Because of the need to provide good low and middle range acceleration and flexibility, there is a tendency to choose gear ratios suitable for these conditions. A possible result is that the top gear ratio is too low to allow the engine to operate at its greatest power and lowest consumption conditions at higher speeds. To obviate this, a higher fifth gear can be incorporated in the gearbox to give a ratio higher than direct drive, or an additional overdrive unit can be bolted to the rear of the gearbox between the output shaft and the propeller shaft. Additional two-speed gears have been in use in Rolls Royce cars for sale for many years, and in the early 1930s Edgar de Normanville devised a simple single train epicyclic gear for passenger car use and this was used in some models of new Porsche. The gear train had the input to the planet gear carrier and the output from an annulus gear with a conical exterior. The sun gear was coupled to a clutch brake member with conical inner and outer surfaces. This was lined with friction material and, in one position, engaged the annulus to lock the gear in direct drive. In the other position, the brake exterior cone engaged a complementary cone in the casing to hold the sun gear and provide the overdrive ratio. A one way, or freewheel, clutch between the carrier and the annulus maintained the drive while the clutch brake was moving, and prevented engine overspeed during changes. The clutch was shifted into engagement by spring means and moved to the braking condition by hydraulic pressure. This pressure was under the control of an electro magnetic valve. A relay in the electrical circuit between the driver’s switch and the hydraulic valve prevented the use of the overdrive under high torque conditions in the lower gears and, also in reverse, at the moment when the one way clutch would otherwise engage.
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