Page:Popular Science Monthly Volume 90.djvu/125

 Popular Science Monthly

��To the right: A casing that is badly rim-cut or broken just above the bead. The flap be- came misplaced because the tire was improperly applied

��Below: A tire worn off by ruts. The same condition results if a tire is run on muddy roads that have a frozen crust through which it sinks, or against the curb

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��tire cannot bend without heat- ing the interior. After a time the rubber ce- ment that holds the in- ternal struc- ture together, hardens ; the layers no long- er adhere to- gether; a loud explosion breaks upon the air; it is time to buy a new tire.

DonH Be Afraid of the Stm

Although tires are thus sensitive to heat, it must not be supposed that they are very much affected by the sun on a hot summer day. It is true that heat expands the air within a tire, and that the resultant expansion manifests itself in in- creased air pressure. But no tire manu- facturer makes allowances for the heat of the sun, simply because it is negligible in the life of a tire. At the end of a day's run, with the thermometer over ninety degrees in the shade, the pressure within a tire will never increase six pounds.

Beware of a Little Cut

Everyone knows that sharp stones, nails and glass may cut through the tread and cause a puncture. But other causes of punctures are not so self-evident. The lug of a screw-bolt, a tire-removing lever, the inner head edges of the casing, sand, and ther hard substances which have worked their way into the casing are equally fruit- ful causes of punctures.

Out-and-out punctures, however, occur rarely. A tread one-half an inch thick, very tough, backed up by ply after ply of the finest and strongest Sea Island cotton

��or the stoutest cord, is not easily pierced. It is the little cut, so insig- i nificant ap- ] parently, that is more in- sidious, be- cause it is a less obvious danger. No cut is so small but mud, gravel and dust can be forced into it, and that dirt bores its way into the tread as the wheel turns over hun- dreds and hundreds of times. Soon the original cut lengthens and widens. More dirt enters. Finally, the tread is completely pierced, and the dirt begins to wedge the tread away from the inner fabric. Often "boils" appear as the visible evidence of the process of destruction. Then comes the inevitable blowout, followed by the usual arguments with the tire company.

Water has the peculiar faculty of pene- trating even the minutest crevices. It is rapidly absorbed by the cotton of the tire fabric. A cut, therefore, is dangerous not simply because it may admit sand or mud, but because it will also permit water to collect. Dampness acts on the fabric of stored tires far more quickly than water on canvas wholly immersed. The fabric is broken down and the casing correspond- ingly weakened. Often tires will burst after having been stored through the winter.

Much of the expense entailed by blow- outs can be saved by treating a cut as sensibly as you would a hand injured by a little splinter. Pick out the tacks, glass and the nails, just as you would the splinter. Wash out the injury, and then fill it with

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