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.