Rear-engine, rear-wheel-drive layout

Source: Wikipedia, the free encyclopedia.
RR layout

In

coaches due to the elimination of the drive shaft with low-floor buses, this layout has become increasingly rare[specify] in passenger cars.[1]

Overview

Mercedes-Benz O500LE coach chassis showing the engine located far behind the rear axle

Most of the traits of the RR configuration are shared with the

front-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 forward

brake 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 to

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 to

understeer
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. 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.[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 than

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 has developed the 911 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]

History

One of first RR cars was Tatra 77 of 1934, the first serial-produced aerodynamic car, designed by Hans Ledwinka. Tatra used this layout until end of production of T700 in 1999. In case of T613 and T700 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 the 130H (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 the Type 3 of 1961, which flattened the engine (or 'pancake'), allowing for luggage spaces front and rear.

993 model. Race-oriented models such as the GT3 and twin-turbocharged GT2 remain solely RR, however.[5]

Another manufacturer to implement the RR configuration was 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 materials and used mechanical components made by Renault. (Alpine was eventually acquired by Renault; the A610
was a Renault product that used the Alpine name.)

Early cars using the RR layout included the

.

Present day

Examples of modern combustion engine cars using the layout include the Porsche 911, Mitsubishi i, Smart Forfour, and Renault Twingo.[6]

Many modern electric cars use an RR layout for base variants with a single motor due to the low weight and cooling requirements of the electric motor. The Tesla Cybertruck and GMC Hummer EV will also use this layout for their base variants.

Most modern heavy duty

very low floor level
in the first two-thirds of the bus, thus making disabled access much easier.

Most

luggage
, and sometimes airconditioning equipment.

Examples

References