ABS, the core technology of automotive braking, has now become widely common. For vehicles over 1 ton, it is not offered as an option but is legally required to be installed as standard. Alongside ABS, a system called ASR is also gradually being adopted. Furthermore, there is something called ABD, which we will explore together, and we will also take a look at the similar technology known as TCS.
Written by: BusLife
Date of Publication: June 10, 2001
From Korean, AI-translated
What is ABS?
ABS stands for Anti-Lock Brake System. Its function is to prevent the wheels from locking during sudden braking while driving, so that even during emergency braking, friction between the tires and the road surface is maintained, allowing the driver to retain steering control.
In a vehicle without ABS, if sudden braking is applied on slippery roads (such as wet pavement, snowy or icy roads, or unpaved surfaces), the wheels can lock, causing the tires to slide over the road surface. Wheel lock means that while the vehicle's inertia wants to keep it moving forward, the strong braking force stops the wheel’s rotation. This leaves skid marks on the road as the tires slide. Locked wheels can lead to very dangerous situations. First, braking distance becomes longer because the tires are sliding instead of rolling to a stop. Second, when the wheels are sliding, turning the steering wheel will not change the vehicle's direction. In some cases, the steering wheel itself becomes harder to turn. This can result in being unable to avoid an obstacle and colliding head-on.
How does ABS work?
Each wheel is equipped with a speed sensor and there is an ECS unit, essentially a type of onboard computer. The sensors monitor the wheel’s rotational state, and when they detect a lock-up, they send a signal to the ECS. The ECS then commands the ABS control unit to activate. When the driver applies the brake firmly and the wheels are about to lock, ABS repeatedly releases and reapplies the brakes. In simple terms, although the driver is holding the brake pedal down, the ABS system rapidly simulates a “brake on, brake off” action. This cycle can occur 8 times per second or more. As a result, the wheels keep rotating while braking, which shortens stopping distance and allows the driver to steer around obstacles. ABS truly shows its value on slippery curves: without ABS, each wheel receives the same braking force, so if one side has less grip or if each wheel has different friction levels, one wheel may lock and cause the vehicle to spin. With ABS, braking force is adjusted individually for each wheel, allowing safe cornering.
When ABS activates, the brake pedal may vibrate rapidly and the car body may also shake. This is not a malfunction—it’s proof that the ABS is working correctly. At this point, if the driver panics and releases the brake, the result can be more dangerous. The correct response is to keep pressing the brake firmly.
What is ASR?
ASR stands for Anti-Slip Regulation. Unlike ABS, which works during braking, ASR functions during acceleration, preventing the wheels from spinning. Some manufacturers use different abbreviations—Samsung Motors, for example, refers to ASR as Acceleration Slip Regulation. Depending on the manufacturer and whether the vehicle is a passenger car or a commercial vehicle, the naming can vary. In some models (such as the SsangYong Musso), there is a system called TCS (Traction Control System), which functions almost identically to ASR.
ASR, in addition to brake control, adjusts the engine output according to road conditions regardless of how much the accelerator pedal is pressed. This helps prevent the drive wheels from slipping or spinning during starts, cornering, or lane changes on slippery roads, ensuring optimal traction and improving vehicle stability. The key points here are “in addition to brake control” and “according to road conditions regardless of pedal input.” This means that ASR automatically distributes engine power to each wheel depending on how slippery the road is. For example, in winter, a vehicle may remain stuck because the wheels spin without gripping the road. ASR applies brake force to the slipping wheel, preventing spin and enabling the vehicle to move forward. This is not only useful in snow—it also engages at high speeds or during sudden acceleration if wheel spin occurs, allowing safe acceleration.
Along with ASR, there is also ABD, which stands for Automatic Braking Differential. In addition to ABS functions, ABD controls the brakes on one or both drive wheels to adjust power delivery when the left and right drive wheels are on surfaces with different levels of grip—such as slippery roads during starts, hill climbs, or cornering—thereby improving vehicle stability. In short, ABD, ABS, and ASR can be considered “cousins” in terms of function.
Lastly, let’s look at a bonus feature.
You may have seen “LSD” badges on vehicles like Hyundai’s Porter or Grace Starex. LSD stands for Limited Slip Differential, also known as an automatic differential lock. In heavy trucks, this function is adapted as the Inter-Axle Differential Lock (IADL). What does this do?
First, we need to understand what a differential is.
The differential—commonly referred to as the “diff”—is located between the rear wheels, in the round central housing that connects the right and left wheels. Its purpose is to allow the inner wheel to rotate more slowly and the outer wheel more quickly when turning, because the two wheels follow different paths. On trucks with 8x4 or more drive wheels, this becomes even more complex. In short, the differential (or “differential gear”) distributes power differently to each wheel, enabling smooth turns.
However, because of the way differentials work, problems arise when the traction levels of the two sides are not equal. If one wheel is on a slippery surface, it may spin faster to match the speed of the wheel with more grip. In winter, if one wheel gets stuck in snow, it may spin endlessly while the other wheel remains still—leaving the vehicle stranded until the snow melts. This is where the LSD or Inter-Axle Differential Lock comes in.
When engaged, this function “locks” the differential, forcing equal power distribution to both sides. This means the wheel with grip can push the vehicle free, even if the other wheel is stuck. Similarly, before entering uneven terrain, turning on the Inter-Axle Differential Lock ensures equal power delivery to each wheel, preventing slippage on rough or slippery roads.
However, caution is required—this function should not be left on for extended periods or used during high-speed driving.
In summary, ABS, ASR, ABD, and LSD have very similar purposes. Aside from differences in installation location, manufacturer, or vehicle type, they perform comparable functions. Knowing this is enough to sound knowledgeable in conversation. Do buses equipped with ABS/ASR also need LSD? And considering that an 11.5-ton truck is a 6x4 and a 25-ton truck is a 10x6 (8x4), is LSD technology applicable? Thinking about these questions might lead you to the answer.
