DRIVETRAIN
Getting power from the engine to the wheels of an automobile has
provided a seemingly endless challenge for rear-wheel-drive,
front-wheel-drive, 4-wheel-drive, front-engine, rear-engine, and mid-engine
cars, longitudinal, transverse, vertical, slant, and flat engines, plus an
amazing array of hardware in between. George Selden's notorious 1877
patent was for a front-drive carriage with a transverse 3-cylinder engine,
anticipating the Chevy/Suzuki Sprint by over a century. When it comes to
car designs, there are very few new ideas, just progressively successful
adaptations of old concepts.
The heart of the drivetrain is the transmission. Because gasoline engines
develop their torque over a very narrow speed range, several gears are
needed to reach useful road speeds. (Steam engines and electric motors can
be used in cars with no transmissions.)
The modern transmission was introduced by a pair of Frenchmen --
Louis-Rene Panhard and Emile Levassor -- in 1894. The engineers had
invited the press to a demonstration of "the most revolutionary
advancement to date in the brief history of the motor car industry."
Unfortunately, the engine in their demo vehicle died, and they were
reduced to giving a chalk talk on multi-geared transmission theory to a
bored press corps.
One 19th-century newsman reported their invention as "more hocus-pocus
from charlatans trying to cash in on the public's fascination with the new
motor car." Maybe the inventors should have skipped the tech talk and just
used the description later attributed to Panhard: "It's brutal, but it
works!"
Cars of the time transmitted engine power to the wheels in a simple
fashion that was easy for non-engineers to visualize. The engine drove a set
of bevel reduction gears that drove a shaft and pulley. Leather belts
extended between the pulley and geared wheels on an axle. One wheel, the
small one, got the car going by meshing with a ring gear on one of the
driving wheels. The big wheel then took over to get the car to hustle along
at a top speed of 20 mph. If the car encountered a hill that it did not have
the power to climb, the driver would come to a dead stop so he could
engage the small wheel.
Thus did British auto pioneer F. W. Lanchester describe the transmissions
in his cars: "One belt-driven HIGH gear that will go over everything and
one bel-driven LOW gear in case the car had to climb a tree."
It was not until a year after their disastrous news conference that Panhard
and Levassor regained their reputations. At this time, they had their first
car ready for the press to drive. With it, they changed a lot of
minds.
That 1895 Panhard-Levassor was revolutionary -- not the transmission
alone, but the whole drivetrain layout. In fact, it has served as the
prototype for most vehicles built in the 90 years since then. Unlike other
cars of that day, it possessed a vertically mounted engine in the front of the
vehicle that drove the rear wheels through a clutch, 3-speed sliding gear
transmission and chain-driven axle. The only modern features missing from
the setup were a differential rear axle and driveshaft. These came along
three years later, in 1898, when millionaire-turned-auto-hobbyist Louis
Renault connected a vertical engine with transmission to a "live" rear axle
by means of a metal shaft.
The live rear axle -- which Renault adapted from an idea developed in
1893 by an American, C. E. Duryea -- was called the differential rear axle.
It used a number of gears to overcome the problem of rapid tire wear,
which resulted on turns with the "dead" axles used by all other carmakers.
"Differential" referred to the ability of the unit to turn the outer driving
wheel faster than the inner driving wheel, eliminating tire scuffing in
turns.
By 1904, the Panhard-Levassor sliding gear manual transmission had been
adopted by most carmakers. In one form or another, it has remained in use
until recent times. Obviously, there have been improvements, the most
significant being the invention of a synchronizing system that permits drive
and driven gears to be brought into mesh with each other smoothly without
gear clashing. This system allows both sets of gears to reach the same
speed before they are engaged. The first of these synchromesh
transmissions was introduced by Cadillac in 1928. An improvement to the
design patented by Porsche is widely used today.
Between the time the sliding gear-transmission was introduced and the
perfection of the synchromesh, there were other attempts at making it
easier for the driver to shift gears. One was the planetary transmission in
the 1908 Model T Ford. It had a central gear, called the "sun" gear,
surrounded by three "planet" gears. Today, planetary gears are more widely
used in automatic transmissions than in manual.
Some pretty elaborate planetary manual transmissions did evolve, however.
One was developed by Walter Wilson and was called the Wilson
Preselector. It came along in 1930.
This gear system, which used four individual planetary gearsets, allowed
the driver to preselect one gear ratio by moving a small lever on the
steering column. the driver could then "order up" the particular preselected
gear by depressing a foot pedal. This caused a camshaft to disengage one
gear and simultaneously allow the preselected gearset to engage.
All transmission designs since the Panhard-Levassor unit have had one goal
in common -- to make shifting easier. Obviously, the easiest to shift
transmission is the automatic. It's strictly an American innovation.
The first automatic was invented in 1904 by the Sturtevant brothers of
Boston. It provided two forward speeds that were engaged and disengaged
by the action of centrifugal weights without need for a foot-operated
clutch. As engine speed increased, the weights swung out to engage bands
-- first the low-gear band and then the high-gear band. The unit failed
because the weights often flew apart.
The next significant attempt at an automatic transmission was by Reo in
1934. Called the Reo Self-Shifter, it was actually two transmissions
connected in series. For ordinary driving, one unit upshifted itself
automatically in relation to car speed through the engagement of a
centrifugal multiple-disc clutch -- much the same idea used by the
Sturtevants. The second transmission was shifted manually and was used
only when a lower gear was needed.
In 1937, Buick and Oldsmobile came out with a transmission called the
Automatic Safety Transmission. it had a conventionally clutch for shifting
the transmission into forward or reverse. Once in forward, the transmission
shifted automatically by using two hydraulically operated planetary units --
one for LOW gear and one for DRIVE. The unit was the forerunner of the
GM Hydra-Matic, which was born in 1938.
The Hydra-Matic consisted of three planetary gearsets that were operated
hydraulically. A fluid coupling was used to connect the engine and
transmission. Credit for perfecting the fluid coupling goes to Chrysler,
which developed the concept in 1937. However, Chrysler did not make use
of it until 1941, when the Chrysler Fluid Drive transmission was
introduced. This was not an automatic unit, but a standard transmission
with a fluid coupling, not a clutch.
By 1948, the automatic transmission had evolved into the hydraulic torque
converter that we know today coupled to a planetary geartrain. The first to
use the converter was Buick. The '48 Buick Dynaflow, as it was called,
was the model for present-day automatic transmissions. Others soon
followed with similar units -- Chevrolet Powerglide, Fordomatic and
Merc-O-Matic in 1950; and the Chrysler M-6 Torque Converter Automatic
in 1951.
These are some other interesting developments in the history of
transmissions and drive units:
- In the early days of transmissions, leather-lined, multiple-disc,
oil-bathed clutches were in common use. Although the first use of a dry
single-plate clutch was by Duryea in 1893, it was not until 1921 that a
design was developed that would not burn out in a few hundred miles,
thanks mainly to Englishman Herbert Frood, who perfected more durable
friction materials.
- Universal joints were first introduced on the 1902 Peerless. The 1908
Franklin was the first car to use roller-bearing U-joints. The 1930
Hupmobile pioneered needle-bearing U-joints, which is the point where we
stand today.
- Although differential locks were first used on a steam lorry in 1903 to
provide wheel traction on slippery roads, it was not until 1956 that the first
production limited-slip differential for a popular car was produced by
Studebaker.
- In 1906, Otto Zachow and William Besserdich of Clintonville,
Wisconsin, built a car with the first successful 4-wheel-drive unit. A year
later, they began a company called the Four Wheel Drive Auto
Co.
- In 1913, Packard made a milestone step in differential development
with the introduction of a spiral-bevel ring and pinion set that cut the noise
level produced in the rear axle. In 1926, with the introduction by Packard
of the hypoid gear rear axle, noise ceased to be a problem altogether,
unless the differential was going bad.
- In 1934, automatic overdrive was introduced on the Chrysler and
DeSoto Airflow.
- The latest development in transmission seems to be the continuously
variable automatic transmission, or CVT. The CVT is driven by a metal
link belt. We've come full circle in 100 years, back to the belt-drive!
