Overdrive, OD

I recently found out that there is a little confusion about OD among some of our members regarding overdrive. I hope this helps to end the confusion and settle few arguments.






OD. OVERDRIVE



a

h2


Usage
/h2

Generally speaking, overdrive (OD) is the highest gear in the transmission. Overdrive allows the engine to operate at a lower rpm for a given road speed. This allows the vehicle to achieve better fuel efficiency, and often quieter operation on the highway. When it is switched on, an automatic transmission can shift into overdrive mode after a certain speed is reached (usually 70+ km/h depending on the load). When it is off, the automatic transmission shifting is limited to the lower gears. For an automatic transmission, it is almost always best to select overdrive and allow the transmission to control engagement of the overdrive. (It may be necessary to switch it off if the vehicle is being operated in a mountainous area or when towing a trailer). With a manual transmission, overdrive should usually be selected when the average speed is above 70 km/h (approx. 43 MPH).



The automatic transmission automatically shifts from OD to direct drive when more load is present. When less load is present, it shifts back to OD. Under certain conditions, for example driving uphill, or towing a trailer, the transmission may "hunt" between OD and the third gear, shifting back and forth. In this case, switching it off can help the transmission to 'decide'. It may also be advantageous to switch it off if engine breaking is desired, for example when driving downhill. The vehicle's owner's manual will often contain information and suitable procedures regarding such situations, for each given vehicle.


Virtually all vehicles (cars and trucks) have overdrive today whether manual transmission or automatic. In the automotive aftermarket you can also retrofit overdrive to existing early transmissions. Overdrive was widely used in European automobiles with manual transmission in the 60's and 70's to improve mileage and sport driving as a bolt-on option but became increasingly more common for later transmissions to have this gear built in. If a vehicle is equipped with a bolt-on overdrive (ie: GKN or Gear Vendors) as opposed to a having overdrive built in you will typically have the option to use the overdrive in more gears than just the top gear. In this case gear changing is still possible in all gears, even with overdrive disconnected. Overdrive simply adds effective ranges to the gears, thus overdrive third and fourth become in effect "third-and-a-half" and a fifth gear. In practice this give the driver more ratios which are closer together providing greater flexibility particularly in performance cars.

h2
How an overdrive unit works
/h2

The overdrive consists of an electrically or hydraulically operated epicyclic gear train bolted behind the transmision unit. It can either couple the input driveshaft directly to the output shaft (or propeller shaft) (1:1), or increase the output speed so that it turns faster than the input shaft (1:1 + n). Thus the output shaft may be "overdriven" relative to the input shaft. In newer transmissions, the overdrive speed(s) are typically as a result of combinations of planetary/epicyclic gearsets which are integrated in the transmission. In these cases, there is no separately identifiable "overdrive" unit. A number of such transmissions and transaxles are manufactured by Aisin, for use in vehicles produced by many different manufacturers. In older vehicles, it is sometimes actuated by a knob or button, often incorporated into the gearshift knob, and does not require operation of the clutch. Newer vehicles have electronic overdrive in which the computer automatically adjusts to the conditions of power need and load.




h2
Fuel economy and drivetrain wear
/h2

Using overdrive gearing, the car's engine RPM drops, reducing engine wear and normally saving fuel. Since 1981 U.S. CAFE (corporate average fuel economy) legislation virtually all domestic vehicles have included overdrive to save fuel. One should refer to the car's owner's manual for the proper speed to run at overdrive. All engines have a range of peak efficiency and it is possible for the use of overdrive to keep the engine out of this range for all or part of the time of its use if used at inappropriate speeds, thus cutting into any fuel savings from the lower engine speed.


There is some debate on the overall efficiency of overdrive gearing, as it requires more moving parts than direct 1:1 drive, but most will agree that within the transmission, this effect is minimal. The other difficulty can be in the drive shaft rotation speed.


Overall drivetrain reduction comes down to three basic factors: transmission gearing (including overdrive), differential gearing (in the axle), and tire size. The rotation speed problem comes into effect when the differential gearing is a high ratio and an overdrive is used to compensate. 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.


The impetus is to minimize overdrive use and provide a higher ratio first gear, which means more gears between the first and the last to keep the engine at its most efficient speed. This is part of the reason that automobiles tend to have larger numbers of gears in their transmissions nowadays. It is also why more than one overdrive gear is seldom seen in a vehicle except in special circumstances ie. where high (numerical) diff gear is required to get the vehicle moving as in trucks or performance cars though double overdrive transmissions are common in other vechiles, often with a small number on the axle gear reduction, but usually only engage at speeds exceding 100 km/h or about 60mph.