When braking heavily in a car you get a dramatic forwards weight transfer. The body pivots forward, the front suspension is compressed, and more weight is transferred to the front tyres. More weight acting on the front tyres means more grip is available, and conversely the more lightly laden rear wheels have less grip at their disposal. If an equal amount of braking force was applied to the front and rear wheels in a heavy braking scenario, the lighter rear wheels would run out of grip first and be much more prone to wheel lock (see Figure 1). Figure 1: A car with an equal front to rear braking bias braking heavily, resulting in rear wheel lock
Most cars however, have a brake bias which sends the majority of the braking effort to the front wheels to counter this effect, however in this case the ratio of front versus back is fixed. Electronic Brake Force Distribution (EBD) provides an automatically adjusting bias depending on conditions to ensure the front and rear axles exert the most effective braking possible without wheel lock. EBD systems work in conjunction with ABS and use an electronically controlled valve which diverts hydraulic pressure to the front and rear axles in real time according to the conditions and scenario.