Rear-wheel drive

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Rear-wheel drive (or RWD for short) is an engine/transmission layout used in automobiles. RWD typically places the engine in the front of the vehicle, but the front mid-engine, rear mid-engine and rear engine layouts are also used.

Rear-wheel drive is used almost universally for driving motorcycles, whether by driveshaft, chain, or belt.

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 transmission at the rear, though most attach it to the engine at the front.

Rear wheel drive has fallen out of favor in passenger cars since the 1980s[citation needed], due in part to higher manufacturing costs, and that front wheel drive is safer to drive[1], and that it performs better on slippery roads.[2][3] However, some automobile brands, including Cadillac, Mercedes-Benz, BMW and Porsche continue to use rear wheel drive platforms. Rear wheel drive (or all-wheel drive) is still the preferred choice for high performance automobiles, due to the fact that very powerful front wheel drive cars tend to understeer.[4] The general limit of controllable power for a front wheel drive car is generally placed between 230 and 300 horsepower.

It still sees heavy use in taxi and police fleets, due to lower maintenance costs.[citation needed][dubious — see talk page]

Contents

[edit] 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.[5]
  • Weight transfer during acceleration - During heavy acceleration, weight is placed on the rear, or driving wheels, which improves traction.
  • No torque steer[4] (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.[1]
  • Better braking - the more even weight distribution helps prevent lockup from wheels becoming unloaded under heavy braking.[1]
  • 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.[1]
  • 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 perform servicing.[citation needed]
  • Robustness - due to geometry and packaging constraints, the CV joints attached to the wheel hub have a tendency to wear out much earlier than the universal joints typically used in their rear-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.[citation needed]

[edit] Disadvantages

  • 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.
  • Decreased interior space - In a passenger car, rear wheel drive means: 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), and sometimes less trunk space (since there is also more hardware that must be placed underneath the trunk). There are some exceptions to this as rear engine designs do not take away interior space. (See Porsche 911, and Volkswagen Beetle)
  • 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. 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.[2]
  • Higher initial purchase price - Probably due to more complex assembly (the powertrain is not one compact unit) and added cost of materials, rear wheel drive is typically slightly more expensive to purchase than a comparable front wheel drive vehicle. This might also be explained by production volumes, or the fact that the majority of rear-wheel cars are in the sports/performance/luxury categories. Few "family" sedans have rear-wheel drive, so a cost comparison is not necessarily possible.[citation needed]
  • 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 - these losses are highly dependent on the individual transmission, and the source should be cited). Cars with rear engine or mid engine configuration and a transverse engine layout do not suffer from this.
  • The long driveshaft (on FR cars) adds to drivetrain elasticity.[2]

[edit] Current or recent rear wheel drive cars to 2006

While the popularity of rear wheel drive has declined, it is still relatively prevalent, and has been making something of a resurgence. Here is list of current or recent rear wheel drive vehicles. See also Category:Rear wheel drive vehicles.

[edit] References

  1. ^ a b c d The Family Car: Front wheel drive, rear wheel drive, or all wheel drive?
  2. ^ a b c The Hidden Virtues of Front Wheel Drive
  3. ^ SUVs carry the load
  4. ^ a b Che Chou. FWD vs. RWD. Forza Motorsport 2.
  5. ^ Jim Kerr. Front wheel drive vs rear wheel drive. Canadian Driver.

[edit] External links

 v • d • e Automobile layout

Layouts: FF, FMR, FR, MF, RMR, RR
Engine positioning: Front-engine, Mid-engine, Rear-engine
Drive: Front-wheel drive, Rear-wheel drive, Four-wheel drive, Six-wheel drive
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