When we buy a car, we often pay special attention to the size of a car, especially when we often need to seat multiple people, a large car can bring more comfort and will not feel too oppressive. Of course, the bigger the ride, the better, but for the driver who drives, the larger the vehicle, the worse the handling. It's particularly inconvenient to walk on some narrow roads or park in a place where space is tight, imagine if a car can go around in circles, whether it can be translated, even if there is little space, and it can be turned around or reversed into a parking space. In order to make the body more agile, is it possible to steer the rear wheels as well? Like the front wheels, it has a large steering angle, which can also be highly agile in small spaces.
In fact, there are now some cars with steering functions on the rear wheels, such as Xpeng X9 and Zhiji L7, which have an overall active steering system, that is, the rear wheels can also achieve steering. The BMW 7 Series and Mercedes-Benz S-Class can be optional, but the price is very high. More than 100 years ago, the idea was to turn both the front and rear wheels, providing stability at high speeds and flexibility in tight spaces. However, at that time, the rear-wheel steering was connected to the steering wheel by some connecting rods, and when the driver turned the steering wheel, the front wheels turned and the rear wheels also turned, but at different angles. Later, because of the high failure rate and immature technology, there was little need for its existence, so it was not developed.
This was followed by a simpler rear-wheel steering, passive rear-wheel steering. The rear wheels of the car that cannot be steered are fixed to the suspension system, and the rear wheels are firmly locked, so there can be no steering angle at all. In the case of a vehicle with passive steering, the rear wheels are not locked with screws and are not hard-connected. It is between the contact point of the body and the suspension, adding some flexible materials, which can be deformed when extruded by external forces. When the vehicle turns, due to the influence of centrifugal force, the rear wheel will be subjected to a lateral force from the road surface, due to the influence of this force, the flexible material will be extruded and deformed, and the angle of the tire can be changed to achieve steering.
However, the steering angle is very small, about 2-3 degrees ±, which is not noticeable at low speeds, and does not add much to the agility of the car. However, it is more obvious at high speeds, when turning at high speeds, the rear wheels are assisted at a small angle, and the vehicle will be clearly felt to be very stable, which improves the turning limit of the whole car. Even a small turn of direction such as changing lanes will make the vehicle very smooth and the handling will be greatly improved. Because this passive steering cannot meet the requirements for luxury performance cars, it is not necessary for ordinary cars, and the steering is not so intelligent, it is gradually eliminated and replaced by an active steering system.
As the name suggests, the active steering system controls the steering itself. Of course, it is not connected with the steering wheel, its principle is to use a set of lead screw nut mechanism, add a steering motor on the rear axle, and the motor drive nut drives the lead screw to produce axial movement, and this axial movement will drive the rear wheel to produce a small amount of steering. That is to say, the steering angle controller of the front wheels is the driver, while the steering controller of the rear wheels is the motor. It uses the control system to receive various dynamic driving signals of the vehicle, according to the speed at that time, the size of the steering angle of the front wheel, etc., through the calculation of the computer module, and then comprehensively judges the output of an optimal steering angle, and the motor performs the operation. Therefore, it is technically complex, but at the same time it must be very delicate and there must be no mistakes. The angle of active steering is much larger than that of passive steering, generally at ±5 degrees, and high-end models can achieve ± 10 degrees.
When the speed is above 60km/h, the rear wheels and the front wheels turn in the same direction, that is to say, the front wheels turn to the left, and the rear wheels turn to the left, which can improve the stability of high-speed cornering and prevent understeer and tail flicking. At 60 km/h or less, the steering direction is opposite to the front wheels, i.e. the front wheels turn to the left and the rear wheels turn to the right, which increases the agility of the vehicle and greatly reduces the turning radius.
Although the rear wheels can steer, they will not be as large as the front wheels, otherwise the vehicle may lose control. A small amount of steering can greatly improve the agility and stability of the vehicle. However, today's rear-wheel steering requires a high level of technology, while also avoiding the technical requirements of tyre, eccentric wear, and suspension durability and stability. Due to technical problems and practical considerations, rear-wheel steering will not be widely popularized, and only large vehicles with higher prices will be equipped or optional. For an ordinary family car, there are no extremely high performance requirements, and there is no need to increase the steering of the rear wheels to drive the vehicle normally. After the increase, the maintenance cost will also be greatly increased, and there are not many cases that are usually used.