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Front-engine, front-wheel drive layout: Wikis

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FF transversely-mounted engine layout
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.

Contents

Usage implications

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 which may aid 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.[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. Electronic traction control can avoid wheel-spin but largely negates the benefit of extra power.[3] This was the reason for the adoption of the four wheel drive quattro system by previously front wheel drive specialist Audi with the 1980 Audi Quattro for road cars. The pioneer of four wheel drive road cars was the Jensen FF in the 1960s for the same reasons.

Historical arrangements

Early cars using the FF layout include the 1931 DKW F1, the 1948 Citroën 2CV, 1949 Saab 92 and the 1959 the original Mini. In the 1980s, the traction and packaging advantages of this layout caused many compact and mid-sized vehicles to adopt it. In transversely-mounted engine FF designs the requirement of a bevel gear to change the direction of the final drive is not needed so coastdown losses are reduced by approximately 2-3%[citation needed] of flywheel power and hence overall efficiency is slightly higher than with an FR design.

There are four quite different particular arrangements for this basic layout, according to the location of the engine, which is the heaviest component of the drivetrain, with respect to the front wheels:

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Mid-engine / Front-wheel drive

The earliest such arrangement was not technically FF, but rather mid-engine, front-wheel drive layout (MF) and had the engine mounted longitudinally (fore-and-aft, or north-south) behind the wheels, with the transmission and differential in front. It was designed by Walter Miller, who had the drivetrain double back to put the differential in the middle, with brakes mounted inboard. E.L. Cord took the easier method of putting the differential in front. With the engine so far back, the weight balance of the L-29 Cord was unwieldy; the driven wheels did not have enough weight upon them. His later 810 and 812 cars were similar. The Citroën Traction Avant used the same MF layout, but solved the weight distribution issue with a new, low slung unibody design, resulting in remarkable handling for the era. This layout was adopted by Renault from the 1960s until the end of the 1980s.

Front-engine longitudinally-mounted / Front-wheel drive

The Grégoire Sport, (designed by Jean-Albert Grégoire), amongst other cars by that firm, had the engine longitudinally in front of the front wheels, with the differential in the middle. This became quite popular, as the German Ford Taunus 12M and the Lancia Flavia used it as well. This is the standard Audi front wheel drive configuration.

Front-engine transversely-mounted / Front-wheel drive

The VW Scirocco from the 1970s is considered the benchmark of today's sporty cars.[4]
Under the bonnet compact transversal layout shows how Mini maximizes passenger space.

Issigonis's Mini and a few successor cars had the engine transversely mounted (east-west), with the transmission in the sump below the crankshaft, with power transmitted by transfer gears. This was as near as possible to putting the entire weight of the drivetrain on the front wheels.

Dante Giacosa put the transmission on one side of the transversely mounted engine, and doubled back the drivetrain to put the differential just behind it, but offset to one side. Hence the driveshafts to the wheels are longer on one side than the other, something which was avoided in the past. This located the weight just a bit in front of the wheels. This arrangement was first tried out on the Autobianchi Primula, next on the Fiat 128, and finally on the Fiat 127, which became car of the year. 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] For this reason, the Issigonis design (in which the two driveshafts are equal in length) is still preferred by many performance drivers and accounts for much of the Mini's success in rally and short-track circuit racing.

Front-wheel drive design characteristics:

  • 1] Mid-engine, front-wheel drive (MF layout): Renault 4 mid-engine, front-wheel drive layout allows greater distance between front doors and wheelwells, and short front overhang.
  • 2] Longitudinally front-mounted engine, front-wheel drive (FF longitudinal layout): The Auto Union 1000, (today Audi) longitudinal layout superseded the DKW F89 front transversal engines in the 1950s.
  • 3] Transversely front-mounted engine, front-wheel drive (FF transversal layout): Fiat 128, followed the foot steps of the rear engined Fiat 600.

References

  1. ^ Hillier, Victor; Peter Coombes (2004). Fundamentals of motor vehicle technology. Nelson Thornes. pp. 9. ISBN 9780748780822. 
  2. ^ "Engine & Driveline Layouts". Drivingfast.net. http://www.drivingfast.net/track/engine-driveline.htm#2. Retrieved 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 reading

  • 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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