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Friday, October 27, 2017

What effect does speed have on tire failure?

Question:
"What effect does speed have on tire failure?
I have an 8000 lb trailer with 4 Goodyear Marathons. The truck is a Dodge 2500 diesel so I can cruise at 75 mph subject to wind and hills.

How much if any does speed factor into tire failure? I replace tires every 4-6 years regardless of what the look like, and keep them inflated at the PSI marked on the tires."

Basically, increased speed means increased temperature in the critical areas of a tire. Here is a graphic showing the relative temperature in different areas of a tire.
with Red being the hottest and dark blue being the coolest.
When you increase the temperature of rubber you are effectively increasing the rate the rubber is aging. I believe we all understand that old rubber is not as flexible as new rubber. If it isn't as flexible you end up with cracks or cracks that grow every time the rubber is flexed. More cracks and larger cracks are what result in tire failure.

For the above reasons plus others, the major tire companies suggest that in RV operation tires never be driven any faster than 75 and you can see this max speed stated in their literature for many of their RV tires.

But wait, you say your tires have a "Speed Symbol" that relates to 87 or 99 or maybe even 130. HERE is some information on speed ratings. Just because you have a tire with a fast symbol doesn't mean it can be driven at those speeds for 20 or 40,000 miles. About the only thing you can count on is that tires with a "higher" symbol will provide better steering response than a tire with a lower rating.

Now back to the question of how fast can you drive or tow your RV. I previously mentioned 75 as the upper limit but for some tires, there are other things to consider. With ST type tires as found on many trailers, we need to remember that the load capacity is based on a formula that originally specified a max speed of 65 mph. In the past, there were Technical Bulletins that advised that inflation needed to be increased (but not above the sidewall max) by 10 psi to go 70 mph. In addition, load needed to be decreased by 10% if you want to go 75. Still, there was that 75 max.

Many trailer owners know that they need to replace their tires before they wear out. If they don't they are probably going to have some type of failure. But those same people will not replace the tires on their pick-up till they are significantly worn. Why?
The answer can be seen if you simply compare the load and speed ratings of an LT tire and an identical size ST type tire.

LT235/75R15 101Q LRC1985# 50 psi 99mph

ST235/75R15 110R LRC2340# 50 psi 106mph

So exactly what type of magical engineering has tire company X used to achieve both increased load capacity plus increased speed capacity when it is the air pressure that supports the load? If they have the ability to make that ST tire really perform at those speed and load conditions what is wrong with their LT type tires?

IMO what we are looking at is a marketing plan taking over the engineering reality.

Bottom line
You may be able to pull the trailer at speeds above 65 or drive the Class-A diesel pusher faster than 75 but you will be consuming the finite life of the tire and can expect a failure before you wear it out no matter how you maintain your tires.

##RVT817



Friday, October 20, 2017

Will confusion on how much inflation be resolved?

Hi Roger.

Do you think the tire pressure issue will ever be resolved to people accepting the values recommended by the tire manufacturers based on actual loading, or will some always be confused between the tables, the placard and quotes of Federal FMVSS Regulations from some on the forum? It seems so simple and logical, but some just don't get it. I still think a sticky written by you could settle the issue, but the moderators disagree. I suspect a concern for liability for the forum to advise something that contradicts the MH placard.

I have a question for you about my RV. I carry 5 psi above the minimum for the load on my steer tires on a 60* morning @ 1100' altitude. When traveling, I had a morning @ 5000' and temp in the upper 30's. This produced a pressure of 1-2 psi below the minimum. I was going to be traveling into a warmer climate (95*), so did not adjust pressures. This type of condition can happen when we are traveling. I don't want to chase the pressures, so if traveling into warmer weather, I just go and watch the TPMS. I don't feel this is a problem but wanted your thoughts.

Thanks for your thoughts and time.

Not sure if there ever will be a resolution to the Inflate to the placard vs Inflate to the actual tire load. I would liken this to the change oil every 3,000 miles vs change when the car's computer advises.

DOT has a goal of trying to make things simple and keep people safe. DOT also knows about the data that indicates that over half of the RVs on the road today have one or more tire overloaded (either too much load or too little air for the actual load) I really can't fault their approach as IMO many RV owners aren't willing to make the effort needed to learn the proper inflation and to then maintain it.

We have had tire inflation stickers in cars for many decades and there have been massive vehicle recalls because their tires were underinflated, I have read that many as much as 30% low. We don't hear anyone making a case that the accidents and fatalities were the driver's fault. That would be blaming the victim which isn't acceptable, even if true.

OK to your question. I am guessing your CIP is in the 90 - 110 psi range so IMO +5psi isn't enough to avoid the pressure fluctuation that results in your overload. This is why I suggest a +10% value for inflation margin over the table minimum number..
In my Class-C RV, with LT225/75R16 LR-E tires, I am lucky as my RV is rather light. I really only need 60 psi F & R based on my "4 corner scale readings. My certification sticker says 65 / 80 and my dealer delivered it at 64 psi all around. 

For my application +10% would indicate 66 psi and a  +15% margin > 69 psi. I run 70-75 
Even though I can be a bit "anal" about inflation I also wrote THIS post and I really do not mess around with my tire inflation.

My TPMS is set to warn before I would be in overload and I run across a scale once a year just to confirm I am not too far from my original "4 corner weights".

In 2014 I drove Ohio > OR > Calgary > Yellowstone >OH over a seven-week period with temperatures ranging from 90's to snow and elevation of 20' to 8,000' and never had to adjust inflation.

Thursday, October 12, 2017

Can Black tire "Covers" be used without causing damage?

Back in June 2011 I did a post that asked the question  "Tire Covers - Do they do any good?"  In that post I showed the numbers from a test I had run where I collected tire temperatures showing the effects of shielding my tires with white vinyl covers vs the tire temperature when exposed to direct sunlight for just a couple hours.

 I also covered the science of the damage that excess heat can do to our tires. The bottom line of that post was that white vinyl can help extend the life of tires by protecting them from both the affects of UV and the IMO more serious damage done to the internal structure of tires from excess heat.

I also did a rough check using dark trash bag covering a tire and in that case I found that the black cover actually resulted in a tire being hotter than when it was just in the Sun with no cover. Based on that limited data I have recommended against the use of Black or dark color vinyl tire covers.

On more than one occasion I have observed some Class-A RVs with what appears to be a mesh shield than hangs down off the side of the RV. This is different than the vinyl "bag" that hangs directly over the outside of my tires. I was able to collect a few data points while in Redmond , OR in 2014 at a large RV Convention, and that data suggested it might be possible to use this mesh material and not increase the temperature of the tires. Finally this Summer while at another RV Convention I struck up a conversation with a representative of ShadePro Inc who offered to send me a Tire Shade to test. In Aug & Sept i had some health issues and then I ran into difficulty with clouds here in NE Ohio but I was finally able to collect the data I felt comfortable with that would allow me to reach a conclusion of if this black mesh material could be used.
 
Here is a shot of my test set-up with a white vinyl on front, control sidewall in center and the black mesh shielding the rear. After 2 hours in the full sun 
In the shade a tire gave 92°F







In the sun the white cover was 126°F







The reference tire sidewall registered at 147°F







and the black mesh shade showed 136°F








Under the cover the front tire was at 114°







While behind the mesh shade the rear tire was only 101°F




Conclusion:
The data shows that in this test the black mesh did a better job of keeping the tire cool than the white vinyl.

I can think of a couple of reasons for this.
1. The vinyl cover was in direct contact with the front tire so heat was being directly transferred to the tire.
2. The mesh allowed better air circulation around the rear tire.
3. The fact that the black plastic was also in direct contact with the tire probably contributed to the poor results.

Observation:
I was wrong to suggest that all black shields were worse than white covers, as this test shows that data is better than opinion when it comes to facts. This is one of the wonderful things about Science. 



Friday, October 6, 2017

"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.