auto.howstuffworks.com/four-wheel-drive.htm/printable
As soon as they start to slip, the torque drops down to almost zero. 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 wont stick to the ground there is simply no way to harness that power. Traction For the sake of this article, well define traction as the maximum amount of force the tire can apply against the ground or that the ground can apply against the tire - theyre 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. For instance, when a car makes a turn, weight shifts to the outside wheels. 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 wouldnt 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.
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 dont 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. In the next section, well see how four-wheel-drive systems can help in these situations. Four-wheel Drive and Low Traction The benefit of four-wheel drive is easy to understand: If you are driving four wheels instead of two, youve got the potential to double the amount of longitudinal force the force that makes you go that the tires apply to the ground. 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 cant 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. 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 wont help you stop on slippery surfaces. In the next section, well take a look at the parts that make up a four-wheel-drive system. The Parts 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. Differentials 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. In all-wheel drive, the speed difference between the front and rear wheels is handled by the transfer case - well discuss this next.
Back to our corner-turning example: While the differentials handle the speed difference between the inside and outside wheels, the transfer case in an all-wheel-drive system contains a device that allows for a speed difference between the front and rear wheels. This could be a viscous coupling , center differential or other type of gearset. These devices allow an all-wheel-drive system to function properly on any surface. 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. 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. 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. Whether manual or automatic, these systems generally use a sliding collar that locks the front half-shafts to the hub. Advanced Electronics On many modern four-wheel and all-wheel-drive vehicles, advanced electronics play a key role. Some cars use the ABS system to selectively apply the brakes to wheels that start to skid - this is called brake-traction control . Others have sophisticated, electronically-controlled clutches that can better control the torque transfer between wheels. Well take a look at one such advanced system later in the article. First, lets see how the most basic part-time four-wheel-drive system works. A Basic System The type of part-time system typically found on four-wheel-drive pickups and older SUVs works like this: The vehicle is usually rear-wheel drive. From there, one driveshaft turns the front axle, and another turns the rear axle.
When four-wheel drive is engaged, the transfer case locks the front driveshaft to the rear driveshaft, so each axle receives half of the torque coming from the engine. Although this system provides much better traction than a two-wheel-drive vehicle, it has two main drawbacks. Weve already discussed one of them: It cannot be used on-road because of the locked transfer case. The second problem comes from the type of differentials used: An open differential splits the torque evenly between each of the two wheels it is connected to see this page of How Differentials Work for more details. If one of those two wheels comes off the ground, or is on ...
|
http://csua.com/feed/