D Car — November 2003

When tires are kept at the correct pressure, the tread should wear evenly and they should last their full service life. But, as mentioned earlier, pressure that is too low or too high causes abnormal wear. Conversely, if the center of the tire is worn or the shoulders are worn, that indicates the pressure is not correct. Tires can also wear abnormally even when the pressure is right—for example, one side may wear much faster than the other. In such cases, the wheel alignment is often out. Inspect the tread for…

…and if you see abnormal wear, have a specialist shop with a four-wheel alignment tester inspect and adjust the alignment. Underinflation is by far the most common cause of tire blowouts while driving, but foreign objects such as nails lodged in the tire can also cause them. Damage or cracks in the tread or sidewall can be another cause. Inspect the tread and sidewalls regularly, and watch carefully for foreign objects or damage.

A device called a Chisso-man separates out nitrogen from the air and fills tires with concentrated nitrogen gas. You may have heard that nitrogen is good for tires. The air normally put in a tire is atmospheric air: about 78% nitrogen, 21% oxygen, and 1% other gases. Compared with ordinary air, nitrogen is said to leak out more slowly, helping the tire retain its pressure longer once it has been set correctly. This is related to oxygen being reactive and nitrogen being inert. The gases have different permeation properties: although the same number of molecules enter the rubber’s molecular lattice, oxygen passes through about three times faster than nitrogen because of oxidation reactions. That is why air escapes more readily. Less leakage means fewer problems caused by low pressure and less effort spent checking pressure. One supplier promoting nitrogen tire fills is Tire-kan, a Bridgestone-affiliated tire-shop chain. The machine shown separates concentrated nitrogen from atmospheric air…