Car layout in automotive design
RMR layout; the engine is located in front of the rear axle.
Rear Mid-engine transversely-mounted / Rear-wheel drive
In
automotive design
, an
RMR
, or
rear
mid-engine
,
rear-wheel-drive
layout
is one in which the rear wheels are driven by an engine placed with its
center of gravity
in front of the rear axle, and thus right behind the passenger compartment. Nowadays more frequently called 'RMR', to acknowledge that certain sporty or performance focused front-engined cars are also "mid-engined", by having the main engine
mass
behind the front axle, RMR layout cars were previously (until ca. the 1990) just called
MR
, or
mid-engine, rear-wheel-drive layout
), because the nuance between distinctly front-engined vs. front
mid-engined
cars often remained undiscussed.
In contrast to the fully
rear-engine, rear-wheel-drive layout
, the
center of mass
of the
engine
is in front of the rear axle. This layout is typically chosen for its favorable
weight distribution
. Placing the car's heaviest component within the wheelbase minimizes its
rotational inertia
around the vertical axis, facilitating turn-in or
yaw angle
. Also, a near 50/50% weight distribution, with a slight rear weight bias, gives a very favorable balance, with plenty of weight on the driven rear axle under acceleration, while distributing the weight fairly evenly under braking, thereby making optimal use of all four wheels to decelerate the car rapidly as well.
The RMR layout generally has a lower tendency to
understeer
. However, since there is less weight over the front wheels, under acceleration the front of the car can be prone to lift and still have
understeer
. 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 of
2+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 its
weight distribution
and the favorable
vehicle dynamics
it produces, this layout is heavily employed in open-wheel
Formula racing
cars (such as
Formula One
and
IndyCar
) as well as most purpose-built
sports racing cars
. This configuration was also common in smaller-engined 1950s
microcars
, 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 similar
mid-engine, four-wheel-drive
layout gives many of the same advantages and is used when extra traction is desired, such as in some
supercars
and in the
Group B
rally cars.
History
[
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]
The 1900
NW Rennzweier
was one of the first race cars with mid-engine, rear-wheel-drive layout. Other known historical examples include the
1923 Benz Tropfenwagen
. It was based on an earlier design named the 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 the
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 1930s which became the first winning RMR 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 RMR
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 late 1950s that
RMR reappeared in Grand Prix
(today's "
Formula One
") races in the form of the
Cooper
-
Climax
(1957), soon followed by cars from
BRM
and
Lotus
.
Ferrari
and
Porsche
soon made Grand Prix RMR attempts with less initial success. The mid-engined layout was brought back to
Indianapolis
in 1961 by the
Cooper Car Company
with
Jack Brabham
running as high as third and finishing ninth. Cooper did not return, but from 1963 on British built mid-engined cars from constructors like
Brabham
,
Lotus
and
Lola
competed regularly and in 1965 Lotus won Indy with their
Type 38
.
Rear mid-engines were widely used in microcars like the
Isetta
or the
Zundapp 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, the
Vallelunga
, 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-engined
Lotus Europa
, built from 1966 to 1975.
Finally, in 1966, the
Lamborghini 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 a
V12
transversely mounted between the rear wheels, solidal to the
gearbox
and
differential
.
[2]
This represented an extremely innovative sportscar at a time when all of its competitors (aside from the rear-engined Porsches), from
Ferraris
to
Aston Martins
, were traditional
front-engined, rear-wheel-drive
grand tourers.
The
Pontiac Fiero
was a
mid-engined
sports car
that was built by the
Pontiac
division of
General 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.
Gallery
[
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]
Mid-engine transversely-mounted, rear-wheel-drive layout
[
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]
-
NW Rennzweier, first of the long line of
Tatra
racing cars
-
The
Lamborghini Miura
, incorrectly accounted as the first mid-engined roadcar
-
The
Lancia Stratos HF
was powered by a mid-transverse mounted
Dino Ferrari
V6
, and proved to be very successful as a rally car.
-
The
Fiat X1/9
was designed around the all-new
front-wheel drive
Fiat 128
, but used these parts in a radical way, moving the entire transverse drive train and suspension assembly from the front of the 128 to the rear of the passenger cabin.
-
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]
-
Toyota MR2
, Japan's first rear mid-engined production sportscar, sold internationally over three generations (1984?2007)
-
The
Consulier GTP
incorporated a mid-transverse mounted Chrysler 2.2 Turbo III engine; it was successful in IMSA competition until it was banned in 1991.
-
The
Lancia Montecarlo
sports car, marketed in the US as the Lancia Scorpion, was developed as part of the Beta range and was powered by a transverse twin-cam, 4 cylinder engine.
-
The
Mitsubishi i
is powered by a 3-cylinder engine mounted behind the rear seats.
-
The
Lotus Evora
uses a transversely mounted Toyota V6 engine.
Mid-engine longitudinally-mounted, rear-wheel-drive layout
[
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]
References
[
edit
]
- ^
Hillier, Victor; Coombes, Peter (2004).
Fundamentals of motor vehicle technology
. Nelson Thornes. p. 9.
ISBN
978-0-7487-8082-2
.
- ^
"History"
.
Official Lamborghini website
. Archived from
the original
on 2012-12-20
. Retrieved
2012-12-04
.
- ^
"Matra-Simca Bagheera"
.
Simca Talbot Information Centre – Simca Club UK
. Retrieved
2006-08-19
.
External links
[
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]