All-Wheel Drive (AWD) layouts
All-Wheel Drive (AWD), Four-Wheel Drive, 4WD, 4×4:
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.Main article: Four-wheel drive
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’s 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 1989 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.
- A ‘through the road’ hybrid vehicle uses a conventional piston engine to power two wheels, with electric motor/generators on the other two wheels, giving a form of part-time 4WD.
- The 2005 Jeep Hurricane concept had an all-wheel drive layout that featured two V8 engines powering a single driveshaft, with a gearbox mounted in the center of the vehicle. The gears connected to two additional driveshafts, one on each side of the vehicle, that delivered power to the wheels via driveshaft joints. This was designed in order to accommodate the vehicle’s unique steering system.
Porsche 959 (R4 layout)
Notable vehicles with the R4 layout include several high-performance Porsche sports cars, including the 959, the 911 Turbo since the introduction of the turbocharged version of the 993 series in 1995, and the 911 Carrera 4 introduced with the 964 series in 1989.
Some Volkswagen Kübelwagen (the rear-engined beetle-based military vehicle used by Germany in World War II) variants were produced with 4-wheel or all-wheel drive, including the Type 86, Type 87, Type 98.
Front-engine, Four wheel drive (F4 layout):
In automotive design, an F4, or Front-engine, Four-wheel-drive layout places the internal combustion engine at the front of the vehicle and drives all four roadwheels. This layout is typically chosen for better control on many surfaces, and is an important part of rally racing as well as off-road driving.
Most four-wheel-drive layouts are front-engined and are derivatives of earlier front-engined, two-wheel-drive designs.
This layout is also the drive train of choice for off-road pickup trucks and SUVs. It allows these vehicles to get the most traction without sacrificing precious cargo or passenger room. The center differential is often not present in these vehicles, meaning the 4WD system does not allow any difference in front and rear axle speeds. For normal road driving, these vehicles are shifted into 2WD mode, preventing damage to the transfer case; though full-time systems cannot go to 2WD mode.
The engine could be longitudinally/transversely-mounted:
Mid-engine, four wheel drive (M4 layout)
It is a type of car powertrain layout. Although the term “mid-engine” can mean the engine is placed anywhere in the car such that the centre of gravity of the engine lies between the front and rear axles, it is usually used for sports cars and racing cars where the engine is behind the passenger compartment. The motive output is then sent down a shaft to a differential in the centre of the car, which in the case of an M4 layout, distributes power to both front and rear axles.
The centre differential contained within many 4 wheel drive cars is similar to the conventional differential in a 2 wheel drive car. It allows torque to be distributed to both drive axles whilst allowing them to spin at different speeds, which vastly improves the cornering of a 4 wheel drive car on surfaces with high grip such as tarmac. However, unlike the differentials on the drive axles which are configured to provide torque equally to both wheels, the centre differential is usually set to have a bias to one set of drive wheels or the other, dependng on application of the car.
Some 4 wheel drive cars use a centre viscous coupling unit that will provide most power to the rear wheels unless the amount of torque being supplied to the rear wheels is in excess of the traction limits of the rear tyres, such as in a Lamborghini Murciélago. Others contain a computer that will decide how much power to distribute to any wheel at any time depending on the circumstances of each wheel. In general the M4 system is not widely used as it is suited toward sports cars and some off-road racing vehicles.
The engine is usually where the weight of a car is most concentrated so placing it between the front and rear axles gives a car a much better handling balance. Assuming the engine is behind the passenger compartment, the engine will also be pushing down on the rear wheels. Because the weight of a car is shifted toward the rear under acceleration in all cars as a rule, this further improves the amount of grip on the rear wheels, increasing the amount of torque that can be supplied to the rear wheels before wheelspin occurs. Because the engine is not in the front, the car can be designed with a minimum amount of frontal area perpendicular to the wind, greatly increasing aerodynamic efficiency.
A computer-controlled four-wheel-drive differential system allows a car to both accelerate and corner more quickly, since it can vary the amount of torque going to the front and rear wheels, and therefore vary how much the car behaves like a front- or rear-wheel-drive car. This means that through a fast corner the car is able to display more “neutral” handling – with less oversteer or understeer. This is a much more efficient means of turning and allows for faster cornering speeds as opposed to a two-wheel or conventional four-wheel-drive system. 
Such a system is very difficult and expensive to design and engineer, which is why it is only usually found on race cars and very expensive sports cars.
Most mid-engine cars, because of the size and position of the engine and transmission, compromise heavily on both passenger and boot/trunk space.
Four-wheel-drive systems tend to be quite heavy and some of the engine’s power can be lost through the various differentials in the car, in addition to the frictional losses of the powertrain.
The variable handling characteristics of a four-wheel-drive car mean that when travelling round a corner at high speeds the car may enter the corner and understeer and then half-way through the corner suddenly start to oversteer.
Posted on July 22, 2012, in General, Uncategorized and tagged 4wd, 4x4, all wheel drive, audi quattro, awd, cars, F4 layout, four wheel drive, front engine all wheel drive, jensen ff, lamborghini sesto elemento awd, lancer evolution, lancia, m4 layout, mercedes benz w124, mid engine all wheel drive, mitsubishi 3000gt, mitsubishi 3000gt vr, quattro, R4 layout, racing all wheel drive, rally, rb4, rear engine all wheel drive, subaru impreza wrx sti, transportation, wheel drive 4wd. Bookmark the permalink. 1 Comment.