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Wednesday, August 30, 2017

Why does a sidewall "blow out" when you run on low inflation?

The title for this post is essentially the exact question I was asked.
"Tireman9, can you explain why, when the tire pressure drops below a rating for a tire, that the sidewall of the tire blows out with great force? What pressure drop will cause this type of damage? When the tire blew it had enough force to damage the motorhome with the rapid escape of air that made a large bang when it blew."

My answer:
OK, let's see if i can cover the details of why and how a tire sidewall fails due to being run without proper inflation.

The mechanics are essentially the same, be it a textile (usually Polyester) tire as are most P, LT, and ST type tires, or for tires with steel body cord as most commercial grade LT tires and "TBR" Truck-Bus Radial" tires. These cords are referred to as the "body ply".

I think we all realize that tire sidewalls bend when loaded. This can be observed by simply looking at the bottom (near the road) vs. the rest of the tire sidewall. The amount of bending is essentially just a function of tire size, load and inflation. This bending includes some stretching of the outer surface of the tire and of the rubber surrounding the body ply. This stretching results in some heat being generated. You can test/experience this heat generation yourself with a simple test of holding a rubber band against your lip and stretching and releasing the rubber band rapidly. Your lip is sensitive enough to feel the temperature rise of the rubber band.

Now the rubber used in tire construction can tolerate some temperature rise. The heat generated can transfer to outside air at about the same rate it is being generated. This is what happens for hopefully tens of thousands of miles and hundreds of thousands of revolutions, i.e., sidewall flexes.

So what happens if there is a leak of inflation air? Or if the tire was not properly inflated in the first place? With lower air pressure the amount of bending increases and with an increase in bending we see more heat being generated. Increased heat generation means increased temperature of the rubber internal to the tire structure. Since rubber is a good insulator, heat transfer can be slower than heat dissipation to the outside air so the temperature can continue to rise ever faster.

The strength of the rubber decreases with an increase in temperature which allows more bending. With slower heat transfer from the internal structure to the outer surface and increased heat generation as more air leaks out, I think you can see how it is possible to get to a point where there is something like a chain reaction or "runaway" temperature increase.

The above heat generation can also result in the polyester experiencing a rise in temperature with the associated loss of strength. You have seen the effect of high heat by holding a match near the end of a piece of Nylon or Polyester rope and see the textile melting. In the steel body ply tire the increased bending can result in a fatigue failure of a steel cord. You can test the fatigue with a steel paperclip. Simply bend the paper-clip a few times and it will break. In the case of a tire the number of bends to failure can easily be in the thousands.

So what you are seeing when the sidewall fails is the result of dozens, hundreds or thousands of miles of excess rapid flexing of sidewall rubber and sidewall cords. If the bending is great enough the materials that are being bent simply fail. An explosive loss of air can occur if/when the force of the remaining inflation exceeds the strength of the body cord. Since there is an almost infinite combination of load, inflation, bending and speed involved, it is impossible to give an exact answer as to when the sidewall will finally fail.

If we have a large tire we can have significant force involved even at relatively low pressure. 20 or 30 psi can easily generate over 30,000 pounds of force.


Tuesday, August 22, 2017

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. 

Tuesday, August 15, 2017

"Run Flat" vs "Blowout" protection

Saw a thread on run flat fixtures to be used on the front of an RV to provide some protection against losing control if a front tire were to fail. A lot of discussion and conjecture but not many facts. Before I start it's important to be sure we all are on the same page with these terms.

"Run Flat" capability in today's tire market is a reality for passenger car tires. Tires designed and so labeled can provide mobility at reduced speed for various number of miles even after a complete loss of air. Some advertise 50 miles at 50 mph. I know of no true run flat tires for LT, Truck or RV application.

"Blowout" is a generic term used by many to describe a tire failure. This term does not provide information to tire engineers on the condition or possible cause of the failure.

"Rapid Air Loss" or "RAL" is any sudden loss of all the air in a tire.

"Belt or Tread Separation" -- This is when the belt and or tread portion of a tire separates from the rest of the tire. The carcass or body of the tire many times will still hold air.

"Run Flat Device" -- This is some equipment that fits inside a tire. It may prevent the beads from De-seating from the rim or may limit the collapse of the tire after the loss of air.

"Steering Stabilizer" -- This is vehicle equipment that may lessen the "jerk' of the steering in the event of an RAL.

I do understand the concern of some about the loss of control of a large RV in the event of a front tire failure. Yes, I have seen some of the videos and they are spectacular, but a front tire failure does not have to mean you will have a crash.

Back in the early '70s I worked extensively on a new type of tire from Firestone, the LXX, which was intended to replace 10.00-20 and 11-22.5 size tires but used a 26.5 rim. I even spent a week in Texas at the test track intentionally puncturing front tires with a 2" diameter hole to test vehicle control. While the tire worked as designed, the cost of new wheels made the tire design not viable in the truck tire market.

Here is a picture in the Akron newspaper at the time showing the tire with my then boss Ed Henry.

The main challenge for vehicle control is to either limit the "drop" when a tire suffers an RAL or to limit the steering response which can send the vehicle into another traffic lane.

There are a number of systems on the market:

Tyron is designed to keep the tire on the wheel. It does not appear to lessen the collapse of the tire.

Rodguard inserts for 14" to 17" tires appears to partially support a flat tire.

Safe-T-Plus steering stabilizers take a different approach to vehicle control.

Hutchenson seems to focus on extreme situations, such as military applications.

I recall hearing about multi-piece inserts like this one from TAC but was not able to find current info for 22.5 RV size tires.

IMO the best approach to preventing a loss of vehicle control is three steps:

1. Use a TPMS and have it set to tightly monitor your inflation. Test your sensor to confirm it reports pressure loss at the level you expect. It may not be set correctly and a test can confirm.

2. Closely inspect your tires at least annually and if possible even have your tire dealer check for out-of-round or lateral wobble of the tread and bulges in the tire sidewall. These can be early warning of separations or other structural damage to a tire.

3. Study and review the safety videos on how to maintain vehicle control in the event of an RAL.
   Here is one from Michelin, and another, and yet another.


Wednesday, August 9, 2017

New Goodyear Endurance pressure & temperature question

Saw this question on temperature and pressure increase on a new set of Goodyear Endurance tires.

"Took our first trip with the new Goodyear Endurance tires.
Trailer rode great, better than the old tires.
So, on this first outing I closely monitored the TPMS to see if there were any differences in the readings I had with the old tires.
Obviously, the first difference was the starting "cold" tire pressure. (65 psi vs the old 50 psi)
The first thing I noticed was how quickly the tire pressure increased once we got on the road.
87 degree day, 60-63 mph, tire pressure on sun load side got to 75psi (73 - 74 psi on shaded side). Tire temp got to 96 - 97 degrees (99 degrees on one tire briefly).
This compares to the average 5 - 8 psi increase I experienced on a typical trip with the old tires.
This brings me to my question.
Is a 10 - 12 psi increase in pressure considered acceptable? If so, what should I set the high pressure alert at on my TPMS?

My reply:
The temperature and pressure changes are reasonable for most 14" - 16" RV tires, especially trailer  applications, but I will focus on your question.

Not sure if your "test" is "scientifically" sound -- You were comparing two different tires (LR-C vs. LR-D) and a new tire vs. old used tire.

New tires will always run hotter than old tires.

With nominally dry air the pressure will increase by about 2% for each +10F. One other related item is that aftermarket external TPMS are not reporting the temperature of the hottest areas of the tire which is internal to the structure. Also while it may not seem reasonable, it has been demonstrated that the temperature of the air inside a tire is not uniform. It is also known that the metal body of valve stems and the metal body of the sensor will be cooled as it whips around in outside ambient air which will result in a lower reading.

In all probability your tire is actually 20 F to 30 F hotter than the TPMS is reporting, but this is within the expected design limits considered by tire engineers.

New tires do run hotter than old for a couple reasons. Extra stress as initial cross-link chemical bonds are broken as tire is "broken in." This takes about 100 miles. New tires have deeper tread depth, which will also make a tire run hotter, which means more pressure growth.  Your GYE have extra components (mass) in the tread. Read the sidewall and I believe you will see Nylon cap plies in the tread which were not in the old GYM tires. This is another contributor to more heat.

To your question on pressure increase setting for TPMS. As I covered in my blog post on TPMS settings I figure that +25% from baseline is a reasonable number, but obviously the pressure you select for your baseline may affect the top number.

Bottom line. Your numbers look normal and acceptable.


Tuesday, August 1, 2017

How I program my TPMS

As you know I am a strong supporter of having all RVs being equipped with Tire Pressure Monitor Systems. Since 2004 cars and many pickups with ratings less than 10,000# GVWR have come with TPMS installed by the factory.

RV companies have not been providing TPMS even as optional equipment from the factory, so we owners have had to make the purchase and install the warning systems ourselves. This means we need to ensure the warning levels are appropriate for our RV.

While I can't address how to program every different type or brand system, I can tell you how I have the system I purchased set up. I hope that after reading this post you can sit down with the information that is specific to your RV,  i.e., load on each tire position, tire size, Load Range and inflation info, and ensure your settings are appropriate for your personal situation. So I suggest you dig out the instructions that came with your TPMS and review your settings.

First off, some terms:

MOP - Minimum Operating Pressure - This is the minimum pressure that I think you should ever operate your tires. This pressure is found in the Load/Pressure tables and is the minimum pressure needed to support the actual measured tire load.

CIP - Cold Inflation Pressure. This is the pressure you would set your tires to before driving on them. The tire should not be warmed by either driving on them or having the tire in direct sunlight for the previous two hours. Some folks call this their "Set Pressure" or "Baseline Pressure". I suggest the CIP be set to MOP + 10%, or +10 psi, but not to exceed the pressure molded on the tire sidewall associated with its max load, as this tire pressure may be the intended max CIP for the wheel.

Tire Load - This is the actual measured load on the tires when the RV is at the heaviest you would ever expect to be. Ideally this has been learned by having individual scales under each tire position on your RV. If you can't get this number then at least get the RV on a truck scale such as a CAT scale at a truck stop and learn the actual load on each axle. We want to know each tire position or each axle, as loads are almost never completely evenly distributed. It is not unusual to have tire on an axle or on one end of an axle to have 500# to 1,000# more than another position. There are a number of posts in this blog on load and how to get the loads on each tire position. Note duals are considered one position. If you can't get individual axle ends measured then I suggest you use the measured axle load and assume one end has 53% of the axle load. This would provide at least a partial margin for error.

Hot Running Inflation - This is the pressure we see on a tire after it has been running down the road. I have previously covered the relation between Temperature and pressure in THIS post. Normally for properly inflated tires this will be 10% to 20% above your CIP.

When I bought my Class-C unit in 2008 the first "add-on" was a TPMS. At the time I was not really aware of the selection of TPMS available for the RV aftermarket but I found a system made by German electronics company Hella. It was an internal system but it was designed for passenger cars so I had some problems as my hot tire pressures occasionally exceeded the high pressure warning level programed by Hella. I learned to ignore the occasional high pressure warning levels. In 2009 I started to go to FMCA RV conventions and rallies where I discovered the aftermarket systems designed for the RV use. Based on features I felt important, I purchased a TireTraker TPMS.
When we bought our new 2016 Class-C coach, I needed a new TPMS and selected the TireTraker TT500 as I felt the features plus the lifetime guarantee made it the best option available for me.

Programing the TPMS:
This is usually a two-step process. First, you need to have the monitor/display "learn" which sensor is on which tire. For the TT500 it involves stepping through the monitor settings, identifying a position and installing a sensor at the appropriate position. Other brand TPMSs may have different setup steps or even may come pre-programed with each sensor marked by position. It's the next step that I think is also important to get right for your specific needs.

Setting the warning levels:
Before we start the actual process of setting the levels, it is important to know what the various levels should be. You also need to know which pressure warning levels you can set and which may need extra work if you can't set the levels yourself. Again I can't address every brand TPMS so you may need to re-read your manual or even contact the support people at your brand TPMS.

1. Low Pressure warning level. Some systems have a fixed % below your baseline pressure; others may allow you to set this level independent of the CIP. You need to know how to set this level or what the % below the baseline your system works on. My TT500 is pre-programed to warn at -15% from the "Baseline" or CIP.

2. High Pressure warning level. The TireTraker is pre-programed at "Baseline" + 25%.

3. High Temperature level. The TireTraker, as are many TPMS, is set to warn of high temperature at the 70 C to 75 C (158 F to 167 F) range. IMO it should be remembered that it is low pressure that generated high tire temperature. If you get a High Temp warning but the pressure is OK, then your sensor has detected high temp from metal components that transfer heat faster than rubber. This might indicate a bearing or brake problem which should be checked as soon as possible.

Here things get a bit complex:
We do not want to operate tires when under-inflated but if we set the CIP or Baseline pressure to just what is needed to support the load the TPMS will not warn till we are 15% underinflated. If our CIP is MOP + 10% (See definition of MOP above) we still could end up with an underinflated tire. So to meet my goal of never operating in an underinflated condition, I will need to set my TPMS "Baseline" pressure to MOP + 15%.

My situation and solution:
As measured by RVSEF (RV Safety & Education Foundation):
My front tires are loaded LF 1,900#   RF 2,100#   and
Rear Duals  LR 3,550#   RR  3,850#
Tires are LT225/75R16 LR-E

Based on Load & Inflation tables for my size tires, my MOP for my front tires on my Class-C is 60 psi based on the 2,100 load and also happens to be 60 psi based on my read dual load of 3,850.

I add 10 psi to 60psi and get 70 psi for my CIP.

If I want my TPMS to warn me before I get to my MOP, I need to set the "Baseline" pressure on my TPMS to MOP + 15%, which for 60 psi is 9 psi. So in my case my "Baseline" for the TPMS and my CIP are within 1 psi of each other. I don't worry about trying to measure or set my tire pressures closer than +/- 1 psi. I would say that if you are within +/- 2 psi when setting your CIP you are in good shape.

We need to be careful when discussing safety margins or pressure changes as people with Load Range C, D and E tires will have different calculations than the folks with LR-G & H tires where 15% might be 18 psi.

The High Pressure warning level for TireTraker is Baseline +25%, so with a Baseline of 69 to 70 psi, that equates to 87 psi. On a recent trip to Indianapolis in the heat of the day I noted my tire pressure had gone up about 15%, so my settings worked out for me.