Inautomotive design, anRR, orrear-engine,rear-wheel-drive layout places both theengine and drive wheels at the rear of the vehicle. In contrast to theRMR layout, thecenter of mass of the engine is between the rear axle and the rear bumper. Although very common intransit buses andcoaches due to the elimination of thedrive shaft withlow-floor buses, this layout has become increasingly rare[specify] inpassenger cars.[1]
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Most of the traits of the RR configuration are shared with themid-engine rear-wheel-drive, or MR. 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 adriveshaft, and thedifferential can be integrated with the transmission, commonly referred to as atransaxle. Thefront-engine front-wheel-drive layout also has this advantage.
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. Typical weight bias for an FF (front engine, front-wheel-drive) is 65/35 front/rear; for FR, 55/45; for MR, 45/55; for RR, 35/65. A static rear weight requires less forwardbrake bias, as load is more evenly distributed among all four wheels under braking. Similarly, a rear weight bias 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 tooversteer, especially when decelerating (whether braking or lifting off the throttle; seelift-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.
Under hard acceleration, the decreased weight over the front wheels means less traction, sometimes producing a tendency for rear-engined cars toundersteer out of a corner.
In these respects, an RR can be considered to be an exaggeration of MR - harder braking, faster and earlier acceleration, and increased oversteer.
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. Manydune 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.[2][3]
Where RR differs from MR is in that the engine is located outside the wheelbase. The major advantage of MR - low moment of inertia - is negated somewhat (though still lower thanFR), 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 is technically more straightforward to drive all four wheels, than in a mid-engined configuration (though there have been more high-performance cars with theM4 layout than withR4). Finally, a rear-mounted engine has empty air (often at a lower pressure) behind it when moving, allowing more efficient cooling forair-cooled vehicles (more of which have been RR than liquid-cooled, such as theVolkswagen Beetle, and one of the few production air-cooled turbocharged cars, thePorsche 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 ofliquid-cooled engines in modern cars, most manufacturers have abandoned the RR layout. The major exception isPorsche, who has developed the911 for over 40 years and has taken advantage of the benefits of RR while mitigating its drawbacks to acceptable levels, lately with the help of electronic aids.[4]
One of first RR cars wasTatra 77 of 1934, the first serial-produced aerodynamic car, designed byHans Ledwinka.Tatra used this layout until end of production ofT700 in 1999. In case ofT613 andT700 Tatra used layout with engine above rear axle, which reduced some disadvantages of RR layout. Mercedes-Benz also produced several models of RR cars in this period, starting with the130H (1934). The radical 1930s Tatra format (air-cooled, rear engine and streamlined, teardrop design) was an influence on Ferdinand Porsche's 'People's Car' (Volkswagen) for Adolf Hitler. As well as being the most produced car ever, it set a trend for RR small cars that lasted well into the 1960s. The final form of the RR Volkswagen was theType 3 of 1961, which flattened the engine (or 'pancake'), allowing for luggage spaces front and rear.
Porsche has continued to develop its911 model as a rear-engined vehicle, although they have introduced multipleall-wheel-drive models. Most notably, the 911 Turbo has been sold as AWD-only since the release of the993 model. Race-oriented models such as theGT3 and twin-turbochargedGT2 remain solely RR, however.[5]
Another manufacturer to implement the RR configuration was theDeLorean Motor Company with itsDeLorean sports car. To compensate for the uneven (35/65) weight distribution caused by the rear-mounted engine, DeLorean used rear wheels with a diameter slightly greater than the front wheels. Before that was the rear-engined Škoda's fromŠkoda 1000MB (produced from 1964) toŠkoda 130/135/136 (produced until 1990) or thePolski Fiat 126p (produced until October 2000).
A range of sports road cars and racing cars with the RR layout was produced by the French companyAlpine. These had bodies made ofcomposite materials and used mechanical components made by Renault. (Alpine was eventually acquired by Renault; theA610 was a Renault product that used the Alpine name.)
Early cars using the RR layout included theTucker,Volkswagen Beetle,Porsche 356,Chevrolet Corvair,NSU Prinz,ZAZ Zaporozhets andHino Contessa.
Many modernelectric cars use an RR layout for base variants with a single motor due to the low weight and cooling requirements of theelectric motor. TheTesla Cybertruck andGMC Hummer EV also use this layout for their base variants.
Most modern heavy dutybuses use an extreme RR layout. Intransit buses this can be used to make avery low floor level in the first two-thirds of the bus, thus making disabled access much easier.
Mosttour buses andcoaches also employ a similar design, however the free space is usually used forluggage, and sometimes air conditioning equipment.
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