How to tell if your tires need more air
Prior to the growth in popularity of the radial ply tire, the experienced motorist could tell by the tire's appearance that it was in need of some attention. An underinflated bias ply tire took on the appearance of a modern radial with bulging sidewalls. Today the appearance of a radial tire can be deceiving, especially some of the more expensive "run flat" tires. Tire appearance is no longer a substitute for a good tire gage.
The National Highway and Traffic Safety Administration (NHTSA) issued a regulation governingpneumatic tire identification, including the location and information on the placard affixed to every approved vehicle. This placard may be located on a door, door jamb or the glove box cover. The placard must list gross vehicle weight (GVW) limits and recommended tire pressure. The tire pressure gage is the logical tool for use by the motorist to check the air pressure in his tires.
Manufacturers have devised any number oftire pressure gages, from the least expensive pencil-like device to expensive electronic devices that read and record tire pressure. Some include a small measuring device to determine tread depth as well.
Tire pressure should be checked when the tires are cold. Air expands when heated and a tire that has been running down the highway will become hot; a pressure reading at this point would be misleading. It is a simple matter to check the cold pressure reading for each tire and compare it to the car manufacturer's recommendation. It is not unusual for the tire pressure recommended for the front and rear tires to be different.
If the motorist has gotten on the road without having checked the tire pressures recently and the pressures are low, the vehicle handling may alert him to a problem. As tire pressure decreases the sidewalls soften and the car becomes less responsive to steering inputs. The driver may notice the handling has become "mushy". This is because turning the steering wheel actually moves the wheel upon which the tire is mounted. When properly inflated the tire is as one with the wheel. When underinflated the sidewalls of the tire flex when the wheel is turned to steer the vehicle and so the tire lags behind the wheel.
Finally, the on-board tire pressure monitor requirement became law in 2005, affecting new cars beginning with model year 2007. NHTSA required a system that would alert the driver when the tire pressure dropped more than 25% below the recommended pressure. This was in response to a series of roll-over accidents experienced primarily in Ford SUVs. Ultimately the blame was assigned to Firestone, the tire manufacturer. The scenario assumed by NHTSA was that tire failures andinappropriate driver responseto the failure resulted in a series of roll-over accidents. Because SUVs tend to have a high center of gravity (CG) and narrow track, proper driver response is critical in maintaining control. The NHTSAengineering reportpointed to defective tires as the root cause of the accidents. NHTSA apparently believed that underinflated tires were a contributing factor in the accidents and their solution was to require the on-board system. So a driver who cannot check the tire pressures or feel the instability resulting from low pressure will be informed via a pictogram illuminated on the instrument panel.
A great deal of time, engineering and testing is invested in the design of the modern vehicle suspension system. The pneumatic tire is a critical component of this system. The recommended tire pressure is researched and developed to yield acceptable ride quality, traction, handling, and tire life. Failure to perform this simple maintenance task negatively impacts these performance features.
