automobile layout 03

8/3/2019 Automobile Layout 03

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In automotive design the automobile layout describes where on thevehicle, the engine and drive wheels are found. Many differentcombinations of engine location and driven wheels are found in practice,and the location of each is dependent on the application the vehicle willbe used for. Factors influencing the design choice include cost,

complexity, reliability, packaging (location and size of the passengercompartment and boot space), weight distribution and the vehicle'sintended handling characteristics.

Layouts can roughly be divided into two categories: front- or rear-wheeldrive. Four-wheel drive vehicles may take on the characteristics of either,depending on how the system is set up to distribute power to the wheels.

Front-engine, front-wheel drive layout

In automotive design, an FF, or Front-engine, Front-wheel drive layoutplaces both the engine and drivenroadwheels at the front of the vehicle.

This layout is typically chosen for itscompact packaging, allowing the rest ofthe vehicle to be designed moreflexibly. In contrast with the front-engine, rear-wheel drive layout (FR), the

FF layout eliminates the need for a central tunnel or a higher chassisclearance to accommodate a driveshaft providing power to the rearwheels. Like the rear-engine, rear-wheel drive layout (RR) and rear mid-engine, rear-wheel drive layout (RMR) layouts, it places the engine overthe drive wheels which may aid traction in many applications. As thesteered wheels are also the driven wheels, FF cars are generallyconsidered superior to FR cars in conditions where there is low tractionsuch as snow, mud, gravel or wet tarmac. When hill climbing in lowtraction conditions RR is considered the best two wheel drive layout.However, very powerful cars rarely use the FF layout because weight istransferred to the rear wheels under acceleration, while unloading thefront wheels and sharply reducing their grip, effectively putting a cap onthe amount of power which could realistically be utilized. Electronictraction control can avoid wheel-spin but largely negates the benefit ofextra power. This was the reason for the adoption of the four wheel drivequattro system by previously front wheel drive specialist Audi with the1980 Audi Quattro for road cars.

Mid-engine, front-wheel drive layout

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In automotive design, a MF, or Mid-engine, Front-wheel drive layoutis one in which the front roadwheels are driven by an internal-combustionengine placed just behind them, in front of the passenger compartment. Incontrast to the Front-engine, front-wheel drive layout (FF), the center ofmass of the engine is behind the front axle. This layout is typically chosen

for its better weight distribution (the heaviest component is near thecenter of the car, lowering its moment of inertia).However, the mid-engined layout does use up central space, making the resulting vehiclerather long. This may be why nomanufacturer currently offers the MFlayout.Examples of road cars using theMF layout include the Cord 810, CitronTraction Avant, Citron DS, Renault 4,Saab Sonett mk1, and the Citron SM.

Front-engine, rear-wheel drive layout

In automotive design, an FR, or Front-engine, Rear-wheel drive layoutis one where the engine is located at the front of the vehicle and drivenwheels are located at the rear. This was the traditional automobile layoutfor most of the 20th century.

The FR layout is often chosen for its simple design and good handlingcharacteristics. Placing the drive wheels at the rear allows ample room for

the transmission in the center of the vehicle and avoids the mechanicalcomplexities associated with transmitting power to the front wheels. Forperformance-oriented vehicles, the FR layout is more suitable than front-wheel drive designs, especially with engines that exceed 200 horsepower. This is because weight transfers to the rear of the vehicle duringacceleration, which loads the rear wheels and increases their grip.

Another advantage of the FR layout is relatively easy access to the enginecompartment, as result of the longitudinal orientation of the drivetrain, ascompared to the FF layout (front-engine, front-wheel drive).

Some manufacturers, such as Alfa Romeo, Porsche (944, 924, 928) andChevrolet (C5 and C6 Corvettes), retained this layout but moved thegearbox from behind the engine to between the rear wheels, putting moreweight over the driven axle. This configuration is often referred to as atransaxle since the transmission and axle are one unit.

Front mid-engine, rear-wheel drive layout

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In automotive design, a front mid-engine, rear-wheel drive layout(FMR) is one that places the engine in the front, with the rear wheels ofvehicle being driven. In contrast to the front-engine, rear-wheel drive (FR)layout, the engine is pushed back far enough that its center of mass is tothe rear of the front axle. This aids in weight distribution and reduces the

moment of inertia, improving the vehicle's handling. The mechanicallayout of a FMR is substantially the same as a FR car. Some models of thesame vehicle can be classified as either FR or FMR depending on thelength of the installed engine (e.g. 4-cylinder vs. 6-cylinder) and its centreof mass in relation to the front axle.

FMR cars are often characterized by a long hood and front wheels that arepushed forward to the corners of the vehicle, close to the front bumper.Grand tourers often have FMRlayouts, as a rear engine would notleave much space for the rear seats.

Rear mid-engine, rear-wheel drive layout

In automotive design, a RMR or RearMid-engine, Rear-wheel drive layout is one in which the rear wheelsare driven by an engine placed just in front of them, behind the passengercompartment. In contrast to the rear-engined RR layout, the center ofmass of the engine is in front of the rear axle. The layout has a tendencytoward being heavier in the rear than the front, which allows for bestbalance to be achieved under braking. However, since there is little weightover the front wheels, under acceleration, the front of the car is prone tolift and cause understeer. The mid-engined layout also uses up centralspace, making it impractical for any but two-seater sports cars.

In modern racing cars, RMR is the usual configuration and is usuallysynonymous with "mid engine". Due to its favorable weight dynamics, thislayout is heavily employed in Formula racing cars (such as Formula One).

This configuration was also common in very small engined 1950smicrocars, in which the engines did nottake up much space. Because ofsuccesses in racing, the RMR platformhas been popular for road-going sportscars despite the inherent challenges ofdesign, maintenance and lack of cargospace.

Rear-engine, rear-wheel drive layout

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In automotive design, a RR, or Rear-engine, Rear-wheel drive layoutis one which places both the engine and drive wheels at the rear of thevehicle. In contrast to the RMR layout, however, the center of mass of theengine itself is actually past the rear axle. This is not to be confused withthe center of mass of the whole vehicle, as an imbalance of such

proportions would make it impossible to keep the front wheels on theground.

Most of the traits of the RR configuration are shared with the MR. Placingthe engine near the driven rear wheels allows for a physically smaller,lighter, less complex, and more efficient drivetrain, since there is no needfor a driveshaft, and the differential can be integrated with thetransmission, commonly referred to as a transaxle.

Since the engine is typically the heaviest component of the car, putting itnear the rear axle usually results in more weight over the rear axle thanthe front, commonly referred to as a rear weight bias. The farther back theengine, the greater the bias. As a very general trend with numerousexceptions, typical weight bias for an FR is 55/45 front/rear; for MR, 45/55;for RR, 35/65. Rear weight bias reduces forward weight transfer underbraking, and increases rear weight transfer under acceleration. The formermeans that traction is more evenly distributed among all four wheelsunder braking, resulting in shorter stopping times and distances. The lattermeans that the driven wheels have increased traction when accelerating,allowing them to put more power on the ground and accelerate faster. Thedisadvantage to a rear weight bias is that the car can become unstable

and tend to oversteer when decelerating (whether braking or lifting off thethrottle). When not braking, the decreased weight over the front wheelsmeans less traction, resulting in the car having a tendency to understeer,which is safer and more stable (further allowing a driver to get on fullthrottle sooner) but is undesirable in cornering.

Due to the handling difficulty, the need for more space efficiency, and thenear ubiquitous use of liquid-cooled engines in modern cars, mostmanufacturers have abandoned the RR layout. The major exception isPorsche, who have developed the 911 for over 40 years and have takenadvantage of the benefits of RR while mitigating its drawbacks to

acceptable levels, lately with the help of electronic aids.

The Tata Nano, the cheapest ($2500)production car in the world introducedin 2008 and built in India also featuredthis layout.

Rear-engine, front-wheel

drive layout

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A rear engine, front wheel drive layout is one in which the engine isbehind the rear wheels, but drives the front wheels via a driveshaft, like aconventional front-engine rear-drivevehicle traveling in reverse.Althoughuncommon in todays' cars, this drive

layout has been used in the past, byBuckminster Fuller in his Dymaxion car.

Front-engine, four-wheel drive layout

In automotive design, an F4, or Front-engine, Four-wheel drive layoutplaces the engine at the front of the vehicle and drives all four roadwheels.This layout is typically chosen for better control on many surfaces, and isan important part of rally racing as well as off-road driving.Probably thebest known user of this layout is Audi with its quattro four wheel drivesystem, which first appeared in the 1980 Audi Quattro road car.

This layout is also the drive train of choice for off-road pickup trucks andSUVs. It allows these vehicles to get the most traction without sacrificingprecious cargo or passenger room. The center differential is often notpresent in these vehicles, the 4WD system is constantly splitting powerbetween both axles when engaged.For normal road driving, these vehiclesare shifted into 2WD mode, preventing damage to the transfer case.

Mid-engine, four-wheel drive layoutIn automotive design, an M4, or Mid-engine, Four-wheel drive layoutplaces the engine in the middle of the vehicle, between both axles anddrives all four roadwheels. Although the term "mid-engine" can mean theengine is placed anywhere between the front and rear wheels, it is usuallyused for sports cars and racing cars where the engine is behind thepassenger compartment. Such a system is expensive to design andengineer, which is why it is only usually found on race cars and veryexpensive sports cars.Most mid-engine cars, because of the size andposition of the engine and transmission, compromise heavily on both

passenger and boot/trunk space.Four-wheel drive systems tend to be quite

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heavy and some of the engine's power can be lost through the variousdifferentials in the car, in addition to the frictional losses of the powertrain.

Rear-engine, four-wheel drive layout

In automotive design, an R4, or Rear-engine, Four-wheel drive layoutplaces the engine at the rear of the vehicle, and drives all fourroadwheels.This layout is typically chosen to improve the traction or thehandling of existing vehicle designs using the rear-engine, rear-wheeldrive layout (RR).Notable vehicles with the R4 layout include several high-performance Porsche sports cars, including the 959, the 911 Turbo sincethe introduction of the turbocharged version of the 993 series in 1995.

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automobile layout 03

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