How Anti-Lock Brakes Work Stopping a car in a hurry on a slippery road can be very challenging. Anti-lock braking systems (ABS) take a lot of the challenge out of this sometimes nerve-wracking event. In fact, on slippery surfaces, even professional drivers can't stop as quickly without ABS as an average driver can with ABS. Location of anti-lock brake components In this article, the last in a six-part series on brakes, we'll learn all about anti-lock braking systems -- why you need them, what's in them, how they work, some of the common types and some associated problems. Getting the ABS Concept The theory behind anti-lock brakes is simple. A skidding wheel (where the tire contact patch is sliding relative to the road) has less traction than a non-skidding wheel. If you have been stuck on ice, you know that if your wheels are spinning you have no traction. This is because the contact patch is sliding relative to the ice (see Brakes: How Friction Works for more). By keeping the wheels from skidding while you slow down, anti-lock brakes benefit you in two ways: You'll stop faster, and you'll be able to steer while you stop. Inside ABS There are four main components to an ABS system: Speed sensors Pump Valves Controller Anti-lock brake pump and valves Speed Sensors The anti-lock braking system needs some way of knowing when a wheel is about to lock up. The speed sensors, which are located at each wheel, or in some cases in the differential, provide this information. Valves There is a valve in the brake line of each brake controlled by the ABS. On some systems, the valve has three positions: In position one, the valve is open; pressure from the master cylinder is passed right through to the brake. In position two, the valve blocks the line, isolating that brake from the master cylinder. This prevents the pressure from rising further should the driver push the brake pedal harder. In position three, the valve releases some of the pressure from the brake. Pump Since the valve is able to release pressure from the brakes, there has to be some way to put that pressure back. That is what the pump does; when a valve reduces the pressure in a line, the pump is there to get the pressure back up. Controller The controller is a computer in the car. It watches the speed sensors and controls the valves. ABS at Work There are many different variations and control algorithms for ABS systems. We will discuss how one of the simpler systems works. The controller monitors the speed sensors at all times. It is looking for decelerations in the wheel that are out of the ordinary. Right before a wheel locks up, it will experience a rapid deceleration. If left unchecked, the wheel would stop much more quickly than any car could. It might take a car five seconds to stop from 60 mph (96.6 kph) under ideal conditions, but a wheel that locks up could stop spinning in less than a second. The ABS controller knows that such a rapid deceleration is impossible, so it reduces the pressure to that brake until it sees an acceleration, then it increases the pressure until it sees the deceleration again. It can do this very quickly, before the tire can actually significantly change speed. The result is that the tire slows down at the same rate as the car, with the brakes keeping the tires very near the point at which they will start to lock up. This gives the system maximum braking power. When the ABS system is in operation you will feel a pulsing in the brake pedal; this comes from the rapid opening and closing of the valves. Some ABS systems can cycle up to 15 times per second. Types of Anti-Lock Brakes Anti-lock braking systems use different schemes depending on the type of brakes in use. We will refer to them by the number of channels -- that is, how many valves that are individually controlled -- and the number of speed sensors. Four-channel, four-sensor ABS - This is the best scheme. There is a speed sensor on all four wheels and a separate valve for all four wheels. With this setup, the controller monitors each wheel individually to make sure it is achieving maximum braking force. Three-channel, three-sensor ABS - This scheme, commonly found on pickup trucks with four-wheel ABS, has a speed sensor and a valve for each of the front wheels, with one valve and one sensor for both rear wheels. The speed sensor for the rear wheels is located in the rear axle. This system provides individual control of the front wheels, so they can both achieve maximum braking force. The rear wheels, however, are monitored together; they both have to start to lock up before the ABS will activate on the rear. With this system, it is possible that one of the rear wheels will lock during a stop, reducing brake effectiveness. One-channel, one-sensor ABS - This system is commonly found on pickup trucks with rear-wheel ABS. It has one valve, which controls both rear wheels, and one speed sensor, located in the rear axle. This system operates the same as the rear end of a three-channel system. The rear wheels are monitored together and they both have to start to lock up before the ABS kicks in. In this system it is also possible that one of the rear wheels will lock, reducing brake effectiveness. This system is easy to identify. Usually there will be one brake line going through a T-fitting to both rear wheels. You can locate the speed sensor by looking for an electrical connection near the differential on the rear-axle housing. ABS Questions Should I pump the brake pedal when stopping in slippery conditions? You absolutely should not pump the brake pedal in a car with ABS. Pumping the brakes is a technique that is sometimes used in slippery conditions to allow the wheels to unlock so that the vehicle stays somewhat straight during a stop. In a car with ABS the wheels should never lock in the first place, so pumping the brakes will just make you take longer to stop. In an emergency stop in a car with ABS, you should apply the brake pedal firmly and hold it while the ABS does all the work. You will feel a pulsing in the pedal that may be quite violent, but this is normal so don't let off the brake. Do anti-lock brakes really work? Anti-lock brakes really do help you stop better. They prevent wheels from locking up and provide the shortest stopping distance on slippery surfaces. But do they really prevent accidents? This is the true measure of the effectiveness of ABS systems. The Insurance Institute for Highway Safety (IIHS) has conducted several studies trying to determine if cars equipped with ABS are involved in more or fewer fatal accidents. It turns out that in a 1996 study, vehicles equipped with ABS were overall no less likely to be involved in fatal accidents than vehicles without. The study actually stated that although cars with ABS were less likely to be involved in accidents fatal to the occupants of other cars, they are more likely to be involved in accidents fatal to the occupants of the ABS car, especially single-vehicle accidents. There is much speculation about the reason for this. Some people think that drivers of ABS-equipped cars use the ABS incorrectly, either by pumping the brakes or by releasing the brakes when they feel the system pulsing. Some people think that since ABS allows you to steer during a panic stop, more people run off the road and crash. Some more recent information may indicate that the accident rate for ABS cars is improving, but there is still no evidence to show that ABS improves overall safety. When used properly, an antilock brake system (ABS) is a safe and effective braking system. ABS allows the driver to maintain directional stability,control over steering, and in some situations, to reduce stopping distances during emergency braking situation, particularly on wet and slippery road surface. To gain this safety advantage, drivers must learn how to operate their ABS correctly. What is ABS? An antilock braking system works with the regular or foundation brakes on your vehicle. ABS simply keeps your base brakes from locking up. In vehicles not equipped with ABS, the driver can manually pump the brakes to prevent wheel lockup. In vehicles equipped with ABS, the driver's foot remains firmly on the brake pedal, allowing the system to automatically pump the brakes. Why is that important? When your brakes lock up on wet and slippery roads or during a panic stop, you lose steering control and your vehicle can spin. Rear wheel ABS prevents wheel lockup so that your car stays in a straight line. If your car has ABS control on all four wheels, you also keep steering control. If you have steering control, it is possible to avoid a crash by steering around hazards if a complete stop cannot be accomplished in time. How do I know whether my vehicle has ABS? Most newer car models offer ABS as either standard or optional equipment. There are different ways to find out whether your car has an antilock brake system: * Read your owner's manual * Check your instrument panel for an amber ABS indicator light after you turn on the ignition. * When you buy, lease or rent, ask your dealer or rental car company. Will I notice anything when the ABS is working? In many vehicles, drivers may experience a rapid pulsation of the brake pedal--almost as if the brakes are pushing back at you. Sometimes the pedal could suddenly drop. Also, the valves in the ABS controller may make a noise that sounds like grinding or buzzing. In some cars you may feel a slight vibration--this means the ABS is working. It is important NOT to take your foot off the brake pedal when you hear noise or feel pulsations, but instead continue to apply firm pressure. Does ABS change the way I should use the brakes? You should not pump your brakes if you have ABS. Just hold your foot firmly on the brakes pedal and remember that you can still steer. How does ABS work? What ABS does is similar to a person pumping the brakes. It automatically changes the pressure in your car's brake lines to maintain maximum brake performance just short of locking up the wheels. ABS does this very rapidly with electronics. Do cars with ABS stop more quickly than cars without? ABS is designed to help the driver maintain control of the vehicle during emergency braking situations, not make the car stop more quickly. ABS may shorten stopping distances on wet or slippery roads and many systems will shorten stopping distances on dry roads. On very soft surfaces, such as loose gravel or unpacked snow, an ABS system may actually lengthen stopping distances. In wet or slippery conditions, you should still make sure you drive carefully, always keep a safe distance behind the vehicle in front of you, and maintain a speed consistent with the road conditions. Are all antilock systems the same? They are all very similar in the way they control brake pressure, but some sytems are designed to prevent only the rear wheels from locking up. These rear-wheel-only systems are found on pickups and sport-utility vehicles. Rear-wheel ABS keeps your vehicle from spinning out of control, but you will not have steering control if the front wheels lock up. All other ABS systems-including those for cars and minivans-- are designed to keep all four wheels from locking up. If you own a pickup or sport-utility vehicle, you can check your owner's manual to see what type of ABS you have. How can I familiarize myself with ABS? Read your owner's manual for more details on the complete operation and benefits of ABS. The antilock brake system is speed sensitive, and will not activate at very slow speeds. One way to familiarize yourself with the operation of ABS is to test drive the vehicle at a speed above which the ABS activates (usually above 10 mph) in an unobstructed parking lot and apply the brakes firmly. It is easier to activate the ABS on a wet and slippery road surface. The antilock system should prevent the wheels from skidding. Pulsation may be felt in the brake pedal and you may hear a clicking sound. Avoid pumping the brake, even if the pedal is pulsating. Some basic terminology to get started: ABS - Stands for Anti-Lock Braking System. ABS Actuator - The work horse of the ABS system, it's the piece that actually releases pressure in the ABS channel to modulate the brakes. ABS Channel - The channel is the hydraulic line(s) from the ABS actuator to the wheel(s). ABS channel can consist of 1 or 2 wheels that will pulse/modulate when the ABS actuator performs its work. ABS Sensor - Measures the speed of a wheel. Consists of gear-shaped sensor rotor and a sensor element. The element contains a bar magnet around which a coil is wound. The sensor is installed on the back side of the brake rotor. Sine-wave current is generated by the sensor as the wheel rotates. The frequency and voltage increase(s) as the rotating speed increases. ABS Pulse or modulation - When the ABS Actuator engages and pulses/modulates the brake pressure being applied to the ABS channel. This is the method by which the system actually works. When the brakes pulse, they are prevented from locking because they rotate for a split second. ABS comes in about 4 different flavors: 1 or 2 Channel 2-wheel (Rear ABS) - This ABS is usually prevalent on trucks. It consists of 2 ABS sensors on the rear wheels and one or two ABS channels to pulse the rear wheel together (1 channel) or separately (2 channels). 2 Channel 4-wheel Criss-cross - This is the ABS system present on the 91-94 Sentra. It consists of 4 ABS sensors (one on each wheel) and 2 ABS channels arranged in a Criss-cross (Left Front & Right Rear, Right Front & Left Rear). When the right rear wheel locks up, the left front wheel & right rear wheel are pulsed together. 3 Channel 4-wheel - This is the more common ABS system in cars. It consists of 4 wheel sensors and 2 channels in the front (LF, RF) and one channel for the rear wheels. When one of the front wheels locks up, it pulses independently of the other wheels. When one of the rear wheels locks up, it is pulsed together with the other rear wheel, similar to a very fast pulling and releasing of the emergency brake. 4 Channel 4-wheel - This is the ABS system present on the 95+ 200SX/Sentra. It consists of 4 ABS sensors and 4 ABS channels. All wheels pulse independently of each other, like it should be. So, the long and short of it is that a 4 channel system will work better because only the wheel that is locked pulses (loses braking power), while the other 3 wheels continue to do braking for the car. This results in more stability while braking and possibly shorter braking distances. Some myths about ABS: With ABS I came stop faster. - Not necessarily. On a wet road, you MAY be able to stop quicker because your wheels are not locked up. However, in several conditions you can stop faster by locking up the wheels. One example is in deep snow. If you lock the brakes you create a wedge effect and stop quicker. Usually ABS distances are a tad shorter (10 feet) than non-ABS brakes, however, ABS gives you more steering ability then non-ABS brakes, the real advantage. Pumping the brakes will help ABS - NO. You will probably confuse the ABS system more than anything else. If you need to stop in a hurry then press firmly on the brakes and DON'T pump them. The ABS system will pump the brakes for you. With ABS I can steer around obstacles while braking - Most of the time. This is the theory, but in practice, if you don't have traction, you can't steer. Take for example ice. You could press on the brakes all you want but the car may not turn, because there is very little traction. Why buy a car equipped with ABS? Yes, it does cost $500 - $1000 more to add ABS to a car. With it I've been able to steer around dogs that decided to cross the road (one on snow, one on dry pavement). I've also avoided many accidents with other cars because I could brake and steer away from the other vehicle(s). So, I'm willing to pay for it because it's saved my butt several times, not to mention some peace of mind knowing that I can just stomp on the brakes and not have to worry about pumping the ol' brakes.