ABCs of Car Tech: Safety tech
This week, we'll be changing gears and clarifying the alphabet soup surrounding passenger-car safety tech.
You have heard gear heads brag about their car's horsepower, handling, and modifications, but when was the last time you've heard anyone brag about their new whip's crash-worthiness?
This week, we'll be taking a look at the unsung heroes of the automotive industry. Some of this hardware could save your life in the event of a crash, while other elements are there to keep you from dinging up your paint job in the first place. That's right. This week, I'll be explaining passenger safety tech.
Seat belts and SRS: Supplemental restraint system
Seat belts are the oldest bit of passenger safety tech and are basically fabric straps that keep you from bouncing around the cabin in the event of an accident. In the earliest days of motoring, you got a two-point lap belt and padded dashboard, but in the 80s, we saw widespread use of three-point safety belts for all passengers.
Working in tandem with the modern seat-belt system is the "supplemental restraint system," which is basically a technical term for airbags. In the beginning, only the front passengers got airbags--usually located in the steering wheel and dashboard. Modern airbags are triggered by sensors in the car that measure vehicle deceleration. When a car runs into something, it decelerates at a terrific rate, triggering the system's deployment. Gasses from a small chemical explosion are captured by the nylon fabric airbag, creating a cushion of air in as little as 8/100ths of a second. I can tell you from experience that the airbag slap stings, but it's definitely softer than the steering wheel.
These days, seat-belt systems include progressive tension that spread the shock of collision forces over time, while SRSes feature as many as six to 10 airbags. There are the standard front airbags, knee airbags, side curtain airbags, and rear curtain airbags. These new airbag systems are custom-shaped for maximum coverage, feature sensors to prevent injury to smaller passengers, and progressive inflation systems to reduce the sting of the airbag slap. Some newer cars feature airbags inside of the seat belts to better distribute collision forces across the the passengers' chests and center curtain airbags that prevent passengers from knocking their heads together in the event of a side collision.
AHR: Active head restraints
In the event of a rear collision, inertial forces cause the passenger's head to snap backwards before bouncing off of the headrest, which can put tremendous strain on the neck. An active head restraint (or active head rest) moves forward during a collision to meet the back of the head and reduce the whiplash effect. The way these systems work can be as simple as a lever or can be spring-loaded. Either way, these systems are usually triggered by a sudden increase in lower back pressure against the seat. In the case of Volvo's Whiplash Protection System, the entire seat back moves to cushion the passenger.
Crumple zones and safety cells
Besides being one of the hundreds of lesser-known Decepticons, a crumple zone is an area of a vehicle designed to deform in the event of a collision. By spreading the impact forces over time, crumple zones minimize the amount of energy transferred into the cabin of the vehicle and, ultimately, to the passengers. Where older cars with rigid bumpers bounced off of obstructions, newer models are engineered with materials designed to deform around an obstruction and reduce vehicle speed in a controlled manner, while also diverting crash forces around the passenger safety cell rather than through it. Some vehicles are also equipped with motor mounts designed to push the engine beneath the vehicle in a worst-case scenario, rather than through the firewall and into your knees. Likewise, sealed polymer fuel tanks are often placed outside of the crumple zone to reduce the likelihood that fuel will ignite in the event of a fender bender.
However, you can't just have the whole car going Silly Putty soft in the event of a crash. This is where the safety cell, a rigid section of the vehicle's chassis surrounding the passenger compartment, comes into play. In the event of a rollover, side collision, or front or rear collision to catastrophic for the crumple zone to absorb, the safety cell remains stiff prevents the passenger from being simply crushed by intruding metal. Automakers design and optimize the safety cell of each vehicle work with its crumple zone, which in turn in optimized to work with the SRS in the cabin.
Launchpad McQuack used to say, "Any crash that you can walk away from is a good one." While that may be true for scarf-wearing cartoon pelicans, in the real world, the best kind of crash is the one that you avoid altogether. This next batch of automotive safety systems are designed to help you keep your car in one piece, shiny side up, and pointed in the right direction.
ABS: Antilock braking system
You're driving down the road, you round the corner and suddenly the cutest puppy you've ever seen is in the middle of your lane. What do you? The obvious answer in most cases is stand on the brakes. However, simply maxing the brakes and locking up the front (or all four) wheels is not the most efficient or safe way to bring a car to a halt. I could launch into a long discussion about static versus kinetic friction (and trust me, I will in a later edition of the ABCs of Car Tech), but suffice it to say that if your car is sliding then it is not taking full advantage of available grip and odds are that you're not in control anymore. (Unless, of course, you're one of these guys.) It may seem counterintuitive, but the fastest way to stop a car is to keep the wheels rolling.
Rather than trying to teach your mom the ins and outs of threshold braking, odds are that her Camry is equipped with ABS. This system monitors the rotational speed of each wheel as well as the amount of input applied to the brake pedal hundreds of times per second. If it detects that one or more wheels is spinning significantly slower than it should be for the vehicle's speed, the system will assume that a slide is imminent and respond by rapidly pulsing the brake calipers. This keeps the wheels spinning, which maximizes the grip available and significantly shortens stopping distances. (If you got out of the car after an ABS stop, you'd notice the skid marks left by your tires form a dotted line.) Additionally, by preventing lock up, ABS allow the driver to continue to steer while slowing, so you'll be able to swerve around the puppy if necessary.
ESC: Electronic stability control
The same sensors that are used to monitor wheel speed while braking (and then some) can also be used to monitor your vehicle and its wheels as you corner to detect excessive wheelspin or slip. This is the vehicle's electronic control system or ESC.
A system of gyrometers, accelerometers, and the aforementioned wheel sensors monitor whether the vehicle is actually headed in the direction that the driver intends. If there is a discrepancy (either wheel spin, under-, or over-rotation), ESC intervenes. Most systems apply ABS braking pressure to specific wheels to "steer" the vehicle back into its intended line--what is known as bias braking.
TCS: Traction control system
I'm sure that I'll get some mail over this one because there's a bit of a gray area between the functions of the TCS and ESC systems. Simply put, if ABS manages traction while braking and TCS manages cornering grip, then TCS is designed to maximize grip for acceleration by monitoring and influencing the flow of power from the engine to the wheels.
How TCSes work varies by manufacturer and by model. The most rudimentary systems work simply by cutting engine output when slip is detected, while more sophisticated systems bring the ABS into the mix to quell wheelspin on a per-wheel basis. Still more sophisticated systems work through active differential systems to divert power away from slipping wheels and to the rollers with grip, straddling the line between boosting performance and safety. I'll dig into the pros and cons of each in a later feature.
TPMS: Tire pressure monitoring system
In a perfect world, we'd all drive 550-horsepower cars that got 50 mpg while pulling 1.3g 'round a skidpad. In this perfect world, everyone would also break out a pressure gauge every morning and check the inflation level of their tires before the commute. Sadly, we don't live in a perfect world and most tire kickers couldn't tell you the first thing about PSI or inflation levels. Thankfully, every vehicle sold after September 1, 2007, has been equipped with a TPMS keeping tabs on the inflation levels at all four corners.
The most important real estate on any car is the contact patch of the tires--the one square foot (give or take) where the rubber meets the road. Properly maintaining tire pressure optimizes the contact patch to the manufacturer's specification. Overinflate and you risk compromising grip (particularly in the wet) and ride quality. Underinflate and reduce fuel efficiency takes a hit. More importantly, prolonged driving in a state of underinflation puts undue stress on the tread and sidewall of the tire, which can cause failure. (Trust me, you don't want a front tire blowing out at 70 mph due to chronic underinflation.)
Most TPMSes are simply idiot-light setups that only notify the driver in the event of gross underinflation, never actually displaying a numeric reading. So, periodic, manual checks with a pressure gauge and the owner's manual are still a good idea. However, the best TPMSes (often found on vehicles making sporting or off-road pretenses) will display a reasonably accurate measurement of tire pressure for individual wheels.
That's it for this week's installation of the ABCs of Car Tech and, as always, there's so much more that I didn't have space to define and explain. Think of this series as a marathon and not a sprint--there will be plenty of time to double back and fill in the details in a subsequent feature. Next week, we'll be sticking with passenger-car safety tech, but focusing on driver aid technology. Stay tuned.