How Vehicle Brakes Work

Inertia or momentum, as described by Newton’s Law of Motion, is the tendency of an object to remain in its current state and resist change. It is measured by an object’s mass when stationary, its mass and velocity when in motion. How do these laws of physics relate to the average commuter? It’s simple. Engineers use algebraic formulas relating to the Law of Motion to design braking systems that will arrest the forward momentum of a 4, 000 pound vehicle traveling at 88 feet per second. Therefore, you have a reasonable expectation that your car will stop when you step on the brake pedal.

Keep in mind, however, that brakes are a wearable item, meaning that certain components of the braking system will wear out before the rest of the car. These components are:

• Brake Rotors - Brake rotors are flat steel discs mounted vertically to the vehicle’s axel. They are approximately nine to ten inches in diameter and one inch thick on most domestic, passenger vehicles. Many rotors are designed with vanes sandwiched between two friction surfaces. This is to dissipate excess heat when the brakes are actuated. Each manufacturer publishes the minimum rotor thickness designed for a particular vehicle. When rotors wear to this minimum, they need to be replaced. Ask your service professional to measure the rotor thickness each time your brakes are inspected.

• Brake Pads - Brake pads consist of friction material on a steel backing plate. The pads are mounted to the stationary brake caliper. When the brakes are actuated, the stationary pads clamp both side of the spinning rotor. This causes friction which in turn produces thermal energy, or heat. The frictions of the pads on the spinning rotors cause the rotors to slow which in turn slow the vehicle’s wheels. This friction and heat cause microscopic particles to be lost on both pads and rotors each time the brakes are actuated. Pads should be replaced when 75% to 80% of the friction material is worn away.

• Brake Fluid - Brake fluid is a misnomer for hydraulic oil especially formulated to be used in certain types of hydraulic brake systems. Hydraulic brake components consist mostly of steel. Brake fluid is hygroscopic. This means that all water molecules have been removed in order to prevent internal corrosion of the brake parts. However, this causes brake fluid to attract moisture. Over time the brake fluid will become saturated with water. This is the primary reason to periodically replace the brake fluid. Check the owner’s manual for the replacement schedule or have your service professional check for moisture or rust scale in the fluid.

Your ability to commute safely day in and day out is literally riding on the condition of your car’s brakes. All highway systems and accompanying traffic laws are designed on the premise that vehicles can stop at certain points. Without this ability our roadways would be dangerously chaotic. Have your car’s brakes checked often. A good rule-of-thumb is to perform a visual brake inspection each time the tires are rotated, or every 5,000 miles.