جامعة الزقازيق. كليــــــة الهندســــــــــة. قسم القوى الميكانيكية. 4th year Mechanical Power Students. REAR WHEEL DRIVE. supervised by. Proff . Dr. Saad Abd Elhameed. بسم الله الرحمن الرحيم. و قل ربى زدنى علماً. صدق الله العظيم. REAR WHEEL DRIVE. DRIVESHAFTS PARTS AND COMPONENTS
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قسم القوى الميكانيكية
Mechanical Power Students
REAR WHEEL DRIVE
Proff . Dr.
Saad Abd Elhameed
بسم الله الرحمن الرحيم
و قل ربى زدنى علماً
صدق الله العظيم
REAR WHEEL DRIVE
DRIVESHAFTS PARTS AND COMPONENTS
3- End Yokes
5- Slip Joints
8- Tube Yokes
9- U Joints
Rear-wheel drive - Definition
Rear wheel drive was a common form of engine/transmission layout used in automobiles throughout the 20th century. RWD typically places the engine in the front of the vehicle, but the mid engine and rear engine layouts are also used.
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, due in part to higher manufacturing costs, and a (largely erroneous) perception by many car buyers that front wheel drive is safer, and that it performs better on slippery roads.
It still sees heavy use in taxi and police fleets, due to cheaper maintenance, and in the case of police fleets, better performance.
Current or recent rear wheel drive cars to 2004
While the popularity of rear-wheel drive has declined, it's still around, 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.
Almost all non-4wd trucks and most SUVs are rear wheel drive.
The overwhelming majority of sports cars are rear wheel drive, although some also have four wheel drive options. It would be redundant to list them all here.
All BMWs except the Mini, and all wheel drive variants.
Cadillac STS, CTS, SRX
Chevrolet Camaro/Pontiac Firebird (all years until the end)
Chevrolet Caprice - discontinued in the late 1990s. Was a popular taxi and police car.
DodgeMagnum/Chrysler300 (2004 and up)
Ford Crown Victoria, a popular police car and taxi, and its near-twins the Mercury Grand Marquis and Marauder
Ford Mustang (all years) - this is debateably a sports car.
Ford Thunderbird (all years) / Mercury Cougar (up to 1997)
Holden Monaro (Australia) and versions sold overseas: Pontiac GTO (USA) and Vauxhall Monaro (United Kingdom)
Honda - S-2000
InfinitiG35, M45, Q45
Jaguar - All except X-Type
Lexus - All cars except ES
Lincoln Town Car
Mazda - Miata, RX-7, RX-8
All Mercedes-Benz sold in North America (except all wheel drive models)
Nissan - 350Z
Toyota - Trueno Sprinter AE86, Supra , MR2
Rear wheel drive layouts
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. Rear-wheel drive is used almost Sketch of FR layout
Rear-wheel drive (RWD) typically places the engine in the front of the vehicle and the driven wheels are located at the rear, a 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.
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 why luxury/sports cars almost never use the FF layout.
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.
Weight transfer during acceleration — During heavy acceleration, weight is placed on the rear, or driving wheels, which improves traction.
No torque steer (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.
Better braking — the more even weight distribution helps prevent lockup from wheels becoming unloaded under heavy braking.
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.
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.
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.
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
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 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.
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. Few modern "family" cars have rear-wheel drive as of 2009, so a direct cost comparison is not necessarily possible.
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). 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. The driveshaft must also be extended for cars with a stretched wheelbase (e.g. limousines, minivans).
Cardan shaft including Cardan joints
Cardan shaft including Cardan joints
Exposed drive shaft on a classic BMW motorcycle
A drive shaft, driving shaft, propeller shaft, or Cardan shaft is a mechanical component for transmitting torque and rotation, usually used to connect other components of a drive train that cannot be connected directly because of distance or the need to allow for relative movement between them.
Drive shafts are carriers of torque: they are subject to torsion and shear stress, equivalent to the difference between the input torque and the load. They must therefore be strong enough to bear the stress, whilst avoiding too much additional weight as that would in turn increase their inertia.
Automotive drive shafts
An automobile may use a longitudinal shaft to deliver power from an engine/transmission to the other end of the vehicle before it goes to the wheels. A pair of short drive shafts is commonly used to send power from a central differential, transmission, or transaxle to the wheels.
A truck double propeller shaft
Front-engine, rear-wheel drive
Main article: Front-engine, rear-wheel drive layout
In front-engined, rear-drive vehicles, a longer drive shaft is also required to send power the length of the vehicle. Two forms dominate: The torque tube with a single universal joint and the more common Hotchkiss drive with two or more joints. This system became known as Système Panhard after the automobile company Panhard et Levassor patented it.
In British English, the term "drive shaft" is restricted to a transverse shaft that transmits power to the wheels, especially the front wheels. A drive shaft connecting the gearbox to a rear differential is called a propeller shaft, or prop-shaft. A prop-shaft assembly consists of a propeller shaft, a slip joint and one or more universal joints. Where the engine and axles are separated from each other, as on four-wheel drive and rear-wheel drive vehicles, it is the propeller shaft that serves to transmit the drive force generated by the engine to the axles.
A drive shaft connecting a rear differential to a rear wheel may be called a half shaft. The name derives from the fact that two such shafts are required to form one rear axle.
Several different types of drive shaft are used in the automotive industry:
1-piece drive shaft
2-piece drive shaft
Slip-in-tube drive shaft
The slip-in-tube drive shaft is a new type that also helps in crash energy management. It can be compressed in the event of a crash, so is also known as a collapsible drive shaft.
Drive shaft for Research and Development (R&D)
The automotive industry also uses drive shafts at testing plants. At an engine test stand a drive shaft is used to transfer a certain speed / torque from the combustion engine to a dynamometer. A "shaft guard" is used at a shaft connection to protect against contact with the drive shaft and for detection of a shaft failure. At a transmission test stand a drive shaft connects the prime mover with the transmission.
How Torque Converters Work
If you've read about manual transmissions, you know that an engine is connected to a transmission by way of a clutch. Without this connection, a car would not be able to come to a complete stop without killing the engine. But cars with an automatic transmission have no clutch that disconnects the transmission from the engine. Instead, they use an amazing device called a torque converter. It may not look like much, but there are some very interesting things going on inside.
In this article, we'll learn why automatic transmission cars need a torque converter, how a torque converter works and what some of its benefits and shortcomings are.
A cut-away model of a torque converter
A torque converter is a modified form of fluid coupling that is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load. Like a basic fluid coupling, the torque converter normally takes the place of a mechanical clutch, allowing the load to be separated from the power source. As a more advanced form of fluid coupling, however, a torque converter is able to multiply torque when there is a substantial difference between input and output rotational speed, thus providing the equivalent of a reduction gear
Torque converter elements
A fluid coupling is a two element drive that is incapable of multiplying torque, while a torque converter has at least one extra element—the stator—which alters the drive's characteristics during periods of high slippage, producing an increase in output torque.
In a torque converter there are at least three rotating elements: the pump, which is mechanically driven by the prime mover; the turbine, which drives the load; and the stator, which is interposed between the pump and turbine so that it can alter oil flow returning from the turbine to the pump. The classic torque converter design dictates that the stator be prevented from rotating under any condition, hence the term stator. In practice, however, the stator is mounted on an overrunning clutch, which prevents the stator from counter-rotating with respect to the prime mover but allows forward rotation.
Modifications to the basic three element design have been periodically incorporated, especially in applications where higher than normal torque multiplication is required. Most commonly, these have taken the form of multiple turbines and stators, each set being designed to produce differing amounts of torque multiplication. For example, the BuickDynaflow automatic transmission was a non-shifting design and, under normal conditions, relied solely upon the converter to multiply torque. The Dynaflow used a five element converter to produce the wide range of torque multiplication needed to propel a heavy vehicle.
Although not strictly a part of classic torque converter design, many automotive converters include a lock-up clutch to improve cruising power transmission efficiency. The application of the clutch locks the turbine to the pump, causing all power transmission to be mechanical, thus eliminating losses associated with fluid drive.
A torque converter has three stages of operation:
Stall. The prime mover is applying power to the pump but the turbine cannot rotate. For example, in an automobile, this stage of operation would occur when the driver has placed the transmission in gear but is preventing the vehicle from moving by continuing to apply the brakes. At stall, the torque converter can produce maximum torque multiplication if sufficient input power is applied (the resulting multiplication is called the stall ratio). The stall phase actually lasts for a brief period when the load (e.g., vehicle) initially starts to move, as there will be a very large difference between pump and turbine speed.
Acceleration. The load is accelerating but there still is a relatively large difference between pump and turbine speed. Under this condition, the converter will produce torque multiplication that is less than what could be achieved under stall conditions. The amount of multiplication will depend upon the actual difference between pump and turbine speed, as well as various other design factors.
Coupling. The turbine has reached approximately 90 percent of the speed of the pump. Torque multiplication has essentially ceased and the torque converter is behaving in a manner similar to a plain fluid coupling. In modern automotive applications, it is usually at this stage of operation where the lock-up clutch is applied, a procedure that tends to improve fuel efficiency.
The key to the torque converter's ability to multiply torque lies in the stator. In the classic fluid coupling design, periods of high slippage cause the fluid flow returning from the turbine to the pump to oppose the direction of pump rotation, leading to a significant loss of efficiency and the generation of considerable waste heat. Under the same condition in a torque converter, the returning fluid will be redirected by the stator so that it aids the rotation of the pump, instead of impeding it. The result is that much of the energy in the returning fluid is recovered and added to the energy being applied to the pump by the prime mover. This action causes a substantial increase in the mass of fluid being directed to the turbine, producing an increase in output torque. Since the returning fluid is initially traveling in a direction opposite to pump rotation, the stator will likewise attempt to counter-rotate as it forces the fluid to change direction, an effect that is prevented by the one-way stator clutch.
Unlike the radially straight blades used in a plain fluid coupling, a torque converter's turbine and stator use angled and curved blades. The blade shape of the stator is what alters the path of the fluid, forcing it to coincide with the pump rotation. The matching curve of the turbine blades helps to correctly direct the returning fluid to the stator so the latter can do its job. The shape of the blades is important as minor variations can result in significant changes to the converter's performance.
During the stall and acceleration phases, in which torque multiplication occurs, the stator remains stationary due to the action of its one-way clutch. However, as the torque converter approaches the coupling phase, the energy and volume of the fluid returning from the turbine will gradually decrease, causing pressure on the stator to likewise decrease. Once in the coupling phase, the returning fluid will reverse direction and now rotate in the direction of the pump and turbine, an effect which will attempt to forward-rotate the stator. At this point, the stator clutch will release and the pump, turbine and stator will all (more or less) turn as a unit.
Unavoidably, some of the fluid's kinetic energy will be lost due to friction and turbulence, causing the converter to generate waste heat (dissipated in many applications by water cooling). This effect, often referred to as pumping loss, will be most pronounced at or near stall conditions. In modern designs, the blade geometry minimizes oil velocity at low pump speeds, which allows the turbine to be stalled for long periods with little danger of overheating.
Efficiency and torque multiplication
A torque converter cannot achieve 100 percent coupling efficiency. The classic three element torque converter has an efficiency curve that resembles an inverted "U": zero efficiency at stall, generally increasing efficiency during the acceleration phase and low efficiency in the coupling phase. The loss of efficiency as the converter enters the coupling phase is a result of the turbulence and fluid flow interference generated by the stator, and as previously mentioned, is commonly overcome by mounting the stator on a one-way clutch.
Even with the benefit of the one-way stator clutch, a converter cannot achieve the same level of efficiency in the coupling phase as an equivalently sized fluid coupling. Some loss is due to the presence of the stator (even though rotating as part of the assembly), as it always generates some power-absorbing turbulence. Most of the loss, however, is caused by the curved and angled turbine blades, which do not absorb kinetic energy from the fluid mass as well as radially straight blades. Since the turbine blade geometry is a crucial factor in the converter's ability to multiply torque, trade-offs between torque multiplication and coupling efficiency are inevitable. In automotive applications, where steady improvements in fuel economy have been mandated by market forces and government edict, the nearly universal use of a lock-up clutch has helped to eliminate the converter from the efficiency equation during cruising operation.
The maximum amount of torque multiplication produced by a converter is highly dependent on the size and geometry of the turbine and stator blades, and is generated only when the converter is at or near the stall phase of operation. Typical stall torque multiplication ratios range from 1.8:1 to 2.5:1 for most automotive applications (although multi-element designs as used in the BuickDynaflow and ChevroletTurboglide could produce more). Specialized converters designed for industrial or heavy marine power transmission systems are capable of as much as 5.0:1 multiplication. Generally speaking, there is a trade-off between maximum torque multiplication and efficiency—high stall ratio converters tend to be relatively inefficient below the coupling speed, whereas low stall ratio converters tend to provide less possible torque multiplication.
While torque multiplication increases the torque delivered to the turbine output shaft, it also increases the slippage within the converter, raising the temperature of the fluid and reducing overall efficiency. For this reason, the characteristics of the torque converter must be carefully matched to the torque curve of the power source and the intended application. Changing the blade geometry of the stator and/or turbine will change the torque-stall characteristics, as well as the overall efficiency of the unit. For example, drag racing automatic transmissions often use converters modified to produce high stall speeds to improve off-the-line torque, and to get into the power band of the engine more quickly. Highway vehicles generally use lower stall torque converters to limit heat production, and provide a more firm feeling to the vehicle's characteristics.
A design feature once found in some General Motors automatic transmissions was the variable-pitch stator, in which the blades' angle of attack could be varied in response to changes in engine speed and load. The effect of this was to vary the amount of torque multiplication produced by the converter. At the normal angle of attack, the stator caused the converter to produce a moderate amount of multiplication but with a higher level of efficiency. If the driver abruptly opened the throttle, a valve would switch the stator pitch to a different angle of attack, increasing torque multiplication at the expense of efficiency.
Some torque converters use multiple stators and/or multiple turbines to provide a wider range of torque multiplication. Such multiple-element converters are more common in industrial environments than in automotive transmissions, but automotive applications such as Buick's Triple Turbine Dynaflow and Chevrolet's Turboglide also existed. The Buick Dynaflow utilized the torque-multiplying characteristics of its planetary gearset in conjunction with the torque converter for low gear and bypassed the first turbine, using only the second turbine as vehicle speed increased. The unavoidable trade-off with this arrangement was low efficiency and eventually these transmissions were discontinued in favor of the more efficient three speed units with a conventional three element torque converter.
A transmission or gearbox provides speed and torque conversions from a rotating power source to another device using gear ratios. The most common use is in motor vehicles, where the transmission adapts the output of the internal combustion engine to the drive wheels. Such engines need to operate at a relatively high rotational speed, which is inappropriate for starting, stopping, and slower travel. The transmission reduces the higher engine speed to the slower wheel speed, increasing torque in the process. Transmissions are also used on pedal bicycles, fixed machines, and anywhere else rotational speed and torque needs to be adapted.
Often, a transmission will have multiple gear ratios (or simply "gears"), with the ability to switch between them as speed varies. This switching may be done manually (by the operator), or automatically. Directional (forward and reverse) control may also be provided. Single-ratio transmissions also exist, which simply change the speed and torque (and sometimes direction) of motor output.
In motor vehicle applications, the transmission will generally be connected to the crankshaft of the engine. The output of the transmission is transmitted via driveshaft to one or more differentials, which in turn drive the wheels. While a differential may also provide gear reduction, its primary purpose is to change the direction of rotation.
Conventional gear/belt transmissions are not the only mechanism for speed/torque adaptation. Alternative mechanisms include torque converters and power transformation (e.g., diesel-electric transmission, hydraulic drive system, etc.). Hybrid configurations also exist.
Main gearbox of the Bristol 171 Sycamore helicopter
Early transmissions included the right-angle drives and other gearing in windmills, horse-powered devices, and steam engines, in support of pumping, milling, and hoisting.
Most modern gearboxes are used to increase torque while reducing the speed of a prime mover output shaft (e.g. a motor crankshaft). This means that the output shaft of a gearbox will rotate at slower rate than the input shaft, and this reduction in speed will produce a mechanical advantage, causing an increase in torque. A gearbox can be setup to do the opposite and provide an increase in shaft speed with a reduction of torque. Some of the simplest gearboxes merely change the physical direction in which power is transmitted.
Many typical automobile transmissions include the ability to select one of several different gear ratios. In this case, most of the gear ratios (often simply called "gears") are used to slow down the output speed of the engine and increase torque. However, the highest gears may be "overdrive" types that increase the output speed.
Gearboxes have found use in a wide variety of different—often stationary—applications, such as wind turbines.
Transmissions are also used in agricultural, industrial, construction, mining and automotive equipment. In addition to ordinary transmission equipped with gears, such equipment makes extensive use of the hydrostatic drive and electrical adjustable-speed drives.
Tractor transmission with 16 forward and 8 backward gears
Many applications require the availability of multiple gear ratios. Often, this is to ease the starting and stopping of a mechanical system, though another important need is that of maintaining good fuel efficiency.
The need for a transmission in an automobile is a consequence of the characteristics of the internal combustion engine. Engines typically operate over a range of 600 to about 7000 revolutions per minute (though this varies, and is typically less for diesel engines), while the car's wheels rotate between 0 rpm and around 1800 rpm.
Furthermore, the engine provides its highest torque outputs approximately in the middle of its range, while often the greatest torque is required when the vehicle is moving from rest or traveling slowly. Therefore, a system that transforms the engine's output so that it can supply high torque at low speeds, but also operate at highway speeds with the motor still operating within its limits, is required. Transmissions perform this transformation.
Many transmissions and gears used in automotive and truck applications are contained in a cast iron case, though more frequently aluminium is used for lower weight especially in cars. There are usually three shafts: a mainshaft, a countershaft, and an idler shaft.
The mainshaft extends outside the case in both directions: the input shaft towards the engine, and the output shaft towards the rear axle (on rear wheel drive cars- front wheel drives generally have the engine and transmission mounted transversely, the differential being part of the transmission assembly.) The shaft is suspended by the main bearings, and is split towards the input end. At the point of the split, a pilot bearing holds the shafts together. The gears and clutches ride on the mainshaft, the gears being free to turn relative to the mainshaft except when engaged by the clutches.
Types of automobile transmissions include manual, automatic or semi-automatic transmission.
The biggest benefit to rear wheel drive is that it spreads the loads of the car across all four tires of a car. In a rear wheel drive car the rear wheels do the pushing while the front wheels are reserved for the steering duties. In front wheel drive cars the front tires must perform both functions. Each front tire in a front wheel drive car must do two tasks. Both the cornering forces and the engine acceleration/deceleration forces in a front drive car act on the same tire.
So in a front drive the tires capacity can be easily exceeded.In a rear drive car the rear tires handle the engine acceleration/deceleration while the front only need to handle the steering forces. Not only does this balance the load on the tires but it reserves the front tires exclusively for the all important steering duties.
Better weight balance. Most rear wheel drive cars have the engine in the front and the drive components in the rear. Front drive cars have everything up front. By properly balancing the front and rear of the car you can improve the handling, acceleration, braking, and thus safety of a car.
Better acceleration. On all but the slipperiest surfaces rear wheel drive cars accelerate faster than a front drive car from a stop. This is because when you accelerate quickly from a stop the weight of the car transfers to the rear of the car. In a rear drive car this places extra weight on the rear of the car, essentially jamming the tires in to the road greatly increasing traction. In a front drive car, when the weight goes to the rear, weight is taken off of the front wheels. The front wheels spin thus losing traction. If the wheels are spinning not only does this slow you down but it also makes it difficult to steer the car. In the rear drive car the front tires are available for steering even if the rears have lost traction.
Better Road Holding. The better weight balance of rear wheel drive allows the car to handle better. The more even weight allows the car to drive neutrally through a corner. This means both the front and rear of the car have near equal loads acting upon them. In a front drive car the the heavy front end causes the front end to have a higher load on it. This will cause the front tires to eventually lose grip well before the rear tires are fully loaded. Front tires on front drive cars do much more work than the rears causing them to wear out much faster. It is best to balance the load as best you can among the four tires. If you are accelerating or slowing down (engine braking) these forces will act upon the already heavily loaded front tires of a front drive car. In a rear drive car the front tires are left for steering even when accelerating or engine braking. Sharing the work among all four tires is the key.
Better Stopping. Because of the better balance rear drive cars brake better. When you stop a front drive car the excess weight in the front of the car allows the force on the front tires to exceed the limits of the tires. The relatively low weight on the rear of a front drive car does not allow the tires to be used to their maximum ability. When panic stopping weight will transfer to the front in both rear and front drive cars but there is more weight left for rear braking action in the rear drive car.
No Torque Steer. Front wheel Drive cars have a problem known as Torque Steer. This occurs when the acceleration of the engine effects the cars steering. Since the driveline is connected to the steering wheels the torque of the engine applies force to the front wheels causing the car to pull to the right during acceleration. Rear Drive cars do not have this problem since the engine is not connected to the steering gear.
Better Ride and Feel. The light front end of the car allows it to "turn in" to a corner easier. The car feels more nimble and controllable. Since the front is not so heavy it is not burdened by needing strong springs to keep it under control. This allows the suspension to be set up softer while maintaining good control ability. The absence of drive shafts (half shafts) and CV joints in the front of the car allows the front suspension to be designed for maximum steering efficiency. The lower rotating weight of the front wheel assemblies improves steering response and ultimate grip. Granted that a rear drive has more weight at the rear of the car but it can be handled by the underutilized (in a front driver) rear tires.
Better Serviceability / More Rugged. Ever notice that cops and taxis avoid front wheel drive like the plague? That is because rear wheel drive cars are more rugged and easier and cheaper to fix.
Better Ultimate Ability.. Purpose built race cars are almost always rear wheel drive. In production based racing series Front Wheel Drive cars are given a performance advantage to make them equal to Rear Drive Cars. Usually this is in the form of a weight break. Granted we shouldn't be driving our cars like race cars on the street but in an emergency having the extra ability in the car is an advantage I would like to have.
Front Wheel Drive vs. Rear Wheel Drive
Today we find that vehicles come in 3 drive configurations front-wheel drive, rear-wheel drive and 4 or all-wheel drive. In this article, we will focus on only front and rear-wheel drive. Front-wheel drive is where the engine drives only the front wheels. Rear-wheel drive is when the engine drives only the rear wheels.
Front-wheel drive is used in order to provide a compact package. Very little space is necessary since the engine and the wheels being driven are in the front of the vehicle. A center tunnel or a higher chassis is not necessary as they would be in a rear-wheel-drive configuration. The driveshaft of the rear-wheel-drive vehicle has to reach from the engine to the rear wheels causing the need for a tunnel or higher chassis. The front-wheel-drive-configuration also promotes good traction.
Due to their better traction due to weight distribution, front-wheel-drive vehicles are preferred to rear-wheel-drive if one needs to drive in snow, mud or wet roads. However, front-wheel-drive vehicles are not always as quick to accelerate as rear-wheel drive because of weight transference that takes place during acceleration causes the front wheels to sharply reduce their grip and this adversely affects torque.
In addition, front-wheel-drive vehicles feature a transverse engine (facing side to side) while rear-wheel-drive vehicles have a longitudinal engine (facing front to back). Due to the style of the engine, front-wheel drive actually restricts the size of the engine. That is why you will see front-wheel-drive cars with inline 4 and V6 engines. Longer engines such as Inline 6 and big bore V8 engines have a difficult time fitting into the available space. This is why most luxury cars as well as sports cars more often than not have a rear wheel drive configuration.
Advantages to a Front-Wheel-Drive Configuration Include:
More Interior Space
Less Cost to Manufacture
Improved Drivetrain Efficiency
Forward Center of Gravity Which Improves Traction
Disadvantages to a Front-Wheel-Drive Vehicle Are:
Poorer Torque Allowing Front-Wheel-Drive Vehicles to Pull Left or Right when They Are Accelerated with Some Force
Lack of Weight Shifting Which Limits the Acceleration of the Vehicle
Reduced Traction When the Vehicle Is Climbing a Slope in Slippery Conditions
The CV Joints Attached to the Wheel Hub Tend to Wear Out Faster
Increased Turning Circle Because the Transverse Engine Limits the Amount the Front Wheels Can Turn