Lesson 9: The Drivetrain

The drivetrain is a series of parts that transfers the energy from the combustion engine to the wheels. When considered with the engine it’s also called the “powertrain”. If needed, review Lesson 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, the drive line or drive shaft, and a rear differential.

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 “direct” drive. In 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’d have 1,000 RPMs output. Second gear may have a 3 to 1 ratio and third gear a 2 to 1 ratio. 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 large to 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.

In 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 in 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 rear differential. It is situated longitudinally underneath the vehicle.

A differential (or rear differential), has hypoid gears. These are specialized gears which transmit the rotation of the drive shaft 90 degrees in order to turn the axles and rear wheels of the vehicle. 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 center 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 inner wheel travels less.

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 pedal is released. When a gear is selected with the gear shift, the gear is being 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. So the gears are manually selected.

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) 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.

Types of Automatic Transmission Fluid:

There are probably 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 the 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 Lesson 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 is the position of the drive train parts and where the drive train transmits the energy.

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

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

A four wheel drive vehicle transmits power through a transmission, then to a transfer case, which distributes power through front and rear drive shafts, then to front and rear differentials.

Lesson 9: The Drivetrain is one of twelve lessons that are also available as a free ebook download which is complete with illustrations. These lessons are based on the outline for the Boy Scout Automotive Maintenance merit badge and adapted by Crawford’s Auto Repair for a general audience. This article is for informational purposes only and the author does not assume responsibility or liability for any accident that may occur when working on motor vehicles. By reading this page you accept the condition that you are ultimately responsible for your own actions. For additional information on the topics on this page see Drivetrain Services, Differential Repairs, Clutch & Transmission Services, and 4-Wheel Drive and All Wheel Drive Services.

Copyright © 2014, Jeff Crawford. Permission is granted to republish this article for personal or commercial use as long as the content, citation, and copyright notice all remain intact and unchanged. There must be an active “follow” link to CrawfordsAutoService.com.

Lesson 9: The Drivetrain is licensed under a Creative Commons Attribution-NoDerivatives 4.0 International License.