The last time most people saw a wedge brake was on a horse-drawn wagon.

The last time most people saw a wedge brake was on a horse-drawn wagon. The mechanism used to keep the wagon from rolling away was a wedge-shaped piece of wood inserted between the outer rim of the wheel and a steel retainer. The brake was very effective for the simple reason that any movement of the wagon in the direction of travel pulls the wedge tighter. Moving in the opposite direction pulls it away from the retainer, thus releasing the brake.

This simple, yet effective, device could be making a comeback, only this time in automobiles. Siemens VDO Automotive (Auburn Hills, MI; is working on an electronic wedge brake (EWB) that not only is part of a brake-by-wire system, but works with a 12-volt electrical system. Created by a company called eStop that is now owned by Siemens VDO, the new brake design replaces conventional brake calipers with a module consisting of a brake pad, a wedge and wedge-holding mechanism, two electric motors, and a set of sensors that read movement and force at each wheel. When the brake pedal is pressed, the electric motors move the brake pad over a series of rollers located on the wedge-shaped surface. The position of the wedge determines the braking force applied—it takes the EWB 100 ms to reach full braking power, versus about 170 ms for a conventional ABS-equipped disc brake system—and the electric motors either hold the pad at this point or back it down a position that will give the driver the decelerative force required for the situation.

An unnamed European automaker is working with Siemens VDO to develop the system further. Not only does it eliminate the need for brake pipes, booster, and hydraulic fluid, but ABS and vehicle stability functions would be fully software-based. Despite the high braking forces and the ability to intricately control them Siemens VDO claims for the EWB, power consumption is minimal. A flashlight battery is strong enough to run each unit.