21 ANTI-LOCK BRAKES ABS Fig. 14 6. ELECTRONIC BRAKE DISTRIBUTION (EBD) EBD substitutes load conscious reducing valve. Its function is not determined by the brake pressure or the weight transfer but by the amount of wheel slip. EBD function is an additional program to the conventional ABS program. It reduces the pressure at the rear depending upon the wheel slip and pressure reduction is carried out after cer tain pressure-maintaining phase. It maintains the brake pressure to both the rear wheels same. If any of the rear wheels is still over braked, ABS becomes operational. EBD warning lamp is turned on in case of error in the EBD system. EBD failure does not necessarily mean ABS failure. EBD (Electronic Brake-force Distribution) Control Necessity 1. During braking, whenever the rear axle wheels lock before the front axle wheels, the vehicle will loose its control & it may start to spin. This is because, braking is always associated with a certain weight transfer from rear axle onto the front axle wheels. Due to this drift of the vehicle weight, there will be lesser normal force acting on the rear axle when compared to the front axle. Due to lesser normal force, rear axle wheels can support lesser brake pressure. In a passenger vehicle, as a counter measure to this problem, the brake circuit for the rear axle will be fitted with LCRV (Load conscious regulating valve). This LCRV avoids the locking of rear axle wheels before the front axle wheels. 2. But LCRV is a mechanical vehicle component and it cannot result in ideal brake force distribution for the rear axle wheels. LCRV distributes rear brake force according to vehicle load to a certain degree. 3. The brake force required for optimum braking may be different for each wheel. This is depending on the road surface, the condition of the friction material of brake lining/ pad & also the condition of the tyres (old/ new). With LCRV, when the same brake force reaches both the rear axle wheels, ideal braking cannot be achieved by the rear axle wheels. 4. When the LCRV fails in any vehicle, the driver has no chance of knowing about this failure. Under this condition, when braked, the vehicle may deviate/ spin (depending on the brake force applied by the driver) from the intended driving direction. 5. In the Safari vehicle with ABS system, the ECU incorporates a special function by name EBD (Electronic Brake force Distribution). With this EBD, the conventional mechanical LCRV has Outlet Pressure Inlet Pressure (bar) Inlet Pressure EBD start point Vehicle equipped with LCRV Vehicle equipped with EBD system Outlet Pressure Laden Unladen Unladen Laden
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