The Siemens VDO electronic wedge brake is a brake-by-wire unit that does not need a 42-volt car electrical system.
Conventional electromechanical brake calipers need to develop a clamping force of six to eight tons on the disc to slow a car. But the wedge brake uses the vehicle's kinetic energy to mechanically amplify braking pressure, eliminating the need for a huge mechanical effort or strong electric motors.
The system also retracts the friction pad when the brake is off, reducing drag, increasing pad life and allowing faster antilock-brake cycle times. Compared with ordinary brakes, the braking distance at 100 kilometers per hour (about 62 mph) is 2 meters shorter.
A conventional hydraulic braking system has two levels of redundancy. If one hydraulic line fails, the backup circuit still brakes two or three wheels. If boost assistance is lost, the backup works, but the driver must push the brake pedal harder.
Brake-by-wire systems must protect against a total loss of electricity. The Mercedes E class used a "wet" backup system of a conventional hydraulic circuit.
Siemens VDO chose a "dry" method: a backup battery to power the brakes if the main system fails. Each wheel has a separate circuit, so three remain if one fails.