THE OVERDRIVE

Southland Times, Issue 22969, 15 August 1936, Page 19

THE OVERDRIVE

CRUISING ECONOMY NOT WANTED. IN TRAFFIC The reappearance of the overdrive in motor cars recalls that the system was a feature of a pre-war American luxury car. It is by no means new, and has always been popular in Germany. The overdrive is really top gear, the direct drive, where an overdrive is provided, ranking second in point of speed to engine revolutions. Actually, the overdrive reduces the engine speed by about 25 per cent, when compared with the normal top-direct drive. When a car is doing 45 m.p.h. in direct drive engagement of the overdrive would increase the speed to nearly 60 m.p.h. at the same engine revolutions. The overdrive adds complexity and cost to the gearing, but it has very definite advantages. It is a fuel saver, keeping the revolutions of the engine down to more economical working speed, but in doing this it accomplishes much more by saving in wear and tear of the working parts. If the overdrive reduces the engine speed in the ratio of 4 to 3 the inertia loading, which depends upon the square of the speed, will be reduced in the ratio of 16 to 9. There are several systems of applying an overdrive. The most popular is a simple supplementary planetary gearbox behind the normal one. So long as the driver keeps his foot on the accelerator pedal the car will run right up to its possible maximum speed in the direct drive, with the overdrive remaining ineffective, but as soon as the driver lifts his foot momentarily from the pedal at any speed in excess of 40 m.p.h. centrifugal weights fly out, disconnect the straight-through drive, and lock the planetary gears, so bringing the overdrive into action. The reverse takes place when the speed of the car falls to 30 m.p.h., normal direct drive coming back into action quite automatically. This entirely does away with one of the bugbears of motoring to a great many drivers, the handling of gears. There is no gear changing to do in the operation of this type of overdrive. Incidentally, it may be mentioned that the system works in conjunction with any of the gears; it is not limited to the direct drive.

It has a weakness in that, not knowing just when the change is going to take place, the driver may be taken unawares just when he is accelerating to overtake other traffic. With the loss of the direct drive there is a momentary hesitation and instead of rushing by as he intended the driver may find himself checked. Overdrive really is not intended to be used in traffic; the whole idea of it is economy in cruising, and when cruising is not the object the overdrive should be locked out of action. A handle is provided on the dash for this purpose. The overdrive should not be in use if emphatic acceleration is likely to be required.

Another system of overdrive is that incorporated in the dual ratio back axle. This system not only reduces engine speed but also cuts down the speed of the propeller shaft. It is always under direct manual control by the driver, but for several reasons is unlikely to become a common feature of everyday motor cars.