There are so many different types of four-wheel drive vehicles as with this system. It seems every manufacturer has different solutions to bring power to all wheels. The language used by different manufacturers can be a bit tricky. So, before we begin to explain how it all works, let's clarify some terminology:
Temporary and permanent systems of four-wheel drive can be evaluated using the same criteria. The best system will send exactly the right amount of torque to each wheel and is the maximum amount of torque that will not let the tire slip.
In this article, we will explain the basics of four-wheel drive, starting with a few notions about friction and giving a look at the components that make up this system. Then let's look at a few different systems, such as found in Hummer manufactured for GM by AM General.
We need to know a little about torque , friction and wheel slip so we can understand the functioning of the different systems of four-wheel drive found in cars.
Torque is the twisting force that the engine produces. The torque produced by the engine is what makes your car move. The various marches in rates and the reduction in the differential multiply the torque and distribute it to the wheels. Greater torque can be sent to the wheels in first gear than in the fifth, since the former has a higher transmission ratio by which the torque is multiplied.
This bar graph indicates the amount of torque the engine is producing. The point on the graph indicates the amount of torque that will cause wheel slip. A car that has a good start never exceeds this torque, so the tires do not slip; a car that has a bad torn exceeds that torque, causing the tire slippage. Once they start to slip, the torque drops to zero.
The interesting thing about which torque is in a situation of poor adherence, the maximum amount of torque generated is determined by the quality of this adherence and not by the engine. Even if you have an engine of NASCAR in your car, there is simply no way to harness the force that the tires do not stick to the ground.
To better explain this article, we define traction as the maximum amount of force that the tire can apply against the ground (or the ground can apply on the tire - it's the same). These are the factors that affect traction:
Basically, the wheel slip occurs when the force applied to the tire exceeds that available adhesion tire. The force is applied to the tire in two ways:
Say you have a car with a powerful rear-wheel drive and is making a turn on a wet road. Your tires have enough to generate the lateral force required to keep your car on the road while the curve is made grip. Say you accelerate background in the middle of the curve ( do not! ) - your engine sends more torque to the wheels, producing a huge amount of longitudinal force. If you add up the longitudinal force (produced by the engine) to the lateral force (created by the curve) and the total exceeds the available grip, you probably will cause wheel spin and the car skidding.
Most people do not even come close to exceeding the available grip for a dry ground, or even on a wet floor and plan. Systems drive four permanent normal wheels are very useful in low grip, such as snow or slippery slopes.
Four wheel drive and grip
The benefits of four-wheel drive are easy to understand: if you're driving with four-wheel drive instead of two, has the potential to double logitudinal force (the force that makes the vehicle floor) that the tires apply to the soil.
This can be useful in various situations.
There are situations in which the four-wheel drive offers no advantage over the two traction. Particularly, the four-wheel drive systems will not help you stop on slippery surfaces. It is all because of the brakes and antilock brake system (ABS).
Components of a system of four-wheel drive
The main of any system of four-wheel drive are the two parts differentials (front and rear) and the transfer case. Added to this, temporary systems have lockable wheels-free cubes and both systems can include advanced electronics that help them to make even better use of available grip.
Cars four wheel drive have two advantages : one located between the front wheels and one between the rear wheels. They send the torque propeller shaft or exchange the car wheels. They also allow the wheels on the left and right rotate at different speeds when making a turn.
In a curve, the inner wheel are of a different external path, and the front wheels are in a different way back so that each of the wheels rotate at a different speed. Differentials allow the speed is different in internal and external to the curve (in a car with permanent four-wheel drive, the difference in speed between the front and rear wheels is provided by a third differential in the transfer case wheels - we'll talk about the following).
There are several types of differential used in cars and trucks. The type of differential used has a significant effect on the manner in which the vehicle uses the available adhesion. See How Differentials Work for details.
The transfer case
This is the device that divides the power between the front and rear axles in a car with four-wheel drive.
Back to our example of the curve: while spreads allow the difference in speed between the front and rear wheels, the transfer case on a wheel drive system, contains a device that allows different speeds between the front and rear wheels. This can be by means of a viscous coupling , a central differential gear or another type. These devices allow a continuous traction system work perfectly on any floor.
The transfer case on a system of temporary drive four wheel engages the tree front to rear drive shaft so that the wheels are forced to rotate at the same speed transmission. This requires that the tires slip when the car makes a turn. Temporary systems such as these should be used only under conditions of low grip, it is relatively easy to slide the tires. On dry asphalt is difficult to slide the tires, so the four-wheel drive must be disengaged to prevent judder during cornering and premature tire wear and the whole transmission.
Some transfer cases, typically those in temporary systems also contain a number of additional gears in low gear . This relationship provides extra motion to the vehicle speed and the torque output at extremely low transmiossão. At first, using the reduced, the vehicle can reach a maximum of 10 km / h, but it is produced a wonderful torque at the wheels. This allows drivers quiet and slowly climb the steepest slopes. Reduced acts on all marches rates.
Each wheel on a car is attached to a hub. Pickup trucks and utility all-terrain wheels usually have temporary front hubs with freewheel . When the front wheel is not being used, these cubes allow disconnecting the wheels of the front differential, the half-trees (trees that connect the differential to the cubes) and the tree of its transmission. This causes them to stop spinning, protecting against wear and unnecessary effort and decreasing fuel consumption.
The wheel hubs-free manuals were very common. To engage the four-wheel drive, the driver had to get out of the vehicle and a button to trigger the cubes crashes. Newer systems have free-wheel hubs automatic that are triggered when the driver engages four-wheel drive. Generally this system can be operated with the vehicle in motion.
Both the manual and automatic system using a sliding sleeve that connects the front semi-tree to the cube of its wheel.
Advanced electronic equipment
advanced electronic equipment play a vital role in many vehicles all-terrain wheels temporary and permanent. Some cars use the ABS system to selectively apply the brakes to the wheels begin to spin - this is called the brake traction control .
Others have a sophisticated electronic control system electronically controlled clutches that can better manage the transfer of torque between the wheels. We will see later this advanced system.
First, let's see how the drive system works in the four most basic temporary wheels.