Understanding Your Car's Drivetrain

Getting your vehicle moving to tackle the morning commute seems simple enough. Turn the key, fire up the engine, put the car in drive and off you roll, right? In reality, a complex series of systems known as the drivetrain must work in concert to convert engine power into forward motion.

So, what is a drivetrain, and why does it matter to you? Let’s dig in.

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What is a drivetrain?

The drivetrain is a series of components that are designed to transfer power from the engine to a vehicle’s driven wheels. Driven wheels receive engine power from the drivetrain. The number of driven wheels can either be two or four.

Depending on your vehicle, there are several different types of drivetrains on the market. Each has a unique set of components designed to connect the engine to the driven wheels and move the vehicle down the road.

How does the drivetrain work?

In order to transfer engine power to the wheels, drivetrains use a transmission, driveshaft(s), axles, differentials and wheels to transfer engine power to driven wheels.

First, you need power from the engine to spin the gears of the transmission. Depending on your vehicle’s configuration, this kinetic energy from the transmission is transferred to the differential or driveshaft and then to the wheels via the axle. There you have it, the miracle of forward motion.

Let’s discuss what each of these components does.

Drivetrain components

Transmission: The transmission is designed to convert an engine’s power into energy that can be transferred to the rest of the drivetrain. As engine power enters the transmission, it uses gears to ensure that the proper amount of power is transferred along to the drivetrain at a given speed. As your vehicle requires more power to maintain its speed, the transmission will shift gears to maximize engine output. Think of the gears on a bicycle.

Driveshaft: A driveshaft delivers engine torque from the transmission to the differential. There are several different types of driveshafts. Most commonly, you’ll find one-piece and two-piece driveshafts beneath vehicles.

U-joint: The U-joint connects the driveshaft to the differential. This unique component is flexible and designed to move. This prevents damage as a vehicle goes over bumps and dips.

Differential: Differentials take engine torque from the driveshaft and split it to each driven wheel via an axle. Some vehicles have limited slip differentials or LSDs. LSDs can sense when a wheel has lost traction, shifting torque to the wheel with more traction to keep the vehicle moving forward.

Axle: The axle takes engine torque from the differential and moves it to the driven wheels.

Constant velocity (CV) joint: Unique to front-wheel-drive vehicles, the CV joint is designed to function under full wheel articulation. This allows the front wheels on a front-wheel-drive vehicle to both steer and propel the vehicle at the same time.

Transaxle: The transaxle combines the transmission, differential and axle into one unit. This is common on vehicles where the engine is in the same location as the driven wheels, such as on a front-wheel-drive vehicle.

Types of drivetrains

There are a wide variety of drivetrains available on modern vehicles, and each has its own set of pros and cons for tackling everything from the morning commute to an off-road trail.

Front-wheel drive

Most light-duty vehicles on the road today are front-wheel drive.

In this configuration, only the two front wheels are driven, and as a result, they need a complex set of components (transaxle, CV joint, etc.) to achieve this task. Front-wheel-drive vehicles consolidate componentry and allow designers and engineers to use this extra space for additional passenger volume, making it a very popular choice for building everything from SUVs to compact sedans. Front-wheel-drive vehicles are also better in icy or snowy conditions because the weight of the engine is over the driven wheels. This provides more traction in tough conditions.

Rear-wheel drive

Heavy-duty vehicles like trucks or larger, body-on-frame SUVs as well as sports cars are often rear-wheel drive.

Rear-wheel drive powers the two rear wheels of a vehicle, leaving the front wheels to do the steering. This setup is common on trucks because trucks are designed to be weighed down in the rear with either payload or while towing a heavy load. This allows for better weight distribution and better overall handling dynamics. Because trucks are extremely light in the rear, having a rear-drive setup helps balance the weight more effectively. This same principle applies to sports cars that depend on weight distribution to handle tight corners at speed.

Four-wheel drive

Off-road vehicles such as Jeep Wranglers use selectable four-wheel-drive systems to get over rough terrain.

These systems allow the engine to drive all four wheels at the same time and at the same speed for maximum traction in the toughest scenarios. Four-wheel-drive systems also offer the ability to select two-wheel drive (usually the rear wheels) for better fuel economy.

In addition to two- or four-wheel drive, these systems allow drivers to pick from 4-HI and 4-LO modes. 4-HI is for use at higher speed (e.g., on snowy paved roads) while 4-LO is for low-speed crawling. A combination of front, rear and center differentials help four-wheel drive accomplish its job.

All-wheel drive

Not to be confused with four-wheel drive, all-wheel drive (AWD) is not selectable for different scenarios, nor does it always drive all four wheels at the same speed when engaged.

Instead, these systems drive all four wheels at the same time but move power around to wheels that need more in slippery conditions. All-wheel drive relies on sensors that indicate when a wheel is slipping and uses front, rear and center differentials to move the power around appropriately. Manufacturers like Subaru offer full-time AWD across their lineup, while other manufacturers like Honda offer part-time AWD.

Part-time systems are front-wheel-drive biased when the wheels are not slipping and can apportion engine power to the rear wheels when power is needed. Full-time all-wheel-drive systems use a 50:50 distribution bias until a wheel slips. Full-time setups use more fuel, but offer more stable handling and better off-road capability. Part-time systems enjoy better fuel economy and more compact dimensions.

Powertrain vs drivetrain

Drivetrains and powertrains are often misunderstood because many drivers believe that there is no difference between the two. In fact, many people often use the terms interdependently but the easiest way to remember the difference is to look at the names themselves.

Powertrains include all drivetrain components, as well as the engine itself. Since the engine produces power, the name “powertrain” was given to this series of components. If you have a powertrain warranty, all of these components are covered to some degree. On the other hand, drivetrains do not include the engine but include all other components to drive the vehicle forward.

Drivetrain maintenance

Much like the rest of your car, the drivetrain requires specific maintenance to stay in peak condition.

Transmission: Transmissions, whether automatic or manual, require regular maintenance. Common maintenance items include changing out transmission fluid and filters or lubricating the clutch on a regular basis.

Driveshaft: Although most driveshafts rarely give drivers problems, they can warp, fall victim to worn-out U-joints or slip splines or become unbalanced. If this happens, severe vibration can occur and may damage other components that are connected to the driveshaft. Regular maintenance and a keen awareness of vibrations will help ensure this issue is nipped in the bud.

Differential: One of the most overlooked maintenance items on a vehicle is the differential. Much like your engine, it requires regular maintenance to stay in peak condition. Vehicles with a transaxle do not require separate differential maintenance since the differential and transmission share the same fluid. However, vehicles with a rear differential or multiple differentials (AWD) may need more regular maintenance. Check with your owner’s manual for details.

Constant velocity (CV) joint: Generally, CV joints are built to last for the life of the vehicle. However, they can fail if a rubber component known as the CV boot is damaged by either age or road debris. Since the CV joint is a precision component, CV boots are designed to keep dirt and debris out of the CV joint for maximum performance and longevity. Once the boot is penetrated, dirt and other debris can damage the CV joint and cause it to fail.

Regular inspections of the CV boot will ensure that the much more expensive CV joint remains protected.