RR layout
Encyclopedia
In automotive design
, a RR, or Rear-engine
, Rear-wheel drive layout places both the engine
and drive wheels at the rear of the vehicle. In contrast to the RMR layout, the center of mass
of the engine is between the rear axle and the rear bumper.
. Placing the engine near the driven rear wheels allows for a physically smaller, lighter, less complex, and more efficient drivetrain, since there is no need for a driveshaft
, and the differential can be integrated with the transmission, commonly referred to as a transaxle
.
Since the engine is typically the heaviest component of the car, putting it near the rear axle usually results in more weight over the rear axle than the front, commonly referred to as a rear weight bias. The farther back the engine, the greater the bias. As a very general trend with numerous exceptions, typical weight bias for an FR is 55/45 front/rear; for MR, 45/55; for RR, 35/65. Rear weight bias reduces forward weight transfer
under braking, and increases rear weight transfer under acceleration. The former means that traction is more evenly distributed among all four wheels under braking, resulting in shorter stopping times and distances. The latter means that the driven wheels have increased traction when accelerating, allowing them to put more power on the ground and accelerate faster.
The disadvantage to a rear weight bias is that the car can become unstable and tend to oversteer when decelerating (whether braking or lifting off the throttle). In turns, this tendency is much more pronounced, to the point that even letting off the throttle slightly while turning can cause the rear tires to suddenly lose grip, and the vehicle to slide rear-first (see lift-off oversteer
). When this happens, rotational inertia dictates that the added weight away from the axis of rotation (generally the steering wheels) will be more likely to maintain the spin, especially under braking. This is an inherent instability in the design, making it easier to induce and more difficult to recover from a slide than in a less rear-weight-biased vehicle. All cars regardless of drivetrain layout obey the same laws of physics and can do this, but it is much easier to do and harder to correct in MR and RR vehicles, with the result that many are unsafe to drive to their limits by average drivers (notable examples include the early Porsche 911
, earlier years of the Toyota MR2
, and the Chevrolet Corvair
). A skilled driver, however, can corner faster by taking advantage of this tendency to oversteer, and is also more likely to take turns at a correct speed by braking before turning, and maintaining slight acceleration through the turn. At the end of the turn, the rear weight bias allows for increased rear traction when accelerating, allowing the driver to accelerate sooner, a major advantage in racing.
When not braking, the decreased weight over the front wheels means less traction, resulting in the car having a tendency to understeer
, which is safer and more stable (further allowing a driver to get on full throttle sooner) but is undesirable in cornering. To counteract this, it's necessary to induce forward weight transfer to increase front grip, but this significantly destabilizes the car and can be hard to do properly, that is, without spinning. High-performance driving necessitates inducing weight transfer
, regardless of drivetrain layout, but it is more important in MR and especially RR layouts, and the greater effect from it is more difficult to handle.
In these respects, an RR can be considered to be an exaggeration of MR - harder braking, faster and earlier acceleration, and increased oversteer, but faster turning if it can be controlled.
In off road and low-traction situations, the RR layout has some advantages compared to other 2WD layouts. The weight is biased towards the driven wheels- as with FF vehicles. This both improves drive wheel traction and reduces the tendency for the undriven wheels to dig in. In addition, the driving and steering requirements are split between front and rear- as with FR vehicles- making it less likely for either to lose traction. Many dune buggies successfully use a Volkswagen beetle as the donor car for this reason. The relative simplicity and light weight compared to 4WD can therefore sometimes outweigh the disadvantage of only having two driven wheels.
Where RR differs from MR is in that the engine is located outside of the wheelbase. The major advantage of MR - low moment of inertia - is negated somewhat (though still lower than FR), and there is more room for passengers and cargo (though usually less than FR). Furthermore, because both axles are on the same side of the engine, it's technically more straightforward to drive all four wheels, than in a mid-engined configuration (though there have been more high-performance cars with the M4 layout than with R4). Finally, a rear-mounted engine has empty air (often at a lower pressure) behind it when moving, allowing more efficient cooling for air-cooled vehicles (more of which have been RR than liquid-cooled, such as the Volkswagen Beetle
, and one of the few production air-cooled turbocharged cars, the Porsche 930
).
For liquid-cooled vehicles, however, this layout presents a disadvantage, since it requires either increased coolant piping from a front-mounted radiator (meaning more weight and complexity), or relocating the radiator(s) to the sides or rear, and adding air ducting to compensate for the lower airflow at the rear of the car.
Due to the handling difficulty, the need for more space efficiency, and the near ubiquitous use of liquid-cooled engines in modern cars, most manufacturers have abandoned the RR layout. The major exception is Porsche
, who have developed the 911
for over 40 years and have taken advantage of the benefits of RR while mitigating its drawbacks to acceptable levels, lately with the help of electronic aids.
Porsche
has continued to develop its 911
model as a rear-engined vehicle, although they have introduced multiple all-wheel drive
models. Most notably, the 911 Turbo has been sold as AWD-only since the release of the 993
model. Race-oriented models such as the GT3
and twin-turbocharged GT2
remain solely RR, however.
Another manufacturer to implement the RR configuration was the De Lorean Motor Company
with its DMC-12
sports car. To compensate for the uneven (35/65) weight distribution caused by the rear-mounted engine, De Lorean used rear wheels with a diameter slightly greater than the front wheels. Other than the Porsche 911
, the last mass produced RR configured car, at least in Europe, is the Smart Fortwo
. Before that was the rear-engined Škoda's from Škoda 1000MB (produced from 1964) to Škoda 130/135/136 (produced until 1990) or the Polski Fiat 126p (produced until October 2000).
A range of sports road cars and racing cars with the RR layout was produced by the French company Alpine
. These had bodies made of composite material
s and used mechanical components made by Renault. (Alpine was eventually acquired by Renault; the A610
was a Renault product that used the Alpine name.) The Ariel Atom
, a high performance sports car, uses an RR layout as a result of its "exoskeletal" design.
Early cars using the RR layout included the Tucker, Volkswagen Beetle
, Porsche 356
, Chevrolet Corvair
, NSU Prinz
and Hino Contessa.
es use an extreme RR layout. In transit bus
es this can be used to make a very low floor level in the first two-thirds of the bus, thus making disabled access much easier.
Most tour bus
es and coaches
also employ a similar design, however the free space is usually used for luggage
, and sometimes airconditioning equipment.
The Tata Nano
, the cheapest ($2500) production car in the world introduced in 2008 and built in India
, also featured this layout.
Automotive design
Automotive design is the profession involved in the development of the appearance, and to some extent the ergonomics, of motor vehicles or more specifically road vehicles. This most commonly refers to automobiles but also refers to motorcycles, trucks, buses, coaches, and vans...
, a RR, or Rear-engine
Rear-engine design
In automobile design, a rear-engine design layout places the engine at the rear of the vehicle. The center of gravity of the engine itself is past the rear axle...
, Rear-wheel drive layout places both the engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
and drive wheels at the rear of the vehicle. In contrast to the RMR layout, the center of mass
Center of mass
In physics, the center of mass or barycenter of a system is the average location of all of its mass. In the case of a rigid body, the position of the center of mass is fixed in relation to the body...
of the engine is between the rear axle and the rear bumper.
Overview
Most of the traits of the RR configuration are shared with the MRMR layout
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...
. Placing the engine near the driven rear wheels allows for a physically smaller, lighter, less complex, and more efficient drivetrain, since there is no need for a driveshaft
Driveshaft
A drive shaft, driveshaft, driving shaft, propeller shaft, or Cardan shaft is a mechanical component for transmitting torque and rotation, usually used to connect other components of a drive train that cannot be connected directly because of distance or the need to allow for relative movement...
, and the differential can be integrated with the transmission, commonly referred to as a transaxle
Transaxle
In the automotive field, a transaxle is a major mechanical component that combines the functionality of the transmission, the differential, and associated components of the driven axle into one integrated assembly....
.
Since the engine is typically the heaviest component of the car, putting it near the rear axle usually results in more weight over the rear axle than the front, commonly referred to as a rear weight bias. The farther back the engine, the greater the bias. As a very general trend with numerous exceptions, typical weight bias for an FR is 55/45 front/rear; for MR, 45/55; for RR, 35/65. Rear weight bias reduces forward weight transfer
Weight transfer
Weight transfer and load transfer are two expressions used somewhat confusingly to describe two distinct effects: the change in load borne by different wheels of even perfectly rigid vehicles during acceleration, and the change in center of mass location relative to the wheels because of...
under braking, and increases rear weight transfer under acceleration. The former means that traction is more evenly distributed among all four wheels under braking, resulting in shorter stopping times and distances. The latter means that the driven wheels have increased traction when accelerating, allowing them to put more power on the ground and accelerate faster.
The disadvantage to a rear weight bias is that the car can become unstable and tend to oversteer when decelerating (whether braking or lifting off the throttle). In turns, this tendency is much more pronounced, to the point that even letting off the throttle slightly while turning can cause the rear tires to suddenly lose grip, and the vehicle to slide rear-first (see lift-off oversteer
Lift-off oversteer
Lift-off oversteer is a form of oversteer in an automobile that occurs when the vertical load on the tires shifts from the rear to the front quickly due to throttle release while cornering...
). When this happens, rotational inertia dictates that the added weight away from the axis of rotation (generally the steering wheels) will be more likely to maintain the spin, especially under braking. This is an inherent instability in the design, making it easier to induce and more difficult to recover from a slide than in a less rear-weight-biased vehicle. All cars regardless of drivetrain layout obey the same laws of physics and can do this, but it is much easier to do and harder to correct in MR and RR vehicles, with the result that many are unsafe to drive to their limits by average drivers (notable examples include the early Porsche 911
Porsche 911
The Porsche 911 is a luxury 2-door sports coupe made by Porsche AG of Stuttgart, Germany. It has a distinctive design, rear-engined and with independent rear suspension, an evolution of the swing axle on the Porsche 356. The engine was also air-cooled until the introduction of the Type 996 in 1998...
, earlier years of the Toyota MR2
Toyota MR2
The Toyota MR2 is a two-seat, mid-engined, rear wheel drive sports car produced by Central Motors, a part of Toyota, from 1984 until July 2007 when production stopped in Japan...
, and the Chevrolet Corvair
Chevrolet Corvair
-First generation :The 1960 Corvair 500 and 700 series four-door sedans were conceived as economy cars offering few amenities in order to keep the price competitive, with the 500 selling for under $2,000...
). A skilled driver, however, can corner faster by taking advantage of this tendency to oversteer, and is also more likely to take turns at a correct speed by braking before turning, and maintaining slight acceleration through the turn. At the end of the turn, the rear weight bias allows for increased rear traction when accelerating, allowing the driver to accelerate sooner, a major advantage in racing.
When not braking, the decreased weight over the front wheels means less traction, resulting in the car having a tendency to understeer
Understeer
Understeer and oversteer are vehicle dynamics terms used to describe the sensitivity of a vehicle to steering. Simply put, oversteer is what occurs when a car turns by more than the amount commanded by the driver...
, which is safer and more stable (further allowing a driver to get on full throttle sooner) but is undesirable in cornering. To counteract this, it's necessary to induce forward weight transfer to increase front grip, but this significantly destabilizes the car and can be hard to do properly, that is, without spinning. High-performance driving necessitates inducing weight transfer
Weight transfer
Weight transfer and load transfer are two expressions used somewhat confusingly to describe two distinct effects: the change in load borne by different wheels of even perfectly rigid vehicles during acceleration, and the change in center of mass location relative to the wheels because of...
, regardless of drivetrain layout, but it is more important in MR and especially RR layouts, and the greater effect from it is more difficult to handle.
In these respects, an RR can be considered to be an exaggeration of MR - harder braking, faster and earlier acceleration, and increased oversteer, but faster turning if it can be controlled.
In off road and low-traction situations, the RR layout has some advantages compared to other 2WD layouts. The weight is biased towards the driven wheels- as with FF vehicles. This both improves drive wheel traction and reduces the tendency for the undriven wheels to dig in. In addition, the driving and steering requirements are split between front and rear- as with FR vehicles- making it less likely for either to lose traction. Many dune buggies successfully use a Volkswagen beetle as the donor car for this reason. The relative simplicity and light weight compared to 4WD can therefore sometimes outweigh the disadvantage of only having two driven wheels.
Where RR differs from MR is in that the engine is located outside of the wheelbase. The major advantage of MR - low moment of inertia - is negated somewhat (though still lower than FR), and there is more room for passengers and cargo (though usually less than FR). Furthermore, because both axles are on the same side of the engine, it's technically more straightforward to drive all four wheels, than in a mid-engined configuration (though there have been more high-performance cars with the M4 layout than with R4). Finally, a rear-mounted engine has empty air (often at a lower pressure) behind it when moving, allowing more efficient cooling for air-cooled vehicles (more of which have been RR than liquid-cooled, such as the Volkswagen Beetle
Volkswagen Beetle
The Volkswagen Type 1, widely known as the Volkswagen Beetle or Volkswagen Bug, is an economy car produced by the German auto maker Volkswagen from 1938 until 2003...
, and one of the few production air-cooled turbocharged cars, the Porsche 930
Porsche 930
The 930 was a sports car built by Porsche, 930 actually being the "type number" for the pre-964 generation 911 Turbo produced between 1975 and 1989...
).
For liquid-cooled vehicles, however, this layout presents a disadvantage, since it requires either increased coolant piping from a front-mounted radiator (meaning more weight and complexity), or relocating the radiator(s) to the sides or rear, and adding air ducting to compensate for the lower airflow at the rear of the car.
Due to the handling difficulty, the need for more space efficiency, and the near ubiquitous use of liquid-cooled engines in modern cars, most manufacturers have abandoned the RR layout. The major exception is Porsche
Porsche
Porsche Automobil Holding SE, usually shortened to Porsche SE a Societas Europaea or European Public Company, is a German based holding company with investments in the automotive industry....
, who have developed the 911
Porsche 911
The Porsche 911 is a luxury 2-door sports coupe made by Porsche AG of Stuttgart, Germany. It has a distinctive design, rear-engined and with independent rear suspension, an evolution of the swing axle on the Porsche 356. The engine was also air-cooled until the introduction of the Type 996 in 1998...
for over 40 years and have taken advantage of the benefits of RR while mitigating its drawbacks to acceptable levels, lately with the help of electronic aids.
History
One of first RR cars was Tatra 77, first serial-produced aerodynamic car. Tatra used this layout until end of production of T700 in 1999. In case of T613 and T700 Tatra used layout with axle in middle of engine, which reduced some disadvantages of RR layout.Porsche
Porsche
Porsche Automobil Holding SE, usually shortened to Porsche SE a Societas Europaea or European Public Company, is a German based holding company with investments in the automotive industry....
has continued to develop its 911
Porsche 911
The Porsche 911 is a luxury 2-door sports coupe made by Porsche AG of Stuttgart, Germany. It has a distinctive design, rear-engined and with independent rear suspension, an evolution of the swing axle on the Porsche 356. The engine was also air-cooled until the introduction of the Type 996 in 1998...
model as a rear-engined vehicle, although they have introduced multiple all-wheel drive
Four-wheel drive
Four-wheel drive, 4WD, or 4×4 is a four-wheeled vehicle with a drivetrain that allows all four wheels to receive torque from the engine simultaneously...
models. Most notably, the 911 Turbo has been sold as AWD-only since the release of the 993
Porsche 993
The Porsche 993 is the company's internal name for the version of the Porsche 911 model manufactured and sold between late 1993 and early 1998, replacing the 964...
model. Race-oriented models such as the GT3
Porsche 911 GT3
The Porsche 911 GT3 is a higher performance version of the Porsche 911 sports car. It is the latest in a line of high performance models, beginning with the 1973 911 RS...
and twin-turbocharged GT2
Porsche 911 GT2
The Porsche 911 GT2 is a sports car built by the German manufacturer Porsche since 1994. It is based on the 911 Turbo, and uses a similar twin-turbocharged engine, but features numerous upgrades, including engine upgrades, larger brakes, and stiffer suspension calibration...
remain solely RR, however.
Another manufacturer to implement the RR configuration was the De Lorean Motor Company
De Lorean Motor Company
The original DeLorean Motor Company was a short-lived automobile manufacturer formed by automobile industry executive John DeLorean in 1975. It is remembered for the one model it produced — the distinctive stainless steel DeLorean DMC-12 sports car featuring gull-wing doors — and for its brief and...
with its DMC-12
De Lorean DMC-12
The DeLorean DMC-12 is a sports car originally manufactured in Dunmurry, a suburb west of Belfast, Northern Ireland by John DeLorean's DeLorean Motor Company for the American market in 1981-82. Most commonly known as the DeLorean, it was the only model produced by the company which would go into...
sports car. To compensate for the uneven (35/65) weight distribution caused by the rear-mounted engine, De Lorean used rear wheels with a diameter slightly greater than the front wheels. Other than the Porsche 911
Porsche 911
The Porsche 911 is a luxury 2-door sports coupe made by Porsche AG of Stuttgart, Germany. It has a distinctive design, rear-engined and with independent rear suspension, an evolution of the swing axle on the Porsche 356. The engine was also air-cooled until the introduction of the Type 996 in 1998...
, the last mass produced RR configured car, at least in Europe, is the Smart Fortwo
Smart Fortwo
The Smart Fortwo is a rear-engined two-seater city car manufactured by Smart GmbH, introduced at the 1998 Paris Motor Show as the Smart City Coupé, and currently in its second generation...
. Before that was the rear-engined Škoda's from Škoda 1000MB (produced from 1964) to Škoda 130/135/136 (produced until 1990) or the Polski Fiat 126p (produced until October 2000).
A range of sports road cars and racing cars with the RR layout was produced by the French company Alpine
Alpine (car)
Alpine was a French manufacturer of racing and sports cars that used rear-mounted Renault engines.Jean Rédélé , the founder of Alpine, was originally a Dieppe garage proprietor, who began to achieve considerable competition success in one of the few French cars produced just after World War...
. These had bodies made of composite material
Composite material
Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...
s and used mechanical components made by Renault. (Alpine was eventually acquired by Renault; the A610
Renault Alpine GTA/A610
The Alpine A610 was a coupe automobile produced by the Renault-owned French manufacturer Alpine. It was launched in 1991. Due to a limited budget at the beginning of the project, its appearance does not differ much from the GTA, and it looks quite similar to the USA GTA with its pop-up headlights...
was a Renault product that used the Alpine name.) The Ariel Atom
Ariel Atom
The Ariel Atom is a high performance sports car made by the Ariel Motor Company based in Somerset, England and under licence in North America by TMI Autotech, Inc. at Virginia International Raceway in Alton, Virginia....
, a high performance sports car, uses an RR layout as a result of its "exoskeletal" design.
Early cars using the RR layout included the Tucker, Volkswagen Beetle
Volkswagen Beetle
The Volkswagen Type 1, widely known as the Volkswagen Beetle or Volkswagen Bug, is an economy car produced by the German auto maker Volkswagen from 1938 until 2003...
, Porsche 356
Porsche 356
The Porsche 356 was the company's first production automobile. It was a lightweight and nimble handling rear-engine rear-wheel-drive 2 door sports car available in hardtop coupe and open configurations. Design innovations continued during the years of manufacture, contributing to its motorsports...
, Chevrolet Corvair
Chevrolet Corvair
-First generation :The 1960 Corvair 500 and 700 series four-door sedans were conceived as economy cars offering few amenities in order to keep the price competitive, with the 500 selling for under $2,000...
, NSU Prinz
NSU Prinz
The NSU Prinz is an automobile produced in West Germany by the NSU Motorenwerke AG. The car was built from 1957 to 1973, and received a model change in 1961 .-NSU Prinz 30:...
and Hino Contessa.
Present
Most modern heavy duty busBus
A bus is a road vehicle designed to carry passengers. Buses can have a capacity as high as 300 passengers. The most common type of bus is the single-decker bus, with larger loads carried by double-decker buses and articulated buses, and smaller loads carried by midibuses and minibuses; coaches are...
es use an extreme RR layout. In transit bus
Transit bus
A transit bus , also known as a commuter bus, city bus, or public bus, is a bus used for short-distance public transport purposes...
es this can be used to make a very low floor level in the first two-thirds of the bus, thus making disabled access much easier.
Most tour bus
Tour bus
A tour bus service is a bus service that takes visitors sightseeing, with routes around tourist attractions. Double-decker buses and/or open top buses are commonly used, for their good views....
es and coaches
Coach (vehicle)
A coach is a large motor vehicle, a type of bus, used for conveying passengers on excursions and on longer distance express coach scheduled transport between cities - or even between countries...
also employ a similar design, however the free space is usually used for luggage
Luggage
Baggage is any number of bags, cases and containers which hold a traveller's articles during transit.Luggage is more or less the same concept as "baggage", but is normally used in relation to the personal luggage of a specific person or persons Baggage is any number of bags, cases and containers...
, and sometimes airconditioning equipment.
The Tata Nano
Tata Nano
The Tata Nano is an inexpensive, rear-engined, four-passenger city car built by the Indian company Tata Motors and is aimed primarily at the Indian domestic market....
, the cheapest ($2500) production car in the world introduced in 2008 and built in India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
, also featured this layout.