The Different Types of Drivetrain Systems

There are four main types of drivetrain systems available on the consumer market. But to be on a slightly more technical aspect there is actually a fifth type of drivetrain system. For those of you that are curious about the fifth system, it known as the IWD (individual-wheel drive). The IWD is somewhat recent in the automotive platform, but has been around for a number of years in the heavy equipment market. Unfortunately this article will not be discussing the IWD considering it is very rare in the general automotive market to this day. The four systems which will be included in this article is the RWD (rear-wheel drive), FWD (front-wheel drive), 4WD (four-wheel drive), and AWD (all-wheel drive). This article will also be explaining the basic mechanics of how each system works, what they can be most commonly found on, and the advantages and disadvantages of ownership. As for the engine configurations, this can vary between manufacturers, therefore I will be sticking strictly to the most common layouts in the diagrams. Drivetrain configurations will be covered in another article.

RWD (Rear-Wheel Drive)

The RWD system has been the starting platform when vehicles were first introduced into the production world. That does not mean it was the first type of drivetrain system invented with motorized vehicles though. Going back into the history books, there is plenty of personal made vehicles which were the basic definition of being a FWD. The RWD system in today's market is most commonly found on trucks, some larger vans, larger sedans, and some sport coupes. This can include the Chevrolet Silverado, Dodge Challenger, and BMW 3 series just to name a few.


Viewing the diagram above demonstrates the basic and most common layout for a RWD drivetrain. The engine and transmission is located at the front of the vehicle in an inline configuration with the chassis. The transmission is then followed by the driveshaft which transmits power to the differential at the rear of the vehicle. The differential distributes the power from the driveshaft in a perpendicular direction, which in turn supplies it to the rear wheels and propels the vehicle. The engine and transmission is not always located at the front of the vehicle, it some designs the vehicle could either be a mid or rear engine configuration. Not all mid-engine vehicles have a rear axle/differential assembly and driveshaft too. If the engine is closer to the rear wheels, they have a transaxle. The transaxle acts similar to a differential, but also takes in the mechanics of a generic transmission. It is able to covert the motion into a perpendicular direction, which is distributed by axles out to the wheels. In the end, a RWD vehicle always has the drive wheels at the rear of the vehicle. Examples of a mid-engine are the Porsche Boxster and Audi R8. Rear-engine examples would be the classic Volkswagen Beetle and Smart Fortwo.

Advantages
-more of an even wear between all four tires
-handling characteristics tend to be better, but this can be based on personal opinion or vehicle design
-improved weight distribution across the chassis depending on the design
-more room to service the drivetrain
-no torque steer

Disadvantages
-more moving parts than compared to a FWD system
-poor handling in wet or slippery driving conditions
-easier to get stuck in the snow
-increased weight
-decreased interior space due to the tunnel for the transmission and driveshaft (depending on the engine configuration)

FWD (Front-Wheel Drive)

Moving along to the FWD system, this as I mentioned above has been around for approximately the same amount of time as the RWD system. It wasn't something which was introduced in the mass automotive production, so it did take some time before it was available from manufacturers. In today's automotive market, FWD is the most common drivetrain system. FWD systems are found on different applications such as cars and family vans. Some examples of this include Dodge Caravan, Ford Taurus, Volvo V70, and Chevrolet Cobalt.


Shown in the diagram above, you can see that the drive wheels are located at the front of the vehicle. There is also no need for a driveshaft to transfer the power to the rear of the vehicle. The engine and transmission can be in either a transverse layout which is the most common or inline with the chassis. This inline layout can be found on some non AWD Subarus and also the Volkswagen Passat B5.5. Regardless of the configuration, the transmission delivers power to axles which then goes directly to the front wheels and puts the vehicle into motion. Unlike a RWD vehicle, the front wheels on a FWD do both the driving and turning. Vehicle manufactures tend to lend towards this route considering it is easier to increase interior space and installation is much easier. This is turn means a savings in production on the long run.

Advantages
-increased cabin space
-better handling in slippery conditions
-better handling in deeper snow
-reduced vehicle weight which can result in better fuel economy
-less moving parts in the drivetrain which can reduce costs on maintenance

Disadvantages
-poor weight distribution
-torque steer
-reduced quality in handling, but this can be based on personal opinion or vehicle design
-increase front tire wear
-there tends to be more strain on the front suspension and steering which can increase maintenance costs
-less room to service drivetrain

4WD (Four-Wheel Drive)

Not long after RWD and FWD platforms were invented, 4WD systems followed shortly after. It was intended for vehicles that could be used on all types of surfaces or terrains without getting stuck. This system is also noted for being much more robust than the AWD system which will be explained later on in this article. The great part about the 4WD system is that you have different modes which you can selected on how the vehicle operates. You are either able to turn the 4WD system off and only have one pair of wheels driving (two-wheel drive), you can engage the system so all wheels drive the vehicle for highway purposes (four-wheel drive high), or select a lower ranger which changes the output ratio of the one pair of wheels that enables the driver to cross over harsher terrain or increase pulling power (four-wheel drive low). Normally 4WD vehicles are based on a RWD platform. In today's market 4WD is commonly found on sport utility vehicles or trucks. Examples of this can include the Dodge Ram 1500 and Jeep Wrangler.


If you refer to the diagram above, you will see that the engine and transmission is located at the front of the vehicle. The transmission is then followed by a transfer case which is an assembly that allows the driver to select what mode the vehicle is controlled in and as mentioned above this consists of two-wheel drive, four-wheel drive high, or four-wheel drive low. Two driveshafts are outputted from the transfer case which provide power to both the front and rear wheels. There is a differential located at both the front and rear of the vehicle. In some vehicle designs the front differential can be in operation full time, therefore the front differential internals are in motion along with the driveshaft between the front differential and transfer case. If the transfer case is not engaged into a 4WD mode, no power is being supplied by the engine to the front wheels. Other designs, the front differential can be disconnect from the vehicle/turned off by electronic or manual locking hubs.

Advantages
-more robust system compared to AWD
-driver is able to select different driving modes (two-wheel drive, four-wheel drive high, four-wheel drive low)
-great for driving on all road conditions
-increased handling characteristics
-able to drive on a variety of terrains
-great for towing weight
-better weight distribution depending on design

Disadvantages
-higher fuel consumption
-more moving parts which can increase maintenance costs
-decreased interior space
-can have torque steering depending on the design
-increase vehicle weight

AWD (All-Wheel Drive)

AWD system shortly after followed. AWD is becoming more and more popular in today's market. It can be found on suvs, daily commuter cars and also higher performance vehicles. AWD is a similar system to 4WD, but it works on a full time basis and is either controlled by hydraulic or electronics rather than manually controller by the operator. Vehicles which are commonly found in today's market that are equipped with AWD is the Subaru Forester, Volvo XC70, and Audi A4.


When referring to the diagram above, this is found to be the most common configuration of the drivetrain. Besides having the engine located at the front, AWD systems can be found in mid-engine and rear-engine vehicles as well. Based on the engine being located at the front of the vehicle, the engine and transmission can be either in a transverse or inline layout similar to what was mentioned in the FWD section. Instead of only two wheels being used to propel the wheel, four are used instead. Even though four wheels are being used to propel the vehicle, not all AWD systems have an equal split of power to all four. In some applications, there can be a 60% power to the front and 40% to the rear, 70% power to the front and 30% to the rear, or other variations depending on the manufacturer. From the transmission, there are axles which drive the front wheels. As for the rear, a driveshaft disturbs the power similar to a RWD to the rear differential and axle assembly. Now in that path of the driveshaft, there will also be a differential that is able to control the amount of power delivered to the rear wheels. This can be controlled by either a computer which is commonly found on newer vehicles or hydraulically controlled like what was found on the earlier AWD systems.

Advantages
-lighter weight than compared to 4WD
-great for all road conditions
-increased handling characteristics
-great for launching in a racing application
-better weight distribution depending on design

Disadvantages
-tends to be harder on fuel
-more moving parts which can increase maintenance costs
-not as robust compared to 4WD
-cannot turn it off manually
-decreased cabin space
-torque steering