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