Rear-engine, rear-wheel drive layout

In automotive design, an 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.

Most of the traits of the RR configuration are shared with the mid-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 a driveshaft, and the differential can be integrated with the transmission, commonly referred to as a transaxle. 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 FR (front engine, rear drive), 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.

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Wikipedia