In automotive design, a RR, or Rear-engine, Rear-wheel drive layout is one which places both the engine and drive wheels at the rear of the vehicle. In contrast to the RMR layout, however, the center of mass of the engine itself is actually past the rear axle. This is not to be confused with the center of mass 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, since all cars steer with the front wheels, it is better for many applications that the engine to be located in the rear—either an 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. Most of the other air-cooled VW types were also RR, including the Type 1 Volkswagen Beetle.
The disadvantages of the RR configuration is that placing the engine outside the wheelbase creates significant problems for handling, specifically skid recovery. When the car begins to slide on a corner, rotational inertia dictates that the added weight away from the axis of rotation (generally the steering wheels) will be more likely to maintain the spin, 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, an 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 therefore be built into the body of the car to facilitate airflow to the radiator.
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). More recently, the $2500 Tata Nano, the cheapest production car in the world introduced in 2008 and built in India also featured this layout.
A range of sports road cars and racing cars with the RR layout was 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.)