Rear-Wheel Drive (RWD) layouts
Rear-Wheel Drive (RWD):
Rear-wheel drive (RWD) typically places the engine in the front of the vehicle and the driven wheels are located at the rear, a configuration known as front-engine, rear-wheel drive layout (FR layout). The front mid-engine, rear mid-engine and rear engine layouts are also used. This was the traditional automobile layout for most of the 20th century. Nearly all motorcycles and bicycles use rear-wheel drive, either by driveshaft, chain, or belt, since the front wheel is turned for steering, and it would be very difficult and cumbersome to “bend” the drive mechanism around the turn of the front wheel. A relatively rare exception is with the ‘moving bottom bracket’ type of recumbent bicycle, where the entire drivetrain, including pedals and chain, pivot with the steering front wheel.
Front-engine, Rear-Wheel Drive (FR):
In automotive design, an FR, or Front-engine, Rear-wheel-drive layout is one where the engine is located at the front of the vehicle and driven wheels are located at the rear. This was the traditional automobile layout for most of the 20th century. Modern designs typically use the front-engine, front-wheel-drive layout (FF).
In an era when gasoline was cheap and cars were heavy, the mechanical advantages of the FR drivetrain layout made up for any disadvantage in weight terms. It remained almost universal among car designs until the 1970s.
After the Arab Oil Embargo of 1973 and the 1979 fuel crises, a majority of American FR vehicles (station wagons, luxury sedans) were phased out for the FF layout – this trend would spawn the SUV/van conversion market. Throughout the 1980s and 1990s, most American companies set as a priority the eventual removal of rear-wheel drive from their mainstream and luxury lineup. Chrysler went 100% FF by 1990 and GM‘s American production went entirely FF by 1997 except theCorvette and Camaro. Ford’s Mustang has stayed rear-wheel drive, as it must maintain a sporty presence, as were Ford’s full-size cars based on the Ford Panther platform (the Ford Crown Victoria, Mercury Grand Marquis, and Lincoln Town Car) until they were discontinued in 2011 in favour of the Ford Taurus, which has a transverse front-wheel-drive layout.
Front Mid engine, Rear Wheel Drive (FMR):
In automotive design, a front mid-engine, rear-wheel-drive layout (FMR) is one that places the engine in the front, with the rear wheels of vehicle being driven. In contrast to the front-engine, rear-wheel-drive (FR) layout, the engine is pushed back far enough that its center of mass is to the rear of the front axle. This aids in weight distribution and reduces the moment of inertia, improving the vehicle’s handling. The mechanical layout of a FMR is substantially the same as a FR car. Some models of the same vehicle can be classified as either FR or FMR depending on the length of the installed engine (e.g. 4-cylinder vs. 6-cylinder) and its centre of mass in relation to the front axle.
- FMR cars are often characterized by a long hood and front wheels that are pushed forward to the corners of the vehicle, close to the front bumper. Grand tourers often have FMR layouts, as a rear engine would not leave much space for the rear seats.
- FMR should also not be confused with a “front midships” location of the engine, referring to the engine being located fully behind the front axle centerline, in which case a car meeting the above FMR center of mass definition could be classified as a FR layout instead.
- FMR layout came standard in most pre–World War II, front-engine / rear-wheel-drive cars.
Rear mid-engine, rear-wheel-drive (RMR)
This kind of layout is also known as MR (Mid-engine, Rear-Wheel drive).
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.(…)
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 Janusis perhaps unique, in that it is literally mid-engined and nearly symmetrical with passengers on both ends of the engine.There is two configurations with this kind of layout;Rear mid-engine transversely-mounted, rear-wheel-drive layout:Rear mid-engine longitudinally-mounted, rear-wheel-drive layout:
Rear-engine, Rear-Wheel Drive (RR)
The Smart uses a RR layout.
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.
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.
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.
Posted on July 22, 2012, in General, Uncategorized and tagged arab oil embargo, arab oil embargo of 1973, cars, FMR, fmr layout, fr, Fr layout, front mid engine rear wheel drive, fuel crises, honda nsx, honda s2000, oil embargo of 1973, porsche, rear engine, rear wheel drive, renault clio sport v6, rr, rwd, rwd layouts, subaru, toyota mrs, transportation. Bookmark the permalink. 1 Comment.