A preselector gearbox is a type of manual gearbox (US: transmission) used on a variety of vehicles, most commonly in the 1930s. The defining characteristic of a preselector gearbox is that the manual shift lever is used to "pre-select" the next gear to be used, then a separate control (a foot pedal) is used to engage this in one single operation, without needing to work a manual clutch.
Most pre-selector transmissions avoid a manual clutch entirely, some use one solely for starting off.
Pre-selector gearboxes are not automatic gearboxes, although they both share some aspects and past influences of epicyclic gearboxes. A fully automatic gearbox selects the ratio used, with a pre-selector gearbox this remains a manual choice.
There are several radically different mechanical designs of pre-selector gearbox.
Advantages of pre-selector gearboxes
For the driver, there are two advantages:
- Fast shifting, with only a single manual operation. This requires less skill to learn than techniques like double declutching and it offers faster shifts when racing.
- Ability to handle far more engine power, with a lighter mechanism.
In engineering terms, some designs of pre-selector gearbox may offer particular advantages. The Wilson gearbox offers these, although they're also shared by some of the other designs, even though the designs are quite different:
- Their friction components are brakes, rather than clutches. These are simpler to engineer, as the wear components can be arranged to not be the rotating parts.
- The friction wear components can be mounted on the outside of the mechanism, rather than buried within it. This makes maintenance and regular adjustment easier.
They were common on Daimler, Alvis, Talbot-Lago, and Armstrong Siddeley cars as well as on many London buses. They have also been used in racing cars, such as the 1935 ERA R4D, and hillclimbing cars such as Auto Union "Silver Arrows". Military applications included tanks such as the German Tiger I and Tiger II in World War II, through to the current tanks such as Challenger 2.
A multi-clutch gearbox avoids the difficulties of shifting gear by avoiding the need to shift at all. It operates as a number of separate gearboxes, each one controlled by a separate clutch, interlocked to avoid multiple selection. Selecting a gear is a matter of selecting the appropriate clutch. An advantage of this type is that it's simple to arrange remote operation, as there is no gear shift linkage as such, merely duplication of a clutch linkage.
Single ratio per clutch
This type of gearbox appeared in the March 1917 Oldbury gearbox trials between 8 different World War One tanks. Each ratio has its own shaft, and its own clutch. Provided the clutches are interlocked so that only one may be engaged at a time, the system is simple.
More recently (c.1980) it has been used experimentally for urban buses in the UK, with a combined Gardner diesel engine and four-speed gearbox. UK buses are double-deckers with rear-mounted transverse engines. Their use also involves much stop-start driving, thus heavy clutch wear. The advantage of this arrangement was its use of four clutches, all easily replaced from outside the engine assembly, without needing to remove the engine.
The Tiger tank of World War Two used a form of pre-selective gearbox, offering 8 speeds. Clutches were used in combinations, allowing many more ratios than actuators. The shift mechanism was hydraulic, to reduce driver effort. There were three hydraulic cylinders, each with two positions and controlling dog clutches for gear trains arranged on four shafts. The cylinders were controlled by a rotary valve on a simple quadrant gear lever and activated by pushing the lever sideways into its gate. The combination of the three cylinders, effectively a 3-bit binary code, permitted 8 different ratios, although in reverse these were limited to just the lowest 4. When the captured Tiger 131 was studied by the British in 1943, the report on the gearbox was carried out by Armstrong Siddeley motors, as experts in such a field.
The idea of rapid shifting by clutch alone has also been developed as the twin-clutch gearbox, in recent high-performance cars. This combines the simplicity of a shifting gearbox with the rapid shifts of a clutch-based design. There are effectively two separate gearboxes, each offering alternate ratios from the overall set, and the two clutches select which gearbox is in effect. Changes within the gearbox are done automatically, to predict which ratio the driver will require next. Provided that the next ratio has been selected correctly (i.e. the computer guessed correctly as to an up-shift vs. a down-shift) the shift itself is merely a rapid movement of the clutch. Unexpected shifts may confuse the system though and it must first select the correct ratio before engaging the clutch, giving a far slower shift.
Major W. G. Wilson (1874–1957) was rewarded as one of the major co-inventors of the tank after World War One. He had mainly been involved with the development of transmissions for tanks, particularly the problem of their steering gearbox. He had become an advocate for the benefits of the epicyclic gearbox, which allowed large torques to be transmitted whilst still being controllable through a small input force. In June 1917, the first mock-up of the Mark V tank appears with the Wilson epicyclic steering gear. This was the first of the heavy tanks that could be driven by a single driver, without requiring him to signal orders inside to others working the brake levers.
Since 1900, the Lanchester Motor Company had built cars with manually controlled epicyclic gearboxes, first with a cone clutch, then with multi-plate clutches. These formed the ratio-changing gearbox of the transmission. In 1918 an experimental tank "Lanchester Gearbox Machine" or "Experimental Machine K" was tested, fitted with an epicyclic gearbox built by Lanchester.
After the War, Wilson had a considerable reputation as an engineer of genius, particularly for gearbox design. In 1928 he patented his design for a novel pre-selective gearbox. Various manufacturers produced preselector transmissions under licence to the Wilson patents. One of the top manufacturers was a French company called Cotal. Wilson himself formed a partnership with J. D. Siddeley of the car maker Armstrong-Siddeley, first under the name of "Improved Gears Ltd.", then later as "Self-Changing Gears Ltd.".
As its name suggests, gear changes were made by selecting a gear ratio in advance of its being needed. The chosen gear was then brought into operation by pressing and releasing the 'gear change pedal', which was normally the left pedal, installed in place of the usual clutch pedal. It is not to be confused with automatic transmission, in that gear changing is initiated by the driver. Unlike the "crash" gearboxes of the first half of the 20th century, the gearwheels in a preselector box are permanently in mesh in an epicyclic layout.
On some cars, starting off from rest involved using the gear change pedal like a clutch. On others, first gear could be selected but while the engine was still idling the car would not move even after the gear change pedal had been pressed and released. When the accelerator was pressed a centrifugal clutch or fluid coupling would engage and the car would begin to move.
Use in cars
Use in buses
A substantial number of British buses built between about 1935 and 1960 had preselector transmission, particularly those built by Daimler and AEC, and some by Leyland. Some were mechanical, whereas the AEC RT type, universal in London at this period, had the gearbox air-operated (compressed air was also used for the brakes, windscreen wipers, etc), which gave off a characteristic hiss when the change gear pedal was pressed. Typical operation of London buses was they had a very low first gear, only used on hills, so the driver when starting would select second gear, depress and release the change gear pedal to engage the transmission, and then select third ready for changing gear on the move, all this done while the bus was still stationary. On starting just the accelerator pedal was pressed, the gear change to third made at about 15 mph by just pressing the left pedal, and that could well suffice to the next stop where the bus would be stopped in gear and the process repeated.
Use in armoured vehicles
- Some 1950s era James Motorcycles were built using Villiers engines that utilized a Pre-Select gearbox. Press the gear pedal down to select 1st gear, then pull in the clutch lever and on its return the gear engaged and drove forward, press the gear pedal again to select 2nd gear and it will engage only after the lever has been pulled in again.
- "Track test of 1935 ERA R4D vs Porsche 911 GT3", Automobile. 2004, http://web.archive.org/web/20050308215823/automobilemag.com/reviews/0412_put_to_the_test/.
- Fletcher, British Tanks, 1915-1919
- "Technical data on the gearbox and transmission of the Tiger I tank".
- "Rebuilding Tiger tank 131". Bovington Tank Museum.
- Fletcher, David (2001). The British Tanks, 1915-1919. Crowood Press, 190. ISBN 1-86126-400-3.
- #Fletcher, British Tanks, 1915-1919, p.70-74
- #Fletcher, British Tanks, 1915-1919, p.122
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