When we look at a car, we generally consider its looks and power and torque figures. It’s probably safe to assume that most of us spend a lot less time mulling over a car’s braking ability and, doubtless, many of us barely consider what the car will do to help out in an emergency stop.
In this article we’ll take a look at some of the braking assistance technology that’s now employed to help us avoid crashes or mitigate their severity.
In a car without an anti-lock braking system (ABS) if you slam the brakes hard for too long it’s likely that one, or more, of the wheels will lock up and stop turning before the car comes to a halt. When a wheel begins to drag or skid, the driver has effectively lost directional control of the car.
The best way to retain steering ability during a sudden stop in a car without ABS is to brake hard, ease off slightly and repeat. This technique, called threshold braking, works on the principle that a tyre’s maximum stopping ability occurs right before it begins to skid. ABS does this automatically, and faster and more precisely than most humans — unlike us mere mortals ABS is able to modulate the brakes of only the wheel that’s about lock up. Motorsports, like Formula 1 and V8 Supercars, have banned ABS as it reduces the level of skill required from drivers.
A 4-channel ABS system consists of four main components: speed sensors for each wheel, computer controlled valves on the hydraulic lines leading to each brake, a pump for restoring hydraulic braking pressure, and an electronic brain overseeing the whole kit and kaboodle.
When the system notices that one wheel is decelerating much quicker than the others, it knows that that wheel is about to lock up. To prevent that from happening it will modulate that wheel’s braking pressure by the closing and opening the appropriate hydraulic lines.
This rapid opening and closing of valves is what causes the pulsing sensation through the brake pedal whenever ABS comes into play. On some cars there may also be a cacophony of strange sounds from the system and the pedal may drop to the floor; come what may, it’s important to keep braking as hard as one can and to remember that ABS makes it possible to steer yourself out of danger.
Many other safety systems, such as stability and traction control, make use of the sensors and valves that are a part of ABS. Today all new cars sold in Australia feature ABS as standard, because, since late 2011, stability control is legally required on all new passenger cars sold in Australia.
The primary task of ABS is to give the driver steering control during hard braking, not reduce stopping distance, as is the general misconception. That said, given the right type of road and conditions, that may prove to be the case. For example, on wet sealed roads ABS usually shortens the distance required for a stop, but on gravel or dirt tracks, skidding may actually decrease stopping distance as the car’s tyres are able to dig into the surface.
Doctors say that prevention is better than cure. In the case of wheel lockup, ABS can be thought as the cure — it only kicks in when the wheels are at the point of slipping — while electronic brakeforce distribution (EBD) is more of a preventative solution.
For the hardest and gentlest stops alike, not all wheels require the same amount of braking force, because each wheel does a different amount of work.
Consider the most common braking scenario: a straight line stop. In this situation the car’s weight shifts forward. Wheels with more weight have more grip and, consequently, are less likely to lock up. So, in cars without EBD there’s a regulator valve in the hydraulic system that ensures that the front wheels receive more braking power. Here the proportion of braking force distributed to the front and the rear is fixed regardless of the circumstances.
This is a perfectly acceptable solution, but it’s far from ideal as many things can affect a car’s weight balance. For example, an emergency stop will send more weight to the front of the car than a gentle, run-of-the-mill stop. During cornering, weight is distributed to the side of the car furtherest away from corner, meaning that the inside wheels are far more likely to slip under braking. And if there’s a stash of stolen gold bullion from Fort Knox weighing down the boot, the rear wheels will carry much more weight under braking than in the common single driver scenario.
EBD is able to cater for any of these situations; as EBD is an extension of the anti-lock braking system, it can monitor each wheel’s speed and acceleration/deceleration to determine how much load it is bearing. By adjusting the valves in the braking system’s hydraulic lines, EBD can distribute more braking force to wheels that are carrying more of the load and less to those that are doing less work.
Some EBD systems also monitor steering wheel angle and the car’s rate of turn (yaw), so if the car understeers or oversteers during mid-corner braking it can be automatically brought back to its intended path by judicious application of the appropriate brake.
Commonly referred to as either brake assist (BA) or emergency brake assist (EBA), braking assistance does just that — it provides extra braking power when the driver attempts to perform an emergency stop.
As a proportion of our driving lives emergency stops are rare. While that’s undoubtedly a good thing, it’s also a slight curse, as many of us aren’t well trained in what to do when those times arise. Various studies have shown that in the majority of test cases, drivers don’t apply enough brake pressure during emergencies.
For example, we might slam the brakes, but not quite hard enough to prevent a collision. Alternatively we might get freaked out by the unusual pulsing sensation through the brake pedal caused by ABS engagement and release the brakes, even a bit, when more braking pressure is required.
A travel sensor attached to the brake pedal allows the brake assist system to detect when the driver attempts an emergency stop and, regardless of the amount of pressure actually applied, apply maximum braking force until the car is brought to a halt.
Some more recent braking assistance systems can detect a sudden lift off from the gas pedal or receive imminent crash warnings from radar- or camera-guided crash detection systems and prime the braking system. That way, when the driver slams the brakes, maximum braking force is available immediately.
Autonomous emergency braking takes that idea one step further by either applying the brakes automatically without any driver involvement or warning the driver of a possible accident and then applying the brakes automatically if he or she doesn’t respond.
Typically, autonomous emergency braking systems only cater for rear-end collisions, as they are both the easiest form of accident to reliably predict and very common. According the USA’s National Transport Safety Board, one third of fatal collisions are rear-enders.
Some newer and more advanced systems, like Volvo’s Pedestrian and Cyclist Detection with Full Auto Brake, can detect pedestrians and cyclists too.
Manner of operation is almost as diverse as the names bestowed upon these systems. For instance, Volvo’s City Safety and Ford’s Active City Stop only operate under around 30km/h or so, and will simultaneously apply the brakes and warn the driver if it believes that a collision is imminent.
Other implementations, such as Mercedes-Benz’ Active Brake Assist, Volvo’s Collision Warning with Auto Brake and Nissan/Infiniti’s Intelligent Brake Assist with Forward Collision Warning, are not only active at higher speeds, but warn the driver first of a possible rear-end accident and only if no action is taken by the driver will the system automatically apply the brakes.