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, Thu, 24 Feb 2005 17:40:50
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from comsumerreports.org:
http://www.consumerreports.org/main/detailv4.jsp?CONTENT%3C%3Ecnt_id=304957&FOLDER%3C%3Efolder_id=301665&ASSORTMENT%3C%3East_id=333137
"An electronic stability control system is designed to help keep a vehicle on its intended path during a turn, to avoid sliding or skidding. It's especially helpful in slippery conditions and accident-avoidance situations. In the case of tall, top-heavy vehicles like large sport utility vehicles and pickups, it can also help keep a vehicle from getting into a rollover situation. Consumer Reports' auto experts highly recommend stability control where it's offered, and think it should be included as standard equipment on all SUVs.
THE BASICS
A stability control system constantly monitors the vehicle's wheel speed, steering angle, lateral acceleration, and yaw rate while you are driving. If the system determines that the vehicle is beginning to slide or spin, it can brake individual wheels and adjust the throttle in milliseconds, to correct the situation and keep the vehicle headed in the direction it's being steered. That is something even a professional race car driver can't do.
Stability control works in conjunction with a vehicle's antilock brake system, traction control system, and engine management controller without any driver intervention.
Car manufacturers will typically rename the systems with their own proprietary designations to make them stand out. For example, Audi and Mercedes-Benz call their systems Electronic Stability Control; Cadillac and Buick use the name Stabilitrak; the systems from BMW and Jaguar are called Dynamic Stability Control; Ford products use AdvanceTrac; and Volvo calls its system Dynamic Stability Traction Control. So sometimes it can be hard to know what a particular system does. If in doubt, ask the salesperson if a certain feature is indeed stability control, and includes steering and yaw sensors.
GETTING TECHNICAL
There are two ways in which a vehicle typically loses control when cornering: understeer or oversteer. Understeer occurs when the front wheels of a vehicle lose traction in a turn. It feels as though the car is going straight rather than in the direction of the turn. Turning the steering wheel more doesn't help. Backing off the throttle will eventually bring the vehicle back into line.
Oversteer occurs when the rear wheels lose traction and the rear of the vehicle begins to swing out, potentially causing a vehicle to spin or slide sideways. This motion can sometimes be corrected by steering into the direction of the slide.
Stability control helps prevent these situations by taking control of the vehicle before or as the driver loses control. It rapidly applies brake force on one or more wheels, depending on the behavior of the vehicle. In an understeer situation (below right), the vehicle is making a turn to the right, but begins to understeer. Stability control initially increases brake force on the right rear wheel. This causes a rotation in the vehicle, helping the front tire gain grip and helping the driver maintain control in the bend.
In the other example (below left), the vehicle is oversteering through the same turn, so stability control quickly brakes the left front wheel. This causes a rotation in the direction opposite to the slide and brings the car back into line. Engine power is also scaled back while this is taking place, aiding in decelerating and assisting the system in controlling the vehicle.
One vehicle, the Volvo XC90, has an advanced form of stability control, designed to help prevent a rollover. A rollover stability system determines whether a vehicle is in danger of rollover and applies braking to the outer wheels, which are most heavily loaded. It uses the antilock brake system to apply the brakes without the driver's depressing the brake pedal. Rollover stability systems work independently of the stability control, traction control and antilock brake systems."
posted by 66.100....
, Thu, 24 Feb 2005 12:59:24, Thu, 24 Feb 2005 17:40:50
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