Tires work by friction. When you step on the accelerator, the friction between the tires and the road is what makes you move forward. When you step on the brake, the friction between the tires and the road is what slows you down. That's why in icy conditions it's sometimes difficult to get your car to start moving when you want it to -- and even more difficult to make it stop.
Friction is also what keeps the car moving in the direction you want it to go. When we accelerate, we often take for granted that the car will move in the direction that the wheels are pointing; however, if there were no friction between the tire and the road the car could easily slip sideways and go out of control.
It's important for driving safety to maintain the friction between the tire and the road, yet there are a number of conditions under which this friction can be lost. One is acceleration on icy roads, as mentioned above, but friction can also be lost if you brake too hard. The forward momentum of the car can keep it moving at a speed significantly greater than the speed at which the tires are spinning. This is called wheel lock and it's a common cause of driving accidents. Once it happens, the tires no longer grip the pavement and the car continues to travel in the direction it was heading when the skid started.
The key to avoiding a skid is the slip ratio, the difference between the speed at which the car is moving and the speed at which the tire is rotating. Antilock braking systems (ABS) can sense the slip ratio of the individual tires and modulate the brake force applied to each tire so that the slip ratio remains within a safe range, thereby avoiding a skid.
When a car slows down, its weight shifts forward. In a front-engine car, the extra weight up front increases the grip of the front tires while it reduces the grip of the rear tires. This makes the rear tires more likely to lock up during braking. When the rear wheels slip, the car can begin to fishtail or even go into a spin. Traditionally, braking systems included a proportioning valve to allocate the correct amount of brake force to the front and rear tires. However, with ABS the proportioning valve no longer provides an ideal solution to the problem.
This is where electronic brake force distribution (EBD) comes in. With EBD, a computer called an electronic control unit (ECU) determines the slip ratio of each of the tires individually. If the ECU notices that the rear wheels are in danger of slipping, it applies less force to them while maintaining (or, if necessary, increasing) the force applied to the front wheels. EBD is also useful when the car is braking while driving around a corner. While turning, the outer wheels of the car rotate more quickly than the inside wheels. If too much brake force is applied to the inner wheels they can lock, causing the car to oversteer and go out of control. EBD can sense the slippage of the inner wheels and reduce the brake force on those wheels without reducing the force on the outer wheels.
How are these electronic miracles accomplished? On the next page we'll examine the hardware and software that an EBD system uses to individually control the amount of brake force applied to each tire.