By Jim Kerr
Automobile braking systems have become quite complex, yet few of us give any thought to them not working. When you step on pedal, you just expect to stop! Fortunately, brake systems are also very reliable, and almost every vehicle does stop every time. They also stop us in ways that are safer than ever before. Here is what happens when you step on the brake pedal in a newer vehicle.
The brake pedal is a simple lever, multiplying the force of our feet into hydraulic pressure that can stop the vehicle. Power brakes are a method of helping you to press the brake pedal so you don’t have to have the legs of a body builder. When you step on the pedal, several things happen at once. First, switches connected to the pedal arm turn on the brake lights, signal the computer to disconnect the automatic transmission torque converter and cancel cruise control operation.
On many newer vehicles, other sensors connected to the brake system monitor how fast you are pushing the pedal. If you are moving it rapidly, the brake computer program recognizes this as a potential emergency situation and will automatically apply full braking force immediately to shorten stopping distances. This feature is usually called “Brake Assist,” and it can make the difference between stopping in time or being involved in a collision.
As you press the brake pedal, the power brake system will push pistons forward in the brake master cylinder. These pistons push brake fluid out to the wheels to apply the brakes, but it isn’t quite that simple. The harder you step on the brake pedal, the more vehicle weight is transferred onto the front tires. The front tires get better grip and the rear tires have less. If the braking force to all the wheels were the same, the back tires would lock up and slide every time you applied the brakes and it could cause the vehicle to go out of control. To prevent this, modern brake systems include a proportioning valve that limits pressure to the rear wheels. Cars had fixed pressure limits, while many vans and trucks used devices that would regulate the pressure depending on the body height: place some cargo in the vehicle and the back end sits lower, brake pressure can then be increased for the back brakes.
Today, most vehicles use Electronic Brake Force Distribution (EBFD). A computer-controlled solenoid that is part of the antilock brake system varies brake pressure to the rear wheels based on vehicle deceleration rates, steering angle and possibly even lateral acceleration of the vehicle. Braking force can be maximized at each wheel so the vehicle can stop quickly but remain in control.
Two other systems have now become key components of brake system on many vehicles: Antilock Brake System (ABS) and the traction control/stability control systems. Both systems use speed sensors at the wheels to monitor the rotation of the tires. By comparing the speed of one tire to another or to programmed acceleration/deceleration rates in the computer, the system can determine is a tire is sliding or spinning.
When a tire is locked up and sliding, the computer operates the ABS to release the brake fluid pressure at that wheel. The tire now starts to rotate again and the ABS system lets the brake pressure back to the wheel so it can continue braking. This cycle can occur 15 times a second or more, keeping the tire rotating, and a rotating tire will have more braking force than a sliding one.
Traction control does the opposite. It applies the brakes to a spinning wheel so that the tire can be slowed down to gain traction. Often, the computers will reduce engine power while to limit wheel spin so the brakes don’t become overheated.
Stability control combines features of both ABS and traction control. By applying brakes on just one or more wheels, it can cause the vehicle to steer or move in a desired direction. It may move the vehicle sideways to reduce the possibility of rolling (called Roll Stability Control) or it may apply one front or rear wheel to make the vehicle go in the direction the steering wheel is pointing. This can prevent the vehicle from spinning out.
As you can see, there is a lot to modern brake systems. Usually, the only problems are worn brake pads and rotors or a fluid leak because of an old and deteriorated brake hose. Flush the brake system with clean brake fluid every couple of years to help keep the rest of the system operating properly for many years and you can continue to trust your brakes every time you step on the pedal.