The more cars on the road, the less room there is for buses to maneuver. This partly explains the popularity of flexible, compact hatchbacks and even minicars. However, having said that, can the flexibility of a car only rely on shortening the wheelbase and reducing the size? No, there is also a “rear wheel steering”.
Rear-wheel steering is not a new thing. As early as the 1980s, Honda and some car companies brought the first batch of rear-wheel steering models to the automobile market. The four-wheel steering system is mainly based on Nissan’s HICAS system, but they have not adhered to it. General Motors’ Quadrasteer system was available on their full-size pickup trucks in 2000. The Porsche 928 also has a rear-wheel steering function, but it can only control the torque of the rear axle during braking.
In fact, rear-wheel steering is not really rear-wheel steering, but “four-wheel steering”, but the term “Rear-wheel steering” is commonly used internationally. For civilian vehicles, due to cost constraints and vehicle stability reasons, two-wheel steering is often used and is designed so that slight understeer can ensure the stability of the vehicle. However, the vehicle often oversteers when turning at high speed, and When turning in a vulgar way, the turning radius is too large, making it extremely inconvenient to park and turn.
Why use rear wheel steering
First of all, let’s talk about the three turning characteristics of the vehicle, namely understeer, neutral steering and oversteer. I believe everyone understands it. If you don’t understand it, you can understand it literally. For more common front-wheel drive vehicles, if understeer occurs, it can be solved by reducing the vehicle speed. If there is more serious oversteer, you need to reverse the direction and cooperate with refueling to pass. This is very demanding for the driver, so generally Generally speaking, the steering characteristics of civilian vehicles will be biased towards slight steering which is not enough to ensure driving stability.
There are two ways of rear wheel steering, one is active steering and the other is follow-up steering. Rear wheel follow-up steering is the technology used by Fukang. The reason must be that the structure is simple and the cost is low. The principle of rear wheel follow-up steering is a very simple physical structure. It just adds some rubber cushions at the connection between the rear suspension and the body. When the vehicle turns and squeezes one side, the rubber cushions will be squeezed. The pressure deforms and causes the rear wheel to change to a certain angle, thereby achieving rear wheel steering. The change of angle depends on the softness and hardness of the rubber cushion. The specific angle needs to be weighed and considered by engineers. Generally speaking, it is below 3 degrees. However, its effect is not obvious at low speeds, and it only has excellent performance at high speeds. This rear-wheel follow-up steering technology was equipped by Citroën on many of its models at the time.
Several benefits of using rear-wheel steering:
1. During straight driving and emergency braking, the front ends of the two rear wheels retract inward, which is what we often call pigeon-toed, to enhance the stability and resistance of the vehicle in straight driving.
2. When turning at low speeds or moving warehouses, the normal turning diameter may cause us to turn in several directions, but the rear-wheel steering system makes it point in the opposite direction to the front wheels, helping to reduce the turning radius, reduce understeer, and improve the vehicle’s stability. Cornering performance and exit capability.
3. When merging during high-speed driving, the angle change of the rear wheels maintains the same direction as the front wheels, resulting in the effect of increasing the wheelbase, making the vehicle more agile and stable at high speeds. Take the Cadillac CT6 (parameters | inquiry) as an example. It has a rear-wheel steering system and its turning radius is 0.9 meters smaller than that of models of the same level. This is a significant difference because the larger CT6 has a smaller turning radius than the ATS. The benefits of rear-wheel steering are not limited to these. When the car changes lanes, the rear wheels and front wheels move in the same direction, making the car change lanes faster and more stable.
Currently, rear-wheel steering systems are available in mechanical, hydraulic and electric/hydraulic types. Moreover, with mature four-wheel steering, the rear wheels steer differently according to different conditions of high and low speeds. At high speeds, the rear wheels and front wheels turn in the same direction (in-phase) to increase stability. At low speeds, the rear wheels and front wheels turn in different directions (Counter-phase), reducing the turning radius.
Rear-wheel active steering technology is very useful for luxury cars with long wheelbases and large SUVs with high chassis. It reduces the turning radius at low speeds, making the vehicle more flexible; it increases the stability of the vehicle at high speeds, making it easier to control. . In addition, this technology can also be seen on some sports cars.
For example, the rear-wheel steering mechanism of the Porsche 911GT3 weighs less than 3kg, can enable the rear wheels to achieve a maximum steering of 6 degrees, and can provide a steering force of up to 4000N. One thing that is different from the common rear-wheel active steering system is that it uses two sets of internal motors to independently control the two rear wheels, thus achieving independent control of the left and right rear wheels, which allows it to have higher control. limit, of course, this places higher requirements on the control system.