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Four-wheel drive

Four-wheel drive, 4WD, or 4x4 ("four by four") is a four-wheeled vehicle with a drivetrain that allows all four wheels to receive torque from the engine simultaneously. In the United States, these cars are often, but not always, included in the broader sport utility vehicle category. While many people associate the term with off-road vehicles, powering all four wheels provides better control in normal road cars on many surfaces, and is an important part of rally racing. The first figure is the number of wheels. The second is the number of powered wheels. With a 4x2, engine power is transmitted to only two wheels, usually the rear.

There is often confusion as to the difference between 4x4s and SUVs. This leads to criticisms of 4x4 vehicles in the media that should actually be directed at SUVs (see SUV Criticism).

Terminology

The term four-wheel drive describes truck-like vehicles that require the driver to manually switch between two-wheel drive mode for streets and four-wheel drive mode for low traction conditions such as ice, mud, snow, slippery surfaces, or loose gravel.

All-wheel drive (AWD) is often used to describe a "full time" 4WD that may be used on dry pavement without destroying the drivetrain, although the term may be abused when marketing a vehicle. AWD can be used on dry pavement because it employs a center differential, which allows each tire to rotate at a different speed. ("Full-Time" 4WD can be disengaged and the center differential can be locked, essentially turning it into regular 4WD. On the other hand, AWD cannot be disengaged and the center differential cannot be locked.) This eliminates driveline binding, wheel hop, and other driveline issues associated with the use of 4WD on dry pavement. With vehicles with more than four wheels, AWD means all wheels drive the vehicle, to varying degrees of engagement, while 4WD means only four of the wheels drive the vehicle continuously.

Identical drivetrain systems are commonly marketed under different names for upmarket and downmarket branding and, conversely, different drivetrain systems are commonly marketed under the same name for brand uniformity. Audi's quattro, DaimlerChrysler's 4Matic used on Mercedes-Benz products, BMW with the xDrive, and Volkswagen's 4motion, for example, can mean either an automatically-engaging "on-demand" system with a Haldex Traction clutch, or a continuously-operating permanent 4WD system with a Torsen (torque-sensing) differential.

Design

Differentials

When powering two wheels simultaneously the wheels must be allowed to rotate at different speeds as the vehicle goes around curves. The problem is even more complicated when driving all four wheels. 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 requires a differential to distribute power between the left and the right sides. When all four wheels are driven, a third differential can be used to distribute power between the front and the rear axles.

Such a design handles well. It distributes power evenly and smoothly, and makes slippage unlikely. Once it does slip, however, recovery is difficult. If the left front wheel of a 4WD vehicle slips on an icy patch of road, for instance, the slipping wheel will spin faster than the other wheels due to the lower traction at that wheel. Although the amount of torque applied to each wheel will be identical, the amount of traction at each driven wheel will be limited to that of the wheel with the least traction (at least one wheel on ice in this case). This problem can happen in both 2WD and 4WD vehicles, whenever a driven wheel is placed on a surface with little traction or raised off the ground. The simplistic design works acceptably well for 2WD vehicles. It is much less acceptable for 4WD vehicles because 4WD vehicles have twice as many wheels to lose traction, increasing the likelihood that it will happen. 4WD vehicles may also be more likely to be driven on surfaces with reduced traction.

Limiting slippage

Traction control was invented to solve this problem for 2WD vehicles. When one wheel spins out of control the brake is automatically applied to that wheel. By preventing one wheel from spinning freely power is divided between the pavement for the non-slipping wheel and the brake for the slipping wheel. This is an effective solution, although it causes additional brake wear and may cause a sudden jolt that affects handling. By extending traction control to act on all four wheels the simple three-differential 4WD design will see limited wheel spin. This design is commonly seen on luxury crossover SUVs.

Locking differentials work by temporarily locking together a differential's output shafts, causing all wheels to turn at the same rate, providing torque in case of slippage. This is generally used for the center differential, which distributes power between the front and the rear axles. While a drivetrain that turns all wheels equally would normally fight the driver and cause handling problems, this is not a concern when wheels are slipping.

The two most common factory-installed locking differentials use either a computer-controlled multi-plate clutch or viscous coupling unit to join the shafts, while other differentials more commonly used on off-road vehicles generally use manually operated locking devices. In the multi-plate clutch the vehicle's computer senses slippage and locks the shafts, causing a small jolt when it activates, which can disturb the driver or cause additional traction loss. In the viscous coupling differentials the shear stress of high shaft speed differences causes a dilatant fluid in the differential to become solid, linking the two shafts. This design suffers from fluid degradation with age and from exponential locking behavior. Some designs use gearing to create a small rotational difference which hastens torque transfer.

A third approach to limiting slippage is the Torsen differential. A Torsen differential allows the output shafts to receive different amounts of torque. This design does not provide for traction when one wheel is spinning freely, where there is no torque. It provides excellent handling in less extreme situations. A typical Torsen II differential can deliver up to twice as much torque to the high traction side before traction is exceeded at the lower tractive side.

Finally, many lower-cost 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 rear axles. Such systems distribute power unevenly under normal conditions and thus do not help prevent the loss of traction, instead only enabling recovery once traction is 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.

History

The true inventor of four-wheel drive is not really known; the history of such was not well recorded. In 1893, before the establishment of a modern automotive industry in Britain, English engineer Joeseph Bramah Diplock patented a four wheel drive system for a traction engine, including four-wheel steering and three differentials, which was subsequently built. The development also incorporated Bramagh's Pedrail wheel system in what was one of the first four-wheel drive automobiles to display an intentional ability to travel on challenging road surfaces. It stemmed from Bramagh's previous idea of developing an engine that would reduce the amount of damage to public roads.

Ferdinand Porsche designed and built a four-wheel driven Electric vehicle for the k. u. k. Hofwagenfabrik Ludwig Lohner & Co. at Vienna in 1899, presented to the public during the 1900 World Exhibition at Paris. The vehicle was powered by an electric hub motor at each wheel, a design later used by NASA in the Lunar rover. Although clumsily heavy, the vehicle proved a powerful sprinter and record-breaker in the hands of its owner E.W. Hart. Due to its unusual status the so-called Lohner-Porsche is not widely credited as the first four-wheel driven automobile.

The first four-wheel drive car, as well as hill-climb racer, with internal combustion engine, the Spyker 60 H.P., was presented in 1903 by Dutch brothers Jacobus and Hendrik-Jan Spijker of Amsterdam. The two-seat sports car, which was also the first ever car equipped with a six-cylinder engine, is now an exhibit in the Louwman Collection (the former Nationaal Automobiel Museum) at Raamsdonksveer in The Netherlands.

Designs for four-wheel drive in the U.S., came from the Twyford Company of Brookville, Pennsylvania in 1905, six were made there around 1906; one still exists and is displayed annually. The second U.S. four-wheel drive vehicle was built in 1908 by (what became) the Four Wheel Drive Auto Company (FWD) of Wisconsin (not to be confused with the term "FWD" as an acronym for front-wheel drive). 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. Thousands of the Jeffery Quad (1913-1919) were similarly used. The Reynolds-Alberta Museum has a four-wheel drive "Michigan" car from about 1905 in unrestored storage.

Daimler-Benz also has a history in four-wheel drive. In 1907 the Daimler Motoren Gesellschaft had built a four-wheel driven vehicle called Dernburg-Wagen, also equipped with four-wheel steering, that was used by German colonial civil servant, Bernhard Dernburg, in Namibia. Mercedes and BMW, in 1926, introduced a rather sophisticated four-wheel drive, the G1, the G4 and G4 following. The 1937 Mercedes-Benz G5 and BMW 325 4x4 featured full time four-wheel drive, four-wheel steering, three locking differentials, and fully independent suspension. They were produced because of a government demand for a four-wheel drive passenger vehicle. The modern G-series/Wolf such as the G500 and G55 AMG still feature some of the attributes, with the exception of fully independent suspension since it hinders suspension articulation. The Unimog is another Mercedes truck.

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. Possibly beaten by the 1938 GAZ-61.

The Land Rover appeared at the Amsterdam Motor Show in 1948, originally conceived as a stop-gap product for the struggling Rover car company, and despite chronic under-investment succeeded far better than the passenger cars. Land Rover developed a luxury 4WD with the Range Rover in the 1970s, which, unlike some offerings from other manufacturers, was capable of serious off-road use. The inspiration was a Willys MB that was frequently run off-road on the farm belonging to chief engineer Maurice Wilks, and was felt that it needed some refinement.

Kaiser Jeep introduced a 4WD wagon called the Wagoneer in 1963. It was revolutionary at the time, not only because of its technical innovations such as an independent front suspension and the first automatic transmission with 4WD, but also because it was equipped and finished as a regular passenger automobile. The Super Wagoneer (1966 to 1969) was powered by Rambler or Buick V8s. Its high level of equipment made it the first "luxury" SUV. American Motors (AMC) acquired Kaiser's Jeep Division in 1970 and quickly upgraded and expanded the entire line of serious off-road built 4WD vehicles. The top range full-size Wagoneer Limited continued to compete with traditional luxury cars. It was relatively unchanged during its production through 1991, even after Chrysler's buyout of AMC.

Jensen applied the Formula Ferguson full-time all-wheel drive system to 318 units of their Jensen FF built from 1966 to 1971, marking the first time 4WD was used in a production GT sports car. Subaru introduced the mass-produced Leone in 1972 featuring a part-time four-wheel drive systems that could not be engaged on dry pavement. The American Motors Company introduced a full time AWD vehicle the same year as the Subaru in the Jeep Cherokee and Wagoneer with Quadra Trac (1973 model year first models sold in Sept 1972). It dominated all other makes in FIA rally competition, due to the performance of the full time AWD, which did not require the driver to get out of the vehicle to lock hubs or manually select between 2WD and 4WD modes in the car like other American 4-wheel drive vehicles of the period. Drivers Gene Henderson and Ken Pogue won the FIA championship with a quadratrac equipped Jeep in 1972

American Motors introduced the innovative Eagle for the 1980 model year. This was the world's first complete line (sedan, coupe, and station wagon) of permanent automatic all-wheel drive passenger models. The new Eagles combined Jeep technology with an existing and proven AMC passenger automobile platform. They ushered a whole new product category of "sport-utility" or Crossover SUV. AMC's Eagles came with the comfort and high level appointments expected of regular passenger models and used the off-road technology for an extra margin of safety and traction.

The Eagle's thick viscous fluid center differential provided quiet and smooth transfer of power that was directed proportionally to the axle with the greatest traction. This was a true full-time system operating only in four-wheel drive without undue wear on suspension or driveline components. There was no low range in the transfer case. This became the forerunner of the designs that followed from other manufacturers. The automobile press at the time tested the traction of the Eagles and described it as far superior to the Subaru's and that it could beat many so-called off-road vehicles. Four Wheeler magazine concluded that the AMC Eagle was "The beginning of a new generation of cars."

The Eagles were popular (particularly in the snowbelt), had towing capacity, and came in several equipment levels including sport and luxury trims. Two additional models were added in 1981, the sub-compact SX/4 and Kammback. A manual transmission and a front axle-disconnect feature were also made available for greater fuel economy. During 1981 and 1982 a unique convertible was added to the line. The Eagle's monocoque body was reinforced for the conversion and had a steel targa bar with a removable fiberglass roof section.

The Eagle station wagon remained in production for one year after Chrysler Corporation acquired AMC in 1987.

Audi also introduced a permanently all-wheel driven road-going car, the Audi Quattro, in 1980. Audi's chassis engineer, Jorg 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, the resulting rally bred Audi Quattro was a famous and historically significant rally car. This feature was also extended to Audi's production cars and is still available today.

In 1987, Toyota also developed a car built for competition in rally campaigns. A limited number of road-going FIA Homologation Special Vehicle Celica GT-Fours (otherwise known as Toyota Celica Turbo All-Trac in some markets) were produced. The All-Trac system was later available on serial production Toyota Camry, Toyota Corolla, and Toyota Previa models.

Some of the earliest mid-engined four-wheel drive cars were the various road-legal rally cars made for Group B homologation, such as the Ford RS200 made from 1984-86. In 1989, niche maker Panther Westwinds created a mid-engined four-wheel drive, the Panther Solo 2.

Today, sophisticated all-wheel drive systems are found in many passenger vehicles and some exotic sports cars and supercars.

4WD in road racing

Bugatti created a total of three four-wheel drive racers, the Type 53, in 1932, but the cars were notorious for having poor handling.

Ferguson Research Ltd. built the front-engine P99 Formula One car that actually won a non-WC race with Stirling Moss in 1961. In 1969, Team Lotus raced cars in F1, and the Indy 500, that had both turbine engines and 4WD, as well as the 4WD-Lotus 63 that had the standard Cosworth engine. Matra also raced a similar MS84, and McLaren entered their M9A in the British Grand Prix, while engine manufacturers Cosworth produced their own version which was tested but never raced. All these F1 cars were considered inferior to their RWD counterparts, as the advent of aerodynamic downforce meant that adequate traction could be obtained in a lighter and more mechanically efficient manner, and the idea was discontinued, even though Lotus tried repeatedly.

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 without significant off-road capabilities are often referred to as All-Wheel Drives (AWD) or SUVs, while those with off-road 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 four times four) was coined in the 1950s to designate a vehicle, primarily a truck, which was equipped with a transfer case to drive the front wheels in addition to the rear wheels (rear-wheel drive was the norm) and a four-speed transmission (three speeds were the norm). Usually the shifter was mounted on the floorpan (column shifters were the norm). Today, as the "norms" have shifted, the term has evolved with the times and may be 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, and is often used to describe them as a two-wheel drive. In Australia the term is often used to describe a ute that sits very high on its suspension. This is to avoid the confusion that the vehicle might be a 4x4 because it appears to be otherwise suited to off-road applications.

Large American trucks with dual tires on the rear axles and two driven axles are officially designated 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 axles 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, as is the Pinzgauer, which is popular with defense forces around the globe.

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. (In CB slang, truckers refer to any two-axled vehicle as a "four-wheeler," sometimes in a derogatory sense, as distinguished from an "eighteen-wheeler" or tractor/trailer.)

Unusual four-wheel drive systems

Prompted by a perceived need for a simple, inexpensive all-terrain vehicle for oil exploration in North Africa, the French motor manufacturer Citroën developed the 2CV Sahara. Unlike other 4x4 vehicles which use a conventional transfer case to drive the front and rear axle, the Sahara had two engines, each independently driving a separate axle, with the rear engine facing backwards. The two throttles, clutches and gear change mechanisms could be linked, so both engines could run together, or they could be split and the car driven solely by either engine. Combined with twin fuel tanks and twin batteries (which could be set up to run either or both engines), the redundancy of two separate drive trains meant that they could make it back to civilization even after major mechanical failures. Only around 700 of these cars were built, and only 27 are known to exist today. Enthusiasts have built their own "new" Saharas, by rebuilding a 2CV and fitting the modified engine, gearbox and axle onto a new, strengthened chassis.

BMC experimented with a twin-engine Mini Moke (dubbed the "Twini Moke") in the mid-1960s, but never put it into production. This made advantage of the Mini's 'power pack' layout, with a transverse engine and the gearbox in the engine sump. Simply by fitting a second engine/gearbox unit across the rear, a rudimentary 4x4 system could be produced. Early prototypes had separate gear levers and clutch systems for each engine. Later versions sent for evaluation by the British Army had more user-friendly linked systems.

Suzuki Motors introduced the Suzuki Escudo Pikes Peak Edition in 1996. Earlier Suzuki versions were twin engined, from 1996 on the engine is a twin-turbocharged 2.0 L V6, mated to a sequential 6-speed manual transmission.

Nissan Motors has developed a system called E4WD, wherein the rear wheels, in a car that is normally front-wheel drive, are driven by electric motors. This system was introduced in some variants of the Nissan Cube and Tiida. (This is similar to the system used on the Ford Escape Hybrid AWD.)

Chrysler's Jeep Division debuted the twin engine, Jeep Hurricane concept at the 2005 North American International Auto Show in Detroit. This vehicle has a unique "crab crawl" capability, which allows it to rotate 360° in place. It also has dual Hemi V8s.

Some hybrid vehicles such as the Lexus RX400h provide power to an AWD system through a pair of electric motors, one to the front wheels and one to the rear. In the case of the AWD model version of the Lexus RX400h (and its Toyota-branded counterpart, the Highlander hybrid), the front wheels can also receive drive power directly from the vehicle's gasoline engine as well as via the electric motors, whereas the rear wheels derive power only from the second electric motor. Transfer of power is managed automatically by internal electronics based on traction conditions and need, making this an all-wheel drive system.

4WD and AWD systems by design type

Center differential with mechanical lock, or other torque transfer features

  • Alfa Romeo 164 Q4 (central viscous coupling, epicyclic unit and Torsen rear differential)
  • Alfa Romeo 155 Q4 (central epicyclic unit, Ferguson viscous coupling and Torsen rear differential)
  • Audi - Quattro Coupé, 4000, 5000 (locking center and rear differentials)
  • Audi Q7 -double pinion 50/50 with lockup clutch pack
  • Ford - Escort (RS 2000 16v 4x4 models and RS Cosworth), Sierra Cosworth, Sierra and Granada 4x4 models, Expedition(new): center mechanical differential/locking center differential(hi-lo)
  • Mercedes GL-Class
  • H1 & HMMWV NVG 242HD AMG open center differential, locked center differential, Neutral, low range locked. Also Torsen1 differential at the front and rear axle, The H1 moved to Torsen2 when ABS was added. The H1 Alpha had optional locking differentials in place of torsens
  • Hummer H2, H3 40/60 planetary with lock
  • Jeep Grand Cherokee, Commander (Quadra-Drive 2 version only for both vehicles)
  • Jeep Liberty, Jeep Cherokee, Dodge Durango (Select-Trac)- NV 242 transfer case- rear drive, open center differential, locked center differential, Neutral, low range
  • Full size Jeeps with Borg Warner QuadraTrac: limited slip center differential, 50/50 locked center differential. Low range could be used in locked or unlocked mode, allowing for use of low range on pavement.
  • Land Rover Defender (and Series III V8 models)
  • Land Rover/Land Rover Discovery/LR3
  • Lada Niva - full-time 4WD using open center diff with manual lock
  • Lexus LX470 -open with lock
  • Lexus RX300 -viscous coupling across the otherwise open center differential.
  • Mercedes-Benz Unimog (locking center and rear with up to 10 low range gears).
  • Mercedes-Benz G-Class (locking center and lockers on both front- and rearaxle)
  • Mitsubishi Pajero (also known as Montero or Shogun)
  • Nissan Patrol Pathfinder/Armada (All-Mode) multi clutch based with locking center differential
  • Porsche Cayenne (Porsche Traction Management) 38/62 planetary with lockup clutch pack
  • Range Rover
  • Suzuki Grand Vitara -full-time 4WD using limited-slip center differential, off-road 4WD with selectable center differential lock and low range transfer case, traction control and electronic stability control
  • Subaru Basic manual transmissions have a 50/50 center differential with viscous clutch, performance models have a planetary differential with computer regulated lockup. Automatic transmission models have ~45/55 planetary with computer controlled lock up
  • Toyota Sequoia (Multi-mode)
  • Toyota Land Cruiser
  • Volkswagen Touareg -double pinion 50/50 with lockup clutch pack

Torsen center differential

Non-locking center differential

  • Cadillac Escalade, STS AWD, SRX AWD (The first two generations had a viscous clutch on the center differential)
  • Chrysler 300C AWD
  • Dodge Magnum, Charger AWD
  • GMC Yukon Denali, XL Denali, Sierra Denali
  • Mercedes 4MATIC cars, R class, and ML class (note some MLs had low range)

The above systems function by selectively using the traction control system (via ABS) to brake a slipping wheel.

Multiple-clutch systems

Clutch pack coupling

Note the above all function like 2WD when clutch pack not engaged, and like 4WD highrange in a part time 4WD system when the clutch is engaged (usually by computer although some allow manual control). Some in this category have varying degrees of control in the torque distribution between front and rear via allowing some of the clutches in a clutch pack to engage and slip varying amounts. An example of a system like this is the BorgWarner i-Trac(TM) system. Note: the Haldex Traction based car list was created from the list on Haldex Traction corporate web site: | Haldex Cars Interestingly, a version of the BorgWarner ITM3e system is used on 2006 and up Porsche 911TT's. These Borg Warner systems were the forerunner of the popular Volkswagen Group Direct-Shift Gearbox (DSG) gearbox.

Off-road drive

These are vehicles with no center differential, also known as selectable 4WD. Off-road drive systems may not be driven in 4WD mode on dry pavement, as damage to the transfer case will occur.

  • Chevrolet Trailblazer and GMC Envoy
  • Chevrolet Tahoe Z71, Suburban Z71, Silverado Z71, Colorado Z71, S-10 series(Not all Z71's are 4-wheel drive)
  • Chevrolet Tracker
  • Dodge Power Wagon (a Ram version with front and rear differential locks)
  • Dodge Ram, Dakota
  • Dodge Nitro (Quadra-Trac 2)
  • Ford F series FX4, Explorer, Sport Trac (all control Trac 1)
  • Ford Ranger (Torsen rear differential)
  • Geo Tracker
  • GMC Yukon Z71, Sierra Z71
  • Infiniti QX56 (All-mode 4WD) Auto-engages 4WD with slip
  • Isuzu i-series
  • Jeep Cherokee (Quadra-Trac 2)
  • Jeep Cherokee, Jeep Wrangler, Jeep Comanche, Jeep Grand Cherokee (ZJ), Jeep Liberty (Command-Trac)
  • Jeep Wrangler (Rubicon has a locking front as well as rear)
  • Land Rover Series (except V8 models)
  • Lincoln Navigator (slip sensing can shift automatically into 4-high)
  • Lincoln Mark LT
  • Mazda B-series
  • Mitsubishi Raider
  • Nissan Patrol
  • Nissan Armada, Pathfinder (All-mode 4WD) Auto-engages 4WD with slip
  • Nissan Titan, Xterra, Frontier (rear locker an option)
  • Subaru Loyale, GL/DL, Brat Front/4wd/4wd lo, Justy
  • Suzuki Sidekick
  • Toyota Tacoma (rear locking differential optional)
  • Toyota Tundra TRD
  • Toyota FJ Cruiser (automatic transmission models)

See also

References

External links

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