CHAPTER XXXI

HOT-WEATHER TIRE EXPANSION

There is considerable question whether the pressure on tires should be decreased during the hot summer days. Tire manufacturers claim, as a rule, that tires heat more on low pressure than when run at the full pressure given by them. They claim that even a slight decrease to offset any increase from heating, because of warmer temperature of the air, friction, and the hot pavements, will injure the tires. On the other hand, drivers of long experience insist that the car should be started out with the tires softer than in cold weather. Still others insist that the tire pressure demanded by the manufacturer makes the tire too hard for comfort and that when the pressure is increased by heat it makes the tire as hard as one of solid rubber. There is some truth on both sides.

There has been considerable criticism of the conclusions drawn from the writer’s experience with tires while on a summer tour, related in the previous chapter. He stated that there was a twelve-pound increase in a forty-mile run. This was disputed. Tests prove that he was right and that in city driving tires ordinarily heat up even when the conditions are not extreme.

To determine what effect the hot weather has on tires in the city, the author drove a 3200-pound Model L Locomobile, equipped all around with 34 × 4-1/2 inch tires, through the park and on Riverside Drive for fifteen miles at ordinary city-traffic speed, which always is under twenty miles an hour. The thermometer registered 86 degrees and the day was partly overcast, so that the full effect of the sun on the pavements was not obtained. The tires were inflated to full 90 pounds, which is the pressure recommended by many manufacturers.

At the end of the run, one tire registered 101 pounds, two were 100 pounds, and one was 99 pounds. The conditions were not excessive in any way and the weather was not abnormal, the road was smooth except for very short stretches, the speed was low, and the tires were of ample size for the weight of car and load.

After the test run, a gauge was made to test the flattening effect on the tire from a lowered pressure. With the car loaded it was necessary to decrease the pressure in the tire to 65 pounds to have 1/16 inch bulge of the side of the tire. At 70 pounds pressure there was no flattening perceptible by the use of the gauge.

It is certain that more tires are damaged by under-inflation than by over-inflation, also that perfect new tires will stand more than 20 pounds over-inflation without damage to them. But it is reasonable to believe that, if a tire has become weakened from any cause, it will blow out quicker under increased pressure. It is also a fact that a tire inflated to the pressure recommended by many manufacturers has very little resiliency, and the riding qualities of the car are very greatly impaired, and when the pressure rises slightly higher the tire loses its chief function, that of absorbing the small road shocks. Anyone who has had the experience of riding on hard tires knows the discomfort of it; one might about as well ride on solid tires.

But all this comes back to the question: What is under-inflation? A tire with ten pounds less pressure than recommended probably will heat more than when carrying a full pressure; but it will very quickly come up to the required pressure when the car is run.

As the pressure recommended by many manufacturers is the extreme so far as comfort in riding is concerned, the car owner must take his choice between comfort and tire mileage, assuming that a slightly less pressure does decrease the life of the tire. Presumably the average owner would rather have comfortable riding, even at the expense of tire mileage, and we are not prepared to admit that decreased mileage would be an inevitable result with ten pounds less pressure. Beyond question the average man would not use a substitute for gasoline which gave him all sorts of discomfort because it did not run the engine smoothly, even if it gave a slightly greater mileage to the gallon.

One manufacturer of fabric tires recommends 56 pounds pressure for a 4-inch tire and guarantees 5000 miles, and makes the claim that this pressure makes riding much more comfortable. Another well-known manufacturer advises inflating the tire only to the point where there is no bulge or flattening, and another type of pneumatic tire has become very popular because it requires still less pressure and makes riding that much more pleasurable.

Where tires are hard and roads are rough, the machine jumps around so that it is more likely to jump off the road, therefore there is an element of safety as well as comfort involved.

For all around comfort, though possibly with lessened tire mileage, the conclusion is reached that a ten-pound decrease in pressure in hot summer weather is desirable. The tire cannot be damaged much, because after a very few minutes on really hot pavements the pressure will equal that recommended by the most exacting manufacturer and any overheating due to this cause would be a matter of very short duration.

As was said in the previous chapter, the wise owner will test out his car and tires and find out just the proper amount of decrease the tires will stand. He will then have to decide whether he wants the highest amount of tire mileage or the highest amount of comfort and act accordingly.

It is beside the argument, but a matter of fact, that a certain tire salesman, a fellow who is quite insistent upon a certain pressure being maintained, makes it his invariable practice to deflate his tires ten pounds in hot weather; he wants tire mileage, too.

All of which goes to show that some people do not always practice what they preach, or take the medicine they give to others.