cause of tire failure. I dare say that each major tire company has a handful of engineers capable of correctly identifying the actual root cause of a tire failure 70% of the time. While they may not be able to identify the actual Root Cause they probably have been trained and have enough experience to know if the tire should be "adjusted" as a "customer satisfaction" case but maybe only one engineer that can properly identify 90% of tire failures and be able to issue a technical "white paper" offering the evidence to support their detailed opinion of the originating root cause for the eventual failure. Too often there is too much missing evidence or accurate reporting to tire operation history for anyone to identify the reason why each and every tire failed. 

Since learning all the important facts of the tire's operating and life experience it is very difficult to be able to  identify the reason for every failure. Even if there is physical evidence of the tire hitting a pot hole you can see in the data of the Jan 2020 blog post that you still can't provide a reliable answer such as. You hit a pot hole 5,678 miles ago and that is why your tire failed.

Back to the question of "Safety Factor or Margin". All tires sold for use on US public highways are to meet a series of tests such as speed, load endurance, staying on the wheel with low inflation etc. Tire companies also have their own minimum performance standards that tires are suppose to meet. Tires are not graded such that they pass 85% of the tests or that 90% of all tires made must be capable of passing all the tests. The laws for tire safety are written such that 100% of the tires sold must be capable of passing 100% of the tests. 

Each tire company tests tires beyond the minimums and using statistical data analysis they can predict that the performance required on any individual test tire such that 99.96% or some similar number, of all tires made will be capable of passing the DOT testing when new. Tire companies have whole departments constantly looking at test tire results and result variation in an effort to be confident they are making good tires.

What tire companies can't do is predict how much any individual RV owner will overload or under inflate their tires so it is impossible to come up with some specific "margin" of acceptable overload because the other factors such as speed, ambient temperature during operation and level of under-inflation are unknown and too variable to allow prediction of durability.

##RVT972

How much Air Compressor do you need?

 Some things to consider before making a purchase.

When making a purchase decision for air compressor the number one feature to consider is the max pressure capability. You must be able to get up to the max for your tires. You can learn that number by simply reading the sidewall of your tires.

All tires will have a statement that reads something like "65 psi ( 450kPa) air pressure Max load 2,500 lbs (1,135 Kg)". That tire is telling you the MINIMUM pressure needed to support the stated MAX load for that tire.

RV Trailers usually come with Load Range C (50 psi) to LR-E (80 psi) tires. Some large Class-A Motorhomes may say 100psi but there are a few tires that have higher inflation pressure associated with the Max load.
Whatever the highest inflation number on any of your tires, you need to have a bit more capability. I suggest at least +10 Psi with + 30 psi being desirable.
The reason for the extra is that if you need 80 psi in your tire and the compressor is only rated for 80 psi you may never actually get to 80 because the rate of inflation slows down for all compressors as you approach their upper rating.

The second number to look at is the Rate of inflation which will be something like 1.5 CFM (Cubic Feet per Minute) @ 100 psi or maybe 3.0 CFM @ 50 psi. Now you need to pay attention to the "At" number because all compressors will put out more air volume at lower pressure but that 3.0 CFM @ 50 psi unit may only be capable of 0.1 CFM @ 100 psi which means if you are inflating to 105 it might take you a long time ( 20 minutes?). Some compressors, may mislead you with high CFM number but state it at a low inflation number so read the fine print.

Finally do not "over-buy" more capability than you need. If you do you may be wasting money on the purchase and also end up with a physically larger and heavier compressor than you realistically need.

Hopefully you will only need to "top off" a tire by adding 5 psi. If you need to add more than 20% of the goal inflation you may have a problem because if a tire has lost 20% of its air and it was driven there may be damage to the tire structure. Re-inflating a tire that has been damaged could result in a tire explosion. If you need that much air I strongly recommend you call a professional. They should have the training and tools that might even include a "Safety Cage" designed to prevent injury.

##RVT971

Load Range E or F or even G. What's the difference?

I found a discussion on Load Range in a Forum where the question was: How strong of a tire do I need? was being discussed. Here is some of the discussion:

Below you see what is stamped on my tire sidewall:
Load Range is rated at "H" 4940# single tire That's at max pressure!!
Load Range 4675 Dual tires.
Tread is 5 ply of steel. Good puncture protection with 5 steel treads.
Sidewall is 1 ply of steel. Maybe this is why they ride smoother with only 1 steel side wall.
Am I missing something since your Load Rating info does is not the same as mine. There's not enough plies to be rated an "H" tire???

Well, I felt that a better understanding of the "strength" of the Steel ply, in the subject tires, might help people understand the concept of different tire constructions 

The number of 'Ply" or layers of cord (textile or steel) are not in themselves any proof of strength. Individual cords of steel are made up of many strands. The steel or other material used to make the strands can have a wide range of strength. Also the number of strands and even the "twist" of the strands can affect the strength and flexibility in the end product. Here are some basic examples of steel cord/cable.

Each has a different configuration. Without more information it would be impossible to know which is "stronger". Don't forget tires have to flex and bend millions of times so just max strength may not be the best choice as you need flexibility too.

If we get to more complex cords we "twist" cords together and can get different properties. As seen here.

So an obvious question is how is the material selected by the tire engineer? 

There are a number of different tests conducted on tires to establish their "strength" rating and the different materials can help a tire meet the different tests. It is completely possible for a given tire to pass some tests associated with a given level. Lets say "G". BUT if a given tire only passes the "F" level of one test, then the tire would be rated as "F" even if it passed the "G" level in the other tests.

Now if the sales dept wanted a "G" rated tire then they would ask the engineer to change the specification so the tire could pass all the "G" level testing. This change or "improvement" may or may not result in more layers of steel. I can relate to an actual example of such a process in a tire I developed. It turned out in this case that the only change I needed to make was the wire in the "Bead" of the tire. This is the "cable" of wire that holds the tire on the wheel. It is kind of an anchor for the body ply. Here is a very basic image of tire components. The feature I want you to understand is where the "bead" is located.

As you can see the 'Ply" can be very complex and simply looking at the number but not the Load range can be misleading.

The bottom line is you need to know the Load Capacity in pounds that you need for your specific application. In the same size you may have different Load Range such as E, F, or G. Each Load Range has a different inflation level for the size tire you are considering and a different load capacity. So you need to consider much more than just the number and type of "Ply".

 

##RVT970

Valve stem extender potential problems

Sometimes my wife accuses me of being too negative because I seem to always come up with something negative about almost any topic. I really don't consider it negative when I see there might be a way to make something or a situation better.

I guess it's in my DNA to never be satisfied and always want things to be better, easier, safer or more durable.

The simple act of checking tire air pressure is an example.

 Each time you use a hand gauge to check air (morning of every travel day) you are pushing on the valve stem. If you have a standard short (less than 2") valve stem, no problem, BUT for dual tire positions as seen on almost all Class-A and Class-C RV motorhomes there are either valve stems that have a bend in them or there are  extenders  of some type. Pushing on a bent or angled stem will place a torque on the stem mount in the wheel. This can lead to eventual degradation of the rubber seal between the wheel and the stem.  Here is what happened a few years ago to the tire of a friend of mine. He was not running a TPMS so got no warning that the valve stem developed a leak at the rubber gasket between the wheel and the stem.

 

 The tire lost air and the steel body cords fatigued due to over-flexing of the sidewall which resulted in the sidewall "blowing out". Initially he thought it might be a "defective tire" but when inflating the new tire it was discovered that the valve stem no longer had a solid rubber gasket at the wheel. So obviously this was not a "defective tire" as any tire can fail if you do not keep the air in it. Soon after this he installed a TPMS.

 If you have some type of extenders, flexible hose or hard line, you might end up moving or bending re even loosening the extender if it isn't supported when you push a gauge or air chuck on the outer end.

One advantage of running TPMS that few people consider is that the TPMS gives you a pressure check each morning, as well as continuous as you drive down the road so this eliminates the need to push on the stem or extender.

TPMS eliminates this torque force on the valve stem mount along with saving you time to go out, get down on your knees, remove the metal valve cap, push on the stem and get a reading. Lots of fun if it is cold or raining.

 

I prefer to just turn on my TPMS in the morning and after my cup of coffee look at the TPM monitor and in a minute or two know the state of inflation for every tire.

 

No Muss or Fuss and the additional benefit of no torquing the valve stem mount. 

##RVT969

 

What inflation should I run ? Motorized vehicles

 The question as asked is simple but of course I have to make the answer complex.

Not really. However there are two different answers. One is for "Motorized vehicles"  Class-A, Class-C Class-B and tow vehicles. The answer for trailers be they tear drop or triple axle 5th wheel trailers has some minor but important differences. This post will address the Motorized Vehicles so I don't have to keep switching back and forth.

Step 1: Learn the ACTUAL load on each end of each axle. This should be done when the Motorhome or truck is loaded to the heaviest you would ever load it.

Step 2: Using the heavier end load number for each axle, consult the Load & Inflation tables for your size and Load Range tire, to learn the MINIMUM inflation required.

Step 3: Add 10% to the number is step 2. This so you are not chasing inflation every time the ambient temperature changes.

Step 4: Set your LOW Pressure warning level on your TPMS to the pressure in #2 minus 2 to 4 psi. (Note this variation is to account for gauge and TPMS pressure reading variation). We want the warning to sound as soon as we might be overloading any tire.

Step 5: Using your hand gauge, that you have checked against your personal digital "Master Gauge", set the "Cold Inflation" pressure for all tires on an axle to the same pressure, which would be the pressure in Step #3. NOTE "Cold" only means the tire has not been driven on or been in direct sunlight for the previous two hours. This means the tire is at "air temperature" or the "Temperature in the shade".

Step #6: Get in your vehicle and you can now drive.


Information. The +10% is to give you "wiggle room" for day to day changes in air temperature. We want to "Protect" the "Minimum" inflation number we learned in Step #2 while at the same time not have to get out and fiddle with tire pressure every morning. Each morning when you get up, you can turn on your TPMS and make your coffee. After a few minutes you can then read the TPMS monitor and as long as there is no unexpected pressure change (drop), you can be satisfied that all tires are sufficiently inflated. You may see a slow drop in pressure over time even when there is no change in temperature. A drop of 1 to 2% per month is normal fol all tires, but once you follow this guide you can probably expect to only need to add air once every two to three months. For folks needing High Pressure ( 80 to 130 psi ) this means you can plan to "top off" your tires at the next truck stop where they should have plenty of air at the pressure need.  I will cover adding pressure to a hot tire in a separate post.

Example: Following the above over my travels from Ohio to Oregon to Calgary to Glacier and home over a two month period, I only needed to add air one time at Yellowstone and I only needed to add about 3 -5 psi to my LR-E tires to get back to my +10% number in Step #3.

Comment: After a while you will learn that the inflation numbers in your tires are reported as slightly different than seen with your hand gauge. This is normal as most TPMS are rated at +/- 2% for pressure accuracy. The primary job of a TPMS is to report a pressure drop Not to report extremely accurate pressure. I would feel that if your hand gauge reads +/- 2 psi from a reference gauge that is good enough. If your gauge is off by 5% of your tire pressure goals you might want to get a better gauge.

Please do not forget the above is specific for "Motorized Vehicles" and not for trailers or dollies you pull.

If you pull a dolly I might treat it more like a trailer with higher inflation than Minimum +10% but that is a separate topic.

##RVT968