Electronic ABS (Anti-Lock Brake System) has been on cars since the 1971 Chrysler Imperial, with every car built since 2013, is mandated to have ABS and Vehicle Stability Control Systems. The question is how ABS works?
To which you need to understand the history of ABS, the Types of ABS, and the individual components before you can understand why and how ABS works. Just understanding ABS prevents wheels from locking up on hard braking isn’t enough.
History of ABS
ABS wasn’t originally designed for cars. In fact, the first concept of ABS was for airplanes in France. Although invented in the 1920’s the first widely used system was in the 1950’s invented by Dunlop Maxaret in the UK. Maxaret’s system used a flywheel and valve to control hydraulic brake fluid pressure. Thus preventing plane wheels from locking up and saving on tires. In the late 1950’s ABS was used on some motorcycles and race cars. However, the technology wasn’t reliable and very expensive. Thus it was put aside. The first electronic ABS was used on the Concorde supersonic passenger jet.
It wasn’t until 1971 that ABS was used on production cars. Even then only on the select Chrysler, Ford, General Motors, and Nissan cars. Not wildly available and very rare on most vehicles.
Components of How ABS Works
Aside from the standard brake components. Which include, the brake master cylinder, lines, and disc or drum brakes. ABS systems also have wheel speed sensors, HCU(hydraulic control unit), and an ECU( Electronic Control Unit). This is how ABS works to limit wheel lock-up.
The wheel speed sensors register the speed the wheel. Most commonly using a magnet and tonewheel to create a signal the ECU reads. There are a few very rear sensors that use an optic sensor and a gated wheel. Which is used by the ECU to determine wheel speed. The downside an optic sensor is they are very subjective to dust and dirt, making them impractical for most cars and trucks.
The ECU then compares the speedometer signal and other wheel speeds to determine if any wheels are locking up or slowing down faster than the car. Once it’s been determined that a wheel or wheels are starting to lock up, the ECU tells the HCU to open its valves.
The HCU opens and closes hydraulic valves to bleed off hydraulic pressure in the brake system, thus preventing wheel lock-up. As soon as the wheel starts to move again the HCU applies brake pressure again. It does this 100’s of times a second. Essentially pumping the brakes faster than a person ever could.
Types of ABS
There are three basic types of ABS. Of course, it gets more complicated on who makes the system and what all it entails, but we will stick with the basics. Every system type is based on how many sensors it uses.
4 channel
4 channel ABS uses an ABS sensor on each wheel. This allows for the car to control each wheel individually. It also means with some extra software and a few extra sensors you get traction control and Vehicle Stability Control Systems. Because of this, every car made since 2013 will have a 4 channel system. Due to a government mandate that all cars newer than 2013 have Vehicle Stability Control Systems.
3 channel
3 channel systems have a sensor on both front wheel, and one sensor that covers both rear wheels. These are commonly found trucks and SUVs with a rear axle. This is due to the sensor being placed in the rear differential.
The biggest downfall to these systems is the idea that if just one rear wheel is locking up the system will control both rear wheels. Potentially making the truck take longer to stop.
Single-channel
Single-channel ABS systems have one sensor in the rear differential. These were typically used on rear-wheel drive and 4wd trucks. Due to the lack of weight over the rear of these vehicles and the way drum brakes work. The rear brakes on these vehicles can lock up before the fronts. Thus, the want for rear wheel only ABS.
Other Features using ABS
ABS sensors are used for more than just Anti-Lock brakes in cars today. There are is traction control, Vehicle Stability Control Systems, and select Tire Pressure Monitoring systems. All which use the ABS wheel speed sensors.
Traction control uses when speed sensors and the speedometer signal to determine if the wheels are spinning. Then it either applies brakes to individual wheels and/or controls the engine speed to limit wheel speed.
Vehicle Stability Control Systems use wheel sensors, a steer angle sensor, and a yaw-rate sensor. With this, it determines if the car is going in the direction it’s being steered. When it detects a problem it will apply brakes to different wheels, limit power steering, and limit engine power to prevent overcorrection from the driver and get the car in control again.
Tire Pressure Monitoring with ABS sensors is based on the idea that an under-inflated tire has a smaller diameter. So the system is set with the tires inflated. Then if one tire seems to be rotating faster than the others it will turn on a light letting the driver know to check their air pressure. This system is not very accurate and doesn’t necessarily take into account all tires being low on tread.
Why have ABS
Now you understand how ABS works, the question is why have ABS? To which, the answer is safety. ABS prevents wheels from locking up during hard braking. This will stop a vehicle faster than if the wheels lock up. It also allows better control of your vehicle. As it’s hard to steer when you have no traction from skidding wheels.
The downsides of ABS
The biggest downfall to ABS is that is can take longer to stop your vehicle on slick roads. It can also mean it takes longer to stop a vehicle. Which could mean the difference between hitting someone or just stopping before you hit someone. The other downside is cost. There are more things to go wrong in your brake system thus more costly to fix when they do go wrong.