Rear-wheel drive

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Rear wheel drive layouts

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.[1] 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.

Characteristics

The vast majority of rear-wheel drive vehicles use a longitudinally-mounted engine in the front of the vehicle, driving the rear wheels via a driveshaft linked via a differential between the rear axles. Some FR layout vehicles place the gearbox at the rear, though most attach it to the engine at the front.

The FR layout is often chosen for its simple design and good handling characteristics. Placing the drive wheels at the rear allows ample room for the transmission in the center of the vehicle and avoids the mechanical complexities associated with transmitting power to the front wheels. For performance-oriented vehicles, the FR layout is more suitable than front-wheel drive designs, especially with engines that exceed 200 horsepower. This is because weight transfers to the rear of the vehicle during acceleration, which loads the rear wheels and increases their grip.

Another advantage of the FR layout is relatively easy access to the engine compartment, as result of the longitudinal orientation of the drivetrain, as compared to the FF layout (front-engine, front-wheel drive). Powerful engines such as the Inline-6 and 90° big-bore V8 are usually too long to fit in a FF transverse engine ("east-west") layout; the FF configuration can typically accommodate at the maximum an Inline-4 or V6. This is another reason luxury/sports cars almost never use the FF layout.

Advantages

  • Even weight distribution — The layout of a rear-wheel drive car is much closer to an even fore-and-aft weight distribution than a front-wheel-drive car, as more of the engine can lie between the front and rear wheels (in the case of a mid engine layout, the entire engine), and the transmission is moved much farther back.[2]
  • Weight transfer during acceleration — During heavy acceleration, weight is placed on the rear, or driving wheels, which improves traction.
  • No torque steer[3] (unless it's an all-wheel steer with an offset differential).
  • Steering radius — As no complicated drive shaft joints are required at the front wheels, it is possible to turn them further than would be possible using front-wheel drive, resulting in a smaller steering radius for a given wheelbase.
  • Better handling in dry conditions — the more even weight distribution and weight transfer improve the handling of the car. The front and rear tires are placed under more even loads, which allows for more grip while cornering.[4]
  • Better braking — the more even weight distribution helps prevent lockup from wheels becoming unloaded under heavy braking.[4]
  • Towing — Rear wheel drive puts the wheels which are pulling the load closer to the point where a trailer articulates, helping steering, especially for large loads.[5]
  • Serviceability — Drivetrain components on a rear-wheel drive vehicle are modular and do not involve packing as many parts into as small a space as does front wheel drive, thus requiring less disassembly or specialized tools in order to service the vehicle.Template:Citation needed
  • Robustness — due to geometry and packaging constraints, the universal joints attached to the wheel hub have a tendency to wear out much later than the CV joints typically used in front-wheel drive counterparts. The significantly shorter drive axles on a front-wheel drive car causes the joint to flex through a much wider degree of motion, compounded by additional stress and angles of steering, while the CV joints of a rear wheel drive car regularly see angles and wear of less than half that of front wheel drive vehicles.Template:Citation needed
  • Can accommodate more powerful engines as a result of the longitudinal orientation of the drivetrain, such as the Inline-6 and 90° big-bore V8, making the FR a common configuration for luxury and sports cars. These engines are usually too long to fit in a FF transverse engine ("east-west") layout; the FF configuration can typically accommodate at the maximum an Inline-4 or V6.

Disadvantages

  • Under heavy acceleration, oversteer and fishtailing may occur.
  • On snow, ice and sand, rear-wheel drive loses its traction advantage to front- or all-wheel drive vehicles, which have greater weight on the driven wheels. Rear-wheel-drive cars with rear engine or mid engine configuration do not suffer from this, although fishtailing remains an issue.
  • Some rear engine cars (e.g. Porsche 911) can suffer from reduced steering ability under heavy acceleration, because the engine is outside the wheelbase and at the opposite end of the car from the wheels doing the steering although the engine weight over the rear wheels provides outstanding traction and grip during acceleration.
  • Decreased interior space — Though individual designs vary greatly, rear wheel drive vehicles may have: Less front leg room as the transmission tunnel takes up a space between the driver and front passenger, less leg room for center rear passengers (due to the tunnel needed for the drive shaft), often no seat for a center rear passenger, and sometimes less boot space (since there is also more hardware that must be placed underneath the boot). Rear engine designs (such as the Porsche 911 and Volkswagen Beetle) do not inherently take away interior space.
  • Increased weight — The components of a rear wheel drive vehicle's power train are less complex, but they are larger. The driveshaft adds weight. There is extra sheet metal to form the transmission tunnel. There is a rear axle or rear half-shafts, which are typically longer than those in a front-wheel drive car. A rear wheel drive car will weigh slightly more than a comparable front wheel drive car (but less than four wheel drive).
  • Improper weight distribution when loaded — A rear wheel drive car's center of gravity is shifted rearward when heavily loaded with passengers or cargo, which may cause unpredictable handling behavior.[6]
  • Higher initial purchase price — Modern rear wheel drive vehicles are typically more expensive to purchase than comparable front wheel drive vehicles. Part of this can be explained by the added cost of materials and increased complex assembly of FR layouts, as the powertrain is not one compact unit. However, the difference is more probably explained by production volumes as most rear-wheel cars are usually in the sports/performance/luxury categories (which tend to be more upscale and/or have more powerful engines), while the FF configuration is typically in mass-produced mainstream cars.
  • The possibility of a slight loss in the mechanical efficiency of the drivetrain (approximately 17% coastdown losses between engine flywheel and road wheels compared to 15% for front wheel drive — however these losses are highly dependent on the individual transmission).Template:Citation needed Cars with rear engine or mid engine configuration and a transverse engine layout do not suffer from this.
  • The long driveshaft (on front engine cars) adds to drivetrain elasticity.[6] The driveshaft must also be extended for cars with a stretched wheelbase (e.g. limousines, minivans).
  • When shifting down at high RPM, rear-wheel drive vehicles tend to oversteer if the clutch is not released correctly.

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