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Pascal Hayek
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THE BATTLE OF THE DRIVE-TRAIN SYSTEMS

FRONT WHEEL DRIVE vs. REAR WHEEL DRIVE vs. ALL WHEEL DRIVE

 

First let me explain what is a “drive-train”. It is the system that transmits the engine power to the wheels via the gearbox, differential and the axles.

 The first wheel drive system is as old as the first car itself. Over the years, as cars have evolved into more complex machines, so have wheel drive systems grown in sophistication to the point of being identified as an individual science in car technology. This has resulted in each unique type of wheel drive system being identified by a different name and classification based on its inherent characteristics.

Are you considering the type of drive-train for your incoming vehicle? Are you confused between choosing a front-wheel drive (FWD) or a rear-wheel drive(RWD) ? Or do you think you require an all-wheel drive (AWD)? Nowadays, the automotive marketplace offers a lot of drive-train choices that it could be quite confusing for buyers. Choosing which drive-train you will use is an essential decision when you are in the process of selecting another vehicle.

This article will explain the difference between the drive systems, their advantages and disadvantages. There are 3 different types of drive-train technologies and they all work differently. By initially knowing and understanding them, you could have a more guided decision on which drive-train you want for your incoming car.

Front Wheel Drive (FWD)

The majority of small and medium sized cars are equipped with FWD system. As of the early eighties FWD started gaining popularity and for more than 2 decades dominated the car market.

 In a front wheel drive car, all the power generated by the engine is used to drive only the two front wheels. The rear wheels act as dummies whereby their only purpose is to support the rear portion of the car and they just roll around to keep the car moving forward as the engine forces the front wheels to rotate. In a car with FWD, the entire drive mechanism including the engine and transmission systems is located to the front of the car.

Handling of FWD vehicles is generally considered stable, predictable, and good for winter weather, as the majority of the vehicle’s weight is located over the driven wheels, which increases traction. Front wheel drive is the cheapest option when buying a new car because it is the simplest and less cost expensive for manufacturers to produce; less moving parts means cheaper production. FWD yields excellent gas mileage because the entire drive-line aside from the motor is much lighter than in all wheel drive and rear wheel drive cars; so being lightweight is a good benefit for fuel economy. Reducing a car's weight benefits fuel economy and enables better braking.

 The other advantage that front wheel drive has is that it has less distance and fewer moving parts to transfer power to the front wheels.

There are disadvantages to FWD vehicles. One such disadvantage is that the design performs poorly in straight-line acceleration. If you're looking for a performance car you may want to stay clear of FWD powered vehicles.

 Front wheel drive cars under heavy acceleration shift all of their weight towards the back of the vehicle; when this happens it normally means a loss of traction and potential wheel hop from this inconsistent transfer of weight. Then there's the poor front drive fun factor; you can't simply slide the rear end of a FWD vehicle around a corner unless you're very talented with the E-brake handle, this can also be very dangerous and is not a wise decision. FWD puts the stress of steering, braking, and accelerating all on the front wheels, this basically means that there is a lot going on in the front making alignments and tire balancing crucial.
Also, some FWD vehicles exhibit “torque steer” (the steering wheel twists or resists the driver under hard acceleration).
 

Rear Wheel Drive (RWD)

 RWD was once the automobiles’ standard driveline. Since the invention of the automobile, RWD was dominant up till the early seventies.

A rear wheel drive system is the opposite of front wheel drive. In this type of drive system, the engine and transmission system drive the two rear wheels of the car. So the engine power is transmitted to the rear wheels via the gearbox, propeller shaft, differential and the axles. With RWD vehicles performance is greatly increased; now when all the weight shifts to the back of a RWD vehicle under heavy acceleration it's actually dynamically being transferred to the wheels that are supplying power for better traction.

 Rear wheel drive cars can be a lot of fun on twisty and winding roads, using the throttle one can control the sliding of the rear of the car with proper steering. There's nothing like fishtailing in a rear wheel drive car.

 In a RWD system, as some of the mechanical parts are removed from the front of the car and moved to the rear, the weight distribution is better between the two ends of the car. RWD vehicles are typically well-balanced and offer superior handling and braking for enthusiast drivers.

Actually, most luxury cars have rear wheel drive, as well as all race cars and most performance-oriented sports cars.

Some disadvantages of RWD are the fact that there is additional weight when compared to FWD, this tends to hinder fuel economy. In combination with the added weight of rear wheel drive there is also a loss of power through the rest of the drive-train since there are more components involved in transferring power to the rear axles. It is unlikely that you will ever see a rear wheel drive vehicle on a list of most fuel efficient cars. Also your choices for fuel efficient motors are fairly limited in a RWD configuration, most manufacturers of rear wheel drive drivelines place either V6, V8, or V12 power plants in RWD vehicles; so that's definitely going to be a problem when it comes to fuel mileage in your rear wheel drive car. Loss of traction and loss of control is very common on rear wheel drive cars because of the light weight nature of the rear of the car. 

All wheel drive (AWD)

All-wheel drive (AWD) – With all-wheel drive, one of the most sophisticated drivelines available today, the engine sends power to all four wheels. Advanced electronics, gears, or fluid-filled differentials can send power to all wheels equally, or transfer torque to the wheels (or wheel) with the most traction. Modern engineering has advanced AWD systems to the point where they are nearly seamless to the driver. In fact, AWD systems can be designed with a front- or rear-wheel bias, allowing engineers to improve driving dynamic. AWD systems offer vastly improved capabilities for winter driving, on wet slippery roads and off road driving.

The best traction in adverse weather conditions comes with all-wheel-drive vehicles. All-wheel-drive passenger cars and SUV’s in fact, handle better than many rear drive cars and are as safe and predictable during everyday driving situations as front wheel drive cars.

AWD use to be exclusive to vehicles that were off road, but all wheel drive has recently found its way into many performance cars such as the Porsche turbo, Subaru Impreza STI and Mitsubishi Lancer EVO. AWD seems to be best suited for performance.


 

 AWD does have quite a few disadvantages when it comes to your pocket book. With the added weight of an all wheel drive driveline and the loss of power from the engine while transferring energy to the AWD system fuel mileage is greatly affected. Another problem with all wheel drive is the cost of repairs over time. There are many moving parts in an AWD system that can go bad from all wheel drive center differentials to driveshaft universal joints. The cost of the all wheel drive parts may not be much more expensive when compared to other cars on the road, but the labor involved in repairing an all wheel drive car can become quite costly.

 

We cannot come across AWD without discussing 4WD and part time 4WD.

Part-time all-wheel drive (Part-time AWD) – Essentially a two-wheel-drive vehicle, this driveline configuration sends power to just two wheels (front or rear) until additional traction is needed. The system is designed to automatically transfer power to the other two wheels once the driven wheels slip, effectively becoming all-wheel drive for a limited period of time. Because some loss of traction must occur before the system intervenes, this type of driveline isn’t completely seamless to the driver. Very effective in preventing complete traction loss, this type of drive-train is not recommended for heavy off-road use.

Full-time four-wheel drive (4WD) – Operating much like all-wheel drive, this type of driveline is usually more robust because it is designed for serious off-road use. Engine torque is usually evenly distributed to each wheel on dry surfaces, and varies automatically according to conditions in low-traction situations. Like part-time four-wheel drive, these vehicles are equipped with a “low” range, and many feature locking differentials (the ability to make all wheels rotate at the same speed) for added traction. Extremely capable off-road and in winter weather, these full-time 4WD systems add weight, at the expense of fuel economy.

Part-time four-wheel drive (Part-time 4WD) – This drive-train configuration typically sends power to the rear wheels all the time. If traction is lost, power can be sent through an electric, mechanical, or hydraulic switching system to all four wheels. In many cases, driver intervention is required to change into 4WD. This drive-train is considered part-time because it often can’t be used on dry pavement for fear of damaging the mechanicals. Part-time four-wheel-drive vehicles are equipped with a low range for off-road use, and many have locking differentials for even more grip in extremely slippery conditions.

 

It is not fair to discuss various wheel drive systems without saying a word about the rear engine RWD and mid engine RWD systems, like the ones installed on sports cars and super sport cars. Porsche 911 is an example of a car with rear engine configuration. On the other hand Ferrari 458 Italia is equipped with a mid mounted engine.

In rear engine configuration, the engine is situated on top of the rear driven wheels. This improves traction when accelerating but the disadvantage lies in the weight distribution since the front end is very light compared to the rear, this affects the stability especially when cornering at high speed.

To conclude this article, nowadays with the presence of electronic stability systems and traction control being equipped on most of the new cars sold on the market, the handling, stability and braking are improved drastically. Recently, the market share of AWD cars and SUV’s (sport utility vehicles) has been increasing worldwide due to the bigger demand for safety.

 

Written by: Pascal Hayek

 

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