Braking, on its best day, is a concession to practicality. In a perfect world, all vehicles would forever maintain whatever momentum they've built, preserving the energy they used to get up to speed and rolling smoothly on a clean, dry road. But as the heavy aircraft engineers who first devised antilock brakes discovered, the real world is a place of constant dangers, inclement conditions and runways that are a bit shorter than the machine itself would prefer.
Brakes are, by far, the single most powerful part of your car. Consider this: If a typical car has 200 horsepower and takes 10 seconds to get to 60 mph under full throttle, and it only takes 2.5 seconds to stop it from 60 mph, then the brakes are putting out about 800 horsepower's worth of reverse thrust. Assuming the tires were engineered to provide just enough grip to keep the car from spinning its tires under acceleration, then the brakes would be easily capable of overcoming the tires' grip. An antilock braking system compares the speeds of all four wheels with electronic sensors; if, during braking, one wheel stops moving or moves much slower than the others, the ABS computer opens a valve to temporarily relieve some of the fluid pressure to that wheel. This reduces braking power, allowing the wheel to spin again. ABS generally reduces stopping distances for street-driven vehicles, and almost always allows a driver to maintain better control while stopping -- but there are a few circumstances where it's less than ideal in terms of stopping distances.
An antilock system is reactive, meaning that it can only prevent wheel lockup after the brakes have already locked the wheel. That sounds bad, but it isn't for two reasons. First, modern ABS systems read wheel speed and cycle hundreds or even thousands of times per second, which is so fast that you won't even notice the cycling. Second, while intuition might indicate full grip on the road would be best for performance, tires actually exert the most grip when they're sliding just a little bit. For most tires, maximum braking performance occurs when the tire is sliding by about 15 to 20 percent. Grip generally drops off precipitously just after that degree of slip, so extracting maximum performance means keeping the tire right at that grip threshold. For that reason, the fact that ABS systems allow a certain degree of slip -- "threshold brake" -- is actually a boon to braking performance.
You don't need a computer to use threshold braking, and many performance drivers opine that a talented foot can beat an ABS system. Racing drivers use manual threshold braking by lightly pumping the brakes, feeling for a loss of grip through the brake pedal. This type of manual threshold braking is an evolution of the pre-ABS technique of pumping the brakes to stop the car faster. The ABS system does exactly the same thing, but automatically keeps the brakes near their grip threshold, and does it a lot faster and more accurately than the average driver. A skilled driver can still beat the ABS system on speed, but he can't control all four wheels independently. In that regard, ABS has an inherent advantage in fast stopping and threshold braking. The best way to use an ABS system under almost all conditions is to simply stand on the brakes and let the ABS computer deal with keeping the tires' 15 to 20 percent grip threshold. It's as simple as that: just stand on the brakes, and keep the car pointed straight. This goes in the dry or rain, as long as all four tires don't lock up. Once all four tires lock up, the ABS system will think the car has stopped; more on that in a moment.
Theoretically speaking, in a highly tuned race car on a race track, with a talented driver, a non-ABS car can be more nimble and predictable while cornering. If ABS has one major drawback in use, it's that it can make the car too stable, making it more likely to plow straight ahead while turning rather than to rotate in the middle of a turn as a racing driver might prefer. This is well outside the realm of the average driver, but it must be said: If your car has ABS, you'll see the greatest braking performance if you adopt a cornering strategy called "trail braking." Trail braking involved quickly stabbing the brakes just before turning into a corner -- while the car is still pointed straight -- transferring the car's weight to the front wheels to enhance turn-in and cornering grip. This can be especially helpful for front-drive cars, which generally don't rotate under cornering, and can use that extra bit of turn-in. But trail braking is difficult to master, potentially dangerous, and probably best left for weekends spent on the race track.
Two sections up, you might have noticed the use of the phrase "you probably can't turn the ABS off anyway." That "probably" is important, because some vehicles -- particularly older vehicles intended for use off road and in severe weather -- had a switch to turn the ABS off. Even some modern vehicles have provisions to almost completely deactivate the ABS system in snow and off road, and for good reason. Hitting the brakes on "deformable" surfaces like heavy snow, sand and gravel causes the material to build up in a wedge shape ahead of the wheel, effectively stopping it by digging in and creating a temporary chock ahead of the tire. This "chock" doesn't happen if the wheel's spinning though, so ABS systems will often increase stopping distances in snow, sand and gravel. However, they'll also keep the vehicle pointed straighter while stopping, which can be more important than stopping shorter. If you can turn your ABS off, experiment with the vehicle in an empty parking lot or field to see how the vehicle reacts with the system off, and decide whether or not you want to hit the switch.
Section 3 mentioned that ABS has issues on ice and potentially in the rain. Most ABS systems are completely comparative, meaning that they only function by comparing the speed of the wheels relative to each other. This works fine as long as at least one wheel is spinning, but locking up all four wheels will effectively deactivate the ABS system. Realistically, this isn't likely to happen unless you're on a long stretch of flat, slick sheet ice and your tires lose anything resembling grip. But if the vehicle stops decelerating on ice and your speedometer drops to zero, let off the brakes and adopt a manual threshold braking technique to keep the tires spinning. Unlike on snow and in gravel, you profit nothing by allowing the tires to lock up completely on ice and on a rain-slicked road. Under these circumstances, it's usually best to use the brakes as though the ABS weren't there. Usually. However, this isn't the case on vehicles equipped with stability control; stability control systems use inertial sensors that tell the car that it's still moving and in what direction, regardless of what the wheels are doing. If you've got stability control, it's best to leave it in its "winter" setting -- if so equipped -- and trust the computer to work out the details for you.
