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Tuesday, March 29, 2016

Safety Margin - Not as meaningful as Reserve Load

I and others really don't like the term "Safety Margin" as it sometimes implies something is safe when in reality all that can realistically be done is to lower the potential for problems.

When it comes to tires the term "Reserve Load" is a more meaningful and descriptive term.

We are discussing the difference between your actual load vs the load carrying capacity of a tire at a specific inflation. If you had a measured load of 4,500# on the heavier end of an axle and your tires was capable of supporting 4,950# at the cold inflation pressure you set the tire to, you would have a "reserve load" of 450# or 10%. To put this figure in perspective most cars have a reserve load capacity of 12% to over 20%.

We have covered proper weighing methods and the importance of having a margin of inflation pressure above the minimum needed to carry the actual measured load you are placing on your tires. So I would ask. Do you know what your reserve load is for the heavier end of each axle?

Despite the above, I occasionally get asked about a tire's "Safety Margin". When asked this question it sounds as if the person is intending to load the tire up to 100% of its rated load plus the additional percent that might be provided by the "safety margin". This is definitely not a good or safe practice.

Yes, some tires may be able to carry more than the max load marked on the tire sidewall (at least for a short time) but I think it is better to think of the average margin rather than the minimum margin.

Federal regulations specify that "all" tires must be able to pass certain tests. So that means 100% of the tires sold are suppose to be able to pass the required tests. Some also want to believe that passing a test that is run on a smooth drum under controlled conditions can always be used to accurately predict trouble free use in real life.

Everyone should understand the concept of product variation. This results from variation in raw materials as well as in variation in the manufacturing process and even variation in testing. 1st tier tire companies have statisticians whose job it is is to target towards minimizing variations mentioned as well as the probability of stacked tolerances to aim for 99.99966% good product hens the term Six-Sigma.  Now not all companies use the concept of six-sigma as this requires a real commitment to ever improving quality from top management all they way down to the night janitor. Such a commitment takes real work, can be expensive and require very involved quality control. Some companies are probably satisfied with 99.9% or maybe 97% or 80% or even lower as they are willing to take a chance that they will not be caught in violation of regulations or that the warranty will expire early enough that there will be no cost to the company for their poor quality. IMO there are companies that no only do not apply statistical quality control such as practiced by 1st tier tire companies and the major car companies of the world, but some companies may not even understand the concept of real quality control.

A company can improve the quality i.e. get closer to 100% good, by over designing the product but if you have large variation you will be building a large number of products that are capable of handling 110% or maybe even 130% of the performance that is needed and this raises cost. Sometimes to the point of pricing yourself out of the market.

Lets assume there was an easy way to measure the quality of tires (remember there is no such test) and we could come up with some numbers. If our target was 7,000 and we were to test a hundred tires from company Q and found that some failed as early as 5000 while others achieved 9000 we could calculate the mean or average and estimate say7500 so some might claim a "safety margin" of 500 (Mean performance minus the goal) when in reality some people would end up with tires that were only capable of 5000. Now if there is a second company Z and with the same test we see that the range was 7000 to a high of 7800 for an average of  7400, would you say the quality or safety margin of these tires was not as good as those from company Q?  After all company Q achieved 7500 while company Z only achieved 7400.
How would you feel if you ended up buying a tire from company Q that only could achieve 5600?

I hope you can see that measuring tire quality can be a very complex issue.  IMO one reasonable way for consumers to estimate the real quality of a product is to look at the basic warranty. Warranty costs are many times greater than the basic cost associated with replacing a product and in the case of tires could amount to as much as three to five times the cost to the corporation of producing a tire in the first place so companies really want to avoid warranty costs.

There are a couple ways to avoid warranty costs. One is to make a good quality product and another is to cut back on the coverage provided by a warranty. If company Z offers say a 3 year warranty while company Q only offers 12 months or maybe as low as 30 day I think it's a safe bet that tires made by company Z have a much better "Safety Margin"  and probably a lower potential for failure.

Rather than worrying about the "margin" of tire brand Q vs brand Z on a theoretical test, I suggest it is better to ensure that your personal "Reserve" is reasonable. To learn what your reserve is you of course need to know your real tire load and that your pressure gauge is accurate and most importantly that at no time you operate your vehicle at a lower than specified pressure and if possible always run a bit more air pressure than the minimum needed to carry the load.

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  1. Since Roger is the expert, why not have him tell us what are the best tires for trailers and motorhomes?

  2. Bob have you read my post on "Best" tire?


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