How Overdrives Work, And Why They re Not A Thing Anymore
How Overdrives Work, And Why They’re Not A Thing Anymore
Only seen now in older cars, the overdrive system was the precursor to transmissions with over four gears
Overdrive as a concept seems pretty awesome – normally displayed as a button or switch, it almost seems like a hidden source of power that can abruptly be pulled out with a press or a flick. Unluckily, it’s the finish opposite, being used in older cars to produce decent fuel economy and less engine noise once up at a cruising speed.
Overdrive as a definition is simply having a swifter transmission output shaft speed than the input rotation speed of the input shaft from the engine. This means torque is effectively being multiplied in a positive sense so being ‘overdriven’, creating a more effortless driving practice.
‘Underdrive’ is therefore the opposite, where the engine is spinning swifter than the resultant rotation of the output shaft. This is experienced in the lower gears (normally up to third gear) which are used for acceleration before cruising gears are employed.
You may associate ‘going into overdrive’ with classic cars, particularly ones that only have four forward gears. This is because, back in the day, top gear in most cars featured a gear ratio of 1:1. This meant that at highway speeds, cars would have to sit at high engine speeds, wasting fuel and making a bit of a racket. Gear ratios are written as the input shaft speed against the output shaft’s constant of one, determining the differences in drive after the power has been sent through the transmission. So a 1:1 ratio is known as ‘direct drive’ as there is no differential in rotation across the transmission.
To solve the issue of inefficient cruising with a 1:1 ratio, engineers determined that it would be lighter to make an extra gearing unit on the back of the gearbox before the propshaft. Redesigning entire transmissions with more gears would be a much more expensive and time-consuming task, so the overdrive unit was born.
An overdrive is effectively a gear-reduction system made up of a combination of sun, planetary and rings gears. When not in use, the overdrive permits direct drive to take place with the sun gear turning the ring gear. Once overdrive is enabled, the sun gear is motionless in place and the planetary gears are brought to life, rotating the ring gear.
The increase in gearing occurs due to the planetary gears having more teeth than the original sun gear, thus turning the ring gear further per rotation. This means that if the planetary gears can rotate the ring gear 1.Two times compared to just one turn for direct drive, the output shaft will have turned twenty per cent further than usual. This will permit the engine to operate at a lower RPM for a given road speed while cruising.
An overdrive system is generally actuated by using a mechanical linkage or electronic solenoid displayed as a button or lever on the gearstick or dashboard. Albeit it could be used as a function for every forward gear, most systems are locked until either third or fourth gear is selected to avoid lugging the engine after each gear switch.
The addition of an overdrive system to a transmission began to fall away in popularity when transmission technology expanded to five- and six-speed gearboxes. The extra gears have ratios higher than 1:1, creating the overdrive feature needed for cruising and motorway driving. This is then further enhanced by the latest crop of transmissions that feature ten or eleven forward gears, all of which can combine to produce fairly nifty fuel consumption figures.
You may not have to look too far back for automatic cars featuring overdrive, but the technology has been out of date for fairly some time in manual transmissions. Telling that, an overdrive unit certainly had its place back in the day and showcased one of the very first ventures towards creating reasonable fuel economy on a long cruise.
