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In automobile design, an 'RR', or 'Rear-engine, Rear-wheel drive' layout places both the engine and drive wheels at the rear of the vehicle. In contrast to the RMR layout, however, the center of gravity of the engine itself is actually past the rear axle. This is not to be confused with the center of gravity of the whole vehicle, as an imbalance of such proportions would make it impossible to keep the front wheels on the ground.
This layout is typically chosen for a combination of several reasons. For optimal handling and to eliminate the phenomenon known as torque steer, the wheels which propel the car should not be the same ones that steer it. For optimum traction, the engine should be nearest to the driven wheels since the engine is typically the densest/heaviest component of the car. Thus, in a car which steers with the front wheels, it is better for the engine to be located in the rear of the car — either a RR or RMR design. It is also advantageous in utility vehicles such as the VW Type 2, since the vehicle can carry a substantial amount of cargo above the engine compartment while retaining a relatively short wheelbase.
The disadvantages of the RR configuration is that placing the engine outside the wheelbase creates significant problems for handling as, when the car begins to slide on a corner, the end of the car will tend to want to swing wide and overtake the front — especially under braking. This tendency is referred to as oversteer and creates potential safety issues in racing applications as well as for ordinary drivers on wet or icy roads, although such behavior is desirable in drifting, a motorsport based on intentional oversteer.
In addition, even though the rear wheels benefit from the additional traction the added weight of the engine gives, the front wheels still need traction in order to steer the car effectively. For this reason, a RR layout car can also be prone to understeer. Most manufacturers have abandoned the RR layout apart from Porsche who has gradually developed their design with improvements to the suspension as well as electronic aids to reduce the shortcomings of the layout to acceptable levels.
Further, problems with engine cooling are commonplace. Since the radiator is not at the front of the automobile, it does not benefit from the airflow a front-engine car would readily avail of. Special ducting must be built into the body of the car to facilitate airflow to the radiator, therefore.
Porsche has continued to develop its 911 model as a rear-engined vehicle, although they have introduced multiple all-wheel drive models. Most notably, the 911 Turbo has been sold as AWD-only since the release of the 993 model. Race-oriented models such as the GT3 and twin-turbocharged GT2 remain solely RR, however.
Another manufacturer to implement the RR configuration was the De Lorean Motor Company with its DMC-12 sports car. To compensate for the uneven (35/65) weight distribution caused by the rear-mounted engine, De Lorean used rear wheels with a diameter slightly greater than the front wheels. Other than the Porsche 911, the last mass produced RR configured car, at least in Europe, is the Smart Fortwo. Before that was the Škoda 130/135/136 (produced until 1990) or the Polski Fiat 126p (produced until October 2000).
A range of sports road cars and racing cars with the RR layout were produced by the French company Alpine. These had bodies made of composite materials and used mechanical components made by Renault. (Alpine was eventually acquired by Renault; the A610 was a Renault product that used the Alpine name.)
Early cars using the RR layout include the Tucker, the Volkswagen Beetle, the Porsche 356, and the Chevrolet Corvair.