The concept of “overdrive” has been around for decades and in theory is the ability of the car to use a gear ratio that allows the engine to be running at a lower rpm to that of the road speed which in turn means fuel savings when cruising at motorway speeds, for example, and therefore lower engine wear and noise.
Mechanically, the overdrive unit is a secondary gearbox that takes input from the gearbox and delivers it to the prop-shaft either as a 1:1 drive or where the output is faster than the input, thus allowing the engine to rev lower at certain speeds. On many of the modern automatic cars, this is now done through the engine management system, however in years gone by, a switch would be activated.
I first experienced a mechanical overdrive in my old 1965 MGB Roadster that had the overdrive switch on the gear lever and could be applied in 3rd and 4th gears. This type of overdrive unit was invented by Captain Edgar de Normanville, a British war veteran, journalist and engineer whose other claim to fame was developing the circular windscreen wiper for ships!
de Normanville had developed a gearbox for Humber in the 1930s and had a small British engineering firm, Laycock Engineering, manufacture his overdrive unit. This was hugely successful in British cars with the two big conglomerates of BMC and Rootes using the unit through most of their brands – hence it being on my old MG! Many of the niche manufacturers like Bristol, TVR, Jensen and even Ferrari fitted them.
The system was based on a combination of systems: solenoids activated by the switch, an oil pressure unit and pistons that effectively swapped the gear ratios. This was done by using a cone shaped clutch that in the “out” position would transfer the input directly to the prop shaft by locking the clutch against the output shaft mechanism. Once the switch was activated (“in”), a solenoid allowed oil pressure to build up and push small pistons to move the cone clutch out of the way which forced the power to a sun and planetary gear system that then sent it to the prop shaft, effectively dropping the rpm by a little over 20% whilst still retaining the road speed.
Laycocks produced several versions starting with the A-type, followed by the D-type and LH-type. The J-type jumped the Atlantic – with Chrysler and American Motors using it in many of their cars. The final version was called the P-type.
Borg Warner had also developed an “automatic” overdrive during the 1930s for the US market that was similar to de Normanville’s design, the main difference being that there was no cone clutch. Borg Warner used a blocker ring and pawl that was activated by a switch in the car and really activated when the car was travelling over 30mph. Using the blocker ring was a simpler but less elegant way than using hydraulic pressure as the European design did. Borg Warner sold their units to Ford, General Motors and some of the brands that became American Motors as well.
Whereas the Laycock-de Normanville design evolved over 40-odd years, the Borg Warner version lasted about 25 years as the company decided to concentrate on developing ever more efficient automatic gearboxes that would in fact make the mechanical external overdrive unit obsolete. It was common in the 1950s for a car to have no more than 4 forward gears – today there are up to 10 in some gearboxes. This means that the overdrive ratios can now be built in to the main gearbox.