In this blog post, we’ll examine the principles behind wheel lockup in cars and the difference between static friction and kinetic friction, and explore the structure and operating principles of the ABS braking system, which automatically replicates the braking actions of a skilled driver.
Imagine you’re driving on a rainy road and suddenly spot an obstacle. Naturally, you’ll hit the brakes. But how should you apply the brakes in this situation? A skilled driver would repeatedly press and release the brake pedal. However, a novice driver is likely to press the brake pedal hard and wait for the car to come to a stop. Is there a problem with this approach? The answer is yes. In a dangerous situation, improper use of the brakes can actually increase the risk of an accident. However, if a car is equipped with ABS brakes, applying the brakes once can produce the same effect as rapidly modulating the pressure multiple times in a fraction of a second—less than one second. This prevents the wheels from locking up and helps the driver maintain control of the car’s direction even during emergency braking. So why is it safer to press and release the brake? And what exactly is an ABS braking system?
A car transmits power generated by an engine or electric motor to the wheels, and the car moves forward as the wheels rotate. When the brakes are applied, the wheels’ rotational speed decreases, causing the car to come to a stop. However, a problem arises when the car continues to slide forward due to inertia even after the wheels have stopped rotating as a result of heavy braking. This phenomenon is called “wheel locking” and occurs primarily during emergency braking on slippery roads caused by rain or snow. When the wheels stop rotating, it becomes difficult to fully utilize the traction generated as the tires roll on the road surface, causing the tires to skid across the road. This wheel locking can increase the car’s stopping distance and may even lead to accidents.
However, an even bigger problem is that when wheel locking occurs, it becomes difficult to change the car’s direction using the steering system—that is, the steering wheel. When steering ability is significantly reduced, it becomes difficult for the driver to control the vehicle’s direction as intended, which can lead to serious accidents such as the vehicle veering out of its lane or colliding with an obstacle. Above all, the risk of an accident increases even further if only one front or rear wheel locks up. This is because the difference in the rotational status of the left and right wheels causes the vehicle’s behavior to become unstable and may result in the vehicle spinning out of control.
So, how can this wheel lockup be prevented? To explain this, we first need to understand friction. Friction can be broadly divided into static friction and kinetic friction. Static friction is the force that acts when two objects in contact do not slide relative to each other, and its magnitude changes in response to an externally applied force. Therefore, static friction increases as the magnitude of the externally applied force increases, but it does not increase indefinitely. When a force exceeding a certain threshold is applied, the object begins to move; the friction just before the object starts to move is the greatest among all forms of static friction and is called the maximum static friction. Next, kinetic friction is the friction that acts when objects slide against each other while in motion. Generally, when the same object moves over the same friction surface, the kinetic friction force is smaller than the maximum static friction force. The key point here is that the maximum static friction force is greater than the kinetic friction force. This is easy to understand if you consider that it is harder to set a heavy object in motion for the first time than it is to keep pushing an object that is already moving. Skilled drivers utilize this principle when applying the brakes. Skilled drivers control the vehicle by applying the brakes appropriately until just before the wheels lock up to ensure sufficient braking force, and then adjusting the braking force when the wheels are about to lock. This method allows the tires to continue rolling on the road surface, thereby maximizing the traction between the tires and the road. Conversely, when the wheels lock up completely and begin to skid, the friction characteristics between the tires and the road surface change, which can impair braking and steering performance.
To prevent accidents, engineers developed the ABS system, which automatically performs the same function as a skilled driver adjusting braking force during emergency braking. ABS is a system that continuously monitors the rotational status of each wheel and rapidly adjusts hydraulic pressure to prevent the wheels from locking up. A typical ABS system consists of an Electronic Control Unit (ECU), a hydraulic control unit, and wheel speed sensors that detect the rotational speed of each wheel. The specific configuration and terminology may vary depending on the vehicle’s design and manufacturer. Even when the driver presses the brake pedal just once, ABS prevents the wheels from locking by repeatedly and rapidly increasing and decreasing brake pressure. In other words, in a car equipped with ABS, even if the driver presses the brake pedal hard just once during an emergency stop, the actual braking system quickly adjusts the braking force as needed while monitoring the status of each wheel. Therefore, the driver no longer needs to repeatedly press and release the brake pedal multiple times, as skilled drivers used to do.
The operating principle of ABS brakes can be thought of as similar to the process by which a person reacts to external stimuli. First, the wheel speed sensors installed on each car wheel correspond to a person’s sensory organs, while the electronic control unit corresponds to the brain. Finally, the hydraulic control unit corresponds to the human musculoskeletal system. Just as when a person touches a hot object with their hand, the information that it is hot is transmitted through the sensory organs, and after the brain analyzes that information, it moves the arm muscles to pull the hand away, the ABS also regulates brake operation based on information collected from the sensors. The wheel speed sensors installed on each wheel continuously measure the wheel’s rotational speed, and this information is transmitted to the electronic control unit. The electronic control unit analyzes the rotational speed of each wheel and any changes in that speed; if it determines that a specific wheel is likely to lock up, it commands the hydraulic control unit to adjust the brake pressure. The hydraulic control unit modulates braking force by rapidly increasing or decreasing brake pressure to prevent the wheel from locking up completely. As this process repeats very rapidly, the vehicle’s wheels are kept rolling as much as possible. This system helps prevent wheel lockup even during emergency braking, thereby helping to maintain the vehicle’s steering capability. Consequently, the driver can steer the vehicle in the desired direction even while the brakes are applied and can maintain stable braking performance depending on road surface and driving conditions.
We have now explored the phenomenon of wheel lockup and ABS brakes. ABS has evolved into one of the key early stages of automotive braking safety technology and is now widely adopted in most passenger cars. Furthermore, with recent advancements in automotive automation and driver-assistance systems, various control devices are being developed, and features such as surround-view monitors (SVM), lane departure warning systems (LDWS), and automatic parking assist systems are now being implemented in actual vehicles. These systems are designed to assist drivers and reduce the risk of accidents that could result from driver error or limitations in driving ability. Furthermore, automotive automation technology is evolving beyond simple driver-assistance functions toward a future where vehicles independently perceive their driving environment and perform some or most of the driving tasks. In fact, companies like Waymo—which originated from Google’s self-driving car project—are testing autonomous driving technology on public roads and developing it into commercial services.
Just as the advent of the automobile greatly expanded people’s spheres of activity in the past, people’s lives and modes of transportation will change significantly once autonomous driving technology advances further and an era arrives where vehicles take over a significant portion of the driver’s role. Various automated systems that function similarly to a skilled driver are gradually being installed in vehicles.
