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Automotive diagrams 10 En

FF transversely-mounted engine layout

Automotive diagrams 08 En

FF longitudially-mounted engine layout

In automotive design, an FF, or Front-engine, Front-wheel drive layout places both the internal combustion engine and driven roadwheels at the front of the vehicle.

Usage implicationsEdit

In contrast with the front-engine, rear-wheel drive layout (FR), the FF 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, FF cars are generally considered superior to FR 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[citation needed]. The cornering ability of a FF vehicle is generally better, because the engine is placed over the steered wheels.[1] 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.[2] High performance vehicles rarely use the FF 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.[3] This was a reason for the adoption of the four wheel drive quattro system by front wheel drive specialist Audi with the 1980 Audi Quattro for road cars.

Historical arrangementsEdit

Early cars using the FF layout include the 1929 Cord L-29, 1931 DKW F1, the 1948 Citroën 2CV, 1949 Saab 92 and the 1959 Mini. In the 1980s, the traction and packaging advantages of this layout caused many compact and mid-sized vehicles to adopt it.

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 driveEdit

The earliest such arrangement was not technically FF, 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. Citroën and Renault used this layout in some models into the 1980s.

Front-engine longitudinally-mounted / Front-wheel driveEdit

The 1946 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 engine was very light, reducing the effect. The engine of the Citroën 2CV was in front of the front wheels, with the transmission behind the axle and the differential between the two. This became quite popular; cars using this layout included the German Ford Taunus 12M and the Lancia Flavia and Fulvia. This is the standard configuration of Audi and Subaru front wheel drive vehicles. The first generation Oldsmobile Toronado and the Saab 99 and “classic” Saab 900 had their engines mounted approximately on the front axle center line, with power being taken by chains or a gear train to a transmission and differential mounted below and beside the engine.

Front-engine transversely-mounted / Front-wheel driveEdit

Scirocco MK I

The VW Scirocco from the 1970s is considered one of the benchmarks of today's sporty hatchbacks.[4]

BMC Mini 021

The bonnet on this older Mini is open, showing the transversely mounted engine that drives the front wheels.

Issigonis's Mini of 1959 and related cars such as the Maxi, Austin 1100/1300 and Allegro had the engine transversely mounted. The transmission was located in the sump below the crankshaft, with power transmitted by transfer gears.

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.

Vehicles with the Giacosa arrangement tend to suffer from torque steer under heavy acceleration.[citation needed] The shorter drive shaft, being stiffer than the longer drive shaft, transmits the motion to the wheels immediately instead of 'winding' up due to the drive torque.[citation needed] The net result is more tractive force at the wheel with the shorter drive shaft and the car tends to pull to the opposite side.[citation needed]

Front-wheel drive design characteristicsEdit

ReferencesEdit

  1. Hillier, Victor; Peter Coombes (2004). Fundamentals of motor vehicle technology. Nelson Thornes, 9. ISBN 9780748780822. 
  2. "Engine & Driveline Layouts". Drivingfast.net. Retrieved on 6 January 2010.
  3. www.motortrend.com Road Test: Rear Drive vs. Front Drive vs. All-Wheel Driv
  4. www.oneighturbo.com Comeback of a sports car legend: Volkswagen Scirocco - accessed 14 March 2010

Further readingEdit

  • 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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