When braking, do you brake all four wheels at the same time or only the front wheels?

There are three basic knowledge about automobile braking system, including active braking and "two kinds of hand brakes". Does the car brake only the front wheel or all four wheels at the same time? I believe many novice drivers will pay attention to this problem. The standard answer is: four wheels brake at the same time, and the state is "front wheel block and rear wheel pull". Only in this state can the vehicle stop at the fastest speed and the shortest distance. It is very poor to "block" the vehicle only by the friction of the front wheel, or to pull the body with the rear wheel, which is easy to cause the body to get out of control.

Knowledge point: linear inertial force! A car may not be in a "straight line" when turning, and it is very normal to brake when turning. However, a moving car has a concept of linear inertial force (influence), which is usually described as: there is invisible force to push the car body to move straight forward before braking. So what happens if you turn at this time and only brake the front wheel? Refer to the figure below.

The inertial force is "pointing to a straight line", but the car body has been tilted at a certain angle when turning. At this time, "pointing straight" means "pushing sideways", which is the so-called lateral force! If the rear wheel has no braking force, the friction coefficient between the tire and the ground will not be very high, and the lateral force will easily push the rear of the car out of control and slip (commonly known as tail flick). Similarly, if the rear wheel brakes when turning, the car body will be pulled by linear force, which will affect the pointing ability of the front wheel after turning. To put it bluntly, the force should be greater than the friction of the front wheel, so that the car can continue to walk in a straight line, otherwise it will also cause the rear of the car to swing and cause the body to cross.

To sum up, the front and rear wheels must be braked at the same time, and the friction when the rear wheels are braked can control the posture of the car body. The TCS/VDC system of ESP body stability program can correct the body posture by adjusting the braking force (friction force) of the front and rear wheels. EBD braking force distribution system can intelligently adjust the braking strength of front and rear wheels, that is, adjust the friction coefficient. So much for the normal braking system. Let's look at two types of handbrake.

0 1 lever _ handbrake

Only the brake rear wheel pull rod controls the hand brake cable (steel cable), which is connected with the rear wheel brake shoe. When parking, tighten the handbrake to stop the rear wheel. As long as the braking force of the handbrake is strong enough, releasing the brake pedal will also cause the car to slip. This kind of infrastructure can also be used as an emergency braking option when the active braking fails, but the use method must be paid attention to!

Knowledge point: the road must be high in the middle and low on both sides, because the drainage in rainy days should be considered. So the car body is also high on the left and low on the right to a certain extent. The skew of the car body causes the center of gravity to the right, and the vertical pressure of the right rear wheel is greater, resulting in greater friction. Directly tightening the handbrake will cause the rear wheel to lock up (stop driving), at which time the right rear wheel will produce "lateral tension" and the friction will be greater. If the speed is too fast, it is likely to shift to the right, leading to sideslip or even rollover (the tail flick stunt is to use the difference between lateral force and friction). The emergency braking of the handbrake can't reach the locking state, and the rapid lifting, releasing and lifting can ensure the effective safe deceleration.

02 electronic control _ handbrake

Normal parking brake rear wheel emergency braking All-wheel brake The electronic button handbrake only brakes the rear wheel during normal parking, but it does not need manual operation after being associated with AUTOHOLD, and will automatically brake when it is engaged in P gear, automatically starts and stops or the door is not turned off. The main function of this system is to improve the convenience of operation and make the central control and handrail area more beautiful.

Key points: When the electronic handbrake button is forcibly lifted for 2~3 seconds, the system will activate "emergency forced braking". The operation step is to press the key to activate the electrical signal and output it to the ECU-driven computer. The module activates the ESP body stability program control unit, and brakes the front and rear wheels simultaneously through ABS/EBD/TCS/ADC. During this period, the engine power output will be cut off (braking priority), and the vehicle will slow down on the premise that ESP controls the vehicle not to lose control. Therefore, the safety level of electronic handbrake is much higher than that of mechanical handbrake, which is the result of intelligent electronic upgrade. So much about braking.