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The Drivetrain: Things You Need to Know

What's the deal with wheels? Why are there a front-wheel, rear-wheel, four-wheel and all-wheel drive choices when you purchase a vehicle? We've gathered information to help you understand drivetrains and help you make informed decisions on which one to avail yourself of.

When you are canvassing for the perfect car to buy, you would come across key technical terms such as transmission, gearbox, front-wheel drive, all-wheel drive and more, which will actually affect your decision to purchase. Knowing a few things about the car's systems and what stuff such as powertrain does will come in handy in your decision making.

Having a car doesn't end with purchasing and driving it, however. To be a really effective driver, you need to understand it – its systems, its parts and how they work together. This way, if you're suddenly caught in a bad situation wherein you don't have access to a mechanic or you are too far away from an authorized service center, you can somehow make quick fixes, or at least know what is the proper thing to do, such as are you going to turn off the engine, or keep it running until you get to a service center.

In this article, we're giving attention to your vehicle's drivetrain. Knowing what it is, what are its parts and the role it plays in the entire mechanics of your vehicle, will help you come up with informed decisions later and will guide you in how to keep it well maintained.

Powertrain versus Drivetrain

Powertrain

If we'll compare a car to a human being, the car's chassis would be the skeleton, while the powertrain would be the muscular system. Every component that converts the power of the engine into movement, from the engine down to the wheels, makes up the powertrain.

Drivetrain

If the powertrain encompasses all parts that power up the car, the drivetrain, as the name implies, cover all components that drive the wheels. Unlike the powertrain, it encompasses only the things that come after the engine, and it is still a subject of debate whether it includes the transmission or not. In a nutshell, the drivetrain is composed of all the bits and pieces in between the engine and the wheels that produce, develop or assist with the drive.

The drivetrain is the vehicle's system that takes power from the engine and sends it via the transmission system to the vehicle's four wheels. During this process, the drivetrain interacts with the engine, transmission, driveshaft, front and rear axles, and the four wheels of the vehicle.

Drivetrains vary because of the different needs to send power from the engine to the wheels.

Purpose of the Drivetrain

As previously discussed, the drivetrain is responsible for transferring torque from the engine to the wheels for the vehicle to move.

For some drivers, there may had been times when you've wondered whether your vehicle could handle a certain type of road or terrain. Subconsciously for sure, you knew the answer to that question, but you may have pushed or stretched your vehicle to go through it anyway, especially if there was no other choice, and later on regret having to pay for damage repair fees.

Your vehicle's drivetrain will confirm the kinds of terrain your vehicle is allowed to navigate.

Parts of a Drivetrain

Before discussing the different types of drivetrain, it is best to familiarize yourself first with its parts in order to understand it better.

Driveshaft. This is the long steel tube linking the vehicle's transmission and the wheels. It transfers mechanical power from the transmission to the other parts of the vehicle.

U-Joint (Universal Joint). This is a pivot point allowing the driveshaft to move at varying angles. It is flexible and helps transmit power.

CV Joints (Constant-Velocity Joints). These are flexible parts of the driveshaft designed to bend in any direction while it turns the wheels at a constant speed.

Differential. This is the power's last stop before it spins the wheels.

Axle Shafts. These compose a single rotating shaft, found on either side of the differential. They supply power from the final drive assembly to the drive wheels.

Types of Drivetrain

Drivetrains commonly come in two different types: the two-wheel drive and the four-wheel drive. A two-wheel drive can either be a rear-wheel drive (RWD) or a front-wheel drive (FWD. The four-wheel drive, on the other hand, comes either as a standard four-wheel drive (4WD) or an all-wheel drive (AWD).

The Rear-Wheel Drive

Vehicles with an RWD system send all the power to the rear wheels. Such a system is best with speed and performance such as in in sports cars; however, fuel economy and traction are greatly reduced with this drivetrain.

The rear-wheel drivetrain has been used the longest and is being used today by many cars and trucks.

Advantages of a Rear-Wheel Drivetrain

This drivetrain provides plenty of advantages over the FWD. One, weight becomes more equally distributed to each tire, allowing for better steering and handling. Next, RWD vehicles offer better braking performance than FWD vehicles. Most importantly, RWD vehicles offer better handling and acceleration because you can split the jobs of steering and driving the vehicle. The back wheels are the only ones moving the car in a rear-wheel drivetrain. Whereas, in a front-wheel drivetrain, you need to both move the car forward or backward and steer it left or right.

A car with a RWD is also less prone to wear and tear of tires and their parts.

The Front-Wheel Drive

Full power is directed to the front wheels in vehicles with an FWD system with the rear wheels essentially being just pulled along. This is most common in sedans and hatchbacks, and cars that aren't built for specific purposes and only for normal driving. Even minivans have an FWD arrangement.

Advantages of a Front-Wheel Drive

Unlike in an RWD, there's no need for the long drive shaft that runs the length of the car to transfer torque to move the wheels of an FWD vehicle. All of the drivetrain's essential components, such as the transmission, differential and drive shafts, are found in the front of the car. And because all the parts of an FWD are in front, the vehicles can be made smaller and lighter. If the FWD cars were made bigger, then there would be more than ample room for passengers.

Because there's more weight at the front of the vehicle as a result of the drivetrain components being in front, FWD vehicles offer better traction especially on slippery surfaces such as in snow, ice and rain much better than the RWD. But this is at lower speeds, as the RWD provides better traction at higher speeds.

More importantly, FWDs are fuel efficient, thus saving you money and they help promote a cleaner and greener environment.

The Four-Wheel Drive

Power is distributed evenly among all the four wheels of a traditional 4WD vehicle. This drivetrain allows for great traction in almost all conditions, especially for off-roads.

The All-Wheel Drive

In an AWD vehicle, power goes also to all of the four wheels but instead of being evenly distributed, the system determines the amount of power to be sent to the front and the back. It is like having both FWD and RWD that can be engaged anytime. The AWD provides the best grip in any road condition and is ideal in snowy situations.

Differences Between a 4WD and an AWD

Deciding to go for a 4WD or an AWD can be a little difficult, so let's differentiate these two four-wheel drivetrains a bit more.

Typically, larger SUVs referred to as 4x4 vehicles have 4WD drivetrains that can be manually selected as they need the extra traction to tackle off-roads. These kinds of vehicles also usually have larger wheels and larger, thicker tires. In traditional 4WDs, the front wheels and rear wheels lock together in situations where you're stuck, such as in a deep mud or snow, so that your vehicle will be able to move forward. It also helps when navigating through difficult terrain such as rocky or rough roads, or steep climbs or descents. There's a direct link from the front to the rear axles in a 4WD but there isn't any mechanism to set any rotational difference in both axles. So, when a 4WD car turns around the bend at a low speed or on a no-slip-possible road surface, the axles, shaft and other related parts of the car will twist and there would be what is referred to as a "wind up". Hence, when cruising on a normal terrain, you must not use all four, but only two wheels, to avoid what is referred to as a "wind up".

Meanwhile, there's no button to press in most modern AWD systems. Commonly seen in small or compact SUVs or crossovers, they're primarily FWD, with power going to the front wheels, but when the vehicle senses that you're losing traction, it transfers power to the rear wheels. Unlike with 4WD, you don't have to turn anything manually on. And unlike 4WDs, the wheels of the vehicle do not lock together. Since all the wheels are always active, the AWD system includes a limited slip differential mechanism that allows rotational difference between the front and rear axles.

True AWD cars are meant for normal driving with a little bit of off-road adventure from time to time. Since they usually have permanently engaged AWD systems, they have twice the grip of a 4WD system. This means that when the car turns at a slippery-than-expected corner, or when instant traction is needed to move safely into a merging traffic, the AWD is already engaged and the required level of traction is available to safely guide the driver through the situation. The driver does not have to do any manual selection to use all the cars' four wheels, as twice the level of traction is always accessible – even in just a split second – to get out of a challenging driving situation.

Vehicles with a drivetrain system that normally runs in 2WD and engages only the 4WD during loss of traction, or one that requires the driver to select 4WD, aren't true AWD vehicles. In such a system, traction is already lost by the time 4WD is engaged and there's a possibility that a potentially dangerous situation has already occurred. A true AWD drives all the wheels at all times.

Drivetrains in Hybrid Electric Cars

In this age and time, hybrid and electric have been key words in automobiles, and could possibly be the future of transport. It is best to have some know-how of these vehicles' drivetrains which vary completely from fuel-powered vehicles. The design of a hybrid-electric car's drivetrain determines how its electric motor works in tandem with the traditional engine. The car's fuel consumption, mechanical efficiency and price all depend on its drivetrain.

The drivetrains available in a hybrid-electric vehicle are the series drivetrain and the parallel drivetrain as well as a combination of both. These drivetrains enable the conventional engine and the electric motor to deliver power either independently or in conjunction with one another.

In a series drivetrain, mechanical power is received only from the electric motor that is run by a battery or by a gasoline-powered generator. In a parallel drivetrain, on the other hand, power is provided by both the electric motor and internal combustion engine at the same time.

The Series Drivetrain

In this simplest hybrid drivetrain configuration, the electric motor is the sole provider of power to the wheels. The electric motor receives electric power from either the car's battery pack or from a gasoline-engine-run generator and transfers this power to the wheels. While there are some hybrids in which power is sent to the front wheels, hybrid and electric vehicle manufacturers are considering AWD as a better fit for such cars. The amount of power coming from the battery or gasoline-engine generator is determined by a computer. The battery pack is recharged by both the engine generator and by means of a regenerative-braking technology.

Hybrids with series drivetrains are exceptional in stop-and-go traffic situations. The built-in computer can opt to power the motor with only the battery pack thereby saving the gasoline-engine run generator for more appropriate situations where such power could be more efficient.

Since hybrid vehicle with this drivetrain has to meet only certain power demands, the engine is typically smaller, yet the battery pack is often more powerful than that in a parallel drivetrain. Vehicles with a series drivetrain, having a larger battery and bigger motor on top of the generator, are however pricier than those with a parallel drivetrain.

The Parallel Drivetrain

In a parallel drivetrain, the vehicle's engine and electric motor work together to generate the power that drives the wheels. Hybrids with this drivetrain has a smaller battery pack than those with a series drivetrain as they rely more on regenerative braking to keep the battery recharged. They however still use the motor as a generator for supplemental recharging in situations where power demand is low, similar to an alternator in a typical fuel-run car.

In parallel drivetrain vehicles, the engine is joined directly to the wheels thus increasing the efficiency of these hybrid cars on the highway.

The Series/Parallel Drivetrain

Hybrid cars that have a series/parallel drivetrain benefit from the advantages, and suffer the same complications, of both the series and parallel drivetrains. The engine can drive the wheels directly as a parallel drivetrain does, but at the same time be effectually disconnected, with power coming only from the electric motor as in a series drivetrain. A popular example of a vehicle with series/parallel drivetrain is the Toyota Prius.

Having gas-only and electric-only options, the engine is quite efficient. When driven at a lower speed, it operates more like a series vehicle, but when driven at high speed, the engine takes over minimizing the loss of energy.

This dual drivetrain arrangement requires a much larger battery pack, a generator and extra computing power to be able to run effectively, hence it is the most expensive among the three drivetrains.

Conclusion

No matter where you are and when, safety should always be a priority. Forcing your vehicle on terrains which it isn't equipped to tackle could be dangerous and potentially life threatening to both driver and passengers. Knowing your vehicle's drivetrain or purchasing the vehicle with a drivetrain appropriate for your needs is key to a safe, effective driving. If you are an adventurer or work in a high-risk environment such as hilly or mountainous areas, a 4WD might be the best for you. However, AWDs provide better maneuvering on slippery roads and is more effective when turning, so if you drive zigzag road often or the road is often slippery especially when it rains, consider spending on an AWD vehicle instead. FWD is enough for city driving, especially if you're using the car to simply drive back and forth from home to work. If performance is important to you and you like speeding on highways, on allowed maximum speed of course, then an RWD may best suit you. No matter which drivetrain you pick, always be a responsible driver, who values life and safety foremost than looking good and cool on the road.