It is an indisputable fact that consumers prefer SUVs, and SUVs account for nearly half of total passenger car sales. The four-wheel drive system is also attracting more and more attention as an important indicator when choosing an SUV. Most ordinary consumers still follow the principle of saving money and may choose a front-wheel drive SUV, but if they want to drive happily, they must have four-wheel drive. The manufacturers' publicity about the four-wheel drive system is so overwhelming that most people really can't understand it. We often see signs saying "Full-time four-wheel drive, please do not tow" on the back of SUVs. So what types of four-wheel drive are there? Why can’t full-time four-wheel drive tow cars? Let’s talk slowly next.
The history of four-wheel drive can be traced back to 1907, when an important customer of Daimler Automobile Company ordered a car and required the car to have excellent off-road performance. Daimler Automobile Company engineers conducted repeated discussions and research, and after a large number of tests, they designed and manufactured the world's first all-wheel drive car, named it "Dernburg Wagen". In the more than 100 years since then, four-wheel drive systems have continued to evolve and are no longer exclusive to off-road vehicles. The application of four-wheel drive systems can also be seen in many cars and SUVs.
Full-time four-wheel drive
Full-time four-wheel drive, as the name suggests, all wheels of the vehicle will receive power in any state without human intervention. It is powerful in all road conditions. Excellent traction performance, suitable for urban road conditions and low-intensity cross-country off-road. Depending on the adjustment, it can be widely used in various types of automotive products, and it also has a place in military vehicles and racing cars. Many people think that full-time four-wheel drive systems can only be equipped on longitudinal engine platforms, but this is not the case. The transverse engine platform can achieve full-time four-wheel drive through a multi-plate clutch center differential and electronic power distribution.
Part-time four-wheel drive
Part-time four-wheel drive means that the driver can switch between two-wheel drive or four-wheel drive by connecting or disconnecting the front and rear axles according to road conditions. All-wheel drive mode enables free conversion between two-wheel drive and four-wheel drive. Part-time four-wheel drive usually only uses the front wheels to drive on urban asphalt roads, and can switch to four-wheel drive on snowy or gravel roads. Of course, this is also the most common driving mode for hardcore off-road vehicles. Its advantages are simple structure, high stability, solid durability, which can not only ensure the vehicle's power and passability, but also take into account fuel economy. It is a very good four-wheel drive solution.
Timely four-wheel drive
The literal meaning of timely four-wheel drive is that the vehicle will only use four-wheel drive when appropriate, and it will still be a two-wheel drive four-wheel drive system under other circumstances. Compared with full-time four-wheel drive, timely four-wheel drive has better fuel economy. However, its physical structure limit is low and it is generally only suitable for urban road conditions. Real-time four-wheel drive can be roughly divided into three types: the first is a hydraulic coupling real-time four-wheel drive, the second is an electronically controlled multi-plate clutch real-time four-wheel drive, and the third is an intelligent electronic real-time four-wheel drive.
Hydraulic coupling type real-time four-wheel drive
The core component of the four-wheel drive is the hydraulic coupling. This closed coupler is filled with silicone oil. One end of the input shaft and the output shaft are in contact with the hydraulic coupling. The impeller, which is immersed in silicone oil, is connected to the front and rear differentials at the other end. During normal driving, the front and rear wheels keep running at the same speed, and there is no speed difference between the two shafts of the hydraulic coupling. When the front wheel slips, the speed will exceed that of the rear wheel, resulting in a speed difference between the two impellers in the coupler. This speed difference will cause the silicone oil to heat up and its viscosity to increase rapidly, thereby transmitting power to the rear wheel.
Multi-clutch real-time four-wheel drive
The core component of this real-time four-wheel drive is a multi-plate clutch limited-slip differential. The multi-plate clutch differential mainly generates differential torque through wet clutch plates, and the compression and separation of the clutch are controlled by an electronic system. When the vehicle is driving normally, the drive mode is front-wheel or rear-wheel. For example, when the system detects wheel slippage, the electronic system controls the clutch compression, thereby transmitting part of the power to the rear or front wheels. In theory, the computer will automatically allocate torque to the front and rear axles based on vehicle speed and road conditions to optimize road holding performance. However, this kind of four-wheel drive system usually only passively intervenes after the main drive wheel loses grip, so its response speed is relatively slow.
Intelligent electronic real-time four-wheel drive
This timely four-wheel drive can accurately control the intelligent transfer case to deliver controllable large and small signals to the four wheels based on the actual adhesion between the four wheels and the ground. of torque, improving vehicle acceleration, stability and fuel economy. Maximum traction is achieved on all 4 wheels when accelerating, resulting in better acceleration. If slipping occurs on a slippery road, ESP can be used to brake the slipping wheels. Eliminates the tendency for the vehicle to lose control by distributing power to wheels that maintain traction, and then redistributes power when the wheels regain traction.
Interpretation on full-time four-wheel drive and please do not tow trailers
For different four-wheel drive structures, there are different things to pay attention to, especially for trailers. We often see "Full-time four-wheel drive, please do not tow" signs posted on the back of SUVs. So why can't full-time four-wheel drive be towed?
With full-time four-wheel drive, all wheels will receive power in any state. The English "All Wheel Drive" is abbreviated as AWD, which is different from 2WD. Full-time four-wheel drive distributes power to the four wheels through a flexibly connected center differential and independent differentials on the front and rear axles. This is different from a purely mechanical differential lock. The differentials of full-time four-wheel drive are mostly hydraulic couplings or multi-plate clutches. The two methods have one thing in common, which is that they can allow the front and rear axles, left and right wheels to There is a speed difference between them. Whether it is full-time four-wheel drive depends entirely on the structure of the transfer case.
A full-time four-wheel drive vehicle maintains four-wheel drive during the entire driving process. The torque output by the engine is distributed to the front and rear axles in a fixed ratio. This drive mode can maintain a relatively high driving speed at any time. Good off-road capability and handling performance. It provides greater traction in all road conditions. However, it cannot adjust torque distribution according to road conditions, and fuel consumption is relatively high.
After understanding the connotation of full-time four-wheel drive, let’s take a look at how to tow a trailer. There are many ways to tow a trailer, such as towing a trailer (using a rope), lifting a trailer (lifting the front or rear wheels), and flatbed trailer (putting all the cars on a flatbed). Of course, there is nothing special about a flatbed trailer, because there is no difference between a vehicle and a vehicle placed stationary on the ground. What we are talking about is lifting the trailer, because the front or rear wheels of the vehicle will still be moving on the ground, so you should treat it with caution.
Full-time four-wheel drive cannot be towed. It refers to lifting the trailer, which is a common towing method used by the traffic police team. It lifts the two front wheels and lets the rear wheels drop to the ground. Because the four wheels of full-time four-wheel drive are all driving wheels, the four wheels are still connected by a transmission shaft when being towed. No matter which set of wheels is mounted on the trailer, the other set of wheels will rotate due to friction with the ground. The most direct consequence is to damage the center differential. If the speed is too fast and the distance is longer, the damage caused will be very large for full-time four-wheel drive.
The gears in the center differential of a full-time four-wheel drive link the front and rear axles of the car. Generally, when towing, the front wheels are suspended in the air and the rear wheels are on the ground. This will create a speed difference between the front and rear axles, and the torque will be concentrated. onto the center differential. If it is a multi-disc clutch type center differential, it will generate a lot of heat. Since the car is not running, the heat cannot be dissipated and the center differential will burn out. If it is a gear type center differential, such as Torsen differential, causing damage to the turbine worm.
The correct way to tow a full-time four-wheel drive is that all four wheels must be off the ground. One is to use a flatbed to carry a truck. This method is also the most commonly used and can be done by general trailers. If the vehicle model is too long and the trailer is too small to carry the vehicle, you can also use the method of mounting the rear wheels.
Summary: Now that you understand the connotations of various four-wheel drive systems, are you interested in buying a four-wheel drive vehicle? In the city, the application of four-wheel drive is not particularly interesting, and full-time four-wheel drive actually consumes more fuel. It is difficult to feel the difference between it and a front-wheel drive vehicle from driving. If you are not a special off-road enthusiast, Four-wheel drive actually doesn't make much sense.
This article comes from the author of Autohome Chejiahao and does not represent the views and positions of Autohome.