Drivetrains ...

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Drivetrain Layout

Introduction
When I open the front hood of my Mazda Protege sedan, I find its engine and many other goodies. Same for my S2000. I also saw many people opening their front hood to inspect the engine. This gave me an impression that all cars have engine stored under their front hood. I used to think this is the truth. But after going to my first car show, I realized that I was like a toad that was living in the bottom of a well.

I am always an admirer of Porsche. They made a lot of uncompromising performance car while maintain the exquisite styling. [Lately they disappointed me with the introduction of automatic 911 Turbo and Porsche SUV(!)] So naturally the first car I wanted to check out at the car show is the legendary Porsche 911 Turbo. After watching it inside out, I lifted the front hood to look for the engine that powers this Godly machine. Heck! There was nothing there, under the hood it is like a front trunk for people to store things. I found the rear trunk too but still no engine. I asked people around and finally someone answered me.

That person says the 911 Turbo is a rear-engine car. The engine is mounted after the rear driving axle. So it is located under the rear trunk. I finally find the engine by unlatching a lock. It is under the smallish rear trunk. Well, why do Porsche put their engines at the back of a car?

There was another thing about cars that bugged me. I am an avid player of the Gran Turismo racing game. Initially, I was baffled by the many designations like FF, FR, MR, RR and so on. I later learned that the first letter of the designation is used to designate the engine location and the second letter is to designate the which pair of wheels the engine drives.

Drivetrain Layout
The car body, the front panel, the rear trunk, the seats and other amenities that come with the car are just distractions to a performance enthusiast. What makes a car move are its engine, its transmission and its wheels. Collectively speaking, these three components comprise the drivetrain.

Engine Positioning
There are two major characteristics of a drivetrain that impacts the performance of a car: the engine placement and the driving wheels location. The engine placement is a big factor to determine the moment of inertia and the weight distribution of car because many other mechanical/electrical components of a car are usually located close to the engine. The driving wheels location determines which wheels the transmission to send the engine-generated torque to. Due to weight transfer, you will soon find out that driving wheels location is a very important factor in car handling.

For simplicity purposes, car nuts classified different engine placements into three types: front-engined, mid-engined and rear-engined. Front-engined cars have their engine placed in front of the passenger seats. Mid-engined cars have their engine placed behind the passenger seats but in front of the rear driving axle. Rear-engined cars have their engine placed behind the rear driving axle. There is also a subclass of front-engined cars called front-mid-engined. In this subclass, the engine is behind the front drving axle but in front of the passenger seats. An example is the Honda S2000.

Driving Wheels
As most people know, the engine isn't necessarily driving all the four wheels. There are actually four types of driving wheels. The first is All Wheel Drive (AWD) in which all four wheels are driven by the engine all the time. Then there is Front Wheel Drive (FWD) in which only the front wheels are driven by the engine. It then follows by Rear Wheel Drive (RWD) in which only the rear wheels are driven. The final one is an odd one. It is called Four Wheel Drive (4WD). It is usually employed by Sports Utility Vehicles (SUV). There is a switch in the car that allows you switch between AWD and FWD mode. Since 4WD doesn't really introduce anything new, so by reading materials related to AWD and FWD should help you completely understand 4WD.

Some common Drivetrain Layouts
In this section, I will give you examples of cars that represents the seven common drivetrain layouts. There will also be some simple description of their handling and acceleration characteristics. Acceleration characteristic will be explained in details at the bottom of this page. However, to understand the handling characteristics, you need to understanding the concepts described in the Cornering Chapter.

Front Engine, Front Wheel Drive (FF)
This is most common drivetrain layout. It is used for all the low cost economy car like Toyota Camry/Corolla, Honda Civic/Accord, Mazda Protege/Millenium, etc.
FF cars are more front heavy. It can counteract with the understeer characteristics exhibited by front wheel drive cars. The overall effect is that it has slight understeer in all acceleration situations. This actually makes the car more stable in city driving.

The main reason for building FF cars is that it is cheaper to build. Steering, engine, transmission, wheels and so on are all very close by, there is no need to build long axles to transmit the engine power to the other end of the car.
Front Egnine, Rear Wheel Drive (FR)

This is most common drivetrain layout for luxury sedans and low end sports car. Examples are Mercedes sedans, BMW sedans, Mazda Miata, Honda S2000.

One of the characteristics of these cars is that they usually have a almost neutral weight distribution due to the driving axle that traverses from the front to the rear.
Since the weight distribution is neutral, to attain higher acceleration potential, the car needs to be RWD or AWD (see the bottom for math-oriented people). For not powerful enough engines in passenger cars, RWD is good enough to exploit the potential.

RWD cars exhibit oversteer under mild acceleration and understeer under heavey acceleration. The oversteer characteristic allows an RWD car to accelerate after exits from the apex and hence attain higher speed when it enters the straight. For details, please refer to the Understeer and Oversteer section and the Cornering Line section.
Front-mid-engined car like S2000 can turn faster than normal FR cars because it has a smaller moment of inertia.

Mid Engine, Rear Wheel Drive (MR)
This drivetrain layout is usually employed by high end sports car and most of the formula one race cars. Notable examples are Porsche Boxster, Ferrari Modena.

Mid-engined is the configuration that has the lowest moment of inertia and hence it turns the fastest.
Weight Distribution is a little bit biased to the rear and hence more prone to oversteer under mild acceleration.

Rear Engine, Rear Wheel Drive (RR)
This is one of the rare drivetrain layouts. Notable examples are Porsche 911 Carrera and the original Volkswagen Beetle.
Rear-engined cars are similar to mid-engined cars but they have higher moment of inertia and are even more prone to oversteer under mild acceleration.

Front Engine, All Wheel Drive (4WD / AWD / FA)
There are two types of cars that employ this drivetrain layout. The first type includes cars that want to provide traction on all four tires such that you can move the car around in snow or unfavorable terrain. Examples are Subarus, Audis, BMW 330xi. The second type includes high power sports car. Examples are Nissan Skyline GTR, Mistubishi Lancer Evolution and so on.

For high power sports cars, the reason for AWD is to exploit all the traction of the four tires to attain the greatest acceleration possible (see bottom for the gory math details).
Most AWD cars are rear biased in which they allocate more torque to the rear than the front. Therefore they all have mild oversteer under mild acceleration.

Mid Engine, All Wheel Drive (MA)
MA car was built in the same spirit as FA cars but the mid engine configuration reduces moment of inertia and hence makes the car turn more quickly. An example is Lamborghini Murcielago.

Rear Engine, All Wheel Drive (RA)
RA cars are an extension of RR cars. They take advantage of the AWD to exploit full acceleration potential. An example is Porsche 911 Carrera 4.

Quantitative Effects of Drivetrain Layout during acceleration
Here we would like to use a simple example to quantify the effects of different driving wheels. We will still use our Skyline GT-R as an example. This time in addition to the default all wheel drive (AWD) setting, we will also study the cases when it is front wheel drive (FWD) and rear wheel drive (RWD). To compare, we will check to see what is the maximum force that can be applied to the ground in each case.

The Math
First, let's assume for a while that our Skyline is FWD. For this kind of car, the maximum force that can be applied to the ground is limited by the limiting friction on the front tires. From the Weight Transfer section, we know that

Wf* = (mgLr - hFf - hFr) / (Lf + Lr)

Since this is FWD, Fr is 0. Note that the maximum of Ff is μWf*. Beyond that, the front tires will slip and hence no force can be transmitted to the ground. By substituting Wf* with Ff/μ, we can solve for maximum Ff:

Ff/μ = (mgLr - hFf) / (Lf + Lr)
Ff = μmgLr / (Lf + Lr + μh)
= 0.94×1550×9.8×1.519 / (1.146 + 1.519 + 0.94×0.34)
= 7267.04N

Next we consider the case when it is RWD. In this case, the maximum force is limited by the limited friction on the rear tires. Again, from the Weight Transfer section, we know that

Wr* = (mgLf + hFf + hFr) / (Lf + Lr)

This time, Ff is 0. The maximum force will be limited by μFr. Beyond that, no force can be transmitted to the ground. Again, by subsituting Wr* with μFr, we can solve for maximum Fr:

Fr/μ = (mgLf + hFr) / (Lf + Lr)
Fr = μmgLf / (Lf + Lr - μh)
= 0.94×1550×9.8×1.146 / (1.146 + 1.519 - 0.94×0.34)
= 6976.75N

For the AWD case, thanks to all the all powerful technology called Limited Slip Differential (LSD), the math is way simpler. LSD can distribute the torque generated by the engine to not yet slipped tires. So during acceleration, when the rear tires start to slip, it can transfer the torque to the front tires that still has traction. Therefore, the car basically won't slip until all four tires slip. To make all four tires slip, you have to apply a force greater than μWf* + μWr* = μmg = 0.94×1550×9.8 = 14278.6N.

Conclusion
As we can see AWD allows the most torque to be transferred to the ground. Then what about RWD and FWD? In our example, FWD allows for great acceleration. But if you look at the formulas more carefully, you can discover that there is a condition for this to happen. Suppose we want to know what is the condition such that RWD works better than FWD, we look at the condition Fr >= Ff:

μmgLf μmgLr
-------------------- >= -------------------
Lf + Lr - μh Lf + Lr + μh
μh >= Lr - Lf

Note that Lr and Lf is determined by the weight distribution of the car. The higher the weight is at the front, the longer Lr is and the shorter Lf is (Note that Lf+Lr is a constant). The converse holds true for rear-heavy cars. For a perfectly balanced (ie 50:50) car, Lr equals Lf and hence this condition holds. But as we can see if the front is heavy enough such that Lr is way longer than Lf, then FWD will prevail. Now we can check whether AWD is the correct configuration for our Skyline GT-R. Recall that our car gives out 392.27Nm of torque at 4400rpm. When it is transmitted to the rear tires at the first gear, it is Γ×g1×G/r = 392.27×3.827×3.545/0.3266 = 16,294.6N. In reality, there should be about 20% loss, so only 13,035.68N can be transmitted to the tires. Note that this is beyond what an RWD GT-R can handle but below what an AWD GT-R can handle. Therefore we can conclude that the engineers at Nissan did the right thing to make GT-R an AWD car.

After all these analysis, a question naturally arises: so if AWD is better than FWD and RWD for powerful engines, why the race cars still use MR configuration? The reason is that installing an AWD system put more weight on the car. For an example, compare the curb weight of BMW 325i and 325xi sedans: 1,463kg vs 1,573kg. There is about 110kg difference. For BMW 325 sedan, there is an increase of 7.5% in weight and hence 7% reduction in acceleration. However, for a 500kg race car, the weight increases by 22% and the acceleration decreases by 18%!

 

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In Automobile design, an FR, or Front-engine, Rear wheel drive means a layout where the engine is in the front of the vehicle and drive wheels at the rear. This was the traditional automobile layout for most of the 20th century.

Overview
This layout is typically chosen for its simple design and weight distribution. Placing the engine at the front gives the vehicle a traditional long hood (in British English "bonnet") and engine cooling is simple to arrange. Placing the drive wheels at the rear allows ample room for the transmission in the center of the vehicle and avoids the mechanical problems of transmitting drive to steered wheels. The layout is still more suitable than front-wheel drive for engine outputs of more than about 150bhp, as the weight transference during acceleration loads the rear wheels and increases grip.

History
The first FR car was an 1895 Panhard model, so this layout was known as the "Systeme Panhard" in the early years. Most American cars used the FR layout until the 1980s, exemplified by the Ford Mustang, Chevrolet Corvette, and vehicles from BMW and Mercedes-Benz. The Oil crisis of the 1970s and the success of small FF cars like the Mini, Volkswagen Rabbit, and Honda Civic led to the widespread adoption of that layout.

Some manufacturers, such as Volvo, retained this layout but moved the gearbox from behind the engine to between the rear wheels, putting more weight over the driven axle. This configuration is often referred to as a transaxle since the transmission and axle are one unit.

Right after the Arab Oil Embargo of 1973 and the 1979 fuel crises, a majority of American FR vehicles (station wagons, luxury sedans) were being phased out for the FF layout - this trend would spawn the SUV/van conversion market. Chrysler went 100% FF by 1990; only GM and Ford retained FR for their luxury and performance vehicles.

GM phased out its FR luxury cars after the 1996 model year, and its F-car (Chevrolet Camaro/Pontiac Firebird) in 2002. GM reintroduced North American FR luxury cars with the 2003 Cadillac CTS. Currently they produce the Pontiac GTO (imported from Australia), Chevrolet Corvette/Cadillac XLR and the Cadillac CTS/STS.

Today, most cars are FF, though the limitations of that layout, such as poor traction under acceleration and excessive nose weight, are beginning to become apparent. Many of the newest models have adopted all wheel drive, and some, like the Chrysler 300 are switching back to the FR layout. Most sports cars and luxury cars have always been FR.

Recently, a new layout called FM for Front-midships has appeared. An FM car places the engine in the front like an FR car, but pushed back enough that the engine's center of gravity is to the rear of the front axle. This aids in weight distribution and handling. Typical FM cars are high performance models such as the Nissan 350Z, Honda S2000, Ferrari 612 Scaglietti, Maserati Quattroporte, Mazda RX-7, Mazda RX-8, and the Mercedes-Benz SLR McLaren.

 

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Front wheel drive is the most common form of engine/transmission layout used in modern automobiles, where the engine drives the front wheels. Most front wheel drive vehicles today feature transverse engine mounting, though many in prior decades were positioned longitudinally instead. Rear wheel drive was the traditional standard while four wheel drive is also sometimes used.

History
Experiments with front wheel drive date to the early days of the automobile. Alvis Cars of the United Kingdom introduced a front wheel drive model in 1928, but it was not a success. The first successful models were the 1934 Traction Avant cars from Citroën in Europe, and the contemporary 810 from Cord Automobile in the United States.

Front wheel drive was further advanced by the now-legendary Mini of 1959 (even if Saab 92 predates it) although its mechanical layout differed from modern vehicles, since the transmission was built into the sump of the engine, and drive was transferred to it via a set of primary gears. This concept was also used by Peugeot and Renault on their jointly-developed "Douvrin" engine of the 1970s. The tendency of this layout to generate unwanted gearbox "whine" has seen it fall out of favour, except at Saab. The first-generation Saab 900 had this layout with helical gears but the thrust bearings involved gave out a characteristic "zing" sound almost like a hairdryer.

Audi pioneered the "U-Drive" layout where the engine is mounted longitudinally, in an "overhung" position over the front wheels. Audi is one of the few manufacturers which still uses this particular configuration. It allows the use of equal-length half shafts and the easy addition of all wheel drive. The Subaru all-wheel-drive layout is simlar but with the gearbox built into the sump of the engine.

The Oldsmobile Toronado, which reintroduced front-drive to the U.S. market in 1966, also used a longitudinal engine placement for its V-8, coupled with an unusual "split" transmission, which turned the engine power 90 degrees. The driveline was set well behind the centerpoint of the wheels, however, for better weight distribution. (Consequently, it was sometimes called the Mafia car, because it was "all hood").

The Cadillac Eldorado, with front wheel drive introduced in 1967, holds the record for the largest engine in a front wheel drive production vehicle, at 8.2 liters (The 1970 edition). The Cadillac Eldorado continued to be the only front wheel drive offering from Cadillac until 1980, when the front wheel drive Cadillac Seville was introduced. Reasons cited for the enormous engine are smoothness due to the inertia of the engine soaking up much of the vibration caused by low milling tolerances of the era, resulting in a very smooth ride.

The changeover of cars in the US to front wheel drive began in earnest in the 1980s, with the introduction of the compact Chevrolet Citation probably marking the real beginning, and the 1986 introduction of the Ford Taurus bringing front wheel drive to prominence in mid-size cars. By the mid 1980s most formerly rear wheel drive Japanese models were front wheel drive, and by the mid 1990s most American brands only sold a handful of rear wheel drive models. Some suggest that the introduction of the Volkswagen Rabbit in 1974 led to Detroit's Big Three to market the FF layout; Chrysler's vehicle lineup was 100% FF by decade's end. GM followed Chrysler in the mid-1990s when their vehicle lineup was FF (with the exception of the Corvette and F-body lineup).

The vast majority of front wheel drive vehicles today use the transversely mounted engine with "end-on" mounted transmission, driving the front wheels via driveshafts linked via Constant Velocity (CV) joints. This configuration was made popular by the Fiat 128 of 1969. The 1959 Mini, while a pioneering transverse front wheel drive vehicle, used a substantially different arrangement with the transmission in the sump of the engine.

Disadvantages of front wheel drive
The center of gravity of the vehicle is typically further forward than a comparable rear-wheel drive layout. In front wheel drive cars, the front axle typically supports around 2/3rd of the weight of the car (quite far off the "ideal" 50/50 weight distribution). This is a contributing factor towards the tendency of front wheel drive cars to understeer.
Torque steer can be a problem on front wheel drive cars with higher torque motors( > 210 Nm ). This is the name given to the tendency for some front wheel drive cars to pull to the left or right under hard acceleration. It is a result of the offset between the point about which the wheel steers (which falls at a point which is aligned with the points at which the wheel is connected to the steering mechanisms) and the centroid of its contact patch. The tractive force acts through the centroid of the contact patch, and the offset of the steering point means that a turning moment about the axis of steering is generated. In an ideal situation, the left and right wheels would generate equal and opposite moments, cancelling each other out, however in reality this is less likely to happen. Torque Steer is often incorrectly attributed to differing rates of twist along the lengths of unequal front drive shafts.
In some towing situations front wheel drive cars can be at a traction disadvantage since there will be less weight on the driving wheels. Because of this, the weight that the vehicle is rated to safely tow is likely to be less than that of a rear wheel drive or four wheel drive vehicle of the same size and power.
Due to geometry and packaging constraints, the constant-velocity joints attached to the wheel hub have a tendency to wear out much earlier than their rear wheel drive counterparts. The significantly shorter drive axles on a front wheel drive car causes the joint to flex through a much wider degree of motion, compounded by additional stress and angles of steering, while the CV joints of a rear wheel drive car regularly see angles and wear of less than half that of front wheel drive vehicles.
The driveshafts limit the amount by which the front wheels can turn, thus increasing the turning circle of a front wheel drive car compared to a rear wheel drive one with the same wheelbase.

 

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In Automobile design, an RR, or Rear-engine, Rear wheel drive, layout places both the engine and drive wheels at the rear of the vehicle. In contrast to the MR layout, however, the center of gravity of the engine is actually past the rear axle.

This layout is typically chosen for a combination of several reasons. For optimal handling and to eliminate the phenomenon known as torque steer, the wheels which propel the car should not be the same ones that steer it. For optimum traction, the engine should be nearest to the driven wheels since the engine is typically the densest/heaviest component of the car. Thus, in a car which steers with the front wheels, it is better for the engine to be located in the rear of the car - either a RR or MR design.

The disadvantage of the RR configuration is that placing the engine outside the wheelbase creates significant problems for handling as, when the car begins to slide on a corner, the end of the car will tend to want to swing wide and overtake the front - especially under braking. This tendency is referred to as oversteer and creates potential safety issues in racing applications as well as for ordinary drivers on wet or icy roads.

In addition, even though the rear wheels benefit from the additional traction the added weight of the engine gives, the front wheels still need traction in order to steer the car effectively. For this reason, a RR layout car can also be prone to understeer. Most manufacturers have abandoned the RR layout apart from Porsche who has gradually developed their design with improvements to the suspension as well as electronic aids to reduce the shortcomings of the layout to acceptable levels.

Another manufacturer to implement the RR configuration was the De Lorean Motor Company with its DMC-12 sports car. To compensate for the uneven (35/65) weight distribution caused by the rear-mounted engine, De Lorean used rear wheels with a diameter slightly greater than the front wheels.

Early cars using the RR layout include the Tucker, the Volkswagen Beetle, the Porsche 911 and the innovative Chevrolet Corvair.

 

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Four wheel drive, 4WD, 4x4 ("four by four"), all wheel drive, and AWD are terms used to describe a four-wheeled vehicle with a drivetrain that allows all four wheels to receive power from the engine simultaneously. While many people think exclusively of off-road vehicles, powering all four wheels provides better control on slick ice and is an important part of rally racing on mostly-paved roads.

Four wheel drive (4WD or 4x4 for short) was the original term, often used to describe truck-like vehicles that required the driver to manually switch between a two wheel drive mode for streets and a four wheel drive mode for low traction conditions such as ice, mud, or loose gravel. The "all wheel drive" term (AWD for short) was invented to distinguish vehicles that are capable of driving all four wheels on normal roads without causing poor control and excessive tire and drivetrain wear. The AWD term is now being used to market vehicles which do not continuously drive all four wheels, but instead switch from two wheel drive to four wheel drive automatically as needed. The terms are thus quite vague in modern usage, with AWD being used to describe vehicles with a wide variety of very different drivetrains.

The buyer must be wary. It is common for identical drivetrain systems to be marketed under different names for upmarket and downmarket branding, and also common for very different drivetrain systems to be marketed under the same name for brand uniformity. For example, both Quattro and 4motion can mean either an automatically engaging system with a Haldex clutch or a continuously operating system with a Torsen differential.

Design
When powering two wheels simultaneously, something must be done to allow the wheels to rotate at different speeds as the vehicle goes around curves. When driving all four wheels, the problem is much worse. A design that fails to account for this will cause the vehicle to handle poorly on turns, fighting the driver as the tires slip and skid from the mismatched speeds.

A differential allows one input shaft to drive two output shafts with different speeds. The differential distributes torque (angular force) evenly, while distributing angular velocity (turning speed) such that the average for the two output shafts is equal to that of the input shaft. Each powered axle requies a differential to distribute power between the left and right sides. If all four wheels are to be driven, a third differential can be used to distribute power between the front and rear axles.

Such a design would handle very well. It distributes power evenly and smoothly, making it unlikely to start slipping. Once it does slip though, recovery will be difficult. Suppose that the left front wheel (of a design that drives all four wheels) slips. Because of the way a differential works, the slipping wheel will spin twice as fast as desired while the wheel on the other side stops moving. (the average speed remains unchanged, and neither wheel gets any torque) Since this example is a vehicle that drives all four wheels, a similar problem occurs between the front and rear axles via the center differential. The average speed between front and rear will not change, torque will be matched, torque goes to zero, speed at the rear goes to zero, and the speed at the front goes to double what it should be... making the left front wheel actually turn four times as fast as it should be turning. This problem can happen in both 2WD and 4WD vehicles, whenever a driven wheel is placed on a patch of slick ice or raised off the ground. The simplistic design works acceptably well for a 2WD vehicle. Since a 4WD is twice as likely to have a driven wheel on an icy patch, the simplistic design is usually considered unacceptable.

Traction control was invented to solve this problem for 2WD vehicles. When one wheel spins out of control, the brake can be automatically applied to that wheel. The torque will then be matched, causing power to be divided between the pavement (for the non-slipping wheel) and the brake. This is effective, though it does cause brake wear and a sudden jolt that can make handling less predictable. By extending traction control to act on all four wheels, the simple 4WD vehicle design based on three differentials can now recover from wheel spin. One nice feature of this design, is that it is traction control, and thus will not work against traction control. This design is commonly seen on car-like luxury SUVs.

Another way to solve the problem is to temporarily lock together the differential's output shafts, usually just for the center differential that distributes power between front and rear. Recall that a drivetrain without differentials will fight the driver, causing tire wear and handling problems. This is of little concern when the wheels are already slipping. One very common design joins the output shafts together via a multi-plate clutch under computer control. This design causes a small jolt when it activates, which can disturb the driver or cause more wheels to lose traction. Another common design uses a viscous coupling unit. A dilatant fluid inside the viscous coupling unit acts like a solid when under shear stress caused by high shaft speed differences, causing the two shafts to become connected. This design suffers from fluid degradation with age and exponential locking (joining) behavior. It can also waste fuel, because it requires that there be a slight shaft speed difference under normal driving conditions (via gearing) to prepare to fluid for operation. Older designs used manually operated locking devices.

Yet another way to solve the problem is via a Torsen differential. When a normal differential is replaced with a Torsen differential, it is possible to drive the output shafts with different amounts of torque. While this is useless in a zero-torque situation, it will help greatly when the slippage is not so extreme. As the slipping side begins to spin out of control, more power is delivered to the other side. A typical Torsen differential can deliver up to twice as much power to the non-slipping side as it delivers to the slipping side. Most Audi Quattro cars, notably excluding the A3 and TT, use a center Torsen differential. For a time, the Volkswagen Passat 4motion shared this design. The HMMWV uses front and rear Torsen differentials, but only has a normal differential in the center. Torsen differentials generally work very well, though they are expensive and heavy.

Many vehicles entirely eliminate the center differential. These vehicles behave as 2WD vehicles under normal conditions. When the drive wheels begin to slip, one of the locking mechanisms discussed above will join the front and real axles. Such systems distribute power unevenly under normal conditions, and thus do not help prevent loss of traction; they only enable recovery once traction has been lost. Most minivan 4WD/AWD systems are of this type, usually with the front wheels powered during normal driving conditions and the rear wheels served via a viscous coupling unit. Such systems may be described as having a 95%/5% or 90%/10% power split. Light trucks and SUVs tend to use multi-plate clutches under computer control, often with 100% of the power going to the rear axle under normal conditions. Sports cars using this type of system always drive only the rear under normal conditions. For example, Lamborghini uses a viscous coupling unit to drive the front, and the Nissan Skyline GT-R uses a clutch. The Audi TT normally powers the front, and has a multi-plate clutch to power the rear.

History
The first ever four wheel drive car (as well as hill-climb racer), the so-called Spyker 60 HP, was built in 1903 by Dutch brothers Jacobus and Hendrik-Jan Spijker of Amsterdam. The first US four wheel drive vehicle was built in 1911 by the Four Wheel Drive auto company (FWD) of Wisconsin. FWD would later produce over 20,000 of its four wheel drive Model B trucks for the British and American armies during World War I. Bugatti created a total of three four wheel drive racers, the Type 53, in 1932, but the cars were legendary for having poor handling. It was not until "go-anywhere" vehicles were needed for the military that four wheel drive found its place. The Jeep, originally developed by American Bantam but mass-produced by Willys and Ford, became the best-known four wheel drive vehicle in the world during World War II. Willys (since 1950 owner of the Jeep name) introduced the CJ-2A in 1945 as the first full-production four wheel drive passenger vehicle.

However, it was not until Jensen applied the Formula Ferguson four wheel drive system to their 1966 Jensen FF that the system was used in a production sports car, but with a total of 320 build units this did not sell in appreciable numbers. The first manufacturer to develop four-wheel drive for road-going cars was Subaru, who introduced the mass-produced 4WD Leone in 1972. This model eventually became the best-selling 4WD car in the world. Audi introduced the first permanently all-wheel driven high volume road-going car, the Audi quattro, in 1980. Audi's chassis engineer, Jörg Bensinger, had noticed in winter tests in Scandinavia that a vehicle used by the German Army, the Volkswagen Iltis, could beat any high performance Audi. He proposed developing a four-wheel drive car, soon used for rallying to improve Audi's conservative image. This feature was also extended to Audi's production cars and is still available nowadays.

Niche maker Panther Westwinds holds the crown for creating the first mid-engined four wheel drive, the Panther Solo 2, in 1989. Today, sophisticated all wheel drive systems are found in many passenger vehicles and most exotic sports cars and supercars.

Terminology
Although in the strictest sense, the term "four wheel drive" refers to a capability that a vehicle may have, it is also used to denote the entire vehicle itself. In Australia, vehicles with offroad capabilities are referred to as "four wheel drives". This term is sometimes also used in North America, somewhat interchangeably for SUVs and pickup trucks and is sometimes erroneously applied to two-wheel-drive variants of these vehicles.

The term 4x4 (read either four by four or full times four) is used to denote the total number of wheels on a vehicle and the number of driven wheels; it is often applied to vehicles equipped with either full-time or part-time four-wheel-drive. The term 4x4 is common in North America and is generally used when marketing a new or used vehicle, and is sometimes applied as badging on a vehicle equipped with four wheel drive. Similarly, a 4x2 would be appropriate for most two-wheel-drive vehicles, although this is rarely used in practice, as vehicles are assumed to be two-wheel-drive unless stated otherwise. A 2×4, however, is unambiguously a piece of lumber.

Large American trucks with dual tires on the rear axles (also called duallys or duallies) and two driven axles are officially badged as 4x4s, despite having six driven wheels because the 'dual' wheels behave as a single wheel for traction purposes and are not individually powered. True 6x6 vehicles with three powered axels such as the famous "Deuce and a Half" truck used by the U.S. Army has three axles (two rear, one front), all of them driven. This vehicle is a true 6x6.

Another related term is 4-wheeler (or four wheeler). This generally refers to all-terrain vehicles with four wheels and does not indicate the number of driven wheels; a "four wheeler" may have two or four wheel drive.

Four wheel drives in Australia
There are two main players in the Australian market: Toyota and Nissan. The typically more massive American four wheel drive trucks and SUVs are generally not as popular among Australian consumers because they are not well suited to the Australian outback. They are often not rugged enough for the harsh conditions, and with their typically larger size they are too wide to fit on the existing wheel tracks created by previous cars (so the driver ends up attempting to carve out his or her own track). As in other countries, four wheel drives have become popular with city-dwelling people, who by and large will never actually drive "off road". This is commonly referred to as driving a Toorak Tractor.

 

[sakuraguy]

In Automobile design, an MR or Mid-engine, Rear wheel drive layout is one in which the rear wheels are driven by an engine placed just in front of them, behind the passenger compartment. In contrast to the rear-engined RR layout the center of gravity of the engine is in front of the rear axle. This layout is typically chosen for its near-ideal weight distribution. The mid-engined layout does, however, use up a lot of space, making it impractical for any but two-seater sports cars. Though it is sometimes advantageous to have more weight in the rear, during cornering it makes entry a lot more difficult as the front tires have less traction, resulting in a slight understeer, and at the exit of the turn, the heavy rear tends to cause oversteer.

Early cars using the MR layout included the 1930s Auto Union racers, Cooper racers, Zündapp Janus (1957), De Tomaso Vallelunga (1965), Lotus Europa (1966), Lamborghini Miura (1966), Ford GT40 (1966), Ferrari Dino 206/246 (1968), and Porsche 914 (1969). Its space-inefficiency means that it is still only used in sports cars and supercars such as the Toyota MR2, the Enzo Ferrari, Honda NSX, Lotus Elise, MGF, and Pagani Zonda. The Zündapp Janus is perhaps unique, in that it is literally mid-engined and nearly symmetrical with passengers on both ends of the engine.

In modern racing cars, MR is the usual configuration and is usually synonymous with "rear engine". Due to its near-ideal weight distribution, this layout is heavily employed in Formula racing cars (such as F1s).

This configuration was also common in very small engined 1950s microcars, in which the engines didn't take up much space.

 

[sakuraguy]

Rear wheel drive was a common form of engine/transmission layout used in automobiles throughout the 20th century. RWD typically places the engine in the front of the vehicle, but the mid engine and rear engine layouts are also used.

The vast majority of rear wheel drive vehicles use a longitudinally-mounted engine in the front of the vehicle, driving the rear wheels via a driveshaft linked via a differential between the rear axles. Some FR layout vehicles place the transmission at the rear, though most attach it to the engine at the front.

Rear wheel drive has fallen out of favor in passenger cars since the 1980s, due in part to higher manufacturing costs, and a perception by many car buyers that front wheel drive is safer, and that it performs better on slippery roads.

It still sees heavy use in taxi and police fleets, due to cheaper maintenance, and in the case of police fleets, better performance.

Advantages
Better handling in dry conditions - less force is applied to the front wheels, allowing more of their friction to be used to steer the vehicle and allowing more power to be applied through the rear wheels, which are unburdened by steering.
Less costly and easier maintenance - Rear wheel drive is mechanically simpler and typically does not involve packing as many parts into as small a space as does front wheel drive, thus requiring less disassembly or specialized tools in order to replace parts.
No torque steer.
Drifting - Drifting is a controlled skid, where the rear wheels break free from the pavement as they spin, allowing the rear end of the car to move freely left and right. This is of course easier to do on slippery surfaces. Severe damage and wear to tires and mechanical components can result from drifting on dry asphalt. Drifting can be used to help in cornering quickly, or in turning the car around in a very small space. Many enthusiasts make a sport of drifting, and will drift just for the sake of drifting. Drifting requires a great deal of skill, and is not recommended for most drivers. It should be mentioned that front wheel drive and four wheel drive cars may also drift, but only with much more difficulty. When front wheel drive cars drift, the driver usually pulls on the 'emergency break' in order for the back wheels to stop and thus skid. This technique is also used for 'long' drifts, where the turn is accomplished by pulling the e-break while turning the steering wheel to the direction the driver desires. With drifting, there is also the importance of 'counter-steering'-where while temporarily out of control, the driver regains it by turning the wheel in the opposite direction and thus preparing for the next turn or straight-away.
Even weight distribution - The division of weight between the front and rear wheels has a significant impact on a car's handling, and it is much easier to get a 50/50 weight distribution in a rear wheel drive car than in a front wheel drive car, as more of the engine can lie between the front and rear wheels (in the case of a mid engine layout, the entire engine), and the transmission is moved much farther back.
Towing - Rear wheel drive puts the wheels which are pulling the load closer to the point where a trailer articulates, helping steering, especially for large loads.
Weight transfer during acceleration.

Disadvantages
More difficult to master - While the handling characteristics of rear-wheel drive may be useful or fun in the hands of someone who knows how to use them, in the hands of most drivers, having the rear wheels move about is unintuitive and dangerous. Rear wheel drive rewards skill, and punishes the lack of it. Other layouts are much more forgiving, but don't offer the same rewards in handling.
Decreased interior space - This isn't an issue in a vehicle with a ladder frame like a pickup truck, where the space used by the drive line is unusable for passengers or cargo. But in a passenger car, rear wheel drive means: Less front leg room (the transmission tunnel takes up a lot of space between the driver and front passenger), less leg room for center passengers (there is also a smaller tunnel for the drive shaft), and sometimes less trunk space (since there is also more hardware that must be placed underneath the trunk).
Increased weight - The components of your typical rear wheel drive vehicle's power train may be less complex, but there are more of them. The driveshaft adds weight. The transmission is probably heavier. There is extra sheet metal to form the transmission tunnel. There is a rear axle or rear half-shafts. A rear wheel drive car will weigh slightly more than a comparable front wheel drive car (but less than four wheel drive).
Higher purchase price - Probably due to more complicated assembly (the powertrain is not one compact unit) and added cost of materials, rear wheel drive is typically slightly more expensive to purchase than a comparable front wheel drive vehicle. This might also be explained by production volumes, however.
More difficult handling on low grip surfaces (wet road, ice, snow, gravel...) as the car is pushed rather than pulled.
Oversteer and the related problem of fishtailing.

 

[SkYwAlKeR]

thanks woo... vr informative...

 

[sakuraguy]

ure welcome buddie, just some of my little contribution to ZTHians :)

Cheers!

 

[DarkChild]

Best quote your sources....

 

[sakuraguy]

Maybe later, i take from various sources last time, it was in my pc.. and lots more too .. later i will post more when im free.