Rear-wheel drive

From Wikipedia, the free encyclopedia

 This article needs additional citations for verification.
Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (April 2008)

Rear-wheel drive (or RWD for short) is a common 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 late 1980s[citation needed]. This was most often due to higher manufacturing costs. Some automobile brands, including Lincoln, Mercedes-Benz, BMW, Lexus, Holden Tomcar, Porsche, and latest, Hyundai retain rear wheel drive platforms. Rear wheel drive is the preferred two-wheel drive choice for high performance automobiles because powerful front wheel drive cars understeer[1] more severely, and the ability to fit a large engine in a front-wheel drive layout while maintaining good fore and aft weight distribution is limited. The limit of controllable power for a front wheel drive car is generally placed between 230 and 300 horsepower[citation needed].

Contents

[edit] Advantages

Merge arrow
 It has been suggested that this article or section be merged into Automobile layout. (Discuss)
  • 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[1] (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.[3]
  • Better braking - the more even weight distribution helps prevent lockup from wheels becoming unloaded under heavy braking.[3]
  • 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.[4]
  • 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.[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

Merge arrow
 It has been suggested that this article or section be merged into Automobile layout. (Discuss)
  • Under heavy acceleration Oversteer and the related problem of fishtailing.
  • 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), and sometimes less trunk space (since there is also more hardware that must be placed underneath the trunk). 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.[5]
  • Higher initial purchase price - Modern rear wheel drive vehicles are typically slightly more expensive to purchase than comparable front wheel drive vehicles. Although the complex assembly (the powertrain is not one compact unit) and added cost of materials, this cost difference is more probably explained by production volumes and the fact that the majority of rear-wheel cars are in the sports/performance/luxury categories. Few modern "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[citation needed] - however these losses are highly dependent on the individual transmission). 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.[5] The driveshaft must also be extended for cars with a stretched wheelbase (e.g. limousines, minivans).

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

While the popularity of rear wheel drive has declined since the early 1980s, it is still relatively common, and has been making something of a resurgence. Here is a list of current or recent rear wheel drive vehicles.

[edit] References

  1. ^ a b Che Chou. FWD vs. RWD. Forza Motorsport 2.
  2. ^ Jim Kerr. Front wheel drive vs rear wheel drive. Canadian Driver.
  3. ^ a b Front wheel drive, rear wheel drive, or all wheel drive?. The Family Car.
  4. ^ Michael Vatalaro. SUVs carry the load. Boat/US Magazine.
  5. ^ a b The Hidden Virtues of Front Wheel Drive. Saab Automobile.

[edit] External links

v • d • e
Automobile layouts
Layouts
FF · MF · FR · FMR · RMR · RR · F4 · M4 · R4
Engine position
Drive wheels