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 autoshifter. 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.
Recently, Ferrari raised the hackles of automotive purists with the announcement that it plans to phase out its conventional manual transmissions in favor of F1-style sequential gearboxes. The announcement has given 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. A gear ratio short enough to accelerate a heavy car from rest would have the engine thrashing its guts out above 15 mph (25 km/h) or so, while a ratio optimized for brisk cruising — say, 40 mph (65 km/h) — would lack the torque multiplication to cope 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.
Shifting an early sliding-gear transmission was seldom a pleasant experience. Even upshifts often required careful timing and patience to avoid grinding gears and downshifts required double-clutching and rev-matching. Few drivers ever mastered those skills, particularly since tachometers were not common on mundane automobiles. The strategy of many drivers was to shift to high as quickly as possible and then stay there, preferably until coming to a dead stop. There were those who prided themselves on being adept with the gearbox, but they were definitely in the minority.
Automakers were not oblivious to this problem and quickly began exploring various ways of alleviating it. Henry Ford preferred planetary transmissions; he allegedly did not even learn to use a conventional gearbox until the mid-1920s. The Model T’s pedal-operated planetary gearbox avoided clashing gears, although it was scarcely less complicated or labor intensive to use.
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 beyond the skill of many mechanics. Cotal and Wilson preselectors were used in a number of British and French cars until the early fifties (and quite a few British buses), but they never quite caught on.
Even the preselector transmissions were not automatic, 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 hydraulics engineer from Portland, Oregon, named Earl A. Thompson. Thompson first started thing simplifying the gear-shifting process while he was in school, although it was another decade before he began to put his ideas into practice. In 1918, he applied for a patent on what he called an “automatic gear-shifting mechanism” using cone-shaped synchronizers to match the speeds of the gears before engaging them. The patent for his invention, issued in 1922, was actually for a preselector transmission, but its most important feature was that it permitted clash-free shifts without double-clutching.
Thompson developed a working prototype and installed it in a new Cadillac donated by his younger brother Kirk, a Cadillac dealer in Spokane, Washington. In 1924, Thompson drove the car to Detroit to demonstrate it to automakers. Although most car companies were intensely resistant to outside inventions, Cadillac chief engineer Ernest W. Seaholm and president Lawrence Fisher thought Thompson’s invention had promise. Seaholm found Thompson’s design crude but intriguing and referred him to GM’s New-Devices Committee. Thompson became so discouraged by the ensuing runaround and general lack of interest that he nearly gave up, but Seaholm finally arranged for Cadillac to take on the development itself, hiring Thompson as a consultant.
Thompson spent the next few years overseeing the development of his synchronizer mechanism, which GM dubbed “Silent Synchro-Mesh.” According to Ernest Seaholm, Cadillac built 25 prototypes, which racked up more than 1.5 million miles (2.4 million kilometers) on the GM Proving Grounds before production began in August 1928. The new transmission debuted that fall on 1929 Cadillac and LaSalle models.
For cost reasons, the early Synchro-Mesh transmissions were synchronized only in second and third gears: Shifting into first generally required coming to a complete stop. Nevertheless, 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 by many other automakers. By the mid-thirties, it was standard on most American and many European passenger cars, although it remained rare on trucks and race cars.
Around the time the first Synchro-Mesh cars went on sale, Larry Fisher and Ernest Seaholm promoted Thompson 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 it automatic.