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Friday, October 17, 2014

What is a good Quality Tire? (part 1)

Simple question that is often asked on various RV and car forums.
The problem is that I seldom recall anyone providing a clear definition of what they mean by the word "Quality".

If we look at Wikipedia we find the following:
    Quality Assurance, methodology of assuring conformance to specifications
    Quality (business), the non-inferiority or superiority of something
    Quality (philosophy), an attribute or a property
    Quality (physics), in response theory
    Quality factor, or Q factor, characterizes a resonator's bandwidth relative to its center frequency
 and others.

A quick look at RV reviews in magazines, finds the word "Quality" used quite often, but I do not recall seeing any evidence, cited by the authors, of said "Quality" of one RV being any better than another. On the contrary if we use length of warranty as a measure of overall quality I would suggest that in 2014 most RVs have quality comparable or worse to 1971 Pinto or Vega cars and many of us remember just how bad they were and the low quality of vehicles produced by "Detroit" till "Japan" beat the pants off US manufacturers, much as Korean car companies are doing now with their 5 and 10 year warranties.

Since this is a blog on tires, let’s limit ourselves to that topic. Even here there are obviously different uses of the word. A recent consumer magazine purported to present some observations on the quality of a group of passenger type tires. The definition of what the author meant was not clearly stated but can be inferred by the importance of certain performance characteristics as measured and ranked by the author. If a tire performed better for one specific characteristic such as tread wear, it was judged as being of better quality than a tire that did not wear as well. I would argue that this is an overly simplistic approach and can easily be misconstrued by readers just as a claim of "better build quality" in an RV review is meaningless if the author does not define what they mean by quality or they fail to present any supporting evidence.

With my background as a tire design engineer for a major tire manufacturer for 32 years and after working in the "Quality Assurance" department for 8 more years, I would like to offer some observations on how the term "Quality" is used in the tire industry. This is a complex topic so it will take more than one blog post but unlike some of my blogs there will be few technical terms used and the few I do use will be clearly defined.

After all I want to make this a "Quality Article".  :-)

Let’s start with a definition of the word "quality" in this post as   "Conformance to Performance Specification".

So a tire that meets or exceeds more "measurable" features as identified in its specification, would be considered to be of better quality than another tire that did not measure up as well for all of the same set of specifications. So what might a list of specifications include? One of the first tasks a design engineer has, is to ensure they understand what the customer wants.

Lets look at a partial list of what might be presented to the engineer. Some performance specifications are very clear such as:
"Pass every government Regulatory test by at least the margin established by company statistical margin as published in Standard Practice".   This quality standard is a must-do. The Pass/Fail levels are published and easily measured using standard tests with conditions for speed, inflation, load, and distance set down in test manuals.

Some other performance specifications are relatively easy to measure so the engineers know if the tire they designed passes or fails to meet the spec and by what margin they pass or fail to meet that particular goal. Some examples of these objectives might be:
"The tire is to weigh no more than 32.5#."
"The High Speed rating will be no less than 75 mph as measured on Society of Automotive Engineers test ABC".
"Rolling Resistance Coefficient (RRC) shall be 0.234 or lower on SAE test XYZ". ( RRC is a measure of fuel economy)
"Cost level shall be no higher than 90% of the reference tire as measured by corporate cost dept."

Others specifications are a little or a lot harder to quantify such as subjective performance
"Steering response shall be judged better than the reference tire".
"Noise shall be better than the specified reference tire".
Part of what makes these performance specs hard to quantify can be seen by looking at the noise evaluation "report card". Some vehicle companies have 10 or more different noise features they may consider important with terms like "Braking Growl", "Expansion Joint Slap", "Smooth Pavement Sizzle" and similar. Even Steering Response is complex with both "On Center Feel" and "Linearity" being a few of the items being evaluated for steering response. We don't need to go into the details of these evaluation features as that would be a post topic unto itself, but you can see these items may be difficult to rate as they are all subjective.

One challenge for the engineer is to get the vehicle evaluator to rank order all of  these various performance characteristic. Many times what we get is "They are all important" so how would you judge the conformance to specification if tire X was rated a +1 for "Expansion Joint Slap" while being rated equal in all other categories to the referenced tire but tire Y was rated a +1 for "On Center Feel" while being rated equal in all other categories to the reference tire? Which tire is the better Quality tire? I think the obvious answer is that despite claiming that all characteristics are "equally important" the evaluator really does have a preference and uses that preference to give the edge to one of the two tires being evaluated.

So why is this all so important? Well in the example of the consumer magazine rating the quality of some tires better than others I think they missed the point that not all tires are designed or even intended to meet identical performance specifications.

Next time we will look at why all tires are not designed to the same Quality Standard and what you can do to try and get the Quality you want in your tires.

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Tuesday, October 14, 2014

Should you reduce pressure when driving on "Hot" Roads?

I got this question and thought that others might be wondering about the same topic.

Jim K asked

 Message: I will be traveling in the desert for the first time and I am
wondering if I should reduce the tire pressure before I go. The hot road will
increase the pressure and I am afraid of damaging my tires.

Hi Jim,

No you don't have to worry about hot roads.
IF you run the correct cold pressure

Now you didn't say if you have a standard RV trailer or a Motorhome so I will give you a summary for each application.

Trailers: You should set the Cold Inflation to the pressure on the tire sidewall. If you look at the sticker on the side of your trailer you should find the tire size, type, Load Range and pressure recommendation from the manufacturer. In almost all cases the recommended inflation is the inflation on the sidewall of the tires.
Have you confirmed you are not overloading any of your tires? Simply guessing or looking at the tires is not good enough you need to get the trailer on a scale and at a minimum get the total load on the tires. Now you can't assume the load is equally distributed side to side or axle to axle Measurements of thousands of trailers suggests you need to assume at least 53/47 to 55/45 split axle to axle and split side to side so you need to calculate the heaviest load based on an estimate of 27% to 30% of the total being on one of the 4 tires. A better method is to get individual tire loading. You can learn more HERE.

Motorhomes are a bit different than towables. Here you need to get the "corner" loading as the side to side difference is affected by the placement of things like generator, water tanks, refrigerators etc. The Front Rear loading is obviously different and for most motorhomes the number of tires on each axle is also different. You can use the information on your placard but a better method is to get the actual tire loading and then using Load/Inflation charts establish the MINIMUM cold inflation then add 10% to get your Cold Set inflation. THIS post has some info and a link in it.

Bottom Line

When tires are designed, we know that some vehicles will be driven on hot roads. Tires will normally run +20°F to +50° above ambient. You should run a TPMS to get warning of air leak due to puncture. If you are driving in the USA you should have no problems.
Most TPMS also have a high temperature warning that is set for 155°F to 160°F. If you get a warning at this temp but the pressure is above your set pressure by about 10%, simply slowing down should lower the temperature. If that doesn't work you can still stop for 10 minutes while you do a walk around to be sure nothing unusuall is going on.
If you are traveling to Saudi Arabia, the Sahara or Australian outback then we need to take some additional steps and precautions.

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