In automotive design , an FWD, or FRONT-ENGINE, FRONT-WHEEL-DRIVE LAYOUT places both the internal combustion engine and driven roadwheels at the front of the vehicle.
* 1 Usage implications
* 2 Historical arrangements
* 2.1 Mid-engine / Front-wheel drive * 2.2 Longitudinally mounted front-engine and front-wheel drive
* 2.3 Front-engine transversely mounted / Front-wheel drive
* 3 See also * 4 References * 5 Further reading
Historically, this designation was used regardless of whether the entire engine was behind the front axle line. In recent times, the manufactures of some cars have added to the designation with the term front-mid which describes a car where the engine is in front of the passenger compartment but behind the front axle. Most pre-World War II front engine cars would qualify as front-mid engine, using the front-mid designation, or on the front axle .
This layout is the most traditional form, and remains a popular, practical design. The engine which takes up a great deal of space is packaged in a location passengers and luggage typically would not use. The main deficit is weight distribution — the heaviest component is at one end of the vehicle. Car handling is not ideal, but usually predictable.
In contrast with the front-engine, rear-wheel-drive layout (RWD), the
FWD layout eliminates the need for a central tunnel or a higher
chassis clearance to accommodate a driveshaft providing power to the
rear wheels. Like the rear-engine, rear-wheel-drive layout (RR) and
rear mid-engine, rear-wheel-drive layout (RMR) layouts, it places the
engine over the drive wheels, improving traction in many applications.
As the steered wheels are also the driven wheels, FWD cars are
generally considered superior to RWD cars in conditions where there is
low traction such as snow, mud, gravel or wet tarmac. When hill
climbing in low traction conditions RR is considered the best
two-wheel-drive layout, primarily due to the shift of weight to the
rear wheels when climbing. The cornering ability of a FWD vehicle is
generally better, because the engine is placed over the steered
wheels. However, as the driven wheels have the additional demands of
steering, if a vehicle accelerates quickly, less grip is available for
cornering, which can result in understeer . High-performance vehicles
rarely use the FWD layout because weight is transferred to the rear
wheels under acceleration, while unloading the front wheels and
sharply reducing their grip, effectively putting a cap on the amount
of power which could realistically be utilized; in addition, the high
horsepower of high-performance cars can result in the sensation of
torque steer . Electronic traction control can avoid wheel-spin but
largely negates the benefit of extra power. This was a reason for the
adoption of the four-wheel-drive quattro system in the high
Jensen FF and
Early cars using the FWD layout include the 1929 Cord L-29 , 1931 DKW
F1 , the 1948
There are four different arrangements for this basic layout, depending on the location of the engine, which is the heaviest component of the drivetrain.
MID-ENGINE / FRONT-WHEEL DRIVE
The earliest such arrangement was not technically FWD, but rather
mid-engine, front-wheel-drive layout (MF). The engine was mounted
longitudinally (fore-and-aft, or north-south) behind the wheels, with
the transmission ahead of the engine and differential at the very
front of the car. With the engine so far back, the weight distribution
of such cars as the Cord L-29 was not ideal; the driven wheels did not
carry a large enough proportion of weight for good traction and
handling. The 1934
Citroën Traction Avant solved the weight
distribution issue by placing the transmission at the front of the car
with the differential between it and the engine. Combined with the
car's low slung unibody design, this resulted in handling which was
remarkable for the era.
LONGITUDINALLY MOUNTED FRONT-ENGINE AND FRONT-WHEEL DRIVE
Left side Right side
Panhard Dyna X , designed by
Jean-Albert Grégoire , had the
engine longitudinally in front of the front wheels, with the
transmission behind the engine and the differential at the rear of the
assembly. This arrangement, used by Panhard until 1967, potentially
had a weight distribution problem analogous to that of the Cord L29
mentioned above. However, the Panhard's air-cooled flat twin engine
was very light, and mounted low down with a low centre of gravity
reducing the effect. The air-cooled flat twin engine of the Citroën
2CV was also mounted very low, in front of the front wheels, with the
transmission behind the axle line and the differential between the
two. This became quite popular; cars using this layout included the
Ford Taunus 12M and the
Lancia Flavia and Fulvia . This is the
standard configuration of
Oldsmobile Toronado (along with its sister models the
Cadillac Eldorado and
Buick Riviera ) used a novel arrangement which
had the engine and transmission in a 'side-by-side' arrangement with
power being transmitted between the two via a heavy-duty chain, and a
specially designed intermediate driveshaft that passed under the
engine sump. This family has the distinction of being the highest
engine capacity (8.2 L) front-wheel-drive vehicles ever built. The
FRONT-ENGINE TRANSVERSELY MOUNTED / FRONT-WHEEL DRIVE
The 1970s Giugiaro styled hatchback coupe
VW Scirocco based on
the contemporary Giugiaro Golf was in MK1 and MK2 forms the most
successful VW coupe with 800,000 made. The bonnet on this
Dante Giacosa 's Autobianchi Primula of 1964, Fiat 128 and Fiat 127 , put the transmission on one side of the transversely mounted engine, and doubled back the drivetrain to put the differential just behind the transmission, but offset to one side. Hence the driveshafts to the wheels are longer on one side than the other. This located the weight just a bit in front of the wheels. It is this system which dominates worldwide at present.
Front-wheel Drive Design Characteristics
MID-ENGINE, FRONT-WHEEL DRIVE (MF layout ):
LONGITUDINALLY FRONT-MOUNTED ENGINE, FRONT-WHEEL DRIVE (FF
longitudinal layout): The
TRANSVERSE FRONT-MOUNTED ENGINE, FRONT-WHEEL DRIVE (FF transverse layout): Fiat 128 , followed the footsteps of the Autobianchi Primula .
Front-wheel Drive Shafts
In front wheel drive vehicles, the drive shafts transfer the drive directly from the differential to the front wheels. A short inner stub shaft is splined to the differential side gear and an outer stub shaft is splined to the front wheel hub. Each stub shaft has a yoke, or housing, to accommodate a universal joint , at each end of a connecting intermediate shaft.
Universal joints let the shaft keep rotating while allowing for changes due to suspension movement, such as shaft length and horizontal angle, and shaft angle as the steering turns. Constant-velocity universal joints are normally used to transfer power smoothly between the components. The inner universal can be a plunge or tripod type joint. The tripod is splined to the intermediate shaft and held by a circlip. A ball, supported on needle roller bearings, is fitted to each post of the tripod, and these slide in a trunion inside the yoke. This caters for changes in shaft length and horizontal angle. The drive is transferred through the trunion and balls to rotate the shaft.
The outer universal joint allows greater angular changes but not changes in shaft length. It is normally a ball and cage type with an inner race splined to the intermediate shaft. An outer race is formed in the yoke. The cage retains the balls in location in grooves in both races. The balls transfer the drive from the shaft to the hub and allow for changes in horizontal angle and for a wide steering angle to be achieved. A flexible rubber boot fitted to each joint retains grease and keeps out dirt and moisture.
Where the differential is not located in the center line of the vehicle, an intermediate shaft can be fitted to maintain equal length drive shafts on each side. This keeps drive shaft angles equal on both sides and helps prevent steering irregularities and vibration. The outer end of the intermediate shaft is supported by a bearing secured to the transaxle case and a universal joint assists with alignment. In some cases a longer drive shaft is used on one side. A rubber dynamic damper may be fitted to absorb vibrations.
* ^ Hillier, Victor; Peter Coombes (2004). Fundamentals of motor
vehicle technology. Nelson Thornes. p. 9. ISBN 978-0-7487-8082-2 .
* ^ "Engine & Driveline Layouts". Drivingfast.net. Retrieved 6
* ^ www.motortrend.com Road Test: Rear Drive vs. Front Drive vs.
* ^ "
* Sedgwick, Michael Cars of the 50s and 60s. Gothenburg, Sweden: A B Nordbok, 1983. (Includes pictures of the engine layouts of the Traction Avant and other designs.)
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