automotive design Automotive design is the process of developing the appearance (and to some extent the ergonomics) of motor vehicles - including automobiles, motorcycles, trucks, buses, coaches, and vans. The functional design and development of a modern m ...

, an M4, or

Mid-engine In automotive engineering, a mid-engine layout describes the placement of an automobile engine in front of the rear-wheel axles, but behind the front axle. History The mid-engine, rear-wheel-drive format can be considered the original layout of ...

, Four-wheel-drive layout places the

internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal co ...

in the middle of the vehicle, between both axles and drives all four road wheels. It is a type of car

powertrain A drivetrain (also frequently spelled as drive train or sometimes drive-train) is the group of components that deliver mechanical power from the prime mover to the driven components. In automotive engineering, the drivetrain is the components o ...

layout. Although the term "

mid-engine In automotive engineering, a mid-engine layout describes the placement of an automobile engine in front of the rear-wheel axles, but behind the front axle. History The mid-engine, rear-wheel-drive format can be considered the original layout of ...

" 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 A sports car is a car designed with an emphasis on dynamic performance, such as handling, acceleration, top speed, the thrill of driving and racing capability. Sports cars originated in Europe in the early 1900s and are currently produced by ...

and

racing cars Auto racing (also known as car racing, motor racing, or automobile racing) is a motorsport involving the racing of automobiles for competition. Auto racing has existed since the invention of the automobile. Races of various sorts were organise ...

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

axle An axle or axletree is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with the wheels rotating around the axle. In the former case, beari ...

s. 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, depending on application of the car. Some 4 wheel drive cars use a centre

viscous coupling unit A viscous coupling is a mechanical device which transfers torque and rotation by the medium of a viscous fluid. Design Rotary viscous couplings with interleaved, perforated plates and filled with viscous fluids are used in automotive system ...

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.

Benefits

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 Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dyn ...

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 Rear-wheel drive (RWD) is a form of engine and transmission layout used in motor vehicles, in which the engine drives the rear wheels only. Until the late 20th century, rear-wheel drive was the most common configuration for cars. Most rear-wheel ...

car. This means that through a fast corner the car is able to display more “neutral” handling - with less

oversteer Understeer and oversteer are vehicle dynamics terms used to describe the sensitivity of a vehicle to steering. Oversteer is what occurs when a car turns (steers) by more than the amount commanded by the driver. Conversely, understeer is what occ ...

understeer Understeer and oversteer are vehicle dynamics terms used to describe the sensitivity of a vehicle to steering. Oversteer is what occurs when a car turns (steers) by more than the amount commanded by the driver. Conversely, understeer is what occ ...

. 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.

Drawbacks

Some cars are designed with a rear wheel drive powertrain where the power is sent to the rear wheels. This design is cheaper than the M4 design because of the limited parts required to make the system function. Manufacturers use a unique layout to balance the weight distributed between the front and rear axels. Having a front engine layout with a rear wheel drive powertrain almost evens out the weight distributed in order to optimize performance and handling. 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

. 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.RallyCars.co
Full time 4 wheel drive turbo charged car dynamics
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Gallery

File:Peugeot_Talbot_Sport_205_T16_(28504430670).jpg,

Peugeot 205 T16 The Peugeot 205 is a supermini (B-segment) car produced by the French manufacturer Peugeot from 1983 to 1999. It was declared "car of the decade" by ''CAR Magazine'' in 1990. It also won ''What Car?s Car of the Year for 1984. The 205 was introd ...

, the first of the M4

Group B Group B was a set of regulations for grand touring (GT) vehicles used in sports car racing and rallying introduced in 1982 by the Fédération Internationale de l'Automobile (FIA). Although permitted to enter a GT class of the World Sportscar ...

rally cars File:Lamborghini_Diablo.JPG, Lamborghini Diablo VT6.0, an early M4

supercar A supercar – also called exotic car – is a loosely defined description of street-legal, high-performance sports cars. Since the 2010s, the term hypercar has come into use for the highest performing supercars. Supercars commonly serve as t ...

File:Bugatti_Veyron_16.4_%E2%80%93_Frontansicht_(3),_5._April_2012,_D%C3%BCsseldorf.jpg,

Bugatti Veyron The Bugatti Veyron EB 16.4 is a mid-engine sports car, designed and developed in Germany by the Volkswagen Group and Bugatti and manufactured in Molsheim, France, by French automobile manufacturer Bugatti. It was named after the racing driver P ...

, the world's fastest production car at the time of its production File:Detroit_NAIAS_2015_2016_Acura_NSX.JPG, Honda/Acura NSX, a novel M4 car using a petrol-electric drive for the front wheels file:Red_2019_Ferrari_SF90_Stradale_%2848264238897%29_%28cropped%29.jpg,

Ferrari SF90 Stradale The Ferrari SF90 Stradale (Type F173) is a mid-engine PHEV (Plug-in Hybrid Electric Vehicle) sports car produced by the Italian automobile manufacturer Ferrari. The car shares its name with the SF90 Formula One car with SF90 standing for the 90 ...

, a twin-turbocharged PHEV M4

References

{{Automobile layouts Four-wheel drive layout