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Inautomotive design, anRMR, orrearmid-engine,rear-wheel-drive layout is one in which the rear wheels are driven by an engine placed with itscenter of gravity in front of the rear axle, and thus right behind the passenger compartment. Nowadays, such cars are more frequently called 'RMR', to acknowledge that certain sporty or performance-focused front-engine cars are also referred to as "mid-engine", the main enginemass being located behind the front axle. Until the early 1990s, RMR-layout cars were just calledMR, ormid-engine, rear-wheel-drive layout), because the nuance between distinctly front-engined vs. frontmid-engined cars often remained rather vague.
In contrast to the fullyrear-engine, rear-wheel-drive layout, thecenter of mass of theengine is in front of the rear axle. This layout is typically chosen for its favorableweight distribution. Placing the car's heaviest component within the wheelbase minimizes itsrotational inertia around the vertical axis, facilitating turn-in oryaw angle. Also, a near 50/50% weight distribution, with a slight rear weight bias, gives a very favorable balance, with significant weight being placed on the driven rear axle under acceleration, while distributing the weight fairly evenly under braking. This arrangement promotes optimal use of all four wheels to decelerate the car rapidly as well.
The RMR layout generally has a lower tendency toundersteer. However, since there is less weight over the front wheels, under acceleration the front of the car can be prone to lift and still haveundersteer. Most rear-engine layouts have historically been used in smaller vehicles, because the weight of the engine at the rear has an adverse effect on a larger car's handling, making it 'tail-heavy', although this effect is more pronounced with engines mounted behind the rear axle.[1] 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 generally only practical for single seating-row sports-cars, with exception to a handful of2+2 designs. Additionally, some microtrucks use this layout, with a small, low engine beneath a flat load floor above the rear wheel-wells. This makes it possible to move the cab right to the front of the vehicle, thus increasing the loading area at the expense of slightly reduced load depth.
In modern racing cars, RMR is a common configuration and is usually synonymous with "mid-engine". Due to itsweight distribution and the favorablevehicle dynamics it produces, this layout is heavily employed in open-wheelFormula racing cars (such asFormula One andIndyCar) as well as most purpose-builtsports racing cars. This configuration was also common in smaller-engined 1950smicrocars, in which the engines did not take up much space. Because of successes in motorsport, the RMR platform has been commonly used in many road-going sports cars despite the inherent challenges of design, maintenance and lack of cargo space. The similarmid-engine, four-wheel-drive layout gives many of the same advantages and is used when extra traction is desired, such as in somesupercars and in theGroup B rally cars.
The 1900NW Rennzweier was one of the first race cars with mid-engine, rear-wheel-drive layout. Other known historical examples include the1923 Benz Tropfenwagen. It was based on an earlier design named the Rumpler Tropfenwagen in 1921 made by Edmund vonRumpler, an Austrian engineer working at Daimler. The Benz Tropfenwagen was designed byFerdinand Porsche along with Willy Walb andHans Nibel. It raced in 1923 and 1924 and was most successful in theItalian Grand Prix inMonza where it stood fourth. Later, Ferdinand Porsche used mid-engine design concept towards theAuto UnionGrand Prix cars of the 1930s which became the first winning RMR racers. They were decades before their time, although MR Miller Specials raced a few times atIndianapolis between 1939 and 1947. In 1953Porsche premiered the tiny and altogether new RMR550 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. The718 followed similarly in 1958. But it was not until the late 1950s thatRMR reappeared in Grand Prix (today's "Formula One") races in the form of theCooper-Climax (1957), soon followed by cars fromBRM andLotus.Ferrari andPorsche soon made Grand Prix RMR attempts with less initial success. The mid-engined layout was brought back toIndianapolis in 1961 by theCooper Car Company withJack Brabham running as high as third and finishing ninth. Cooper did not return, but from 1963 on British built mid-engined cars from constructors likeBrabham,Lotus andLola competed regularly and in 1965 Lotus won Indy with theirType 38.
Rear mid-engines were widely used in microcars like theIsetta or theZündapp Janus.
The first rear mid-engined road car after WW II was the 1962 (Rene) Bonnet /Matra Djet, which used the 1108cc Renault Sierra engine, mated to the transaxle from the FWD Renault Estafette van. Nearly 1700 were built until 1967. This was followed by the first De Tomaso, theVallelunga, which mated a tuned Ford Cortina 1500 Kent engine to a VW transaxle with Hewland gearsets. Introduced at Turin in 1963, 58 were built 1964–68. A similar car was the Renault-enginedLotus Europa, built from 1966 to 1975.
Finally, in 1966, theLamborghini Miura was the first high performance mid-engine, rear-wheel-drive road car.The concept behind the Miura was that of putting on the road a grand tourer featuring state-of-the-art racing-car technology of the time; hence the Miura was powered by aV12 transversely mounted between the rear wheels, solidal to thegearbox anddifferential.[2] This represented an extremely innovative sportscar at a time when all of its competitors (aside from the rear-engined Porsches), fromFerraris toAston Martins, were traditionalfront-engined, rear-wheel-drive grand tourers.
ThePontiac Fiero was amid-enginedsports car that was built by thePontiac division ofGeneral Motors from 1984 to 1988. The Fiero was the first two-seater Pontiac since the 1926 to 1938 coupes, and also the first mass-produced mid-engine sports car by a U.S. manufacturer.
![As with many "rear mid-engine transversely-mounted / rear-wheel-drive layouts", the Matra-Simca Bagheera shared Simcas 1100 and 1307 front-wheel-drive mechanicals, but placed behind the passenger compartment.[3]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3a1974_MATRA_SIMCA_BAGHEERA%2c_pic10.JPG)
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