By:Prayag nao
Before we get started explaining how it work, let's clear up some terminology:
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 bar graph below indicates the amount of torque that the engine is producing. The mark on the graph indicates the amount of torque that will cause wheel slip. The car that makes a good start never exceeds this torque, so the tires don't slip; the car that makes a bad start exceeds this torque, so the tires slip. As soon as they start to slip, the torque drops down to almost zero.
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.
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:
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.
For more details read my other post on differential.
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 How Differentials Work for more details). If one of those two wheels comes off the ground, or is on a very slippery surface, the torque applied to that wheel drops to zero. Because the torque is split evenly, this means that the other wheel also receives zero torque. So even if the other wheel has plenty of traction, no torque is transferred to it. The animation below shows how a system like this reacts under various conditions.
Before we get started explaining how it 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.
Torque, Traction and Wheel Slip:
Torque is the twisting force that the engine produces. The torque from the engine is what moves your car. The various gears in the transmission and differential multiply the torque and split it up between the wheels. More torque can be sent to the wheels in first gear than in fifth gear because first gear has a larger gear-ratio by which to multiply the torque.The bar graph below indicates the amount of torque that the engine is producing. The mark on the graph indicates the amount of torque that will cause wheel slip. The car that makes a good start never exceeds this torque, so the tires don't slip; the car that makes a bad start exceeds this torque, so the tires slip. As soon as they start to slip, the torque drops down to almost zero.
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.
- 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.
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.
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.
For more details read my other post on differential.
Four-wheel Drive Differential:
The front and rear axles each have an open differential. Although this system provides much better traction than a two-wheel-drive vehicle, it has two main drawbacks. We've 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 How Differentials Work for more details). If one of those two wheels comes off the ground, or is on a very slippery surface, the torque applied to that wheel drops to zero. Because the torque is split evenly, this means that the other wheel also receives zero torque. So even if the other wheel has plenty of traction, no torque is transferred to it. The animation below shows how a system like this reacts under various conditions.
Previously, we said that the best four-wheel-drive system will send exactly the right amount of torque to each wheel, the right amount being the maximum torque that won't cause that tire to slip. This system rates fairly poorly by that criterion. It sends to both wheels the amount of torque that won't cause the tire with the least traction to slip.
There are some ways to make improvements to a system like this. Replacing the open differential with a limited-slip rear differential is one of the most common ones -- this makes sure that both rear wheels are able to apply some torque no matter what. Another option is a locking differential, which locks the rear wheels together to ensure that each one has access to all of the torque coming into the axle, even if one wheel is off the ground -- this improves performance in off-road conditions.
The Four-wheel Drive Hummer:
Previously, we said that the best four-wheel-drive system will send exactly the right amount of torque to each wheel, the right amount being the maximum torque that won't cause that tire to slip. This system rates fairly poorly by that criterion. It sends to both wheels the amount of torque that won't cause the tire with the least traction to slip.There are some ways to make improvements to a system like this. Replacing the open differential with a limited-slip rear differential is one of the most common ones -- this makes sure that both rear wheels are able to apply some torque no matter what. Another option is a locking differential, which locks the rear wheels together to ensure that each one has access to all of the torque coming into the axle, even if one wheel is off the ground -- this improves performance in off-road conditions.
the Hummer is equipped with a brake traction control system. When one tire starts to slip, the brake traction control applies the brakes to that wheel. This accomplishes two things:
- It keeps that tire from slipping, allowing it to make maximum use of its available traction.
- It allows the other wheel to apply more torque.
The brake traction control system applies significant torque to the wheel that wants to slip, allowing the Torsen differential to apply two to four times that increased torque to the other wheel.
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