I forgot when it started, the four-wheel drive system is considered to be a magical weapon under all kinds of fanfare, it seems that since the four-wheel drive system, the car has only started to use four wheels to drive.
The development history of the four-wheel drive system will not be pursued for the time being due to the issue of space. Even if the origin of the four-wheel drive system is not known, it will have no impact on the inference of this article. Judging from the current four-wheel drive system, it can be roughly divided into on-road four-wheel drive system and off-road four-wheel drive system. Their functional bias can be seen literally. The former is mostly used for high-performance models, such as Subaru STi, Mitsubishi EVO, Nissan GTR, Lamborghini’s various bar fours, Porsche 911, etc.; while the latter is for non-paved roads. born through sex. Since many people like sports cars, let’s start with the road-type four-wheel drive system.
1. Where does the road-type four-wheel drive system come from?
A long time ago, four-wheel drive systems were used on non-paved roads. In the early 1980s, the World Rally Championship WRC began to enter the glorious Group B era. Audi engineers applied the four-wheel drive system to the WRC car for the first time, which allowed it to achieve very powerful speeds on non-paved roads. There are many mid-engine pure rear-wheel drive racing cars such as Lancia Stratos. Even Audi can rely on the advantages accumulated in these non-pavement stages, even if it does not perform well in the road stage, it can still win the annual championship. The mass-produced prototype of this Audi racing car is the famous Quattro, which means “4” in Italian.
Quattro became famous because of its excellent performance in WRC, and it took a completely different route from the popular sports cars BMW E30 M3 and Mercedes-Benz 190E AMG at the time. The latter two are inseparable in the DTM and even the world’s major venues, while Quattro is the first time that a sports car is equipped with a four-wheel drive system.
Because Quattro performed well in WRC, especially on ice and snow stages and gravel stages, many teams began to follow suit and develop four-wheel drive racing cars. And their mass-produced versions are naturally four-wheel drive models. It can be said that the WRC event itself is a mixture of road and non-paved roads, which gave birth to the road-type four-wheel drive system.
In the 1990s, Japanese automakers entered WRC in order to enter the European market and achieved good results. Toyota, Nissan, Mitsubishi, and Subaru all had their own four-wheel drive high-performance models, and this experience laid a solid foundation for Subaru. Timely four-wheel drive brand image.
2. Can the four-wheel drive system increase the cornering limit?
Perhaps many people still remember the years when Nissan GT-R dominated various competitions like a hooligan in the 1990s, and thus won the title of “God of War”. Many people say that it relies on the ATTESA-ETS four-wheel drive system. In fact, it relies on high horsepower… The four-wheel drive system is just to prevent the more than 500 horsepower (compared to other vehicles in the same field) from Racing cars generally have more than 200 horsepower) It’s just too difficult to control.
The limit of cornering, in the final analysis, is the limit of grip of the tires. So from this point of view, even if all four wheels have grip, it is actually impossible to increase the minimum speed (that is, the cornering limit) through the curve. So if it is the same tire condition, the tire grip to 100 is the cornering limit of this car. If the adjustment of the suspension is not discussed, the “inner fight” between the front and rear wheels will consume the grip of the tires and reduce the minimum speed when cornering.
Let’s go back to the story of the Audi Quattro. At that time, Quattro’s performance on the road stage was terrible. The reason was that the initial form of the full-time four-wheel drive system did not know how to turn on the road!
The principle is this. The original intention of the four-wheel drive system is to lock the front and rear axles at the same speed, so that all four wheels have grip. However, when the car is turning, the trajectory of the front axle and the rear axle are different, so the speed of the front and rear wheels is also different. So this leads to the fact that when the car is turning (here we are talking about extreme control), the front and rear wheels will wrestle with each other in order to achieve the theoretical speed difference, but because there is actually no speed difference. This is why the time-sharing four-wheel drive system must be separated from the four-wheel drive system after entering the road driving: the front wheels refuse to go when entering the corner, and tend to push the head; One can imagine how difficult it is to drive such a car. What kind of four-wheel drive system can increase the tracking and stability of the body, just listen to it.
Therefore, in the current full-time four-wheel drive system, various complex technologies are added to the central differential. more natural.
When a car equipped with a four-wheel drive system launches a new generation of four-wheel drive system, its slogan is “compared with the old model, the flexibility of cornering is improved”, not that it is improved compared with front-wheel drive or rear-wheel drive vehicles. Cornering flexibility. Because the inherent characteristics of the four-wheel drive system are that the front and rear wheels will fight inside, thus consuming the grip of the tires, while this will not happen to front-wheel drive or rear-wheel drive vehicles.
3. So what is the significance of the road-type four-wheel drive system?
Four-wheel drive, as the name suggests, its advantage lies in “drive”, that is, when the power is involved, the advantages of the four-wheel drive system will appear. The biggest function of the road-type four-wheel drive system is to intelligently distribute the power to the four wheels, so that the wheels with better grip conditions can get more power, so that the car can accelerate more efficiently.
If it is transformed into “tactical meaning”, it means that when exiting a corner, all four wheels can have power at the same time. Compared with front-drive or rear-drive, excessive power will cause the wheels to slip, and the four wheels will share the power, reducing the chance of a single wheel slipping, thereby maximizing the effect of cornering acceleration.
Therefore, although the minimum speed of a four-wheel-drive sports car (mechanical four-wheel drive, not a hybrid four-wheel drive like McLaren P1 and Porsche 918 Spyder) is not as fast as that of a two-wheel drive car, if the overall In terms of analysis, the corner entry is slightly slower and the center of the bend is slower, but the time to accelerate out of the corner is earlier, and the accelerator can be fully opened faster, and the corner exit can be much faster than that of a two-wheel drive vehicle (especially low-speed corners). Therefore, after the entire track, the time lost when the corner limit is slightly lower can be offset by a faster corner exit. For the gasoline-electric hybrid four-wheel drive system, the cornering speed is the same as that of a two-wheel drive vehicle (because there is no front and rear wheel interference), and the four-wheel drive mode is switched to the corner exit. It is so fast that there is no solution.
4. What is the difference between front-wheel drive and rear-drive four-wheel drive systems?
This is a very interesting topic. The so-called front-wheel drive and rear-wheel drive four-wheel drive systems can be regarded as a front-wheel drive or rear-wheel drive vehicle, and then when the main drive shaft slips (the front drive shows a push head, and the rear drive shows oversteer) , what percentage of the driving force can be obtained by the other axis.
Typical front-wheel drive four-wheel drive systems such as Golf R/Audi TTS, Mitsubishi EVO are also biased towards front drive; typical rear drive four-wheel drive systems include Porsche 911, Nissan GT-R (strictly speaking, there is a pure rear drive state), Ferrari FF this kind. And their driving characteristics are actually very close to front-wheel drive or rear-wheel drive. Only when the main drive wheel breaks through the limit or is about to break through the limit, the four-wheel drive system will obviously intervene to save the driver.
This then triggers some weird handling characteristics. For example, Golf R, when the car is pushing the head, you can actually continue to step on the accelerator. After the rear wheels share more power from the front wheels, the ratio of grip between the front and rear wheels will change, and the situation of pushing the head will be alleviated; another example is Subaru For the Impreza STi, if the rear wheels oversteer, many people’s first reaction is to release the accelerator and reverse the direction. However, the four-wheel drive system has detected that the car is oversteering, and immediately distributes the power to the front wheels. At this time, what the driver needs to do is to hold the accelerator, give more power to the front wheels, and put the center of gravity on the rear wheels to reduce oversteering. Fix it. You see, with and without a four-wheel drive system, the methods of rescuing a car are completely different. The four-wheel drive system and people’s instinctive reactions have different correction methods, which will interfere with each other. The four-wheel drive system is a double-edged sword. If you are not familiar with the operating mechanism of the four-wheel drive system and the timing of intervention, it will make the car fall into an even more outrageous loss of control.
5. What determines the performance of the four-wheel drive system?
Generally speaking, it is the innate structure and acquired adjustment, that is, the hardware part and the software part. The innate structure includes the structure of the central differential of the four-wheel drive system – the Torsen differential has fast response, fixed intervention ratio, and reasonable cost; the viscous coupling differential has slow response but soft intervention and low cost; multi-disc clutch differential The device response is slower than Torsen, but the intervention ratio has great adjustment flexibility and high cost.
In view of the increasingly complex performance requirements of the four-wheel drive system, among the mainstream road-type four-wheel drive systems on the market, basically only the longitudinal engine platform of the Volkswagen Group uses the Torsen differential. In addition, the adjustment flexibility is much greater. Plate clutch differentials have become mainstream. This leads to another question, the calculation speed of the control unit and the accuracy of the algorithm. Based on these two points, the engineer’s adjustment level. The calculation speed and algorithm of the control unit are the basis. The calculation speed must be fast, and then to control the differential, its intervention speed must be faster than the human reaction time, otherwise this four-wheel drive system will have no meaning.
The algorithm includes body yaw angle (the angle between the actual motion trajectory of the body and the front wheel angle), wheel slip rate (whether there is idling), vehicle speed, steering wheel angle, etc., and these collected parameters are calculated according to a set The program is processed, and then the current state of the car is obtained, and then the work order of the four-wheel drive system is obtained. Whether the algorithm is smart or not determines whether the four-wheel drive system is smart or not. The smart four-wheel drive system is relatively easy for engineers to adjust.
Okay, having said so much, it’s time for a closing statement. Is the road-type four-wheel drive system good or not? First of all, although it can’t increase the cornering speed of cornering, it can help the driver to exit the corner faster; second, it can improve the fault tolerance rate of a car to a certain extent (only when accelerating), However, if you are not familiar with the temperament of this four-wheel drive system, it may add to the confusion; third, not all four-wheel drive systems are so powerful. The structure, response speed, and adjustment level can make different brands, or even different models of the same brand. Four-wheel drive systems perform differently.
Source of pictures and texts: Original article of Xincheping.com.
