How It Works: ABS

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It may not seem the way things should work, but when you hit the brakes hard – especially in a panic stop – sometimes you don’t want them to grab hard and hold on. And that’s where your vehicle’s anti-lock brakes (ABS) come in.

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ABS has been around a long time: there were some experimental versions on cars as far back as the 1930s. It was available on a handful of models around 1970, but it only started becoming more common in the 1980s. Since the 2012 model year, it’s been required on every new passenger vehicle in Canada. Electronic stability control (ESC), which builds on ABS and helps to correct sideways skids, is also mandatory.

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How It Works: ABS Back to video

Anti-lock brakes help tires to maintain their grip. This allows you to steer while you’re braking hard, which might make the difference between avoiding an obstacle or smacking into it.

Your wheels stop turning when you hit the brakes, but it’s your tires gripping the pavement that actually stops your vehicle. If you don’t have ABS and the tires can’t grip, the wheels lock up but the tires slide. Even if you try to steer, the vehicle will keep sliding out of your control.

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Before ABS, drivers were taught to pump the brake pedal when this happened. Each time the brakes are released, the tires turn a little and grip the asphalt. That quick, alternating on-off brings the vehicle to a controlled stop. The anti-lock brake system does the same thing by rapidly applying and releasing the brakes, but much faster than anyone could do it – as much as several times a second when required. Because the tires are maintaining their grip, you can steer while braking.

It’s important to remember that ABS doesn’t come on every time you brake for a stop (of course, if it does, your driving habits need some serious work!), and when it does activate, it can be scary if you’ve never experienced it before. The brake pedal will pulsate, and you’ll probably hear a loud grinding or groaning sound. It’s important to keep your foot firmly on the pedal until your vehicle comes to a complete stop. If you release the brake it’ll deactivate the ABS and you won’t get the benefit.

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If you’ve never experienced your vehicle’s ABS, go into an empty parking lot and try some panic stops, just so you know what to expect. It’s also a good idea to practice the old-fashioned method of pumping and releasing the brake, in case you’re ever in a vehicle that doesn’t have ABS, or if your system ever develops a problem and doesn’t operate. (If this happens, you’ll see a warning light on your dash of a circle with ABS written inside it.)

How ABS actually works involves speed sensors, which determine how fast each wheel is turning. These sensors are vital to the vehicle’s functions, since how fast you’re going affects the operation of everything from the engine and transmission, to the cruise control. For the ABS, the system compares the wheel speeds to determine if one or more wheels might lock up, or are spinning on a slippery surface. The traction control and electronic stability control systems also use this information to determine if they’re needed.

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Brakes bring your vehicle to a stop by using hydraulic pressure to press pads against the spinning brake disc (or, on vehicles with rear drum brakes, against the spinning drum), and the resulting friction stops the wheel from turning. If a wheel locks up, special valves rapidly open and close to control the hydraulic pressure to pump the brakes.

You won’t feel your ABS kicking in at lower speeds, as it generally starts working only around 25 km/h or more (which is why you won’t activate it when backing up, since you’re usually going slower than that). That’s mostly because your wheels frequently turn at different speeds, but it doesn’t mean you’re losing control. For example, when you turn a corner, the outside wheels turn slightly faster than the inside ones to allow you to make the turn – no ABS required.

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Of course, while ABS is an important safety item, you should never drive as if you’re going to have to depend on it. It doesn’t always stop you any sooner, and in some cases, it may slightly increase your stopping distance. Its primary function is to give you steering control as you’re braking, to help you stay in control when you might otherwise run into something in your path, or slide out while navigating a curve.

Since it’s all about letting the tires get a grip during braking, rather than sliding like a toboggan, ABS is most effective when the tires have sufficient tread. An easy test to determine if your tires still have sufficient tread is to put a Canadian dime between the tread blocks, with the Bluenose ship upside down. If you can see all of the ship’s sails, you need new tires. In winter months, you should equip your vehicle with winter or all-weather (not all-season) tires , which will give you maximum grip. And whenever it’s slippery, slow down and reduce your need for your anti-lock brakes.