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Guidelines, Instration, Information & Articles...
Front and Rear Wheel Drive
Front Wheel Drive
Around since the 1920's, front wheel drive didn't catch on with American consumers until the gas crisis in the 1970's. As Americans struggled against high fuel prices, automakers began to seek new ways to increase fuel efficiency. The best way of course was to reduce the size (and thus the weight) of most vehicles. As Detroit aimed to make cars smaller, they needed a more efficient layout that would yield more interior room in a smaller package.
Front wheel drive was the solution. By placing the engine and transaxle in the front, there is no large transmission housing or driveshaft tunnel running through the passenger compartment. In addition, engines were positioned transversely to reduce the size of the engine bay. And there was another advantage as well. With 60% of its weight at the front, 40% at the back, fwd holds an advantage in slippery conditions such as ice or snow as more weight is over the drive wheels reducing slip during acceleration. But most of the advantages end there.
As most of the weight in up front, a fwd car is not as well balanced therefore it doesn't handle quite as well. Also, as vehicles continue to become more powerful, front wheel drive becomes more of a liability. Torque steer (when the steering wheel pulls to one side during acceleration) is a serious issue with many front wheel drive cars that exceed 250hp. As such, we've seen resurgence in the popularity of rear wheel drive in more powerful vehicles.
Rear Wheel Drive
Prior to the fuel crises in the 1970's, rear wheel drive was king. Just about every vehicle, from economy to luxury, came with rear wheel drive. The shift from rear wheel drive to front took about a decade. Since the mid eighties, just about every economy car, family sedan, minivan and even many sport coupes came with front wheel drive. Luxury marks such as BMW and Mercedes-Benz continued on with rear wheel drive but Cadillac eventually moved every vehicle to front wheel drive. Once again, times have changed.
Over the last few years we've seen more and more vehicles (re) introduce rear wheel drive. Why? Well, it simple. As cars become more powerful it is difficult to have one set of wheels doing the steering and the accelerating. By having the front wheels do the steering, and the rear wheels driving the car, you get a better-balanced vehicle. This eliminates torque steer and improves acceleration. Rear wheel drive offers better weight distribution (much closer to 50/50 than fwd), which in turn offers more predictable handling. Finally, with the advent of traction control and stability management systems, the front wheel drive advantage in slippery conditions has been significantly reduced.
More and more rwd vehicles have the option of AWD as well. If nothing else, this is a great way for automakers to hedge their bets. Still, some consumers are skeptical of rear wheel drive. Perhaps they are the victims of clever marketing by Madison Ave. that tried to get people to accept fwd and forget all about the virtues rear wheel drive. They did a great job. Perhaps too good.
Today cars are more powerful yet yield better fuel economy. As such, we can look at fwd and rwd more objectively. Is one better than the other? Fwd still holds an advantage in terms of packaging efficiency, offering greater interior room in a smaller package.
Rear wheel drive provides better handling and acceleration and with the addition of traction control, virtually eliminates the fwd advantage in the snow. In the end, it depends on what you want from your car. If it's performance, you're looking at rwd. If you're indifferent, perhaps looking for a small car with greater interior volume, it's front wheel drive for you.
Over the last 20 years, technology has improved both layouts, reducing the advantages of fwd to a point where rwd is a viable option for most people. Ultimately, you've got more choice, and when more choice is offered we all win.
How Four Wheel Drive Works
There are almost as many different types of four-wheel-drive systems as there are four-wheel-drive vehicles. It seems that every manufacturer has several different schemes for providing power to all of the wheels. The language used by the different carmakers can sometimes be a little confusing, so before we get started explaining how they work, let's clear up some terminology
- Four-wheel drive - Usually, when carmakers say that a car has four-wheel drive, they are referring to a part-time system. For reasons we'll explore later in this article, these systems are meant only for use in low-traction conditions, such as off-road or on snow or ice.
- All-wheel drive - These systems are sometimes called full-time four-wheel drive. All-wheel-drive systems are designed to function on all types of surfaces, both on- and off-road, and most of them cannot be switched off.
Part-time and full-time four-wheel-drive systems can be evaluated using the same criteria. The best system will send exactly the right amount of torque to each wheel, which is the maximum torque that won't cause that tire to slip.
The interesting thing about torque is that in low-traction situations, the maximum amount of torque that can be created is determined by the amount of traction, not by the engine. Even if you have a NASCAR engine in your car, if the tires won't stick to the ground there is simply no way to harness that power.
For the sake of this article, we'll define traction as the maximum amount of force the tire can apply against the ground (or that the ground can apply against the tire -- they're the same thing). These are the factors that affect traction:
- The weight on the tire - The more weight on a tire, the more traction it has. Weight can shift as a car drives. For instance, when a car makes a turn, weight shifts to the outside wheels. When it accelerates, weight shifts to the rear wheels. (See How Brakes Work for more details.)
- The coefficient of friction - This factor relates the amount of friction force between two surfaces to the force holding the two surfaces together. In our case, it relates the amount of traction between the tires and the road to the weight resting on each tire. The coefficient of friction is mostly a function of the kind of tires on the vehicle and the type of surface the vehicle is driving on. For instance, a NASCAR tire has a very high coefficient of friction when it is driving on a dry, concrete track. That is one of the reasons why NASCAR race cars can corner at such high speeds. The coefficient of friction for that same tire in mud would be almost zero. By contrast, huge, knobby, off-road tires wouldn't have as high a coefficient of friction on a dry track, but in the mud, their coefficient of friction is extremely high.
- Wheel slip - There are two kinds of contact that tires can make with the road: static and dynamic.
- static contact - The tire and the road (or ground) are not slipping relative to each other. The coefficient of friction for static contact is higher than for dynamic contact, so static contact provides better traction.
- dynamic contact - The tire is slipping relative to the road. The coefficient of friction for dynamic contact is lower, so you have less traction.
Quite simply, wheel slip occurs when the force applied to a tire exceeds the traction available to that tire. Force is applied to the tire in two ways:
- Longitudinally - Longitudinal force comes from the torque applied to the tire by the engine or by the brakes. It tends to either accelerate or decelerate the car.
- Laterally - Lateral force is created when the car drives around a curve. It takes force to make a car change direction -- ultimately, the tires and the ground provide lateral force.
Let's say you have a fairly powerful rear-wheel-drive car, and you are driving around a curve on a wet road. Your tires have plenty of traction to apply the lateral force needed to keep your car on the road as it goes around the curve. Let's say you floor the gas pedal in the middle of the turn (don't do this!) -- your engine sends a lot more torque to the wheels, producing a large amount of longitudinal force. If you add the longitudinal force (produced by the engine) and the lateral force created in the turn, and the sum exceeds the traction available, you just created wheel slip.
Most people don't even come close to exceeding the available traction on dry pavement, or even on flat, wet pavement. Four-wheel and all-wheel-drive systems are most useful in low-traction situations, such as in snow and on slippery hills.
The benefit of four-wheel drive is easy to understand: If you are driving four wheels instead of two, you've got the potential to double the amount of longitudinal force (the force that makes you go) that the tires apply to the ground.
This can help in a variety of situations. For instance:
- In snow - It takes a lot of force to push a car through the snow. The amount of force available is limited by the available traction. Most two-wheel-drive cars can't move if there is more than a few inches of snow on the road, because in the snow, each tire has only a small amount of traction. A four-wheel-drive car can utilize the traction of all four tires.
- Off road - In off-road conditions, it is fairly common for at least one set of tires to be in a low-traction situation, such as when crossing a stream or mud puddle. With four-wheel drive, the other set of tires still has traction, so they can pull you out.
- Climbing slippery hills - This task requires a lot of traction. A four-wheel-drive car can utilize the traction of all four tires to pull the car up the hill.
There are also some situations in which four-wheel drive provides no advantage over two-wheel drive. Most notably, four-wheel-drive systems won't help you stop on slippery surfaces. It's all up to the brakes and the anti-lock braking system (ABS).
Components of a Four-wheel-drive System
The main parts of any four-wheel-drive system are the two differentials (front and rear) and the transfer case. In addition, part-time systems have locking hubs, and both types of systems may have advanced electronics that help them make even better use of the available traction.
A car has two differentials, one located between the two front wheels and one between the two rear wheels. They send the torque from the driveshaft or transmission to the drive wheels. They also allow the left and right wheels to spin at different speeds when you go around a turn.
When you go around a turn, the inside wheels follow a different path than the outside wheels, and the front wheels follow a different path than the rear wheels, so each of the wheels is spinning at a different speed. The differentials enable the speed difference between the inside and outside wheels.
The transfer case on a part-time four-wheel-drive system locks the front-axle driveshaft to the rear-axle driveshaft, so the wheels are forced to spin at the same speed. This requires that the tires slip when the car goes around a turn. Part-time systems like this should only be used in low -traction situations in which it is relatively easy for the tires to slip. On dry concrete, it is not easy for the tires to slip, so the four-wheel drive should be disengaged in order to avoid jerky turns and extra wear on the tires and drivetrain.
Some transfer cases, more commonly those in part-time systems, also contain an additional set of gears that give the vehicle a low range. This extra gear ratio gives the vehicle extra torque and a super-slow output speed. In first gear in low range, the vehicle might have a top speed of about 5 mph (8 kph), but incredible torque is produced at the wheels. This allows drivers to slowly and smoothly creep up very steep hills.
Locking Hubs
Each wheel in a car is bolted to a hub. Part-time four-wheel-drive trucks usually have locking hubs on the front wheels. When four-wheel drive is not engaged, the locking hubs are used to disconnect the front wheels from the front differential, half-shafts (the shafts that connect the differential to the hub) and driveshaft. This allows the differential, half-shafts and driveshaft to stop spinning when the car is in two-wheel drive, saving wear and tear on those parts and improving fuel-economy.
Manual locking hubs used to be quite common. To engage four-wheel drive, the driver actually had to get out of the truck and turn a knob on the front wheels until the hubs locked. Newer systems have automatic locking hubs that engage when the driver switches into four-wheel drive. This type of system can usually be engaged while the vehicle is moving.
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