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Friday, July 21, 2017

When should you replace your tires?

Saw the following question on an RV forum from a motorhome owner.
"Does anyone ever just replace their front tires after 7 years? It seems that a tire failure on the front is much worse than the back. If the tires have a lot of tread and no sign of cracking, why replace the back ones?"

One reply correctly said, "There's more to consider than age. Were they properly inflated? Did you bang into curbs to damage the sidewalls? There are many failures due to improper operating conditions. Not just age."

I would first ask if you know the "life experience" of the tires:

Did the RV have a previous owner?
While they may claim they always checked inflation, can we be 100% certain?
Did you purchase the tires direct from a major tire company and know the complete use history?
Have you confirmed your load on all the tires from day one?
Did you ever discover one of your tires 20 or 30 psi low one time?

There are many things to consider, but if you are confident the tires have always been properly inflated for the actual load the following should help.

"Blowouts" (look at the picture at the top of my page), or more properly Run Low Sidewall Flex failures, happen because of running on low inflation. This can be avoided with the use of TPMS.

Belt separations, which are much less common on commercial grade tires, may occur after the rubber at the belt edges degrades due to cumulative damage from heat.

UV does not affect the internal structure of tires and external cracking is only one symptom of exposure to sunlight. I consider external cracking like having a person run a temperature. Having a temperature is just a symptom of some other illness, usually an infection. You can be seriously ill but not be running a temperature. Think of heart disease or cancer as two things that can kill you but don't cause you to run a temperature.

The suggestion on when to replace tires is only based on probability, as it is impossible to know just how much damage has been done to the components of a tire. The general suggestion is to have tires closely examined by a tire expert at 5 years and each year after that. Tires should be replaced at 10 years no matter what is visible on the surface of a tire.

I wrote a blog post on a suggested inspection and "step replacement" concept. A version of this suggestion could result in your keeping newer tires on the front, which in theory should improve your chances of not having a belt separation on the front. Of course also running TPMS will improve your chances of not having a Run Low Flex Failure too.


Thursday, July 13, 2017

How are RV tires developed?

 By Roger Marble

If a tire is being designed for a specific vehicle manufacturer such as Ford, Chevy, Toyota, or BMW, there will be a number of tires submitted by competing tire companies all trying to deliver the best overall compromise in performance characteristics. Please note that all original equipment ("OE") vehicle manufacturers have slightly different requirements but all make similar requests for performance improvements in many areas. In the future I will use the term "OE" to include these car and pickup manufacturers.

Compromise: Now is a good time to talk about some of the various trade-offs the engineer is faced with when trying to meet conflicting goals and customer wants. I am sure we would all like an RV that has all the interior space and amenities of a 40’ diesel pusher but gets 25 mpg and can be driven down crowded city streets without knocking off our mirrors. Oh yes, it should also cost under $30k. Well, Bunkie, that just ain’t gonna happen in real life.

The same goes for a tire that handles like an Indy tire, is as quiet as the proverbial mouse, has great off-road traction, is good for 100k miles, and costs $25. One thing few people realize is that most if not all performance characteristics are a compromise. For example, if you improve wet traction you probably hurt fuel economy unless you use a special type of rubber that costs double per pound and is more difficult to process. If you improve handling you might hurt ride and noise. When you improve noise you can significantly increase the cost of making the molds used in manufacturing. The cost of a tire mold can be as low as $10,000 and can approach $100,000 each. Depending on the production volume needs, a tire manufacturer could need 30 or more molds. The list of trade-offs goes on and on.

The competition for a tire application might start three or more years before scheduled start of delivery with two to five tire manufacturers competing for the contract, knowing that only one or two will end up being selected to actually provide tires. The costs associated with building and testing special prototype tires can run in the hundreds of thousands of dollars and are absorbed by the tire company. The only way a tire company can afford this type of activity is by landing a contract for a few hundred thousand tires so the costs can be spread out.

Unlike “OE,” an RV manufacturer may only need a couple thousand tires, so a custom tire designed for a specific RV would be cost prohibitive. Since the RV manufacturer won’t be trying to get custom tires, it doesn’t have staff engineers working on developing specifications for such tires. The RV company will in all likelihood either take what comes already on the cut-away chassis or the bare chassis for Class-C or A vehicles, and in the case of trailers, may buy the tire with the lowest cost that can meet tire size requirements and expected delivery schedule.

For RV applications the one thing that is in the control of the manufacturer is “Reserve Load.” This is the difference between the load placed on each tire with the RV normally loaded and the load capability of the tires at specified inflation.