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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, built by AustrianNesselsdorfer Wagenbau-Fabrik for Baron Theodor von Liebig, was one of the first race cars purpose-built with mid-engine, rear-wheel-drive layout, using a flat-twinBenz & Cie. engine. Among the engineers wasEdmund Rumpler, an Austrian engineer who also worked at Daimler and in aviation, and in 1921 built theRumpler Tropfenwagen. His passenger car design was primarily streamlined teardrop-shaped for a low drag coefficient of only 0.28, the central driver seat and rear mid-engine layout only means to achieve it. The car, built in Berlin, was no success, but it was frequently used as taxi. At the time, there was only theAVUS circuit, and noReichsautobahn network yet on which it could have made good use of its low drag asGrand tourer.Carl Benz & Söhne licenced the Tropfenwagen design in 1922, but the tall curved windscreen was not practical, and work focussed on open top cars.
TheBenz Tropfenwagen race car was designed underHans Nibel in1923 in Germany, a time of crisis, which soon forced Daimler and Benz to cooperate and merge in 1926. Several configurations were tested, with different positions of driver, gas tank and radiators. ForGrand Prix motor racing, then with a 2 liter capacity limit, a new engine had to be developed, still without the new superchargers thatFerdinand Porsche had introduced at Daimler, winning theTarga Florio in 1922 and 1924 for Mercedes. Three Benz RH cars competed in the1923 Italian Grand Prix at the fast new track atMonza whereFerdinando Minoia finished fourth while supercharged FIATs dominated.
With superior traction, the mid engine cars were well suited forHillclimbing and won several races in 1924 and 1925 with Franz Hörner, Willy Walb andAdolf Rosenberger who became business partner of Ferdinand Porsche when he set up his engineering bureau in Stuttgart. Porsche developed a mid-engine design concept for the 1934Grand Prix rules. ThisP-Wagen was adopted for theAuto Union racing cars of the 1930s. The Auto Union became the first Grands Prix winning RMR racers, yet were difficult to drive, on circuits and in hillclimbing. The annualAIACR European Championship comprised only few events, mostly four, of which in the1936 Grand Prix season all were won by Auto Union, with three more wins in other major races.Bernd Rosemeyer became the champion, winning five major Grands Prix in the supercharged V16 mid-engine, rear-wheel-drive Typ C. Mercedes developed new types and dominated the following seasons, though.
In 1946 Ferdinand Porsche was bailed out of prison to design a mid engine race car for Italians, the Porsche Type 360Cisitalia Grand Prix, aVoiturette with supercharged 1500cc engine and even 4WD. Italian funding ran out before the car was raced. In Germany, dealer and racer Glöckler converted the rear engine 2+2 seaterPorsche 356 into mid-engine 2-seat race cars and convinced Porsche to offer such a race-ready car: the tiny RMRPorsche 550 Spyder, which soon got a matching DOHC flat-4 engine. From 1954 onwards the 550A was notoriously winning in the 1500cc sports and endurance race car classes against much larger cars – a sign of greater things to come.
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At the1956 French Grand Prix, the first rear mid-engine single seater appeared in Formula One racing, theBugatti Type 251, which was uncompetitive, though. In the1957 German Grand Prix, 1500cc Formula Two cars were allowed. Six mid-engined Cooper-Climax showed up, and three closed bodyworkPorsche 550 sportscars of which passenger seat and spare wheel were removed, making them single seaters.Edgar Barth qualified and finished 12th, ahead of some 2500cc F1 and all open-wheel F2.
A swift Mid-engine revolution converted Formula One from old front engine glory to new mid-engine dominance in just about a year, between early 1958 and June of 1959.
The mid-engineCooper F2 cars with the 1500ccClimax FPF engine received a 2 litre version for F1 races, and won the first two races of the1958 Formula One season, under rather unusual circumstances, though. Due to lack of power they were not competitive in the remainder of that season, but the warning shots had been fired.
When the Climax engine was bored out to full capacity for the1959 Formula One season,Jack Brabham won the first Grand Prix in Monaco in a Cooper, and defended his Championship lead till the end. The last proper win of a front engine F1 car on a twisty circuit occurred already at the 31 May1959 Dutch Grand Prix, byJoakim Bonnier in aBRM P25. Front engine cars from Scarab and Aston Martin were outclassed, and even Ferraris would win only three more GPs, all on high speed circuits:1959 French Grand Prix at Reims,1959 German Grand Prix on the banked Berlin AVUS. In the1960 Formula One season, Ferrari could score podiums only in the first to races, with Cooper and Lotus leading in the constructor standings. For the1960 Italian Grand Prix, the banked oval version of Monza was chosen by the local organisers, to help Ferrari win a last hurrah with superior engine power, in absence of British teams that did not bother to take part. Ferrari already ran the future mid-engine car and smaller engine.
Thus, with Lotus and BRM adopting the Cooper-Climax layout, the mid-engine revolution of F1 was completed before 1961. It dragged on much longer insports car racing where the underpoweredPorsche 550 since 1953 won many class victories, at the 1954Carrera Panamericana, the final one, and basically every year at the24 Hours of Le Mans. The 1956Targa Florio was the first major overall win for a mid-engine race car since the 1930s, but in a year the event was not part of theWorld Sportscar Championship. Already in 1957, thePorsche 718 was introduced, winning the1959 Targa Florio, providing the first mid-engine win in the1959 World Sportscar Championship. In the1960 World Sportscar Championship, Porsche also won the Sebring 12h, and with two wins and a second place tied Ferrari in the Championships, but the Scuderia won the title due to two third places over one for Stuttgart, which barely made it into the top ten at the1960 24 Hours of Le Mans against cars with double or triple capacity.
For the1961 World Sportscar Championship, Ferrari adopted the mid-engine layout at least with theDino V6-poweredFerrari SP series. With more power from a 2.4 litre engine, but good handling, it won the1961 Targa Florio over the Porsche, and again in the1962 Targa Florio. Front engine Ferrari, Jaguar and other large GT still prevailed at Le Mans, until the1962 24 Hours of Le Mans. For the1963 World Sportscar Championship, the FIA allowed prototypes, and Ferrari finally moved the big V12 behind the driver, opening theFerrari P series with a 3 litre 250P that achieved immediate success on the racetrack, winning the12 Hours of Sebring,1000 km Nürburgring, and the all-important1963 24 Hours of Le Mans. Thus, the mid-engine revolution took place in Sportscar racing, as other manufacturers joined in, putting big US V8s behind drivers, namelyLola and Ford with theFord GT40.
The pioneer of mid-engine racing, Porsche, had won its token F1 Grand Prix in 1962, but was denied an overall 24h Le Mans win until the Porsche 917K finally succeeded in 1970.CanAm wins followed in 1972 when the 917/10 was turbocharged.
The last pure-bred circuit racing series to adopt the mid-engine layout was theIndianapolis 500-style oval racing in the US. Already at the1961 Indianapolis 500 theCooper Car Company withJack Brabham was running as high as third and finished ninth. Cooper did not return, but from 1963 on "British Invasion" built mid-engined cars from constructors likeBrabham,Lotus andLola competed regularly, and driven by the newFord Indy V8 engine, theLotus 38-Ford of Jim Clark won in 1965, followed by the Lola ofGraham Hill in 1966.
In tin-top racing, GT and Touring cars, different layouts still compete. The rear enginePorsche 911, having won the 1973 Targa Florio, Daytona, and as turbocharged 935 even the 1979 Le Mans overall, is still going strong in GT racing in whichBalance of performance ensures parity of different layouts, mostly mid-engine Ferrari and evenCorvette C7, with theFord Mustang GTD and BMW still championing the standard front engine rear drive layout.
Rear mid-engines were widely used in microcars like theIsetta or theZündapp Janus. The very firstPorsche 356/1 was mid-engined, but the regular production 356 used the 2+2 seat rear engine layout of the Volkswagen to be useful as regular car. The series of rear mid-engine two seat sports cars from Zuffenhausen started in 1953 with thePorsche 550 and via 718 and 904 ended with thePorsche 906. They were mainly intended for competition use, but also road legal as this was either required by the rules, or by customers who wanted to drive to and from events. The VW-Porsche 914 was offered as road car, thePorsche 914-6 GT as racer.
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=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3a1974_MATRA_SIMCA_BAGHEERA%2c_pic10.JPG)
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