Under Pressure

Tuesday, May 11, 2010 by Jonas Paeplow

Most vehicle owners do not check tire pressure nearly enough. An American Automobile Association (AAA) poll suggests that 85 percent of motorists do not even know how to check tire pressure.

The most important job a tire has is to support the load to which it is attached. Tires are rated to handle specific loads but only at a predetermined air pressure. By keeping the air pressure set correctly, tire performance, longevity and fuel economy are optimized.

According to tire industry data, 85 percent of all tire air pressure losses are the result of slow leaks that occur over a period of time. Tires typically lose air pressure through natural leakage (permeation) at a rate of about 1 psi per month. In addition, tire manufacturers say that seasonal climatic changes result in air pressure losses of 1 psi for every 10 degrees F decrease in the ambient temperature.


Here in the Midwest, differences between summer and winter temperatures average about 50 degrees F, resulting in a net loss or gain of approximately 5 psi in air pressure. This variation is enough to drastically affect handling, traction and durability of the average tire if the pressure is not adjusted. Even temperature fluctuations during an average day can make a difference. Variations between nighttime and daytime temperatures in this part of the country can average 20 degrees F and result in pressure changes of more than 2 psi.

A tire pressure survey of more than 5,400 vehicles’ conducted in March-May 2009, by the Rubber Manufacturers of America found:

  • Only 9% of vehicles had four properly inflated tires.
  • 50% of vehicles had at least one under inflated tire.
  • 19% of vehicles had at least one tire under inflated by 8 psi

According to government statistics, in the United States, 660 lives are lost and 33,000 are injured every year due to tire pressure related accidents. Improper tire pressure costs an extra $3.7 billion in fuel annually and every year, 4.5 million tires need to be replaced before reaching the end of their designed lifespan. A 10 psi loss of air pressure could result in a corresponding reduction in tire load capacity of 1,000 lbs. Overloading of tires combined with highway speeds will cause tires to overheat and lead to them to fail, prematurely.

Tire inflation pressure should be checked every month and before long trips. To properly check pressure, check tires when cold – before the vehicle is driven. Use the vehicle manufacturer’s recommended pressure found on a label located on the driver’s door or door pillar or check the owner’s manual.

The most accurate way to check your tire pressure economically is with a digital tire pressure gauge. Two of the finest examples available at Tire Rack are:

The Accutire ABS Coated Air Gauge features heavy-duty construction to withstand shifting around in your glove compartment. Designed to last, it has an angled head and rubber coated handle for easy gripping. The LCD display is large and easy-to-read. If you forget to turn the gauge off, don't worry, it is equipped with automatic shut off. The tire gauge will read within 0.05 psi. The lithium battery will never need to be recharged or replaced. This digital gauge measures air pressure from 5-150 psi in 0.5-pound increments.

 

ABS Coated Air Gauge 

Accutire Digital Set Point Programmable Air Gauge w/Light is an easy-to-use, multi-featured gauge with an extra large, blue, backlit LCD screen and ergonomic styling. It measures psi from 5-99 pounds in 0.5-pound units, and includes the patent-protected Set Point programmable feature which allows for recording the factory-recommended tire pressure for both front and rear tires. Other helpful features include a white LED flashlight to make checking tire pressure at night or in the garage a cinch; an audible pressure signal; auto off; and a five-year manufacturer warranty.


Digital Set Point Programmable Air Gauge w/Light


But my car came equipped with a Tire Pressure Monitoring System, why do I still need to check pressures, why not just wait until the light comes on?

The system of computer and sensors to monitor tire pressure is known as Tire Pressure Monitoring System (TPMS). A major concern is that drivers of vehicles equipped with a tire pressure monitoring system will become over confident in the capabilities of their system and will be even less likely to confirm their vehicle's cold tire pressure

In the fall of 2000, following several fatal accidents involving tire inflation, tire failure and vehicle rollover, a bill called the Transportation Recall, Enhancement, Accountability and Documentation Act (TREAD) was signed into law. This law mandates the use of a suitable TPMS technology in order to alert drivers of a severe under-inflation condition of their tires. All new models produced after September 2007 are required to have the system.

There are two types of systems on the market today — indirect and direct. As a tire loses pressure its rotational speed changes relative to the properly inflated tires. Indirect systems use algorithms to interpret signals from wheel speed sensors to detect a deflated tire.

Direct systems use separate pressure transducers mounted in each wheel that detect deflation and then transmit a frequency signal to a control unit which triggers an information lamp on the instrument panel. Both systems still require manual correction of the tire’s air pressure.
 

So why should you still check your tire pressure manually? Well, its a little like waiting until your oil light comes on before you check your oil, in other words, it could be too late.

A passenger car tire that requires 35 psi on a vehicle with TPMS may not trigger the lamp and warn the driver about pressure loss until it drops to 26 psi depending on the type of system used. Under the same circumstances, a driver of a light truck that calls for 80 psi won't be warned until just 60 psi remains. In both of these cases, significant load capacity has been sacrificed before the driver is warned.

Regardless of what type of vehicle you drive or what type of tires you ride, spending a few minutes every month checking your tires is time well spent. You'll save fuel dollars, premature tire replacement costs and who knows, perhaps you might save someone's life in the process. Isn't that worth it?

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