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In automobile design, an MR or 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 gravity 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 does suffer from a tendency toward being heavier in the rear than the front, which is not ideal for handling. However, it is generally felt that the lower polar inertia more than makes up for this. The mid-engined layout also uses up central space, making it impractical for any but two-seater sports cars.

In modern racing cars, MR 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 F1s).

This configuration was also common in very small engined 1950s microcars, in which the engines didn't take up much space.

Because of successes in racing, the MR platform has been popular for road going sports cars despite the inherent challenges of design, maintenance and lack of cargo space. The 1923 Benz Tropfenwagen was the first race car with mid-engine, rear-wheel drive layout. It was based on an earlier design named Rumpler Tropfenwagen in 1921 made by Edmund von Rumpler, an Austrian engineer working at Daimler. The Benz tropfenwagen was designed by Ferdinand Porsche along with Willy Walb and Hans Nibel. It raced in 1923 and 1924 and was most successful in Italian Grand Prix in Monza where it stood fourth. Later, Ferdinand Porsche used mid-engine design concept towards the Auto Union Grand Prix cars of the 1930's which became the first winning MR racers. They were decades before their time, although MR Miller Specials raced a few times at Indianapolis between 1939 and 1947. In 1953 Porsche premiered the tiny and altogether new MR 550 spyder and in a year it was notoriously winning in the smaller sports and endurance race car classes against much larger cars- a sign of greater things to come. The 718 followed similarly in 1958. But it was not until the 1960's that MR reappeared in Grand Prix (todays 'Formula 1') races in the form of the Cooper - Climax (1959), coinciding or soon followed by cars from BRM and Lotus. Ferrari and Porsche soon made Gran Prix MR attempts with less initial success. Dan Gurney brought MR back to Indianapolis in 1963 via Lotus and in a year it was the clear trend, becoming the first mid-engined design to win there in 1965. In 1964 Jim Hall created his MR Chaparrals, pushing certain innovations which have never been duplicated, and over the next several years were probably the fastest road racing cars of any type. The Chaparrals continued in the fantastic, unrestricted Can-Am series from 1966-1974 where all cars were MR. Ford, all but ignoring Can-Am racing in its own country, took advantage of somewhat uncompetitive Le Mans racing to introduce the MR GT40 which not only won, but more importantly dominated Ferrari at Le Mans from 1966-1969. In turn, by 1973 Le Mans and Can-Am were both so dominated by Porsche's blistering MR 917's (leading ultimately to the 936 and 956/962), the fate of Sports Prototypes as MR was sealed and Ferrari lost interest in Le Mans until the 1980's. The skyrocketing speeds and power of the 917 era necessitated restrictions which permanently ended unlimited road racing.

Porsche 550 Spyder (1954) was the first ever production road car with MR layout. A total of 125 including 90 road versions were produced. Examples of other early road cars using the MR layout included , Zündapp Janus (1957), De Tomaso Vallelunga (1965), Lotus Europa (1966) 9300 produced, Lamborghini Miura (1966) 987 produced, Ford GT40 (1966), Ferrari Ferrari Dino 206/246 (1968), Porsche 914 (1969) 118,000 produced, and De Tomaso Pantera (1971). Its space-inefficiency means that it is mainly used in sports cars, for example the Porsche Boxster and Cayman, Toyota MR2, Pontiac Fiero, Honda NSX, Lotus Elise, MGF, and supercars such as the Pagani Zonda, recent Porsche Carreras, Ferrari Enzo, Lamborghinis, McLaren F1, Ford GT, etc. See the List of MR Cars for a complete list. The Zündapp Janus is perhaps unique, in that it is literally mid-engined and nearly symmetrical with passengers on both ends of the engine.


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