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LITTLE HOUSE PROJECT
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SPEEDOMETERS
SOMETIMES
TELL LIES
by Martin Bunn
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Loud voices were coming from the Model
Garage when Gus Wilson arrived one morning. Two customers were in the midst
of a heated argument.
"Hey, you guys," the gray-haired
mechanic shouted as his car rolled to a stop in the driveway, "Pipe down.
You're waking up the neighborhood." Then he asked Joe Clark, his partner,
who was busy unlocking the gasoline pumps. "What's it all about?"
"Got me," the younger garage man said
with a gesture of disgust. "All I know is, they've been arguing steady for
the last half hour."
"It's about speedometers," one of the
men offered excitedly, "Otis, here, says his is right and mine's wrong."
"I'll do more than say it, AL. "the
other broke in. "I'll prove it."
"Hold on there," Gus bellowed. "Two
people talking at once never settled an argument. One of you keep quiet and
let the other fellow talk."
Al Taylor caught his breath first.
"It all started about two weeks ago."
He began, "when Frank Otis and I decided we'd treat our families to a trip
to the Chicago Fair. To make a party of it we planned to stick close
together.
"It worked out fine. I'd
trail him one day and he'd trail me the next. We kept together every inch
of the way, but when we got to
Chicago, my speedometer read 970 miles
while his read 930. Coming back it was the same way."
"Maybe they're both wrong," Gus
suggested.
Both men sputtered indignant replies
as Gus walked to the front curb where the two cars were parked.
"Mine's a newer car than his," put in
Otis. "My speedometer ought to be nearer right."
"I looked up the mileage
according to the roads we took and it checks nearer mine than yours,"
retorted
Taylor.
Gus calmly walked around one car and
then the other.
"Say, if you fellows will can the
chatter for about five minutes I'll tell you something interesting." He said
finally.
"In the first place, now matter how
good a speedometer is, it rarely clocks the exact mileage after a car's been
driven five or six thousand miles."
"Humph!" grunted Otis. "If they go bad
that soon, what's the sense of having one?"
"The speedometers don't wear out," Gus
corrected. "But your tires do."
"What have tires got to do
with it?" asked
Taylor.
Gus walked into the garage office and
beckoned the others to follow.
"See these?" he asked, picking up a
pair of gears that served as paper weights on Joe Clark's desk.
One is larger than the other, isn't
it?" The two men nodded.
"Suppose we roll them along this desk
top for one complete revolution," suggested Gus, demonstrating as he spoke.
"The small one doesn't go quite as far as the large one, does it?
"Now, let's apply that to the wheels
on a car. To start with speedometers tick off the miles according to the
revolutions of the wheels and each one is designed to be used with a certain
size wheel. Naturally, if the wheels are larger or smaller than they're
supposed to be, the speedometer reading will be wrong."
"How can a wheel be smaller or larger
than it's supposed to be"" demanded Frank Otis.
"Easy enough," Gus said with a smile.
"Tires have a bad habit of wearing out and car owners often forget to keep
the air pressure up. In both cases, the tires will be smaller than they
should be. If you fit your car with oversize shoes as you've done, Otis,
the wheels will be larger."
"I can see how it might make a
difference on an old automobile that has the speedometer geared to the front
wheel." Chris replied. "but what about the cars that have it geared to the
drive shaft?"
"Makes no difference where it's
geared." Gus insisted. "It still gets its movement from the wheels and if
the tires don't pace off the distance they're supposed to, the reading will
be wrong."
"Gosh! The size of your
wheels will make a difference, won't it," exclaimed
Taylor, "But forty miles is a big error in
a trip to
Chicago."
"Let's figure it out in black and
white just for the fun of it," Gus suggested as he picked up a scrap of
paper and fished for his favorite pencil stub. "Just to make it easy, let's
take a car with thirty-inch tires.
"Normally, a thirty-inch tire, with a
circumference of about seven feet, nine inches, will make about six hundred
and seventy-five complete revolutions every mile. Since that's the normal
condition, the speedometer will be set to tick off a mile every time the
wheels turn that number of times.
"Now, let's suppose the tires are worn
down a half inch or underinflated the same amount. That'll bring the
diameter down to twenty-nine inches and the tire will pace off three
additional inches for every revolution. In a mile the error is three times
six hundred and seventy five or approximately two thousand inches.
"Changing that into miles," Gus
continued to calculate, "will mean that every time the speedometer clicks a
mile the car will have traveled one hundred and seventy feet less than a
mile. In a trip of one thousand miles, that amounts to about thirty-five
miles error.
"And the same thing happens if the
tires are larger than they should be," Gus added.
"Only the error will be in the other
direction. The speedometer will read low."
"How can we check them?" asked Otis.
"Simple enough," explained
Gus. "Make sure your tires have the right amount of air in them and then go
out on the
Mount
River highway where the
automobile club has that measured mile. Just run the length of it several
times and see what your speedometer reads.
"Of course, if you want to make a
larger test, take a trip some place where you're sure of the mileage. Then,
when you've found out the error, if there is any, you can figure it in
percentage."
"By the way, Gus, how about the
miles-per-hour reading on a speedometer?" asked Otis, "Wasn't the size of
the wheels affect that also?"
"Sure, said Gus, "The speed reading
depends on the revolutions of the wheels just like the mileage. Naturally,
if the wheels are smaller than they should be, the speedometer will show a
higher speed than it should. On the other hand, if the wheels are larger,
it will read lower."
"Oh, so that's it, Al," Otis grinned
with satisfaction. "No wonder you got that bunch of junk you call a car up
to seventy when the best I've been able to do is sixty-seven. Your tires
are worn and mine are oversize."
"Aw, go on,"
Taylor snapped back, "you couldn't get
that crate of yours up to seventy if you used spokes for wheels."
Gus shoved his pencil stub back in his
pocket and walked out of the office.
Soon the door opened with a bang and
the two men, still arguing loudly, appeared.
"What's up now?" Gus asked as they
rushed toward their cars.
"We're going out to that measured
mile," said Frank. "Maybe we can settle two arguments at once."
"You didn't do so well with them," Joe
grinned as the two cars disappeared up the street. "They came here with
one argument and left with two."
"Oh, well," growled Gus, "I started
them thinking anyway. And, if you don't think that's something, you don't
know these two birds as well as I do."
END
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