"Stop the car,
Clem," Mrs. Ferrers commanded.
"I can't stand that awful drumming sound another moment.
My head aches like fury.
It's driving me crazy. Can't you
do something about it?"
Clem Ferrers smiled
placatingly, "Sure, Aggie, we'll stop and I'll see what I can do.
It didn't seem so bad to me."
"It wouldn't!" his wife
snapped. "I declare, Clem, you'd
never notice anything wrong so long as the wheels keep turning around."
With that she sprang out,
plumped herself down on a near by rock, and rested her head in her hand, a
picture of woe.
"Good thing I'm not so
gosh-blinked fussy," Clem murmured to himself as he clawed the tool kit out
from under the rear seat and hesitatingly thumbed over the various articles
in it. Finally his eyes lighted
up as his fingers closed over the handle of a huge monkey wrench.
"Maybe a couple swipes
with this'll bend it so it won't rattle so," he muttered as he climbed up on
the fender and began to inspect the top.
At this particular point
in the proceedings, an auto service car labeled "Model Garage" hove into
sight with the huge hands of Gus Wilson clasped firmly around the steering
wheel. Perched beside the
veteran auto mechanic was his partner Joe Clark.
"What do you reckon he's
fixing to do up there with that big wrench?"
Joe questioned as Gus applied the brakes.
"That's just what I want
to know," Gus said as he climbed out and walked over to Ferrers' car
"What's gone wrong,
mister?" he asked.
"The madam, there, she
says something in the roof is humming so it gave her a headache,"
Clem explained. "I
thought maybe if I gave it a couple of socks here and there, whatever is
loose might get stuck so it couldn't wiggle any more.
Got any good ideas?"
"Sure," said Gus.
"Climb down and let me drive your car so I can see what the noise
sounds like."
Gus took the wheel, with
Ferrers beside him, and drove off.
"There's nothing the
matter with the top," Gus reported when they got back.
"Trouble is, some of the body bolts have come loose.
At certain speeds you get a vibration in the body that seems to come
from the top, I'll tighten up the bolts."
"Beats me how the noise
could sound like it came from the top when those bolts underneath were doing
it," Ferrers puzzled, as he bent down to watch the work.
"Simple enough,"
Gus told him, "All noise comes from vibration.
And vibrations seem around in metal or wood till they reach a place
where there's a broad, flat surface that isn't fastened all over.
Then off they go into the air.
Your car tells you where the noise is coming from, but not how it got
there."
"That was a
funny case," observed Joe as the two garagemen continued on their way home.
"there's a lot about this vibration business I don't savvy at all.
For instance, I can understand how mounting an engine in the chassis
with rubber at every point ought to cut down the vibration, but what's the
difference between just ordinary rubber mounting and this 'floating power'
they talk about? Does 'floating
power' mean anything at all?"
"Did you ever hit a
baseball too near the end of the bat, or maybe too near your fingers?" Gus
countered, "Stung like a flock of bees, didn't it?
When you hit the ball with just the proper spot on the bat, you
didn't feel anything except the club stopping in the air, eh?
Well, floating power is like that.
It's all in the location of the engine supports.
"Suppose," Gus continued,
"you could hang an engine up in the air without any supports and start it
going, what would happen? It
would vibrate a bit - no engine is perfect that way - but if you looked over
the engine, you'd find at least two spots that didn't seem to move at all.
They're like the spot on the baseball bat and the place where your
hands hold it. You could touch
one of those spots on the engine and you wouldn't feel any vibration to
speak of, showing that the whole engine was wobbling back and forth with
that point as one of the centers.
"Floating power means
holding the engine in the frame by those points where there's no vibration -
after you've found where they are.
Of course, you can't hang an auto motor in the frame with only two
supports. Extra rubber mounted
brackets are put in to steady the engine a bit and to keep it from twisting
itself right out of the frame trying to turn the shaft."
"I see," Joe interrupted.
"They don't worry about the motor vibrations as long as they can keep
them out of the rest of the car.
Why didn't somebody think of that before?"
"Probably somebody did,"
Gus suggested. "but he didn't
know how to apply the idea to an auto engine.
Rubber mounting made it real easy and practical.
Don't get the idea that floating power and rubber mounting are so
effective that it isn't necessary to worry about motor vibrations any more.
"Take it from
me, if you tried to apply floating power to some of the unbalanced auto
engines they made years ago, you'd probably have the motor hop right out of
the car if you tried to drive fast.
"No, rubber mounting and
floating power are ornaments that can be added only after you've done pretty
near all the mechanical balancing that's possible.
I can remember when a set of pistons was considered O.K. if they
didn't vary in weight more than a few ounces.
Now they match 'em on the cheapest cars to the fraction of an ounce,
connecting rods the same way. In
the old days crankshafts for cheap cars came through without any balancing
at all and high grade car makers tested them only for static balance.
Now nearly every one, including the cheapest, gets tested for dynamic
balance."
"I understand pistons
should be the same weight and so on." Joe again interrupted, "but I don't
understand that static, dynamic balancing business.
If you set a crank shaft with the end bearings resting on a couple of
knife edges and you take metal off here and there till it doesn't have any
tendency to roll because one throw is maybe, a bit heavier than another, why
shouldn't it be in perfect balance when you run it in the engine?"
"It would if you were
talking about a thin flywheel instead of a crank shaft," Gus explained.
"The static or standing-still balance of the flywheel would be
practically the same as its dynamic or running balance because all the weight is in
one plane."
"How do they do the
dynamic balancing?" "In a
special machine that rotates the crank shaft and registers which way it's
out of balance," Gus explained.
"I think I've
got a glimmer of an idea about that," said Joe after a minute or two.
"Now explain one other thing that's always puzzled me about engine
balancing. Why is it you can't
balance a four-cylinder motor and get it just as smooth as a six?
I never could understand why the two pistons coming down don't equal
the other two pistons going up.
If the pistons all weigh the same and so do the connecting rods, and
the crank shaft is dynamically balanced, what is there left to cause
vibrations?"
"Better brains than yours
or mine have puzzled over that one, Joe," Gus said with a grin.
"The answer is what the engineers call the angularity of the crank
shaft. If you measure the motion
of the piston as it slides down the cylinder, you'll find it goes quite a
bit more than halfway by the time the crank has reached the halfway point.
Sounds impossible, but you can prove it for yourself in two minutes
with a compass and a ruler. The
only way you could get balance would be to put a pair of cylinders on
opposite sides of the crankshaft.
Then the irregular piston motion would balance."
"So that's it, eh?"
said Joe as they reached the Model Garage, "Well, I'll take your word
for it. Just goes to prove
that whatever goes up must come down - but not the same way!"
END
