Some time ago, we conducted a live broadcast of modern N series models at the Guangdong Racing International Circuit, when an audience friend asked if the Elantra N is better than the Megane RS. Although Elantra N refreshed the editor's understanding of Korean performance cars, I personally think that it is still slightly inferior to the Megana RS. Why do I dare to say that? Because Renault and Honda have been competing for the title of "fastest front-drive" in New North for many years, the arms race is quite fierce. To take on the FK8, the latest generation of Megana RS and Trophy-R models are equipped with four-wheel steering, and if I remember correctly, this technology should be the first time in the field of steel cannons.
One might say, isn't it four-wheel steering? It is not four-wheel drive, how much can this thing bring to the performance of the vehicle? It is no exaggeration to say that the highly complete four-wheel steering system can upgrade the grip of the tires to a notch.
In fact, four-wheel drive can not improve the grip limit of the vehicle, because it is determined by the tire material and suspension settings, four-wheel drive is only said to be able to distribute power to the four wheels, increase the utilization of power, in exchange for faster corner acceleration. As long as it has not yet reached the stage of acceleration out of the corner, the four-wheel drive vehicle will not have a speed advantage in the corner than other driving forms of vehicles. Why does four-wheel steering improve a vehicle's grip? It all starts with the yaw angle of the tires.
Because the tire will deform, when the vehicle is steered, its actual steering angle is different from the steering angle of the wheel, and the engineering deviation is called the yaw angle. Experiments have shown that in the case of the same material and shape, the grip of the tire will increase with the yaw angle, and the change curve will increase first and then decrease. This tire characteristic is very common, for example, in racing, many drivers will slightly turn back to the direction when the vehicle pushes the head, this is because the tire yaw angle is too large, the grip is seriously lost, at this time you can slightly correct the direction to reduce the yaw angle of the vehicle and let the wheels restore grip.
When it comes to the yaw angle of the vehicle, one concept that must be mentioned is Ackermann's steering geometry. Theoretically, the outer wheels of the front axle will have a greater steering angle than the inner wheels, which will cause the steering centers of the four wheels to coincide. However, because of the existence of yaw angle, the standard Ackermann geometry can not play the maximum grip of the tire, through a reasonable Ackermann geometry setting, the vehicle will have obvious advantages in some specific sections. For example, in higher-speed Formula races, the car will use a negative Ackerman angle, that is, the front wheel will present an "inner eight" posture, which will make the outer wheel have a larger steering angle when cornering at high speed; and in some low-level touring car races, because the track is more compact and there are more low-speed corners, the car will often use a positive Ackerman angle setting, at this time the front wheel will show an "outer eight" attitude, which will make the steering angle of the outer wheel in the low-speed corner not too large, so as not to exceed the best yaw angle range of the grip.
Then again, while adjusting the car's Ackerman geometry can give it an edge in certain track conditions, conversely, a car with more grip in high-speed corners may lose grip in a low-speed corner because the vehicle's yaw angle is too large, while a car with an advantage in low-speed corners will lose time in high-speed corners because the yaw angle is too small. At this time, four-wheel steering can be used to show off.
The four-wheel steering system can change the wheelbase effect on the handling, which allows the vehicle to have different Ackermann geometry adjustments at the same time. For example, in the high-speed bend, the rear wheel and the front wheel rotate on the same side, in the case of the curve is unchanged, the driver needs to play more directions to enter the corner at the same angle, which changes direction to increase the steering angle of the outer wheel of the front axle, which is equivalent to the negative Ackerman angle of the vehicle; and in the slow bend, the front and rear wheels of the vehicle rotate in the opposite direction, reducing the steering angle of the outer wheels of the front axle, equivalent to the positive Ackerman angle. Therefore, a set of well-adjusted four-wheel steering system can improve the grip limit of the tire.