A parking brake is a separate braking system that holds a stopped vehicle in place, especially on a hill. It matters because gravity can make a parked car roll even when the engine is off and the service brake pedal is no longer pressed. In many vehicles, the parking brake acts on the rear wheels so the car resists rolling forward or backward.
It is also useful as a backup holding system if the main hydraulic brakes lose pressure.
Key Facts
- On a hill, the downhill pull on a car is F_parallel = mg sin(theta).
- The parking brake must create enough braking torque so that tau_brake >= F_parallel r_wheel.
- A manual parking brake uses a lever or pedal to pull steel cables connected to the rear brake mechanisms.
- An electronic parking brake uses a switch, control module, and electric motors or actuators to clamp the rear brakes.
- Friction between brake pads or shoes and the rotor or drum creates the holding force: F_friction = mu N.
- Parking brakes are best applied before shifting fully into Park on a steep hill to reduce load on the transmission parking pawl.
Vocabulary
- Parking brake
- A brake system designed to hold a parked vehicle still by mechanically or electronically applying braking force.
- Service brake
- The main brake system operated by the brake pedal to slow or stop the vehicle while driving.
- Brake cable
- A strong steel cable that transfers pulling force from a manual parking brake lever or pedal to the rear brakes.
- Brake caliper
- A clamp-like part in a disc brake that pushes brake pads against a rotor to create friction.
- Brake torque
- The twisting effect produced by brake friction that resists wheel rotation.
Common Mistakes to Avoid
- Assuming Park and the parking brake are the same, which is wrong because Park locks the transmission while the parking brake clamps the wheels.
- Forgetting the hill direction, which is wrong because the braking torque must oppose the direction gravity would make the wheels turn.
- Thinking the parking brake always uses the front brakes, which is wrong because many vehicles apply it mainly to the rear brakes for stable holding.
- Using only weight mg on a hill calculation, which is wrong because the force pulling the car downhill is the component mg sin(theta), not the full weight.
Practice Questions
- 1 A 1200 kg car is parked on a 10 degree hill. Estimate the downhill force using F_parallel = mg sin(theta) with g = 9.8 m/s^2.
- 2 A car on a hill has a downhill force of 1800 N and wheel radius 0.30 m. What total braking torque at the wheels is needed to hold it still?
- 3 Explain why applying the parking brake before releasing the foot brake on a steep hill can make the vehicle more secure and reduce stress on the transmission.