FOUR-WHEEL DRIVE (4WD)

 

Let’s start with the old-school version. 4WD, sometimes also referred to as Four by Four, or 4×4, is typically used on off-road vehicles – or at least vehicles with all-terrain capabilities.

Unfortunately, 4WD doesn’t fit neatly in a one-sentence explanation, but we’ll stick to the basics here.

Power goes from the transmission to what is known as a transfer case. This system then splits power between the front and rear axles so that maximum torque is going to each wheel. This power delivery process is nothing new, and still manages to propel modern Jeeps over, well, just about anything, but it does have some issues. When the transfer case splits power evenly, it ensures that each wheel turns at the same speed. This is deeply problematic when doing things like turning. You see, for a car to make a turn, the inside wheel has to turn more slowly than the outside wheel, which is covering more ground. If the vehicle can’t do this, the inside wheel loses traction and it spins freely. This, as you might be able to guess, isn’t great for moving forward efficiently.

There are a couple of ways that modern 4WD systems get around this. For starters, most modern 4WD systems are only on when you activate them. This can be done electronically or by using that protruding lever that sits somewhere between your radio and the center console. That way, you can use 4WD at low speed in snow or mud, but enjoy the drivability of two-wheel drive in normal conditions. When left in 2WD, there are fewer moving parts, and therefore fewer restrictions to forward motion. Said a different way, you’ll save fuel when don’t need to engage 4WD.

The other, more refined 4WD systems are activated with buttons or switches, rather than a manual lever, and include multiple settings for the 4WD system. These systems usually have a 4WD ‘High’, which splits power less evenly and allows what’s called ‘limited slip’ between the inside and outside wheels. This corrects the locked, spinning inside wheel problem to a point. Typically, however, High 4WD is recommended only up to around 60 mph. Flip these into ‘Low’, and they act much the same as old, locked systems.

 

 

PROS

  • Traction in off-road conditions. The wheels of the vehicles are better able to maintain a proper grip on the road, especially when the road is wet;
  • Adds weight and complexity car so as to turn off and improve fuel economy.
  • Greater hauling and towing capabilities;
  • Drivers might simply feel more secure driving a vehicle that has four-wheel drive capabilities.

CONS

  • More expensive than the two wheel drive models;
  • Tight corner braking phenomenon: when the propeller shaft between the front and rare axle is connected directly, the differences between the rotations of the front and rear axles cannot be absorbed;
  • Weight is increased due to the number of parts;
  • Construction is complex;
  • Too many moving parts increases vibration and noise.

Examples of four wheel drive vehicles: Subaru Outback, Subaru Forester, Audi Q5, Jeep Grand Cherokee e.t.c.

 

FRONT WHEEL DRIVE

This is a transmission system that provides power to the front wheels of a motor vehicle. This form of engine and transmission layout used in motor vehicles, where the engine drives the front wheels only. Most modern front-wheel-drive vehicles feature a transverse engine, rather than the conventional longitudinal engine, arrangement generally found in rear-wheel-drive and four-wheel-drive vehicles.

Experiments with front-wheel-drive cars date to the early days of the automobile. According to various sources, sometime between 1895 and 1898 Graf and Stiff built a Voiturette with a one-cylinder De Dion-Bouton engine fitted in the front of the vehicle, powering the front axle. It was thus arguably the world’s first front-wheel-drive automobile, but it never saw mass production, with only one copy ever made. In 1898, Latil, in France, devised a front-wheel-drive system for motorizing horse-drawn carts.

PROS

  • Since a single unit contained in the engine compartment of the vehicle, there is no need to devote interior space for a driveshaft tunnel the volume available for passengers and cargo;
  • Cheaper due to the fewer components;
  • Less weight due to fewer components;
  • Fuel economy;
  • Improved drivetrain efficiency;
  • Assembly efficiency;
  • Slippery surface traction
  • Predictable handling characteristics;
  • Better crosswind;
  • Front wheel drive allows the use of the left=foot braking technique;
  • Tactile feedback via the steering wheel informing driver if a wheel is slipping.

CONS

  • The center of gravity of the vehicle is typically farther forward than a comparable rear-wheel drive layout. In front wheel drive cars, the front axle typically supports around two thirds of the weight of the car;
  • Torque steer can be a problem on front wheel drive cars with higher torque engines and transverse layout;
  • lack of weight shifting will limit the acceleration of a front wheel;
  • In some towing situations front wheel drive cars can be at a traction disadvantage since there will be less weight on the driving wheels;
  • Due to geometry and packaging constraint, the constant-velocity joints attached to the wheel hub have a tendency to wear out much earlier than their rear wheel drive counterparts.
  • The driveshaft may limit the amount by which the front wheels can turn, thus it may increase the turning circle of a front wheel drive one with the same wheelbase;
  • In low traction conditions the front wheel lose traction first making steering ineffective.

 

Examples of front wheel drive vehicles: 2015 Toyota Avalon, 2015 Ford Fusion, 2015 Volkswagen,

REAR WHEEL DRIVE

This is a transmission system that provides power to the rear wheels of a motor vehicle. It means that the wheels that receive power from the engine, are the ones in the back. In the rear-wheel-drive vehicles, the front wheels do not drive the car at all.

PROS

  • Better weight balance because most rear wheel drive vehicles have the engine in the front and the drive components in the rear.
  • Better acceleration. This is because when you accelerate quickly from a stop the weight of the car transfers to the rear of the car.
  • The better weight balance of rear wheel drive allows the car to handle better.
  • Better Stopping: Due to the better balance rear drive cars brake better.
  • No Torque Steer,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.
  • Better Serviceability: Rear wheel drive cars are more rugged, cheaper and easier to fix.
  • Traction in Snow and Ice:Front drive cars have more weight over the front wheels. This gives more traction for acceleration in very slippery conditions. This is the biggest advantage in a front drive car.
  • Packaging:In front drive cars the engine and drivetrain can be pre-assembled as one unit and then fitted to the car during assembly. This leads to a more efficient assembly operation.
  • Cost:Due to easier manufacturing and a few less components it may be cheaper to manufacture a front wheel drive car.
  • Passenger/Trunk Space:Since there is no driveshaft running along under the car, front drive cars have more passenger and trunk space. Also, not having a rear wheel drive suspension can allow the trunk area to be larger.

 

Cons

  • Under heavy acceleration (as in racing), over steer and fishtailing may occur as the rear wheels break free and spin. The corrective action is to let off the throttle (this is what traction control automatically does for RWD vehicles).
  • 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. This issue is particularly noticeable on pickup trucks, as the weight of the engine and cab will significantly shift the weight from the rear to the front wheels. Rear-wheel-drive cars with rear engine or mid-engine configuration do not suffer from this, although fishtailing remains an issue. To correct this situation, owners of RWD vehicles can load sandbags in the back of the vehicle (either in the bed, or boot) in order to increase the weight over the rear axle, however speeds should be restricted to correctly predicted available grip of the road.
  • Some rear engine cars such as Porsche 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 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 not inherently take away interior space.
    • A rear-wheel drive vehicle with four-wheel drive, compared to a front-wheel drive vehicle with four-wheel drive, will have a less efficient interior packaging since the transmission is often under the front passenger compartment between the two seats, whereas the latter can package all the components under the hood.
  • 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).
  • Rear biased 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 at the hands of an inexperienced driver. It needs to be noted that rear engine cars are by their very nature, rear weight biased.
  • 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 labor put in to 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% coast down 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).

Examples of rear wheel drive vehicles: Ford Crown, Mazda MX-5 Miata, and Nissan 350Z, Honda S2000 etc.

 

ALL WHEEL DRIVE

All-Wheel Drive is a much more recent innovation, and, as you might expect, much more complicated. It crops up on everything from supercars like the Audi R8 to grocery-getters like the Buick Encore. In fact, a good rule of thumb might be to think of AWD as the “car” system while 4WD is the “truck” system. In this rule, consider crossovers like the Honda CR-V, Toyota RAV-4, Mazda CX-3, etc. to fall under the “car” category while SUVs like the Chevrolet Tahoe and Toyota 4Runner fall under the “truck” category. The biggest difference between 4WD and AWD is that an AWD drive system is on all the time. Well, mostly. But we’ll get to that, as there are two types of all-wheel drive: mechanical and electronic.

The most common way of accomplishing a capable, mechanical AWD system is by using three differentials. A differential is a box of gears, a.k.a. engineering magic, that can take power from the transmission and split it at different levels between two wheels or the front and rear axles (four wheels).

In AWD, this system works to get power to the wheels with the most traction by splitting power between the front and rear axles on the center differential, and the individual wheels by way of the front and rear differentials. This is useful either in slippery conditions when different wheels might be getting different amounts of grip from moment to moment. The Mercedes-Benz E63 AMG is a perfect example. It is now sold only in AWD in the United States because its power can overwhelm the traction of the rear wheels alone. But even when we aren’t talking about 500+ horsepower cars, splitting power evenly means added stability in all types of weather.

AWD isn’t quite as robust as 4WD and it can’t match the same levels of traction in extremely low-speed off-roading that the older 4WD systems provide. But AWD does have some clear advantages.

The pioneer and industry standard for AWD systems, Audi Quattro, distributed torque mechanically. Quattro allowed Audi to dominate rallying for nearly a decade in the 1980s, but heaven help you and your bank account if it went wrong.

These days, computers are involved in most AWD systems. Sensors on each wheel monitor traction, wheel speed, and several other data points hundreds of times a second. An ECU (engine control unit) dictates where power is sent and to which individual wheel depending on whichever has the most grip.

This type of system, usually called torque vectoring, appears on everything from the Subaru WRX to the Dodge Charger these days. Torque vectoring has allowed massive improvements in handling and all-weather capability.

Examples of All wheel Drives: Subaru Outback, Subaru Legacy etc.

 

Rear wheel vs front wheel

It is cheaper to design and build a FWD car. … This is why FWD is most commonly found in economy-type and lower-cost cars. The other FDW plus is better traction than a RWD car can deliver — especially in rain and snow. The front wheels pull the car instead of the rear wheels pushing it.

 

 

Four wheel vs all wheel

All-Wheel Drive (AWD): A drivetrain that employs a front, rear and center differential to provide power to all four wheels of a vehicle. Four-Wheel Drive (4WD): A drivetrain that employs two differentials and a transfer case to provide power to all four wheels of a vehicle.

CONCLUSION

Picking a vehicle is one of the most important decisions one can make, so before picking and settling for one think about, and her components, price and maintenance. Here are some general tips that might come in handy.

  • Low mileage
  • Great headlights
  • Clean, pleasant-smelling interiors
  • Low maintenance needs
  • High trade-in value
  • Low emissions
  • Wide rims, clean tailpipes
  • Room to comfortably fit five
  • Great performance when wet

 

 

Thus avoiding similar problems as depicted in the illustration below. Hopefully all the information provided above will help you understand your vehicle better or will help you in choosing your vehicle, that is what you need to know when choosing the drive system for your vehicle. For more information on choosing your drive system, go to UK Quality Car Export

 

 

 

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