The Drive Train | Beginner’s Auto Maintenance & Repair | Jeff Crawford

The drive train (drivetrain or powertrain) is a series of parts that transfer energy from the combustion engine to the wheels. if needed, review Chapter 5 to recall how the internal combustion engine works to convert chemical energy to rotational energy. The drive train has three main parts: the transmission, drive line or drive shaft, and differential. ln, some four-wheel-drive vehicles a transfer case is also necessary.


The Drive Train | Beginner’s Auto Maintenance & Repair | Jeff Crawford


The Drive Train | Beginner's Auto Maintenance & Repair | Jeff Crawford
The powertrain for a Subaru vehicle. The engine and transmission are seen on the right. The drive shaft is in the middle and the differential is on the left at the center of the rear axle.


The rotation of the crankshaft in the internal combustion engine does not equal the rotation of the wheels. Just like a bicycle, motor vehicles have gears. The lower gears are used for lower speeds, while the higher gears are used for higher speeds. Yet, the RPM which is read on the dashboard tachometer may be relatively constant (or stay within a range) through all speeds. The transmission, which is where the actual gears are located, takes an input speed from the crankshaft and reduces it to the output speed until the vehicle is in a “direct” drive. ln other words, it changes the gear ratios. So, in first gear, you might have an input-to-output ratio of 4 to 1. This means that as the input shaft goes around four times, the output shaft goes around one time. At 4,000 RPMs input, you would have 1,000 RPMs output. The second gear may have a 3 to 1 ratio and the third gear a 2 to 1 ratio. The fourth gear would be direct drive with a 1-to-1 ratio.


Think of the sprockets on a bicycle. There is a large sprocket on the front that rotates as the bike is pedaled, and there are also large and small sprockets on the back wheel with a chain that connects the front sprocket to the rear sprockets. As the gears are changed, so is the mechanical advantage for the person that is pedaling.


ln direct drive, the input speed, and the output speed are the same; there is no gear reduction. Modern vehicles have overdrive, which is a ratio of 0.75 to 1. This means that the input speed is less than the output speed, which results in better fuel economy on modern vehicles.


These numbers and ratios are used to present the general idea. Every vehicle has its own input-to-output ratios.


The drive line (or drive shaft) is a round tube that transfers power from the transmission to a differential. lt is situated longitudinally underneath the car. A front-wheel-drive vehicle won’t have a drive shaft. Some four-wheel-drive vehicles have more than one drive shaft.


A differential (aka rear differential in rear-wheel drive vehicles, rear end, or part of the rear axle) has hypoid gears. These are specialized gears that transmit the rotation of the drive shaft 90 degrees in order to turn the axles and correlating wheels and tires.

There are spider gears inside the center section of the differential that allow the wheels to travel different distances while turning. Consider a car that is turning right. The driver’s side rear wheel travels a farther distance than the passenger side rear wheel. Think of using a protractor to draw a circle. One leg of the protractor simply rotates on one central spot, while the other leg draws the circle. While the rear wheels of a vehicle don’t turn as sharply as a protractor, the principle is the same.


The Drive Train | Beginner's Auto Maintenance & Repair | Jeff Crawford
A differential for a German vehicle with a cutout to see the inside. 


The Drive Train | Beginner's Auto Maintenance & Repair | Jeff Crawford
An illustration of a differential that shows how the gears can be used to change the direction of the rotation. The drive shaft is on the lower right corner of the image. The top right and lower left of the image show the rear axle which extends to the rear wheels.

The Difference Between a Manual and Automatic Transmission:

A manual transmission (which is also called a standard transmission) uses conventional gears. All of these gears are turning at all times when the engine is running and the clutch is released. When a gear is selected with the gear shift, the gear is attached to the output shaft. Then to change gears, the clutch is depressed and the next gear is selected which attaches that gear to the output shaft. The gears are manually selected.


The Drive Train | Beginner's Auto Maintenance & Repair | Jeff Crawford
Illustration of a manual transmission.

An automatic transmission uses “sun and planet” gears. A “sun and planet” gear set has a central “sun” gear with “planetary” gears (typically three or four) located around the “sun” gear. These gears can rotate, turn or hold to create different speeds and directions. To select the gears there are multiple clutch packs and/or bands that are hydraulically applied by a valve body to control gear selection.


The Drive Train | Beginner's Auto Maintenance & Repair | Jeff Crawford
Illustration of sun and planet gears in an automatic transmission. The yellow gear in the illustration is the “sun” gear and the four violet gears surrounding the sun gear are the “planet” gears.


Types of Automatic Transmission Fluid:

There are 30 or more different types of transmission fluid for today’s modern vehicles. All the fluids look the same. Refer to the owner’s manual to select the correct fluid.

Aftermarket suppliers make fluids that are compatible with multiple different vehicles. When choosing aftermarket fluids, be sure that the specified fluid in the owner’s manual is listed on the aftermarket bottle.

Lubricants Used in Standard Transmission:

The lubricants used in a standard (or manual) transmission on modern vehicles have multiple types of fluid. Be sure that the fluid installed in the vehicle meets the manufacturer’s specifications.

Lubricants Used in Differentials:

Most differentials use conventional 80-90 gear oil. The 80-90 numbers indicate the weight and viscosity of the oil. (These are similar to the API and SAE discussed in Chapter 5). Some vehicles require special fluids. Be sure the fluid meets the manufacturer’s specifications. Limited slip differentials may require an additional additive.

The Difference Between Front Wheel Drive, Rear Wheel Drive & Four Wheel Drive:

The main differences between front wheel, rear wheel, and four-wheel drive are the position of the drive train parts and where the drive train transmits the energy.


The Drive Train | Beginner's Auto Maintenance & Repair | Jeff Crawford
Front-wheel drive Includes engine (red), and transaxle.


A front wheel drive vehicle incorporates a differential and a transmission into one piece, called a transaxle. The transaxle transmits power from the engine to the front wheels.


The Drive Train | Beginner's Auto Maintenance & Repair | Jeff Crawford
Rear-wheel drive Includes engine (red), transmission, drive shaft, and rear differential.



A rear wheel drive vehicle transmits engine power through a transmission to the drive shaft, and then to a differential in the rear of the vehicle.


The Drive Train | Beginner's Auto Maintenance & Repair | Jeff Crawford
Four-wheel drive Includes engine (red), transmission, transfer case, front & rear drive lines, and front & rear differentials.


The Drive Train | Beginner's Auto Maintenance & Repair | Jeff Crawford

One type of four-wheel-drive vehicle transmits power through transmission, and then to a transfer case, which distributes the power through the front and rear drive shafts, then to the front and rear differentials. Another type of four-wheel drive has a front transaxle and rear differential.

(The diagrams in this book are not the only configurations. A vehicle can have a rear engine with all three drive types, or even a mid-mounted engine.)

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