Four-wheel drive

Four-wheel drive, All-wheel drive, AWD, 4WD, or 4×4 ("four by four") is a four-wheeled vehicle with a drivetrain that allows all four wheels to receive torque from the engine simultaneously.

While many people typically associate the term 4x4 with off-road vehicles and Sport utility vehicles, powering all four wheels provides better control than normal road cars on many surfaces, and is an important part in the sport of rallying.

In abbreviations such as 4×4, the first figure is normally the total wheels (more precisely, axle ends, which may have multiple wheels), and the second, the number that are powered.

Syntactically, 4×2 means a four-wheel vehicle that transmits engine power to only two axle-ends: the front two in front-wheel drive or the rear two in rear-wheel drive.

By this system, a six wheeled military transport truck would be a 6×6, while the typical American semi-truck tractor unit having two drive axles and a single unpowered steering axle would be a 6×4.

Four wheel drive refers to vehicles that have a transfer case (some of which include a differential that may or may not be lockable) between the front and rear axles, meaning that the front and rear drive shafts will not rotate at different speeds. This provides maximum torque transfer to the axle with the most traction, but can cause binding in high traction, tight turning situations. They are also either full-time or part-time 4WD selectable.

All wheel drive refers to a drive train system that includes a differential between the front and rear drive shafts. This is usually coupled with some sort of anti-slip technology that will allow all wheels to spin at different speeds, but still maintain the ability to transfer torque from one wheel in case of loss of traction at that wheel. All wheels are engaged to the drive full-time.

Differentials

When powering two wheels simultaneously the wheels must be allowed to rotate at different speeds as the vehicle goes around curves. This is accomplished with a differential. A differential allows one input shaft (e.g., the driveshaft of car or truck) to drive two output shafts (e.g. - axles shafts that go from the differential to the wheel) independently 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 differential ring gear. Each powered axle requires a differential to distribute power between the left and the right sides. When all four wheels are driven, a third or 'center' differential can be used to distribute power between the front and the rear axles.

The described system handles extremely well, as it is able to accommodate various forces of movement and distribute power evenly and smoothly, making 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. Since a differential applies equal torque to each half-shaft, power is reduced at the other wheels, even if they have good traction. 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 with which to lose traction, increasing the likelihood that it may happen. 4WD vehicles may also be more likely to drive on surfaces with reduced traction. However, since torque is divided amongst four wheels rather than two, each wheel receives approximately half the torque of a 2WD vehicle, reducing the potential for wheelslip.

In 1893, before the establishment of a modern automotive industry in Britain, English engineer Bramah Joseph 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 Bramah'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. An electric hub motor at each wheel powered the vehicle. 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 the Hague 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 around 15,000 of its four-wheel-drive Model B trucks for the British and American armies during World War I. Approximately 11,500 of the Jeffery or Nash Quad models (1913–1919) were similarly used. The Quad not only came with four-wheel drive and four-wheel brakes, but also featured four-wheel steering.

The Reynolds-Alberta Museum has a four-wheel-drive "Michigan" car from about 1905 in unrestored storage. The Marmon-Herrington Company was founded in 1931 to serve a growing market for moderately priced four-wheel-drive vehicles. Marmon-Herrington specialized in converting Ford trucks to four-wheel drive and got off to a successful start by procuring contracts for military aircraft refueling trucks, 4×4 chassis for towing light weaponry, commercial aircraft refueling trucks, and an order from the Iraqi Pipeline Company for what were the largest trucks ever built at the time.