How the Automatic Transmission Evolved on American Automobiles

by James M. Flammang

Updated: April 2022

As the Great Depression ended and World War II
loomed, only one American model could be fitted with
a fully automatic transmission: the 1940 Oldsmobile.

Gear shifting, a task barely noticed today, was a perpetual battle during the automobile's first half-century. Early motorists had to exercise a taxing combination of muscle and manual dexterity, tempered by ceaseless patience, to get their horseless carriages moving and up to speed.

One inherent "flaw" in the gasoline engine made gearing essential. Torque output (rotational force) is weaker at low engine speed, while starting out or climbing – just when an extra "twist" is most needed to overcome the car's inertia. For acceptable acceleration, a variable-ratio intermediary must be positioned between engine and wheels.

Gears provide a practical, if compromised, solution. If a small gear, driven by the engine, turns a larger gear connected to the car wheels, that driven gear rotates slower – but produces greater torque. Gearsets in varying sizes could offer one ratio for initial takeoff, another for building speed, a third for cruising. The key problem: How to get from one range to the next.

Constant-mesh gears used separate clutches to change ratios. One form, the planetary transmission, is best known for its long service on the Model T Ford. Bands surrounding each planetary gearset, tightened by a foot pedal, activated the desired one. Shifting tended to be jerky, but clash-free.

Clash was the music of the sliding gear transmission, which debuted in France in 1891. Only one foot-operated clutch was needed. But the movable gears were anxious to grind, even "strip," while meshing. As co-developers René Panhard and Emile Levassor allegedly replied to critics: C'est brutal mais ga marche ("It's brutal, but it works."). Sliding gears spread quickly onto gasoline-powered vehicles in both Europe and America, and live on in modem manual transmissions.

The search for simpler shifting brought one tentative solution: friction drive, propelling the 1903 Union and, a few years later, the Gearless, Simplicity, and Cartercar. Infinite ratios were manually selected by shifting the meeting point of twin discs, which rotated perpendicular to each other. As Metz advertised, there was "no clutch to slip – no gears to strip." Disc slippage and wear, however, hindered friction's success.

Actual automatic shifting arrived earlier than is commonly believed: on the 1904 Sturtevant. A foot button controlled everything. Multiple-disc clutches running in oil, each hooked to a constant-mesh gear, activated according to engine speed. Slightly above idle speed, spring-loaded centrifugal weights moved outward, engaging the low-speed clutch to move the car.

At higher rpm, the next clutch kicked in. Downshifting occurred as the engine slowed, the weights retracting to release each clutch.

Lost in a sea of minor makes, the Boston-built Sturtevant, with its primitive yet farsighted transmission, expired after 1908. Observers may have agreed with the 1906 prediction of Winthrop E. Scarritt, first president of the American Automobile Association, that the "present unsatisfactory system of change-speed gears will be supplanted by a variable-speed device... [perhaps] a system of hydraulic transmission." Around that time, in fact, the Barber hydraulic transmission was developed in Britain. But nearly three decades were to pass before another true automatic shift would arrive on the American auto scene.

Not that various inventors weren't trying to ease the task. The 1907 Columbia "Gasolect" used its gasoline engine to run a generator, which fed current to an electric motor that served as a multi-level booster. This Entz magnetic clutch enjoyed more success on the 1915 Owen Magnetic, "Car of a Thousand Speeds" (which actually had six forward ranges at its selector).

Electric shift appeared on the Haynes and other mid-teen makes. A Cutler-Hammer version on the 1919 Premier used a small lever to pre-select each gear, energizing solenoids to move the shift fork as the clutch pedal was pressed. (The preselector idea wasn't new, having been patented in 1898 by Dr. Frederick Lanchester in Britain.) Sliding keys shifted the Chandler Traffic Transmission's constant-mesh gears.

Clutches improved as facing materials grew more durable. Still, transmissions were nicknamed "crash boxes" for good reason. Only by tricky "double clutching," tickling engine speed to match the next desired ratio, could gears be drawn silently into mesh. Close coordination between hand and foot, plus an ear for engine rpm, was mandatory.

Male motorists also perpetuated the myth that gear changing amounted to a form of manly art. That hardly thrilled automakers who wanted to market their wares to more women.

Not until 1928 did a method of eliminating clash appear. Synchro-Mesh used metal cone clutches that brought gears to nearly equal speed before mating. Cadillac, its originator, advertised that "The gears shift instantly, easily, at any speed, without the slightest bit of clashing."

Free Wheeling, introduced on the 1930 Studebaker, was another innovation to ease shift woes. An over-running clutch allowed the engine to relax at road speeds, coasting merrily along, like a bicycle when its rider stops pedaling. Automatic vacuum-actuated clutches, created by Bendix, debuted on the 1932 Buick with "Wizard Control." Pressing and releasing the accelerator caused the clutch to engage and disengage.

William B. Bums, toiling on his own, invented automatic overdrive after experimental work with the Columbia 2-speed rear axle (which served a similar purpose). A primitive version had appeared a quarter century earlier on the Gramm truck. Bums sold his creation to Warner Gear, which developed it for the 1934 Chrysler Airflow. Unlike normal direct drive in high gear, overdrive let the transmission's output shaft rotate faster than the input. The secret? A centrifugal clutch seizing or releasing a 2-speed planetary gearset – descended from the Model T and soon to be adapted for automatics.

Between 1928 and 1932, many cars switched from 3-speed to 4-speed transmissions. Though largely a sales gimmick, their failure to find buyers signalled the need for change.

That drivers are "too shiftless to shift," declared consulting engineer Walter C. Keys in his 1933 address to the Society of Automotive Engineers, "seems to point strongly in the direction to automatic transmissions." At last, the long-awaited demise of the gearshift lever seemed imminent.

Technology was trying hard. Buick began secret work on an infinitely-variable "Roller" friction transmission in 1932. Across the Atlantic, various fluid couplings and hydraulically-operated gearboxes saw action. From Sweden, the flywheel-and-sleeve Spontan torque converter arrived for experiments in American cars. Fluid couplings combined with a preselector and gear-change pedal drove planetary gearsets on the British Daimler and Armstrong-Siddeley.

Other nearly-forgotten attempts include the Constantinesco and Lysholm-Smith torque converters; the De Lavaud; the continuously-variable Hayes; and the German Lentz "hydraulic gear." Among other obstacles, experimental hydraulic transmissions needed extremely high fluid pressures – beyond the capacity of contemporary seals.

Though intrigued, American automakers deemed the embryonic automatics too complex for economical production. Dramatic evidence of the need, though, could be seen in a 1934 survey of downtown Manhattan, which revealed that drivers shifted gears every 11 seconds. Chicago bus drivers fared even worse, "clocked" at up to 12,000 shifts per day. Some would soon get relief from the mechanical Mono-Drive developed by Oscar Banker for GMC buses, with an automatic centrifugal clutch and three forward speeds.

Credit for the "first" passenger car automatic sold in significant numbers must go to Reo, named for founder Ransom E. Olds (head of Oldsmobile early in the century. Actually a semi-automatic. the Self-Shifter was developed by Horace T. Thomas, chief Reo engineer since 1904. Thomas contracted cancer in 1925 and went into semi-retirement, doing much of his work at home. An assistant, Gale Hines, later insisted he had developed the device himself, also at home, but Reo rejected his claim.

Thomas had installed experimental Self-Shifters in Reo cars by 1930. Production cost about $6 million, and it debuted in spring 1933 – followed by a flurry of lawsuits from other inventors. Only one case made it to court, which Reo won. Self-Shifters first were standard on the luxurious Reo Royale, but soon an $85 option on Flying Cloud models.

Operation was similar, at least in principle, to that of the old Sturtevant. Two transmissions stood in series: a 2-speed manual controlled by a dashboard T-handle, plus an automatic section. That section included two gear pairs inside an internal-toothed drum, with an over-running clutch between them. A conventional foot clutch had to be used when starting out, stopping, and shifting between Low and High range.

All told, the Self-Shifter had four speeds: two in Low range, and two in High. To start in High (comparable to taking off in 2nd gear with manual shift), the driver pushed in the dash lever. At about 14 miles per hour, it shifted automatically into direct drive, as eight centrifugal governor weights around the drum moved outward to lock multi-disc clutches. Gears and drum then rotated as a single unit. A downshift came below 10 mph, as spring tension overcame the weights' centrifugal force, releasing the clutches.

Low range, mainly recommended for hills and bad roads, could be used for ordinary starts, shifting (with clutch) to High later. Twisting the lever put the car into reverse.

"It is simplicity itself," insisted 0. G. Poxson, Reo's general sales manager. Operation was smooth and quiet. Owners and experts have declared Self-Shifters "foolproof" and "virtually troublefree." Sluggish acceleration was its prime weakness. Shifts occurred too soon for snappy performance, whether starting in Low or High.

Promoted both as a driving aid and a safety feature (keep both hands on the wheel), Self-Shifters did not catch on as hoped. For 1935, a less durable (cheaper) version was offered, manufactured by Dana-Spicer. It sold no better and gained a poor reputation. In 1936, Reo left the car business and turned exclusively to commercial vehicles.

Even if Reo's Self-Shifter did not last long or sell well, or evolve into an improved version, this pioneering venture drew the attention of industry leaders. They knew that many motorists never became adept with the gearshift, while countless others were kept from car ownership by their fears of the gears.

Among the millions who lacked gearbox expertise were several top General Motors managers, including corporate head Alfred Sloan. By the early 1930s, they were ready to act.

Buick's Roller device was to fizzle (too costly and unreliable); but a new project would result in the Automatic Safety Transmission, an $80 option on the eight-cylinder 1937 Oldsmobile.

AST development began in 1932, directed by Earl A. Thompson. Shifting problems were hardly new to Thompson, who, as Cadillac's assistant chief engineer, had invented synchro-mesh. Beginning in a tiny lab with two assistants, he worked diligently on this secret project that carried the code name "Military Transmission," even though it had nothing to do with the armed services.

Cadillac dropped the project in 1934, and it was taken over by GM's central staff. Oldsmobile's general manager, Charles L. McCuen, heard about the work and wanted the new transmission for his division. While Thompson and his team continued their research, McCuen gave orders to get moving on production.

In order to get the AST into cars by late spring of 1937, no attempt was made to incorporate a fluid coupling or torque converter. Like Reo's, the AST would be a semi-automatic with a regular friction (pedal) clutch. Though installed on Oldsmobiles, the production units were built at a Buick plant.

A selector quadrant on the steering column had two Forward positions (low and high), Reverse, and "Off" (neutral). Two planetary gearsets in series, each with a brake band and disc clutch, delivered four forward speeds. Whereas the Reo had shifted at preset car speeds, AST shifts varied according to road speed and gas pedal position.

A two-stage centrifugal governor determined the best gear for present conditions, controlling the oil pressure that performed the actual shift. Power could travel through either the front or rear gearset, both, or neither – four ratios in all.

In 1st gear, bands for each planetary set held tightly, delivering power through both for maximum gear reduction. In 2nd, the forward band was released, so only the rear gearset operated; in 3rd, the opposite. And in "Super-Drive" 4th, both bands were loose for direct drive.

Starting in high range, the transmission shifted from 1st to 3rd to 4th gear. Low operated only in 1st and 2nd.

Drivers could shift between low and high range without touching the clutch, or even releasing the gas pedal. Only when standing still was the clutch needed. The upshift from 3rd to 4th gear, coming at any speed from 22 to 65 mph, gave acceleration far superior to Reo's. Tromping on the gas in top gear produced a kickdown to 3rd (dubbed a "pick-up gear").

Oldsmobile ads stressed safety and operating ease – plus added passenger room, due to lack of the familiar, long floor lever. On the down side, the AST needed frequent adjustments and tended to "clunk" when downshifted into low. Operation was also touchy, affected by engine tune-up. Dealers were told not to attempt repairs. At the factory, failed units would be dismantled and studied. Such analysis paid off later, but the rather complicated Automatic Safety Transmission developed a reputation for unreliability.

Availability was extended to six-cylinder Oldsmobiles for 1938. It was also offered for that model year only on Buick's cheapest model, the Special, called the "Self-Shifting Transmission." Only about 28,000 units were made between 1937 and 1939, AST's final year, so it found a home on less than 8 percent of potential Oldsmobile and Buick models.

While AST shifted automatically, that annoying pedal still lurked at the floorboard. Earl Thompson finally declared, "Let's get rid of the clutch pedal and make this thing fully automatic." Thus was born 4-speed Hydra-Matic, created for the 1940 Oldsmobile.

Hydra-Matic's fluid flywheel eliminated the mechanical clutch and cushioned the impact of shifting gears. Otherwise, operation was similar to the AST. The shift lever selected a combination of valves to direct oil through the proper paths in the maze, controlling brake bands and clutches on dual planetary gearsets. Again, shift points depended on throttle position and road speed. Low range shifted from 1st to 2nd, while high (Drive) used all four ratios.

Initially priced at only $57, Hydra-Matic carried a tempting warranty: a new transmission would be installed free if the original failed. Factory study of broken-down units, coupled with earlier AST analysis, helped eliminate bugs in a hurry. Hydra-Matic went on Cadillacs in 1941, Pontiacs in 1948, and was later licensed to various non-GM vehicles (Lincoln, Kaiser, Nash, Rolls-Royce, Hudson).

Fluid couplings took a different turn at Chrysler. Fluid Drive was installed on all 1939 Custom Imperial Eights.

The principle, old and simple, may be visualized as two electric fans facing each other. Turn one on, and its mate begins to rotate in tandem. Motion is transmitted through air alone. Substitute oil for that air and – voila – a fluid coupling, with centrifugal force acting on the liquid to transmit engine power with silky smoothness.

Dr. Herman Föttinger had developed a fluid coupling for German boats in 1905. Chrysler began early in the Thirties by entering into an agreement with Harold Sinclair, who had reworked Föttinger's patents for British autos. American development fell to Chrysler's chief transmission engineer, Carl A. Neracher.

His foremost problem: choosing materials. British couplings had been cast in one piece from aluminum, but those were too weak to handle powerful American engines. Forgings were too costly. Many forms were tried before the Chrysler team settled on welded, stamped steel construction. Only the recent advances in spot welding techniques for all-steel car bodies made this manufacturing method possible. Production couplings had 138 spot welds on their 46 fins.

Drawings depicted Fluid Drive as two halves of a metallic grapefruit, with "pulp" removed but "vanes" intact. It had only those two major moving parts. A conventional clutch sat between the coupling and gearbox, but the engine could run while the car stood still, its gears meshed and clutch pedal released.

Engineering critics branded constant-slip fluid couplings as inefficient gas guzzlers. Chrysler insisted that slippage was negligible, dropping from 100 percent when motionless to barely 1 percent at road speeds.

Though described by MoToR magazine as giving the "sensation of gearless" operation, Fluid Drive had no automatic shift. Gear changes demanded clutch activity. Lazy motorists could remain in 3rd all day, however, unless they had to climb a hill or back up – provided they had time to spare, since acceleration from a standstill would be anemic. Its virtues lay elsewhere. Technical editor Harold Blanchard accurately observed that "even the most unskilled operator cannot fail to make a perfectly smooth start nor can the engine be stalled."

Vacamatic, a semi-automatic transmission introduced on the 1941 Chrysler Eight, boosted shiftless performance. Behind the coupling and clutch now stood a 2-speed gearbox with automatic overdrive (more accurately, underdrive) in each range.

The shift pattern was similar to that of a manual transmission, with Low (2nd), High (3rd), and Reverse positions. (No 1st gear.) Clutch action was needed only when selecting a range. After starting out in High, letting up on the gas at 15 mph or more caused an upshift from Lo-Hi to Hi-Hi, marked by a noticeable "clunk." Low range, essential only for quicker takeoffs, had a similar upshift. Downshifts occurred as the car slowed, or by flooring the gas.

DeSoto received an identical apparatus, called Simplimatic, for 1941. Chrysler products retained the semi-automatic through 1952 under various names, including Tip-Toe Shift (DeSoto), Gyromatic (Dodge), Fluid-Matic and Prestomatic (1950-51 Chryslers).

Just before all car production ceased in 1942, some curious semi-automatics made a brief appearance. Studebaker's Turbo-Matic was an overdrive transmission with fluid coupling and automatic vacuum clutch (no pedal), shifting within Traffic or Cruising range. Liquamatic, optional on Lincoln and Mercury, had a clutch pedal but shifted itself between 2nd and 3rd gear. Both are best remembered for notorious undependability and few were sold. Neither reappeared as automobile production resumed in 1946, though Hudson's Drive-Master did.

Only one piece of the automatic transmission puzzle remained. No manufacturer had yet produced a true torque converter for passenger cars. That glory came to Buick with its Dynaflow, unveiled late in 1947 for the Roadmaster. As usual, the principle was an old one, used on prewar Yellow Truck & Coach buses.

At first glance, a torque converter seems similar to Chrysler's fluid coupling. It's much more – actually the infinitely-variable transmission that auto Utopians had envisioned for decades. While the coupling merely transmits power, the converter alters that power. It does the work of steel gears in producing extra leverage, or "push," at the car's wheels; but does so hydraulically, using turbines, with a veritable infinity of potential ratios.

How does torque multiplication work? Between the vaned driving (pump) and driven (turbine) members stand one or more non-rotating wheels (stators). They deflect power back to the driving element, adding to the force already being delivered by the engine. Blades are shaped to cause vigorous rebounding of the oil, boosting its velocity, reusing force that would otherwise be wasted.

Dynaflow creation began during the war, derived from the transmission in the Buick-made Hell-Cat tank. Aluminum construction kept weight down. Twin rigid stators deflected fluid at low speeds. But at higher rpm, when increased torque wasn't needed, a one-way clutch let the stators turn freely. The system then acted as a simple fluid coupling.

Buick ads said it all: "You just start the engine...set a lever...step on the gas...and go!" A low, nasal exhaust drone accompanied the uninterrupted rise in car speed. Even with torque multiplication in a ratio of 2.25-to-1 and Buick's powerful "Fireball" engine, though, takeoffs were tedious. A planetary gearbox added a manual Low range for swifter starts – but even worse fuel economy.

Subsequent automatics were based upon Hydra-Matic's hydraulic shifting or the torque converter. Most combined the two principles in a single device, giving the advantage of torque multiplication along with actual gear changes for snappier pickup.

Chevrolet's 1950 Powerglide was a Dynaflow derivative. Buick added a second turbine to Dynaflow in 1953, then a third, in the race for quicker acceleration. Ford began to produce Ford-O-Matic and Merc-O-Matic, torque converters coupled to gearing, for 1951 models. Packard's 1949 Ultramatic added a direct-drive lockup, bypassing the converter at cruising speed to eliminate slippage. Chrysler abandoned its semis for automatic Powerflite in 1953.

By the mid-1950s, virtually all American autos offered a true automatic shift, either optionally or as standard equipment. Refinements came regularly, but all can trace their origins back to AST and Dynaflow – and indirectly to Sturtevant and Reo. Only with the rise of fuel-efficient imported cars in the late 1970s, and sporty high-performance models in subsequent decades, did the manual shifter experience a rebirth.

Up From the Floor

While automatic transmissions progressed at Oldsmobile and Chrysler, other automakers at least wanted to make manual shifting easier. One noteworthy innovation was the Bendix electro-vacuum "Finger-Tip Gear Shift," called "Electric Hand" as a $20 shifting aid on the 1935 Hudson. The driver pre-selected a gear by moving a miniature lever at the steering wheel. Vacuum cylinders then shifted the internal mechanism as he stepped on the clutch or, with optional vacuum clutch, released the gas pedal. A similar Bendix unit shifted the streamlined 1936-37 Cord. Neither was reliable, demanding frequent adjustment.

During 1938, the final year of old-fashioned floor shifting for most makes, a crop of vacuum boosters from Evans Products Company appeared as options on Nash, Studebaker, and Graham models. A stubby 5-inch shift lever stuck out from the dash. Chevrolet offered vacuum assist in 1939, as a $10 option with its new column shifter. Each used vacuum supplied by the engine to produce "fingertip" manipulation of the shift mechanism. The lever moved only a shoft distance when shifting from 1st to 2nd, or down to 3rd.

"The motoring public accepts new features of design with little thought and still less knowledge of the great amount of detail work ... spent in perfecting them," said Scientific American magazine when describing Pontiac's original column shift. Looking backward, raising the lever off the floor seems so elementary.

Primitive column shifters, in fact, had appeared on Winton and Pierce models around the turn of the 20th century. Since 1931, Pontiac had experimented with some 15 styles, using cables, pins or ball-and-socket joints. "Safety Shift gear control" was a $10 option on 1938 Pontiacs. Cadillac and LaSalle made it standard equipment that year and by 1939-40, virtually all automakers gladly abandoned the floor lever.

Note: A version of this article was originally written in 1986, but never published.

© All contents copyright 1986, 2022 by James M. Flammang
Photo Source: Wikimedia Commons
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