Automobile layout


Automobile layout

In automotive design layout specifies where on the car the engine and drive wheels are found.

Front-wheel drive types

A Front-engine, front-wheel drive layout places both the engine and driven wheels at the front of the vehicle. This layout is typically chosen for its compact packaging - that is, it takes up very little space, allowing the rest of the vehicle to be designed more flexibly. In contrast with the FR layout, the FF layout eliminates the need for a central tunnel or a higher chassis clearance to accommodate a driveshaft providing power to the rear wheels. Like the RR and RMR layouts, it places the heavy engine over the drive wheels which aids traction. As the steered wheels are also the driven wheels, FF cars are generally considered superior to FR 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.

Characteristics

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. [http://www.edmunds.com/insideline/do/Features/articleId=43847 Inside Line: What Wheel Drive?] ] 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 centre of gravity farther forward than a comparable rear-wheel drive layout, improving traction and directional stability on wet, snowy, or icy surfaces.cite book | last = William | first = Milliken | title = Race Car Vehicle Dynamics | year = 1995 | publisher = SAE International | isbn = 1560915269 | pages = 730 | chapter = Merits of Front-, Rear-, and Four-Wheel Drive | quote = Front-wheel drive has been most successful in the lower power/weight range and in sutuations in which superior derectional stability on low coefficients is important. There has never been a successful front-drive Grand Prix car nor a competitive Indianapolis car of more than 300 hp.] [http://mywebpages.comcast.net/cvetters3/test1.htm "What's It Like To Drive"] , describes a test between two Dodge Daytonas, one FWD and one RWD] 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. [http://www.saabnet.com/tsn/models/1984/pr2.html The Hidden Virtues of Front Wheel Drive] ]

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." [http://www.gglotus.org/ggtech/m100-lcu-manual/m100lcu.htm Lotus Elan M100 Sales Manual] ] However, this may only apply for cars with moderate power-to-weight ratio.cite book | last = Frere | first = Paul | title = Sports Car and Competition Driving | year = 1992 | publisher = entleyPublishers | isbn = 0836702025 | pages = 67pp | chapter = From Slipping to Sliding | quote = Front-wheel drive which, due to the reduced front wheel grip under acceleration, is practical only for cars of moderate power-to-weight ratio] cite book | last = Prost | first = Alain | title = Competition Driving | year = 1990 | publisher = Hazelton Publishing | isbn = 0905138805 | pages = 50pp | chapter = Controlling a car at the limit | quote = Front-wheel drive. In this instance, both power and steering are directed through the front wheels, the rears remaining free. Following the principle of weight transfer once more, the lightening of the front wheels under acceleration considerably reduces their effectiveness and thus limits the usable power. Consequentally, this type of transmission is generally less effective on racing circuits, a few rare exceptions notwithstanding, but has its advantages in road events where maximum power is not called into play so often] 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.

In a front wheel drive car it is easier to correct trailing-throttle or trailing-brake oversteer.

Some high-power front wheel drive cars may exhibit torque steer.Torque steer can be addressed by using a longitudal layout, equal-length drive shafts, half shafts, a multilink suspension or centre-point steering geometry.cite web |url=http://www.mscsoftware.com/support/library/conf/adams/euro/2002/papers/005_EUC_008_FORD.pdf |title=Torque Steer Influences on McPherson Front Axles |author=Jens Dornhege] cite web |url=http://www.mph-online.com/mag/streetknowledge/003 |publisher=MPH Magazine |title=What is Torque Steer?] cite web |url=http://www.autozine.org/technical_school/handling/tech_handling_6.htm |title=Handling |publisher=AutoZine Technical School] cite web |url=http://www.se-r.net/about/g20/scc/oct98/tb.html |title=Technobabble: Multilink and the Beam | publisher=Sport Compact Car - November '98] cite web |url=http://www.autozine.org/technical_school/suspension/tech_suspension21.htm |title=Suspension Geometry |publisher=AutoZine Technical School] cite web |url=http://www.quaife.co.uk/Why-use-Quaife |title=Why use Quaife?] cite web |url=http://www.robietherobot.com/storm/geo_storm_mods_transmission.htm |title=Storm Transmission Modifications] cite web |url=http://www-cdr.stanford.edu/dynamic/bywire/acc2004_r1.pdf |title=Vehicle State Estimation Using Steering Torque |author=Paul Yih |publisher=Stanford University]

Lack of weight shifting will limit the acceleration of a front-wheel drive vehicle. During heavy acceleration, weight is placed on the rear, or driving wheels, which improves traction. This is the main reason 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. The weight shifting and weight distribution of rear wheel drive cars cause 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 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 this configuration provides outstanding grip and traction as the engine weight is over the drive wheels. 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.

Advantages

* 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.
* Cost: Fewer components overall.
* Weight: Fewer components usually means lower weight.
* Improved fuel efficiency due to less weight.
* 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.Fact|date=February 2007
* 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.cite book | last = William | first = Milliken | title = Race Car Vehicle Dynamics | year = 1995 | publisher = SAE International | isbn = 1560915269 | pages = 730 | chapter = Merits of Front-, Rear-, and Four-Wheel Drive | quote = Front-wheel drive has been most successful in the lower power/weight range and in sutuations in which superior derectional stability on low coefficients is important. There has never been a successful front-drive Grand Prix car nor a competitive Indianapolis car of more than 300 hp.]
* 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. [http://www.saabnet.com/tsn/models/1984/pr2.html The Hidden Virtues of Front Wheel Drive] ]
* The 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 early Saabs being an exception). [ [http://www.modernracer.com/tips/frontwheeldriveoversteer.html Modern Racer: Front-Wheel-Drive Oversteer] ] Dubious|Intentional skidding|date=March 2008
* 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.Dubious|For a given vehicle weight, power and tire size, a front wheel drive car is always faster over a given section of road|date=March 2008 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.
*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).

Disadvantages

*Torque steer is the tendency for some high power 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 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 all- or 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.
* The driveshafts may limit the amount by which the front wheels can turn, thus it may increase the turning circle of a front-wheel drive car compared to a rear-wheel drive one with the same wheelbase.

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 (FR). 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. [cite web|url=http://www.sae.org/technical/papers/2006-01-1339|title=Development of a New Hybrid Transmission for RWD Car|publisher=www.sae.org|accessdate=2008-01-11|last=|first=] Rear-wheel drive is used almost universally for driving motorcycles, whether by driveshaft, chain, or belt.

Characteristics

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.

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).

Advantages

* 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. [cite web|url=http://www.canadiandriver.com/winter/020619.htm|title=Front wheel drive vs rear wheel drive|author=Jim Kerr|publisher=Canadian Driver]
* Weight transfer during acceleration - During heavy acceleration, weight is placed on the rear, or driving wheels, which improves traction.
* No torque steercite web |url=http://forzamotorsport.net/121306-1.htm |title=FWD vs. RWD |publisher=Forza Motorsport 2 |author=Che Chou] (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.cite web |url=http://www.familycar.com/NextCar/Step3.htm |title=Front wheel drive, rear wheel drive, or all wheel drive? |publisher=The Family Car]
* 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.cite web |url=http://findarticles.com/p/articles/mi_m0BQK/is_5_10/ai_n15393977 |title=SUVs carry the load |publisher=Boat/US Magazine |author=Michael Vatalaro]
* 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.Fact|date=February 2007
* Robustness - due to geometry and packaging constraints, the CV joints attached to the wheel hub have a tendency to wear out much later than the universal 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.Fact|date=February 2007

Disadvantages

* 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), 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.cite web |url=http://www.saabnet.com/tsn/models/1984/pr2.html |title=The Hidden Virtues of Front Wheel Drive |publisher=Saab Automobile]
* Higher initial purchase price - Modern rear wheel drive vehicles are typically slightly more expensive to purchase than comparable front wheel drive vehicles. Although the complex assembly (the powertrain is not one compact unit) and added cost of materials, this cost difference is more probably explained by production volumes and the fact that the majority of rear-wheel cars are in the sports/performance/luxury categories. Few modern "family" sedans have rear-wheel drive, so a cost comparison is not necessarily possible.Fact|date=February 2007
* 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 driveFact|date=March 2008 - 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).

History and current use

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, 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. [ [http://www.popularmechanics.com/automotive/new_cars/1266931.html Comparison Test: Front-Wheel Drive Vs. Rear-Wheel Drive - Popular Mechanics ] ] 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 [ [http://blog.vehiclevoice.com/2005/12/why_good_enough_isnt_1.html VehicleVoice News: Ford Panther Platform - Why Good Enough Isn't ] ] , as was the Ford Mustang [ [http://www.popularhotrodding.com/features/0501phr_2005_ford_mustang/index.html 2005 Ford Mustang - Popular Hot Rodding Magazine ] ] and Lincoln LS.

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. [ [http://www.rearwheeldrive.org/rwd/rwdrate.htm Automaker Ratings ] ] 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.

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 [ [http://dfwdriveyourdream.com/blog/cadillac-dts-to-be-discontinued/1126 CADILLAC DTS TO BE DISCONTINUED | Drive Your Dream Blog ] ] , 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. [ [http://www.slate.com/id/2082761 Chrysler brings back rear-wheel drive. - By Mickey Kaus - Slate Magazine ] ] [ [http://www.leftlanenews.com/2011-chrysler-300-dodge-charger-to-remain-rear-wheel-drive.html 2011 Chrysler 300, Dodge Charger to remain rear-wheel drive ] ] Ford never eliminated FR cars, but is looking to replace the dated designs that it currently has. [ [http://www.detnews.com/apps/pbcs.dll/article?AID=/20080324/AUTO01/803240338/1148 Detnews.com | This article is no longer available online ] ]

References


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