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Wednesday, June 27, 2012

Can you "Limp" home on a failed dual?

I have read a few posts on RV forum sites when the topic of what to do when a tire fails and you don't have a spare.
Many times it has been suggested that the person plans on "limping" home or to the tire service center with just one tire in the dual position of their RV.

While on the surface this might seem to be a reasonable approach, there are a few things that need to be considered.

First, unless you have a TPMS that warned you that one of your duals was loosing air, in all probability you have no idea how long you drove with one dual underinflated. If you don't know when the tire started to loose you also have no idea at what speed you drove on one underinflated tire and also on one overloaded tire.

If a tire looses more than 20% of its air it is considered to be "Run- Flat" by tire industry

If you have driven on a "flat" tire it is considered to have been damaged and should not be considered for future safe usage.

What is not obvious when you read the above is that is one tire of a dual goes flat the mate has now been overloaded. I covered some of the info in a post on the Special Considerations for Duals, but there is more that needs to be considered.

If you have one tire punctured and loosing air the mate is "taking up" the load for both tires until it is 100% overloaded. Driving at highway speed will do serious internal structural damage. Most of which cannot be seen with out special equipment like X-Ray so it is simply considered scrap so now you need to replace two tires.

Trying to "limp" means that you need to reduce your speed and according to the special tables in Tire & Rim Association for adjustments for speed, inflation and load your limp speed is not 30mph or 20 mph or even 10 mph but it is limited to 2 mph. That is TWO miles per hour Maximum. Any more and you are damaging your tire beyond repair.

The solution would be to call for service. I would only drive on a single tire for a hundred yards at most to get to a location where I could safely pull off the roadway.

If you feel your situation means you must drive on a single tire of a dual position then you need to accept that you will need to replace both the tire that suffered the failure and the tire that was 100% overloaded.  I would also remove the flat tire as you don't want it flapping around as it will do damage to your RV.

Are Black Tire covers bad? part 2


June 11 I posted on black tire covers possibly doing more harm than good. There were a number of comments on this topic and I wanted to more than post quick comment replies so here are my responses.

Gareth said "Black VS. White is a no brainer". We all know that black absorbs heat, white tends to reflect heat" and that he hoped that there had been UV resistance testing done. He also wasn't impressed with my use of a dark color trash bag.

Well Gareth I agree that a plastic trash bag isn't the same as a vinyl (plastic) tire cover but I don't have a budget to go and buy products and do testing so I just made a quick comparison. I have learned that even with Apples vs Apples data some people will still question test results.   RE: UV testing. I have never seen tire cover mfg make any claims or provide data so I would not be surprised if they never bothered to pay a lab to do any testing. If anyone knows of such data please let me know.


Bob asked if he was better off with no cover or a black cover until he buys white covers. While I have no data, my opinion would be to use no cover but spray your tires with a protectant that claims to offer UV protection. High heat can damage the tire internals while UV will only attack the surface.


Al said his "shields" are black but since they attach to the side of the RV and not the tire they prevent a temperature increase of more than 2 degrees F.  I would really like to test his product.


Pennierich suggested using Mylar covers and Bill suggested trying "space blankets".
Well the problem is I haven't found tire covers that use the mylar material. I did look at the mylar sheets which are inexpensive but would need some R&D to make into tire covers as a simple sheet of this material would not stay on tires very well.


General comment on UV
A few weeks ago I asked a manager of a lab at a major tire company about UV testing and wondered if I could purchase the special light bulbs needed. To test for UV resistance I would need UV-C bulb and a simple 40w bulb would cost me $1,000 so I will not be setting up a test lab anytime soon.

Anyone thinking they have a better product please feel free to email me at Tireman9   at   Gmail  dot   com   and lets see what can be worked out. I will be happy to conduct a controlled test and publish the results. If there is something better and less expensive than the basic white covers I would like to know about the product.

Saturday, June 23, 2012

Request to all posting questions on tires

Just a quick post for those asking questions about tires. Before you post your question on inflation / size/ speed / load etc on any of the RV forums or on this or other tire related Blogs,

Please provide the complete and accurate tire information.
This can be found on the tire placard. Better yet, if you can get it in focus, post a picture of the placard. The tire engineers that follow these posts seem to have to ask follow up questions most of the time as the information provided is not always sufficiently detailed or accurate for us to do more than take a guess.
Some examples of incomplete information include statements such as "I follow the manufacturer’s recommendation"  Which mfg? The tire or the RV?
It is the RV mfg that has the legal responsibility to provide the minimum inflation recommendation for the tires applied to the RV. The inflation level must be sufficient to carry the load as specified on the placard. The tire manufacturer does not know what the RV weighs or what the axle rating is.

Sometimes we hear  "I had my RV weighed and got the inflation"  Did you get the actual corner loads for each position or did you get the total for each axle and divide by two or did you get the total scale weight of the RV and divide by the number of tires on the ground?
Where did you get the inflation information? From your tire companies load/inflation tables or from some self appointed "expert" who invented their own formula (as has been done by some on the Internet) or by asking the guy parked next to you in the campground?

Missing or incomplete information
While we don't need the details on the make & model of your RV we do need to know if it is a motorized unit or a two or three axle 5th wheel or a simple single axle TT. If you want to help others you can include make & model but it takes a lot of extra time to try and research what the configuration is if the tire engineer is not personally familiar with your model. The inflation recommendations for motorized units can be different than for towables. The number of axles is important as it affects the side loading. Also if the tire in question is in a dual position application will affect our answer. While most ST and LT tires have polyester body ply and most 22.5" size have a steel body ply it again will eliminate guessing if you provide the information molded on the sidewall that tells both the number and materials used in the construction of your tires.

DOT number  Providing the complete DOT number can provide valuable information. We can translate the code to know who, where and when the tire was manufactured. With this extra information we might save us all time by suggesting the tires be dismounted and inspected or simply replaced. We might be able to check to see if there had been a recall or other complaints filed with the National Highway Traffic Safety Administration on your brand and size tire. If you are unsure what we need you can always post a clear, in-focus picture of the DOT.

Thanks in advance for your assistance.
We want to provide clear and correct answers to your questions but we do need your help so that we have the information we need to make an informed answer.

Monday, June 11, 2012

Black tire covers may do damage.

Last June I published the results of a test I ran on tire covers. I used my white vinyl covers as samples in the test. I mentioned that I did see some companies advertising black tire covers. I do not think are a good idea but need data to offer an informed opinion, so here is my test simulating black tire covers.

Here is a shot of my test set-up.
I used a trash bag that was actually dark green to simulate a black tire cover. The tire is just covered by the bag not in it.
Here we see the reference white cover at 93.5°F which is 5 degrees cooler than in the original test but close enough for this test.
The old passenger tire I had registered 113.3°F in the part that was in direct sunlight. This tire was dirty and gray so it did not register as hot as a clean black tire would.

Here we can see that the black (dark green) plastic cover was at 136.4°F which is almost identical to the uncovered tire in the original test at 136.1°F so this dark cover is actual heating the tire in this test.

Here we see that black tires and black covers can run almost 40° hotter than when under a white cover. This means the rate of aging of this tire under the dark cover is 12 to 15 times as fast as when a tire kept is cooler under a white cover.

Some will ask if lowering the temperature for a few hours each day is worth the effort, so lets set up a hypothetical situation.
Assume you spend 6 months out of the year in sunny Florida or Arizona or similar location with lots of sunshine. You are parked so one side of the RV is in the full sunshine for 8 hours a day and the sun shines 25 out of each 30 days. Now to make the math easier let’s assume it is always 85°F ambient. Now based on the original test our tires in full sun would be at 132° but if shielded by white covers would only be 96°.
So the question is: How much “older” would the tires without covers or with black covers be than tires that were covered with white covers?
Tire A has a cover. So lets figure that as our control so there are 25 days times 6 months times 8 hours per day of solar heat exposure for a total of 1200 hours.
Tire B has no cover, so it’s temperature is two times 18° hotter. Remember the “aging” rate doubles (times 2) with each 18° increase so this tire spends 25 days times 6 months times 8 hours times 2 times 2 or 4,800 hours.
The difference is 3,600 hours which equals 150 days or almost a half year. In this example each year your uncovered tire “ages” almost one and a half years vs one year for the covered tires. This means that after 6 years of staying only 6 months in a sunny location your uncovered tires have rubber properties that are more like nine year old tires. If we consider tha additional damage done by UV light I think you can see that the uncovered tires will probably have serious sidewall cracking after only 6 years of real time aging.
Covering tires with dark or black covers may shield the UV but they do nothing for the heat and could make the heat worse through radiation.

Bottom Line. If you want to get the most out of your tires you should cover them with a white or light colored cover when the tire will be exposed to extended time in direct sunlight.

Monday, June 4, 2012

Fire Safety is not a focus of my blog


RV Fire Safety is not a focus of my blog.
However, I was sent a link to a news story about an RV fire that occurred not too long ago involving three fatalities and according to the report there was also a tire failure. Here is the link to the story. It was suggested by a reader that I use this as a topic for my blog.

The main reason I am writing about this tragedy, is to point out the importance of having plans in case of fire. You need a plan on what to do if there is a fire when you are parked and you need a different plan on what to do if the slides are in and you are traveling down the road.
Here is a new video from a friend Mac McCoy, that shows one possible plan that involves using the emergency escape window. Some of you may know him as "Mac the Fire Guy". Check his web site for where he is giving his seminar and fire demo and go see it.
One thing Mac mentions in his seminars on RV Fire Safety, when he covers the use of the escape window, is that some windows like the one in my Coachmen cannot be opened without breaking the window so you need to contact your RV manufacturer to confirm that a “Fire Drill” will not result in a broken, expensive window.  

Now I want to make it clear that it is difficult for a tire failure by itself to start a fire. Once the tire looses enough air to “fail” you can no longer drive on that tire. The temperatures involved do not approach the self ignition point of rubber. According to this web site
the ignition temperature of rubber is 500°F to 600°F. The fact is that once rubber gets to above the 340°F softening or “melting” point, it loses its all its strength, falls apart under load and the tire would “blow out” or shred and scatter the pieces along the highway which would result in the vehicle stopping and the heat would no longer be generated as the failed tire is no longer carrying any load. Every week hundreds of tires fail somewhere yet there is no fire so the numbers support the degree of difficulty in having a tire self ignite.

It is possible that there are other ignition sources such as spark from damaged electrical wiring, grease, oil, fuel or even brake fluid and some of these might ignite if the RV is damaged and then there would be flame which could then ignite the tire. To put the temperatures in perspective, diesel has a flash point of 156°F, brake fluid 374°F, engine oil flash point is 300° to 440°F, wood catches fire at 374°F and fiberglass and many plastics seen in RV construction have ignition point temperatures from 190°F to 450°F. You will note that these temperatures are all lower than the ignition point of rubber.

Some other web sites with information on RV Fire safety; HERE and HERE is one on how to create a plan. RVBookstore has a DVD on RV Safety which is also available in electronic version 
Bottom Line: RVs should not catch fire if there is a tire failure. Maybe the best advice I can give is that you can go a long way by simply preventing tire problems in the first place by being sure your tires are properly inflated all the times. That means knowing the inflation needed, checking the inflation at the start of every travel day and now you can have tools that can warn you if a tire starts loosing air while driving, so I strongly recommend having a TPMS. I have TPMS on my car and on my RV.