All-wheel drive or 4×4 transmission is when the torque from the engine is received not by two front or two rear, but by all four wheels of the car. This is necessary in order to increase its maneuverability on surfaces with low coefficients of adhesion – that is, where there are not enough two driving wheels, because they slip.
A 4×4 transmission, especially a serious one, suitable for difficult conditions, significantly complicates the design of the vehicle, and this is not always acceptable. Therefore, there are several types of all-wheel drive, depending on its capabilities and, accordingly, the complexity of the design.
What cars have 4×4 drive
There are quite a few cars currently being produced that are declared by the manufacturer as all-wheel drive. But, despite the fact that all wheels can actually receive power from the engine, the owner does not always get real benefit from this. The point, as noted above, is in the types of transmission, because very often there is simply no point in installing a full-fledged 4×4 system on a certain model.
Dozens of current crossover models equipped with all-wheel drive have truly simplified designs. The second pair of wheels turns on either untimely or only briefly, that is, in such a way that the driver does not receive effective assistance from them.
But real, effective all-wheel drive is currently found on special vehicles – SUVs and pickup trucks designed on their basis. We’ll talk about them today.
How does an SUV’s all-wheel drive work?
All the fundamental differences between 4×4 transmissions lie in the design of their units, which we will discuss below. But in general the scheme is always more or less the same and looks like this.
First, the engine, located in front under the hood. From it, power is transferred to the clutch (if the gearbox is mechanical) or to the torque converter (if the gearbox is automatic). And from there, after the first or second clutch is closed, the torque goes to the gearbox. In a conventional car, the transmission sends power to the two drive wheels. But for an all-wheel drive car, it must first be divided between the front and rear axles, and then distributed in pairs to the wheels.
Transfer case
It is in the distribution of power between the axles that the secret of the efficiency of all-wheel drive usually lies, and the transfer case (“rozdatka”) is responsible for this. Its presence is the main sign of a serious, highly effective full rein. In addition to the distribution function, in most cases it has a so-called downshift – the second, lowest degree. Which allows the wheels to rotate more slowly at the same engine speed, which is useful for better maneuverability on weak surfaces such as mud, sand, and snow.
But just as important is how the transfer case distributes torque between the two axles. There are two ways she does this.
Temporary all-wheel drive: Part Time
This is a system of the initial, basic level. Most of the time, the car remains single-wheel drive; in the case of SUVs and pickups, it is rear-wheel drive. If necessary, the driver connects the front axle through the transfer case. But it can only be used on bad roads, where the wheels can slip relative to the ground.
The fact is that the Part Time transfer case connects the front end rigidly (there is no differential), without the ability of the axles to rotate at different speeds. If you drive the Part Time transmission in 4×4 mode onto a section of clean, dry road, its components and tires are subject to accelerated wear. True, some of the most expensive models have a system that automatically turns off all-wheel drive when it reaches a certain, in its opinion, high speed.
Constant 4×4: Full Time
In principle, the Full Time transmission is designed in the same way as its cheaper counterpart. But the transfer case has a differential that allows the two axles to rotate at different speeds. Thanks to this, the car can drive in all-wheel drive on any road, including dry asphalt.
In practice, this means confident movement and adequate controllability, for example, on a highway that alternately has sections of ice (snow, mud) and clean asphalt or, say, stones.
But the built-in Full Time differential also has a negative side. On heavy slippery roads, in snow, on clay or on ice, “differential” often leads to excessive slipping of the wheels of one axle and the loss of all torque to it. That is, a car, one of whose wheels is actively slipping in a hole, transverse rut or on a patch of ice, simply stops. To prevent this from happening, the transmission of many SUVs uses a differential lock – one or more.
Lock
The sidewalling of the “diffs” can be automatic or forced, in other words, with the participation of automation or with the participation of the driver (using a button or lever). It is not necessary to install a block on each differential, although such cars do occur. More often the inter-axle “diff” (in the transfer case) is blocked; less common is the inter-axle “self-block” on the rear axle. The front differential lock is not used in factory configurations, but you can install it yourself.
Models and configurations
Regarding the packages. The same model of SUV or pickup truck can be equipped with both Part Time and Full Time transmissions, depending on the level of equipment and the market for which the particular vehicle was intended. Please note that these two types of transmissions from different manufacturers sometimes have their own names that differ from those given above.
In short
The choice of all-wheel drive type for an SUV or pickup truck depends on the conditions in which you plan to operate the vehicle. If the most important thing is off-road capability, then a basic Part Time type design is completely sufficient. Moreover, its technical simplicity will be an additional advantage for a machine that is operated in difficult field conditions far from a company service. And if a car often drives on more or less smooth, but “dirty” roads, it will benefit from a permanent 4×4 so that, among other things, it can maintain high speed in all-wheel drive mode.
2024-03-29 05:25:53
#Allwheel #drive #systems #pickups #SUVs #work
