Como funciona a tração nas quatro rodas

Autor: 
Karim Nice

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:

  • Four wheel drive : usually, when manufacturers say that the car has four wheel drive , are referring to the temporary system . For reasons that we will explore later in this article, these systems are only designed for low grip situations like off-road or on snow or ice.
  • Traction on all wheels : these systems are also called permanent four-wheel drive and all wheel drive . These systems are designed to use on all surfaces, both on roads and outside them. Most of them can not be disabled.

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.

diagram of four-wheel drive

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 torquefriction 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:

  • Weight on the tire - the more weight on the tire, the more traction it has. The weight may change according to the movement of the car. For example, when a car makes a turn, the weight is transferred to the outer wheel. When he accelerates, the weight goes to the rear wheels (see How Brakes Work for details).
  • Coefficient of friction - This factor relates the amount of friction force between two surfaces with a force that unites them. In our case, it relates the amount of grip between the tires and the road to the weight resting on each tire. The friction coefficient is, in most cases, a combination of types of tires of the vehicle and the type of surface on which it is driven. For example, a NASCAR tire has a very high coefficient of friction when being used on a track of dry concrete. This is one reason why NASCAR race cars can make turns at such a high speed. However, the coefficient of friction for the same tire in the mire was almost zero. In contrast, large tires for off-road full of ruts that do not have a coefficient of friction as high on a dry track, the mud would have.
  • Wheelspin - there are two types of contact that tires can make the road: static and dynamic.
    • Static contact - the tire and the road (or the ground) does not slide relative to one another. The coefficient of static friction in contact is higher than the dynamic causing the static provide a better grip.
    • Dynamic contact - tire slips relative to the road. The coefficient of dynamic friction for the contact is smaller, providing less adhesion.

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:

  • Along - the longitudinal strength comes from the torque applied by the engine to the tires or the brakes. It tends to accelerate or decelerate the car.
  • Side - the side force is created when a car makes a turn. Is force needed for a car to change direction - after all, the tires and the ground comes from the lateral force.

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. 

  • In the snow : a lot of traction is required to move a car in the snow. The amount of force available is limited by adherence at the moment. Most cars with traction on two wheels can not move if more than a few inches of snow on the road, in the snow because each tire has only a small amount of traction. A car with four-wheel drive uses four tires.
  • Off the road : the conditions of off-road, it is very common for a pair of tires to be in a situation of low grip, such as crossing a stream or mud puddle. With four-wheel drive, the other pair set of tires still has grip, so that you can take it out of the difficult situation.
  • Climbing slippery slopes : this task requires a lot of grip. A car with four-wheel drive can use all four tires to get the car uphill.

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.

Differential

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).

the differential four-wheel drive
Differential free - the most common type of differential

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.

the gearbox of a car with four-wheel drive
A box of periodic transfer of temporary drive four wheels. The reduction by planetary gear can be used to provide low gear.

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.

Hub-free
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.