Globe Drive

Hitting the brakes in a Depression-era Ford activated a system any driver could understand: the brake pedal was connected to steel rods that pressed friction pads against metal drums in the wheels.

Science soon provided a superior (but harder to understand) technology – hydraulic brakes, which linked the brake pedal to the wheels with fluid-filled hoses. Although Henry Ford distrusted them, hydraulic brakes quickly became an industry standard.

The 2010 Toyota Prius uses a braking system that represents the next leap in braking technology, applying Airbus-style digital controls to the task of stopping the car. Although the Prius has a conventional disc brake in each wheel, that's where the similarity to other cars ends.

There is no physical connection between the pedal and the brakes. Instead, the pedal serves as a pressure-sensitive actuator for a system known as Brake By Wire. Pressing on the brake pedal of a 2010 Prius triggers a complex system of electronic sensors and microcomputers that interprets the driver's braking command, then decides what to do.

Although no conclusions have been reached, it appears that computer-instigated delays in braking may be the culprit in the current Prius debacle. The problem has been discussed extensively on hybrid car websites that attract Prius fans and owners, many of them early-adopter tech buffs and “hyper-milers” who compete for fuel economy records.

Despite the current controversy, the Prius's braking system is part of its core appeal. Each of the Prius's wheels is connected to an electric motor that serves two functions – generating power, and slowing the car.

This offers a number of key advantages. Most important is the way it helps maximize the recapturing of energy that would otherwise be lost. In a standard car, the brakes simply convert kinetic energy into heat. But as the electric motors help slow the Prius, they generate electrical power that is pumped back into the battery, increasing fuel efficiency.

Some Prius buffs have speculated that the car's Brake By Wire system can been confused by slick surfaces and road bumps that supply unexpected wheel-speed readings.

(Mercedes encountered these, as well as other problems, when it experimented with a digitally controlled brake system in 2002, finally abandoning the system.)

At the priuschat.com website, the 2010 Prius's brake issues have been debated for weeks. Several users reported odd sensations while braking, and attributed it to a slight delay that can be produced when the Brake By Wire computers switch back and forth from the regenerative motor system to the disc brakes while deciding how much braking is required.

One online user described what he encountered when his car hit a pothole while he was braking: “It does not seem to make me lurch ahead or make it take longer to brake,” he wrote. “It just feels strange. It only happens when I am going in one direction and this is the only place it happens. It really is a huge hole in what is not even a piece of paved street. I can't wait for them to fix it and then I am sure that the slight change I feel will go away. I find the brakes on my Prius to be the best brakes I have ever had in a car.”