In automotive design, a RMR or Rear Mid-engine, Rear-wheel drive layout is one in which the rear wheels are driven by an engine placed just in front of them, behind the passenger compartment. In contrast to the rear-engined RR layout, the center of mass of the engine is in front of the rear axle. This layout is typically chosen for its low polar inertia and relatively favorable weight distribution (the heaviest component is near the center of the car, making the main component of its moment of inertia relatively low) . The layout has a tendency toward being heavier in the rear than the front, which allows for best balance to be achieved under acceleration. It is felt that the low polar inertia is crucial in selection of this layout. The mid-engined layout also uses up central space, making it impractical for any but two-seater sports cars.
In modern racing cars, RMR is the usual configuration and is usually synonymous with "rear engine". Due to its favorable weight dynamics, this layout is heavily employed in Formula racing cars (such as Formula One). This configuration was also common in very small engined 1950s microcars, in which the engines did not take up much space. Because of successes in racing, the RMR platform has been popular for road going sports cars despite the inherent challenges of design, maintenance and lack of cargo space.
