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 MR 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 vehicle itself, as an imbalance of such proportions would make it nearly 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 MR design.
The disadvantage 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.
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.
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.
A range of sports road cars and racing cars with the RR layout was produced by the French company Alpine (badged as Renaults in some contries, including the UK, to avoid trade mark conflicts). These had bodies made of composite materials and used mechanical components made by Renault.
|FF | FMR | FR | MF | RMR | RR | F4|
|Front-engine | Mid-engine | Rear-engine|
|Front-wheel drive | Rear-wheel drive | Four-wheel drive | Six-wheel drive|