The original Hydra-Matic transmission was one of the most important innovations in the history of the automobile. It wasn’t the first automatic transmission, but it was the first one that really worked and its resounding commercial success paved the for every subsequent auto-shifter. This week, we take a look at the origins of the Hydra-Matic and its originator, Earl Thompson, who also developed the first synchromesh gearbox back in the 1920s.
In 2010, Ferrari raised the hackles of automotive purists with the announcement that it would shortly phase out its conventional manual transmissions in favor of F1-style sequential gearboxes. The announcement gave new fuel to an old debate: whether the sliding-gear manual transmission and separate clutch pedal are fundamentally obsolete.
Outside of a small contingent of enthusiasts and truck drivers, the automotive world has long regarded the manual gearbox as at best a necessary evil. The multi-speed transmission, which dates back to the 1890s, evolved to compensate for the limitations of early engines, which had modest power and narrow rev bands. Getting a heavy car moving from rest required short (high numerical) gear ratios that would have the engine thrashing its guts out above about 15 mph (25 km/h), but a ratio optimized for brisk cruising — say, 40 mph (65 km/h)– wouldn’t have the torque multiplication to deal with steep hills. One of the great attractions of early electric cars, despite their severely limited range, was that they seldom required any gear changes at all, since electric motors produce their maximum torque from 0 rpm. Had early automakers devised a more efficient means of storing electricity, the evolution of passenger-car powertrains might have been very different indeed.
Actually shifting an early sliding-gear multi-speed transmission was seldom a pleasant experience. Even upshifts often required careful timing and patience to avoid grinding gears while downshifts required double-clutching and rev-matching. Few people ever mastered those skills, particularly since tachometers were not common on mundane automobiles, so the strategy of many drivers was to shift to high as quickly as possible and then stay there for as long as possible. There were a few, then as now, who prided themselves on being adept with the gearbox, but they were definitely in the minority.
Automakers were not oblivious to this problem and explored various ways of alleviating it. Henry Ford preferred planetary (epicyclic) transmissions; he allegedly did not even learn to use a conventional gearbox until the development of the Model A mid-1920s. The Model T’s pedal-operated planetary gearbox at least avoided clashing gears, although it was scarcely less complicated or labor-intensive to use than a sliding-gear transmission.
Shortly after the Great War, a number of inventors, including Britain’s Walter Gordon Wilson and France’s Jean Cotal, explored more sophisticated “preselector” planetary transmissions. With a preselector gearbox, you chose the desired ratio with a selector lever and then engaged that gear by pushing the gear selector pedal, which took the place of the traditional clutch. Preselector gearboxes were very convenient when they worked, but were expensive, heavy, consumed a great deal of power, and exceeded the skill of many mechanics. Cotal and Wilson preselectors were used in a number of pricier British and French cars (and quite a few British buses) until the early fifties, but never quite caught on.
Even the preselector transmissions were not automatic (with a number of rare exceptions beyond the scope of this article), although they did make manual shifting relatively painless. There had been experiments with various forms of self-shifting since 1904, although most had limited success. Until the early 1930s, even hydraulic brakes were considered rather advanced and automatic shifting represented a much greater technological challenge. The sliding-gear transmission, whatever its limitations, was a known quantity.
Among the many people searching for easier ways to change gears was a young hydraulics engineer from Oregon named Earl A. Thompson. In 1918, Thompson applied for a patent on a three-speed preselector transmission that used a drum-shaped synchronizer to match the speed of each newly selected gear with the speed of the transmission output shaft prior to engaging that gear. The idea was that the driver would select the desired gear and then press the clutch pedal, which would disengage the clutch, match the speed of the selected gear with the speed of the output shaft, complete the shift, and then reengage the clutch automatically.
Thompson continued to develop and refine this idea, filing a second patent application in 1923 that included a new synchronizer mechanism using cone clutches to match the speeds of gears to be meshed. He managed to build a functional prototype of his preselector transmission, which he installed in a new Cadillac donated by his younger brother Kirk, a Portland-area Cadillac dealer. That September, Thompson and his brother drove the car to Detroit, where Thompson hoped to sell his invention to the auto industry.
Although Detroit’s usual reaction to outside inventions bordered on the categorically hostile, the Thompson brothers managed to secure an audience with Cadillac chief engineer Ernest W. Seaholm and division president Lawrence Fisher. Fisher referred Thompson to the General Motors New-Devices Committee, but the ensuing bureaucratic runaround left Thompson so discouraged that he almost gave up and went home. Seaholm, who had found Thompson’s model crude but promising, finally convinced Fisher that Cadillac itself should take on the development of Thompson’s invention and hire Thompson as a consultant.
Thompson resettled in Michigan to work on developing his ideas into production form. His preselector transmission concept was discarded — judging by the patent filings, it added complexity to no obvious benefit — but Cadillac remained very interested in his gear synchronizer mechanism, which would work just as well in an otherwise conventional transmission. In August 1928, after 25 prototypes and 1.5 million miles (2.4 million kilometers) of road testing, Cadillac finally put Thompson’s invention into production. The new transmission, dubbed “Silent Synchro-Mesh,” debuted that fall on the 1929 Cadillac and LaSalle.
For cost reasons, the early Synchro-Mesh transmissions provided synchronization only between second and third gears, so shifting into first still generally required coming to a complete stop to avoid clashing. Still, the system was a considerable improvement on earlier ‘crashbox’ transmissions and made driving a great deal easier. Synchro-Mesh quickly spread to other GM divisions and was subsequently licensed or imitated by many other automakers in the U.S. and Europe. By the mid-thirties, most passenger cars had some form of synchronized transmission, although until the late sixties, many still had unsynchronized low gears.
Thompson became a Cadillac employee roughly a year after the first Syncro-Mesh cars debuted. About a year after that, Fisher and Seaholm promoted him to assistant chief engineer. Thompson had taken much of the teeth-gnashing (both literal and figurative) out of shifting. The next step was to make the process automatic.