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In automotive design the automobile layout describes where on the vehicle the engine and drive wheels are found. Many different combinations of engine location and driven wheels are found in practice, and the location of each is dependent on the application the vehicle will be used for. Factors influencing the design choice include cost, complexity, reliability, packaging (location and size of the passenger compartment and boot space), weight distribution and the vehicle's intended handling characteristics.
Layouts can roughly be divided into two categories: front- or rear-wheel drive. Four-wheel drive vehicles may take on the characteristics of either, depending on how the system is set up to distribute power to the wheels.
Front-wheel drive layouts
Front-wheel drive layouts are those in which the front wheels of the vehicle are driven. The most popular layout used in cars today is the front-engine, front-wheel drive, with the engine in front of the front axle, driving the front wheels. This layout is typically chosen for its compact packaging; since the engine and driven wheels are on the same side of the vehicle, there is no need for a central tunnel through the passenger compartment to accommodate a prop-shaft between the engine and the driven wheels.
As the steered wheels are also the driven wheels, FF (front-engine, front-wheel drive layout) cars are generally considered superior to FR (front-engine, rear-wheel drive layout) cars in conditions such as snow, mud or wet tarmac. However, powerful cars rarely use the FF layout because weight transference under acceleration unloads the front wheels and sharply reduces their grip, effectively putting a cap on the amount of torque which could realistically be utilized. Electronic traction control can avoid wheelspin but largely negates the benefit of extra torque/power.
A transverse engine (also known as "east-west") is commonly used in FF designs, in contrast to FR which uses a longitudinal engine. The FF layout also restricts the size of the engine that can be placed in modern engine compartments, as FF configurations usually have Inline-4 and V6 engines, while longer engines such as Inline-6 and 90° big-bore V8 will rarely fit. This is another reason why luxury/sports cars almost never use the FF layout; one exception is the Volvo S80 (FWD/4WD) which uses transversely mounted inline 6 and V8 engines.
Front wheel drive gives more interior space since the powertrain is a single unit contained in the engine compartment of the vehicle and there is no need to devote interior space for a driveshaft tunnel or rear differential, increasing the volume available for passengers and cargo. There are some exceptions to this as rear engine designs do not take away interior space. (See Porsche 911, and Volkswagen Beetle) It also has fewer components overall and thus lower weight. The direct connection between engine and transaxle reduces the mass and mechanical inertia of the drivetrain compared to a rear-wheel drive vehicle with a similar engine and transmission, allowing greater fuel economy. In front wheel drive cars the mass of the drivetrain is placed over the driven wheels and thus moves the center of gravity farther forward than a comparable rear-wheel drive layout, improving traction and directional stability on wet, snowy, or icy surfaces. Front-wheel drive cars, with a front weight bias, tend to understeer at the limit, which according to for instance Saab engineer Gunnar Larsson is easier since it makes instinct correct in avoiding terminal oversteer, and less prone to result in fishtailing or a spin.
According to a sales brochure for the 1989 Lotus Elan, the ride and handling engineers at Lotus found that "for a given vehicle weight, power and tire size, a front wheel drive car was always faster over a given section of road." However, this may only apply for cars with moderate power-to-weight ratio. According to road test with two Dodge Daytonas, one FWD and one RWD, the road layout is also important for what configuration is the fastest.
The lack of weight shifting limits the acceleration of a front-wheel drive vehicle. During heavy acceleration, weight is shifted to the back, improving traction at the rear wheels at the expense of the front driving wheels; consequently, most racing cars are rear-wheel drive for acceleration. However, since front-wheel drive cars have the weight of the engine over the driving wheels, the problem only applies in extreme conditions. The weight shifting and weight distribution of rear wheel drive cars make them more likely to oversteer and the related problem of fishtailing. On snow, ice, and sand, rear-wheel drive loses its traction advantage to front or all-wheel drive vehicles which have greater weight over the driven wheels. Rear wheel drive cars with rear engine or mid engine configuration retain traction over the driven wheels, although fishtailing remains an issue. Some rear engine cars (e.g. Porsche 911) can suffer from reduced steering ability under heavy acceleration, since the engine is outside the wheelbase and at the opposite end of the car from the wheels doing the steering. A rear wheel drive car's center of gravity is shifted rearward when heavily loaded with passengers or cargo, which may cause unpredictable handling behavior.
On FR cars, the long driveshaft adds to drivetrain elasticity.
- Interior space: Since the powertrain is a single unit contained in the engine compartment of the vehicle, there is no need to devote interior space for a driveshaft tunnel or rear differential, increasing the volume available for passengers and cargo.
- Instead, the tunnel may be used to route the exhaust system pipes.
- Weight: Fewer components usually means lower weight.
- Improved fuel efficiency due to less weight.
- Cost: Fewer material components and less installation complexity overall. However, the considerable MSRP differential between a FF and FR car cannot be attributed to layout alone. The difference is more probably explained by production volumes as most rear-wheel cars are usually in the sports/performance/luxury categories (which tend to be more upscale and/or have more powerful engines), while the FF configuration is typically in mass-produced mainstream cars. Few modern "family" cars have rear-wheel drive as of 2009, so a direct cost comparison is not necessarily possible. A contrast could be somewhat drawn between the FF Audi A4 and the FR BMW 3-Series, both which are in the compact executive car classification.
- Improved drivetrain efficiency: the direct connection between engine and transaxle reduce the mass and mechanical inertia of the drivetrain compared to a rear-wheel drive vehicle with a similar engine and transmission, allowing greater fuel economy.
- Assembly efficiency: the powertrain can often be assembled and installed as a unit, which allows more efficient production.
- Placing the mass of the drivetrain over the driven wheels moves the centre of gravity farther forward than a comparable rear-wheel drive layout, improving traction and directional stability on wet, snowy, or icy surfaces.
- Predictable handling characteristics: front-wheel drive cars, with a front weight bias, tend to understeer at the limit, which (according to e.g. SAAB engineer Gunnar Larsson) is easier since it makes instinct correct in avoiding terminal oversteer, and less prone to result in fishtailing or a spin.
- A skilled driver can control the movement of the car even while skidding by steering, throttling and pulling the hand brake (given that the hand brake operates the rear wheels as in most cases, with some Citroen and Saab models being notable exceptions). A small car with the FF layout is superior for motor sport events focusing on manouvreability such as Autotesting
- It is easier to correct trailing-throttle or trailing-brake oversteer.
- The wheelbase can be extended without building a longer driveshaft (as with rear wheel driven cars).
- Torque steer is 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, canceling each other out, however in reality this is less likely to happen. Torque steer can be addressed by using a longitudinal layout, equal length drive shafts, half shafts, a multilink suspension or centre-point steering geometry.
- Lack of weight shifting will limit the acceleration of a front-wheel drive vehicle. In a vehicle, the weight shifts back during acceleration, giving more traction to the rear wheels. This is one of the main reasons why nearly all racing cars are rear-wheel drive. However, since front-wheel drive cars have the weight of the engine over the driving wheels, the problem only applies in extreme conditions.
- 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.
- Traction can be reduced while attempting to climb a slope in slippery conditions such as snow or ice covered roadways.
- Due to geometry and packaging constraints, the CV joints (constant-velocity joints) attached to the wheel hub have a tendency to wear out much earlier than the universal joints typically used in their rear-wheel drive counterparts (although rear-wheel drive vehicles with independent rear suspension also employ CV joints and half-shafts). 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.
- They cannot compete in the sport of drifting due to the need of RWD to push rear of the vehicle in a circle. This is not necessarily an issue to most car manufacturers.
- Turning circle — FF layouts almost always use a Transverse engine ("east-west") installation, which limits 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. A notable example is the original Mini. It is widely misconceived that this limitation is due to a limit on the angle at which a CV joint can be operated, but this is easily disproved by considering the turning circle of car models that use a longitudinal FF or F4 layout from Audi and (prior to 1992) Saab
- The FF transverse engine layout (also known as "east-west") restricts the size of the engine that can be placed in modern engine compartments, so it is rarely adopted by powerful luxury and sports cars. FF configurations can accommodate at most Inline-4 and V6 engines, while longer engines such as Inline-6 and 90° big-bore V8 will rarely fit, though there are exceptions. One way around this problem is using a staggered engine.
Rear wheel drive layouts
Rear-wheel drive (RWD) typically places the engine in the front of the vehicle and the driven wheels are located at the rear, a configuration known as front-engine, rear-wheel drive layout (FR layout). The front mid-engine, rear mid-engine and rear engine layouts are also used. This was the traditional automobile layout for most of the 20th century. Nearly all motorcycles use rear-wheel drive, either by driveshaft, chain, or belt.
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 gearbox at the rear, though most attach it to the engine at the front.
The FR layout is often chosen for its simple design and good handling characteristics. Placing the drive wheels at the rear allows ample room for the transmission in the center of the vehicle and avoids the mechanical complexities associated with transmitting power to the front wheels. For performance-oriented vehicles, the FR layout is more suitable than front-wheel drive designs, especially with engines that exceed 200 horsepower. This is because weight transfers to the rear of the vehicle during acceleration, which loads the rear wheels and increases their grip.
Another advantage of the FR layout is relatively easy access to the engine compartment, as result of the longitudinal orientation of the drivetrain, as compared to the FF layout (front-engine, front-wheel drive). Powerful engines such as the Inline-6 and 90° big-bore V8 are usually too long to fit in a FF transverse engine ("east-west") layout; the FF configuration can typically accommodate at the maximum an Inline-4 or V6. This is another reason why luxury/sports cars almost never use the FF layout.
- Even weight distribution — The layout of a rear wheel drive car is much closer to an even fore and aft weight distribution than 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.
- Weight transfer during acceleration — During heavy acceleration, weight is placed on the rear, or driving wheels, which improves traction.
- No torque steer (unless it's an all wheel steer with an offset differential).
- Steering radius — As no complicated drive shaft joints are required at the front wheels, it is possible to turn them further than would be possible using front wheel drive, resulting in a smaller steering radius for a given wheelbase.
- Better handling in dry conditions — the more even weight distribution and weight transfer improve the handling of the car. The front and rear tires are placed under more even loads, which allows for more grip while cornering.
- Better braking — the more even weight distribution helps prevent lockup from wheels becoming unloaded under heavy braking.
- 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.
- Serviceability — Drivetrain components on a rear-wheel drive vehicle are modular and do 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 service the vehicle.
- Robustness — due to geometry and packaging constraints, the universal joints attached to the wheel hub have a tendency to wear out much later than the CV joints typically used in front-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.
- Can accommodate more powerful engines as a result of the longitudinal orientation of the drivetrain, such as the Inline-6 and 90° big-bore V8, making the FR a common configuration for luxury and sports cars. These engines are usually too long to fit in a FF transverse engine ("east-west") layout; the FF configuration can typically accommodate at the maximum an Inline-4 or V6.
- Under heavy acceleration oversteer and fishtailing may occur.
- On snow, ice and sand, rear-wheel drive loses its traction advantage to front or all-wheel drive vehicles which have greater weight on the driven wheels. Rear wheel drive cars with rear engine or mid engine configuration do not suffer from this, although fishtailing remains an issue.
- Some rear engine cars (e.g. Porsche 911) can suffer from reduced steering ability under heavy acceleration, because the engine is outside the wheelbase and at the opposite end of the car from the wheels doing the steering although the engine weight over the rear wheels provides outstanding traction and grip during acceleration.
- Decreased interior space — Though individual designs vary greatly, rear wheel drive vehicles may have: Less front leg room as the transmission tunnel takes up a space between the driver and front passenger, less leg room for center rear passengers (due to the tunnel needed for the drive shaft), often no seat for a center rear passenger, and sometimes less trunk space (since there is also more hardware that must be placed underneath the trunk). Rear engine designs (such as the Porsche 911 and Volkswagen Beetle) do not inherently take away interior space.
- Increased weight — The components of a rear wheel drive vehicle's power train are less complex, but they are larger. The driveshaft adds weight. There is extra sheet metal to form the transmission tunnel. There is a rear axle or rear half-shafts, which are typically longer than those in a front-wheel drive car. A rear wheel drive car will weigh slightly more than a comparable front wheel drive car (but less than four wheel drive).
- Improper weight distribution when loaded — A rear wheel drive car's center of gravity is shifted rearward when heavily loaded with passengers or cargo, which may cause unpredictable handling behavior.
- Higher initial purchase price — Modern rear wheel drive vehicles are typically more expensive to purchase than comparable front wheel drive vehicles. Part of this can be explained by the added cost of materials and increased complex assembly of FR layouts, as the powertrain is not one compact unit. However, the difference is more probably explained by production volumes as most rear-wheel cars are usually in the sports/performance/luxury categories (which tend to be more upscale and/or have more powerful engines), while the FF configuration is typically in mass-produced mainstream cars. Few modern "family" cars have rear-wheel drive as of 2009, so a direct cost comparison is not necessarily possible.
- The possibility of a slight loss in the mechanical efficiency of the drivetrain (approximately 17% coastdown losses between engine flywheel and road wheels compared to 15% for front wheel drive — however these losses are highly dependent on the individual transmission). Cars with rear engine or mid engine configuration and a transverse engine layout do not suffer from this.
- The long driveshaft (on front engine cars) adds to drivetrain elasticity. The driveshaft must also be extended for cars with a stretched wheelbase (e.g. limousines, minivans).
Four wheel drive layouts
Front-engine, rear-wheel drive derived “F4” layout
Note: in North America, Australia and New Zealand the term "four-wheel drive" usually refers only to drivetrains which are primarily two-wheel drive with a part-time four-wheel drive capability, as typically found in pickup trucks and other off road vehicles, while the term "all-wheel drive" is used to refer to full time four-wheel drive systems found in performance cars and smaller car-based SUVs. This section uses the term four-wheel drive to refer to both, as per the rest of the world.
Most 4WD layouts are front-engined and are derivatives of earlier front-engined, two-wheel drive designs. They fall into two major categories:
- Front-engine, rear-wheel drive derived 4WD systems, standard in most sport utility vehicles and in passenger cars, (usually referred to “front engine, rear-wheel drive/four-wheel drive”), forerunners of today models include the Jensen FF, AMC Eagle and Mercedes-Benz W124 with the 4Matic system and Suzuki Grand Vitara with/without 4 mode transfer case.
- Transverse and longitudinal engined 4WD systems derived almost exclusively from front-engined, front-drive layouts, fitted to luxury, sporting and heavy duty segments, for example the transverse-engined Mitsubishi 3000GT VR-4 and Toyota RAV4 and the longitudinal-engined Audi Quattro and most of the Subaru line.
For a full explanation of 4WD engineering considerations, see the main article on four-wheel drive
In terms of handling, traction and performance, 4WD systems generally have most of the advantages of both front-wheel drive and rear-wheel drive. Some unique benefits are:
- Traction is nearly doubled compared to a two-wheel drive layout. Given sufficient power, this results in unparalleled acceleration and driveability on surfaces with less than ideal grip, and superior engine braking on loose surfaces. The development of 4WD systems for high performance cars was stimulated primarily by rallying.
- Handling characteristics in normal conditions can be configured to emulate FWD or RWD, or some mixture, even to switch between these behaviours according to circumstance. However, at the limit of grip, a well balanced 4WD configuration will not degenerate into either understeer or oversteer, but instead break traction of all 4 wheels at the same time into a four-wheel drift. Combined with modern electronic driving aids, this flexibility allows production car engineers a wide range of freedom in selecting handling characteristics that will allow a 4WD car to be driven more safely at higher speeds by inexpert motorists than 2WD designs.
- 4WD systems require more machinery and complex transmission components, and so increase the manufacturing cost of the vehicle and complexity of maintenance procedures and repairs compared to 2WD designs
- 4WD systems increase power-train mass, rotational inertia and power transmission losses, resulting in a reduction in performance in ideal dry conditions and increased fuel consumption compared to 2WD designs
- The handbrake cannot be used to induce over-steer for maneuvering purposes, as the drivetrain couples the front and rear axles together. To overcome this limitation, some custom prepared stage rally cars have a special mechanism added to the transmission to disconnect the rear drive if the handbrake is applied while the car is moving.
Unusual 4WD layouts
- From 1993 onwards, some models of Porsche 911 feature a rear-engined 4WD layout, which is akin to a longitudinal front-engine 4WD layout installed backwards with the engine at the rear of the car
- From 2007 onwards, the Nissan GT-R features a front-engine 4WD longitudinal layout, but with the gearbox at the rear of the vehicle. This provides a more ideal weight balance, and improves directional stability at very high speeds by increasing the vehicle's moment of inertia around the vertical axis. This layout necessitates a second prop-shaft to carry power to the front wheels.
- Some types of farm tractors and construction site machinery use a 4WD layout where the wheels on each side are coupled together, rather than the wheels on each axle, allowing these vehicles to pivot about their center point. Such vehicles are controlled in a fashion similar to a military tank.
- The Citroën Sahara had a 4WD system using complete Citroën 2CV drivetrains at both ends of the car, such that the engine at the front powered the front wheels and the engine at the back powered the rear wheels.
History and current use
FMR layout, standard in most Front-engine / Rear-wheel drive cars pre-World War II, where the engine was located behind the front axle.
The first FR car was an 1895 Panhard model, so this layout was known as the "Système Panhard" in the early years. Most American cars used the FR layout until the mid 1980s. The Oil crisis of the 1970s and the success of small FF cars like the Mini, Volkswagen Golf, Toyota Tercel, and Honda Civic led to the widespread adoption of that layout.
After the Arab Oil Embargo of 1973 and the 1979 fuel crises, a majority of American FR vehicles (station wagons, luxury sedans) were phased out for the FF layout — this trend would spawn the SUV/van conversion market. Throughout the 1980s and 1990s, most American companies set as a priority the eventual removal of rear-wheel drive from their mainstream and luxury lineup. Chrysler went 100% FF by 1990 and GM's American production went entirely FF by 1997 except the Corvette and Camaro. Ford's full-size cars (the Ford Crown Victoria, Mercury Grand Marquis, and Lincoln Town Car) have always been FR, as was the Ford Mustang and Lincoln LS. In 2008 Hyundai introduced its own rear-wheel drive car, the Hyundai Genesis.
In Australia, FR cars have remained popular throughout this period, with the Holden Commodore and Ford Falcon having consistently strong sales. In Europe, front-wheel drive was popularized by small cars like the Mini, Renault 5 and Volkswagen Golf and adopted for virtually all mainstream cars.
Upscale marques like Mercedes-Benz, BMW, and Jaguar remained mostly independent of this trend, and retained a lineup mostly or entirely made up of FR cars. Japanese mainstream marques such as Toyota and Nissan became mostly or entirely FF early on, while reserving for their latterly-conceived luxury divisions (Lexus and Infiniti, respectively) a mostly FR lineup. While many automakers lost sight of the true sports car, Mazda introduced the highly successful Miata roadster in 1990, a true 2-seater sports car using the traditional FR layout which led to other compaines such as General Motors to produce a FR sports car based on their Kappa platform.
Currently most cars are FF, including virtually all front-engined economy cars, though FR cars are making a return as an alternative to large sport-utility vehicles. In North America, GM returned to production of the FR luxury car with the 2003 Cadillac CTS, and with the removal of the DTS, Cadillac will be entirely FR (with four-wheel drive available as an option on several models) by 2010, and the 2010 Camaro returns as a FR sports car. Chrysler returned its full-size cars to this layout with the Chrysler 300 and related models. Ford never eliminated FR cars, but is looking to replace the dated designs that it currently has. Nissan are also bringing back the Silvia to their line-up, Mazda is said to be releasing a new rotary-powered FR car in their RX line-up by 2010 and there are rumours Toyota are bringing a successor to the AE86, in other words, an affordable RWD car. Hyundai introduced their affordable RWD car being the 2009 Hyundai Genesis and 2010 Hyundai Genesis Coupe
In the 21st century, with solutions to the engineering complexities of 4WD being widely understood, and consumer demand for increasing performance in production cars, front-engined 4WD layouts are rapidly becoming more common, and most major manufacturers now offer 4WD options on at least some models. Manufacturers with a notable expertise and history in producing 4WD performance cars are Audi and Subaru.
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- ^ 2011 Chrysler 300, Dodge Charger to remain rear-wheel drive
- ^ Detnews.com | This article is no longer available online