New-York tribune., 1919-04-06
There's a Reason
For Gears; You
Should Learn 1t
Expert Explains Function of
Change Speed Mechanism
in the Automobile-No
Car Can Be Run Success
fully Otherwise
II. Clifford Brokaw
Technical Director West Side Y "

eCNnlcnL DtreCtOT west SIDE \ N.
c. A. Automobile School.
Perhaps you have wondered why it
was necessary to change speed gears
on hill. 1t may be you have thought
it useless custom and have tried to
take hills on high, regardless of what
1t does to your motor If so, pause long
enough to learn the why of speed gears
on the car and the reason why second,
or even first, speed is necessary in hill
climbing. 1t may save you fuel and
upkeep charges.
If automobiles were driven entirely
on smooth and level roads at an even
speed there would be no need Of a..
gear case; all that would be needed
would be clutch to connect and dis
connect tho engine from tho road
wheels. No other device would be
needed to meet the conditions Of
possible and desirable. But for sandy
roads and steep hills. IL there were no
change speed gears it would be neccs-
sary to have very high power en-
g!ne under the hood and the maximum
of such power would be used only at
rare intervals. At all other times the
motor would be very wasteful in oper-
ation.
A gasolene engine does not operate
efficiently at low speed; to get cconom-
tcal power it must be run at compare
lively high speed. On heavy grade
or sandy road 1t Is not speed but
power that is needed, and if the en-
gine does not develop it under the
slowed down conditions Of the grade
or heavy road, it follows that the motor
must be spceded up and run at high
speed. The only way to accomplish
this Is to introduce a device which
will permit the engine to run at nor
mally high speed to develop the power
and to transmit this power to the road
wheels at lower speed. This device
IS called the transmission gearset.
This .gearset is merely an arrange
ment of gears on parallel shafts, which
permits a selective meshing at varying
ratios. 80 that without decreasing the
engine speed a lower speed is trans
mitted to the drive shaft Most per
sons are familiar with the principle
of reducing speed by gears in machin-
cry. The gearset usually is arranged
for three speeds-low, intermediate
and high forward, as we'll as reverse
The high speed gear is known as the
direct drive, which means that the
propeller shaft, leading from the trans
mission case to the rear axle, is di-
rectly connected to the engine shaft
and makes the same number of revo-
lutions.
The intermediate speed gear will
probably give about two revolutions of
the motor shaft to one OF the propeller
shaft, while the low speed ratio is
3 Or S1a to /. The reverse gear is usu-
ally q trifle lower, or about to /.
These figures will vary in different
cars, but are approximately correct
Besides the transmission gear re-
duction there Is a reduction at the rear
axle, through the differential gears,
= II.
usual figure not far from Az to /
that is, the propeller shaft makes Az
revolutions to each one revolution of
axle and road wheels.
Probably it will give some idea of
how the power is increased by changing
to lower speed If we take, as an ex-
ample, q car with four-cylnder en-
gine, with wheels equipped with "a
inch tires, and with reduction at the
rear axle of Az to /. With 34-inch
tires the car will travel forward ap-
proximately feet with each revolu-
ton. If the car is being driven on
nigh speed gear, for each 9 feet of
travel on the ground the engine shaft
will make a u revolutions, and as there
are two power impulses to each shaft
revolution of 4-cylinder engine there
will be power impulses, or l to a
foot
Changing to the Intern ediate speed
gear, and assuming that It's reduction
IL 1.S to l, in addition to the Az reduc-
tion at the differential total of 8 to /,
there will be 16 power impulses for
each feet of travel, or nearly
a foot. This should make it clear that
by shifting the gears and increasing
the number of power impulses we have
nearly doubled the pulling force ex-
erted.
On the low speed gear, with re-
ductton ratio Of 9.2 and the "a reduc-
ton Of the rear axle, total ratio Of
1aa to l is obtained, which gives 20
power impulses for each 8 feet, or more
than to a foot.
With 12-cylinder engine there are
6 power impulses for each revolution
Of the crankshaft, and with 36-inch
tires there would be approximately
9h feet Of travel each revolu-
tion. With q az ratio on the differ
entail. on high speed gear we would
get 27 power impulses for 9M feet Of
travel, or about 3 each foot; on inter
mediate speed gear there would be
As power impulses, or each foot; and
on low speed gear 81 power impulses,
or 10 each foot. This does not neces-
sarliy mean that 12-cylinder motor
is more powerful than 4-cylinder;
that depends on other things, and the
IL explosions might develop much less
power than the four. But the compara-
tive pulling power Of the several gears
IS the same no matter how many cylin-
ders there are
1t should be plain from this timo
that in climbing a hill where the grade
is so steep that the motor cannot make
it on high speed gear, with one power
impulse each foot of travel, it is pos
sible to change to intermediate and
obtain 2 power impulses, or to low
speed and 3.
Grades usually are overestimated
we often hear persons speak of 80
per cent grades, when 1t is doubtful If
they ever have seen a road with that
grade. Fifteen per cent seems to be