Quote:
Originally Posted by hackish
I think it's very important to point out to readers that tires are tested to a maximum recommended pressure by the tire manufacturer. They have a significant amount of wiggle room in this. A little birdy told me that premium brands will typically over-do this by 50% this and "drive" the tire at max weight and 25% over max speed to achieve a statistically small chance of a blowout.
Over-inflation can lead to less traction, especially during a panic stop. It can also lead to blow-outs at high speed. Ask firestone about that. I wouldn't recommend going over the max on the tire. Suppose you're in a crash due to a blowout or loss of control. If someone is injured they will check the tire pressures among other things. Just make sure you understand that if you're going to over-inflate, there are risks.
-Michael
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It occurred to me that my previous post shouldn't have just stated there was a problem. It should have pointed out what was wrong and explained why. So here goes.
What follows is about Standard Load Passenger Car tires. Extra Load PC tires and other types of tires are different in the details, but the general idea is the same.
There are load tables that delineate the relationship between inflation pressure and load. These max out at some inflation pressure. That point is called the rating point - although the terms "Rated Load" and "Rated Inflation Pressure" are the way it is usually stated.
The rated load will be imprinted on the sidewall of the tire. However, the rated inflation pressure may or may not be - usually not. If the term "Max Pressure" is used, it usually means this is NOT the rated pressure.
For Standard Load Passenger Car tires built to the US tire standard, the rated inflation pressure is 35 psi. For Standard Load Passenger Car tires built to metric standards, the rated inflation pressure 2.5 bar (36 psi) with appropriate difference in max load.
There are lots of tire tests, but the one of most interest here would be the industry standard "Step Load" test. The test is performed by loading a tire against a wheel. I hope you'll forgive me if I get the details wrong. I'm doing this off the top of my head, but the jist is correct.
The starting point is 80% of the rated load with the tire at the rated pressure with the surface speed of the wheel at 50 mph. The test is run for 2 hours (100 miles), then the load is increased 5% and run for another 2 hours, and so forth until the tire completes the 100% step. At that point, the tire passes the government minimum.
It is common for this test to be continued by increasing the load in the same stepwise manner. Tire manufacturers have a minimum value they use for QA purposes - say 125% of the rated load. They may stop the test at that point.
But sometimes the test is run to failure. Again, the load is increased in a stepwise manner - until failure.
When the tire is removed (whether at the QA limit or at the point of failure), it is examined. If there was a failure, was it heat related failure (not the goal of the test)? Was it a stress type failure (the intented result)?
My point? The standard industry (and government) test is NOT run at the maximum pressure (with a few exceptions).
There are a ton of other types of tests: Ride, handling, rolling resistance, force and moment, etc. Very few are run at the pressure listed on the sidewall. They are run at the pressure prescribed by the test.
For example, ride and handling tests are run at the pressure listed on the vehicle tire placard.
I hope this clarifies things. Oh, and you need to stop listening to that bird.