All-Wheel Drive

By SRT-Tom · Nov 22, 2019 ·
  1. SRT-Tom
    Four-wheel drive, 4WD, 4x4 ("four by four"), or AWD ("all-wheel drive") is a four-wheeled vehicle with a drivetrain that allows all four wheels to receive torque from the engine simultaneously. While many people associate the term with off-road vehicles and sport utility vehicles, powering all four wheels provides better control in normal road cars on many surfaces, and is an important part in the sport of rallying.

    The first application of four-wheel drive, occurred in 1893. In 1937, the Mercedes-Benz G5 and BMW 325 4x4 featured full time four-wheel drive, four-wheel steering, three locking differentials, and fully independent suspension.

    1937 Mercedes G5

    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 WWII.


    1943 Willys Overland Jeep

    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-World Championship race with Stirling Moss in 1961. In 1969,Team Lotus raced cars in the Indy 500 and two years later in Formula 1 with the Lotus 56, that had both turbine engines and 4WD, as well as the 4WD-Lotus 63 that had the standard Cosworth engine. Nissan and Audi had success with all-wheel drive in road racing with the former's advent of the Nissan Skyline GT-R in 1989.


    In abbreviations such as 4x4, the first figure is normally taken as the total number of wheels and the second is normally taken as the number of powered wheels (the numbers are actually axle-ends to allow for more than one wheel on each end of an axle). 4x2 means a four-wheel vehicle in which engine power is transmitted to only two axle-ends: the front two in front-wheel drive or the rear two in rear-wheel drive.

    Four Wheel Drive
    "A" points at the engine, "B" points at a driving wheel, "C" points at the transfer case or center differential depending on system

    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 and classification purposes, and are not individually powered. True six-wheel drive vehicles with three powered axles such as the famous "duece 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..

    4WD versus AWD

    The term four-wheel drive typically describes truck-like vehicles that may allow the driver to manually switch (sometimes with an automatic option) between two-wheel drive mode (if available) 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 damaging the differentials, 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 axle to rotate at a different speed. This eliminates driveline binding, wheel hop, and other driveline issues associated with the use of 4WD on dry pavement. For 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.

    Because all 4 tires in a full time AWD system are connected by a system of differentials, they are potentially very susceptible to torque reduction when a wheel loses traction. Without sophisticated traction control systems, they would become immobilized when any one of the four tires lost traction. A traditional part-time 4WD system does not connect the front and rear via a differential, and therefore does not suffer any front/rear torque reduction - if a front tire loses traction, it does not reduce torque delivered to the rear tires, even without traction control systems.

    Part-time 4WD systems are therefore mechanically simpler, cheaper, and tougher than AWD systems, and inherently better at making use of available traction. Part time 4WD transfer cases are also usually equipped with a gear reduction setting that multiplies torque for greater power at lower speeds, a vital feature for vehicles that will see much off road use. The drawback is that because it lacks a center differential, a part time 4WD system can only be used in low traction situations where the wheels have the ability to slip as needed.

    For these reasons, full time AWD is appropriate for improving on road handling and is seen on cars and car-based crossover SUVs, while traditional part-time 4WD systems, without center differentials, or with locking center differentials, or with locking center differentials, are better for heavy duty use, such as off- roading or deep snow, and are commonly seen on trucks and truck based SUVs.

    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 becoming commonplace on vehicles, including muscle cars like the Challenger (Note- Traction control can be disengaged for racing by pressing a button in the dash).

    Challenger Traction Control Switch

    A second method to reduce slippage is a locking differential. It works 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.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.

    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.

    2009 Ford Edge with Selectable AWD

    Challenger AWD GT

    The 2017 Dodge Challenger GT became the only car in its class to offer AWD. It featured the 305 horsepower, 3.6-liter Pentastar V6 that makes 268 lbs-feet of torque.

    2017 Challenger GT

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