Overdrive (mechanics)

Calculating these from first principles is generally difficult due to a variety of real-world factors, so this is often measured directly in wind tunnels and similar systems.

[b] At even slightly lower speeds than maximum, the total drag on the vehicle is considerably less, and the engine needs to deliver this greatly reduced amount of power.

In an era when cars were not able to travel very fast, the maximum power point might be near enough to the desired speed that additional gears were not needed.

The reason for this separation of duties between the front and back of the car was to allow the drive shaft to run at lower torque, by using higher RPM.

This was particularly important in the early days of cars, as their straight-cut gears were poorly finished, noisy and inefficient.

Final drive ratios of 4:1 were common,[c] meaning that the wheels would turn at one fourth the rate they would if directly connected to the engine.

Under certain conditions, for example driving uphill, or towing a trailer, the transmission may "hunt" between OD and the next highest gear, shifting back and forth.

In practice this gives the driver more ratios which are closer together providing greater flexibility particularly in performance cars.

Newer vehicles have electronic overdrive in which the computer automatically adjusts to the conditions of power need and load.

De Normanville overdrives were found in vehicles manufactured by Standard-Triumph, who were first, followed by Ford, BMC and British Leyland, Jaguar, Rootes Group and Volvo to name only a few.

Another British company, the former aircraft builder Fairey, built a successful all-mechanical unit for the Land Rover, which is still in production in America today.

Several famous marques used A-type overdrives, including Jaguar, Aston Martin, Ferrari, Austin-Healey, Jensen, Bristol, AC, Armstrong Siddeley and Triumph's TR sports car range, from the TR2 through to the end of the 1972 model year of the TR6.

In 1959, the Laycock Engineering Company introduced the D-type overdrive, which was fitted to a variety of motor cars including Volvo 120 and 1800s, Sunbeam Alpines and Rapiers, Triumph Spitfires, and also 1962–1967 MGBs (those with 3-synchro transmissions).

From 1967 the LH-type overdrive was introduced, and this featured in a variety of models, including 1968–1980 MGBs, the MGC, the Ford Zephyr, early Reliant Scimitars, TVRs, and Gilberns.

The J-type overdrive was introduced in the late 1960s, and was adapted to fit Volvo, Triumph, Vauxhall/Opel, American Motors and Chrysler motorcars, and Ford Transit vans.

The Volvo version kept the same package size as the J-type but with the updated 18 element freewheel and stronger splines through the planet carrier.

The Gear Vendors U.S. version uses a larger 1.375 outer diameter output shaft for higher capacity and a longer rear case.

In 2008 the U.S. company Gear Vendors, Inc.[3] of El Cajon, California purchased all the overdrive assets of GKN to continue production of the U.S. version and all spares for J and P types worldwide.

With substantial improvements developed in Muncie, Indiana, by William B. Barnes for production by its Warner Gear Division, BorgWarner provided the box that was factory-installed between the transmission and a foreshortened driveshaft.

Since the overdrive function, if enabled, could be shifted by simply easing up on the accelerator without depressing the clutch pedal, the action was much like a semi-automatic.

Also, an electrically operated solenoid would deactivate the unit via a switch under the accelerator pedal providing the equivalent of the kickdown of the automatic.

A knob connected to a bowden cable, similar to some emergency brake applications, was also provided to lock out the unit mechanically.

This may create unpleasant vibrations at high speeds and possible destruction of the driveshaft due to the centripetal forces or uneven balance.

The driveshaft is usually a hollow metal tube that requires balancing to reduce vibration and contains no internal bracing.

The higher speeds on the driveshaft and related parts can cause heat and wear problems if an overdrive and high differential gearing (or even very small tires) are combined, and create unnecessary friction.

This is especially important because the differential gears are bathed in heavy oil and seldom provided with any cooling besides air blowing over the housing.