Braking systems in vehicles have been recognized to engage and lock up during sudden application. This locking of the brakes may cause steering by the driver to be quite difficult and in extreme conditions may cause the driver to lose control of the vehicle.
Under hard and sudden braking, most automobiles tend to lock-up the rear brakes first (versus the front brakes). The more heavily the driver brakes, the greater the vehicle experiences a dynamic shifting of the weight of the vehicle from the rear axle to the front axle. This shifting typically causes a front/rear brake bias to be skewed toward the rear of the automobile, causes the rear wheels to lock-up, and causes the automobile tires to skid on the travelling surface.
Racing automobiles are even more prone to rear brake lock-up than standard automobiles due to the reverse torque and the higher revolutions per minute ("RPM's") of the engine. When a driver releases the throttle of the car, the reverse torque of the engine applies a major portion of the braking effort to the rear wheels. The rear braking effort of the braking system coupled with the reverse torque of the engine further causes the rear wheels to lock-up. Therefore, the driver typically must apply the brakes slowly to prevent this lock-up of the rear brakes.
Automobile manufacturers have approached solving this brake lock-up problem by using various systems and techniques. For example, proportioning valves have been installed on automobiles which allow the front and rear brakes to have proportionally the same brake fluid pressure up to what is referred to as the "knee" point. At this "knee" point pressure, the brake fluid pressure for the rear brakes increases at a fixed percentage as compared to the front brakes. This is conventionally accomplished with a valve consisting of a step piston and a spring. The step piston is restrained by the spring until it is overcome by the brake fluid pressure at the "knee" point. Then, the rear brake fluid pressure increase is reduced an amount equal to the difference in the area of the piston step. An example of such a system may be seen in U.S. Pat. No. 5,222,787 by Eddy et al. titled "Electro-Hydraulic Braking System." These proportioning valve systems, however, fail to be effective primarily because the initial brake fluid pressure locks the rear wheels, and such a system has no effect on the initial brake fluid pressure.
Also, computerized anti-locking braking systems ("ABS") have been developed which sense the rotation of the wheels of an automobile. In such systems, when the wheel rotation stops, the brakes are released. Examples of such systems may be seen in U.S. Pat. No. 5,192,120 by Reinartz et al. titled "Brake Pressure Control System With An Electrical Motor Operating A Pump And Control Valve"; U.S. Pat. No. 5,178,442 by Toda et al. titled "Brake Pressure Controlling Apparatus"; and U.S. Pat. No. 5,219,210 by Maehara titled "Brake Fluid Pressure Controller For Vehicle." The computerized ABS, however, have at least two major disadvantages: (1) such systems are very expensive and complicated; and (2) such systems only work when the car is travelling in a relatively straight line, e.g., a car cornering can be sliding the tires from excessive braking force, yet the wheels will still be rotating.
Further, restricting fluid flow from the rear brakes to prevent initial rear brake lock-up has also been attempted in the past. Problems arose in such systems, however, due to the small orifice size in the brake lines necessary to make these fluid restriction systems effective during application of the brakes by a driver of a vehicle with such a system. In this type of system, the brake fluid is forced through an orifice in the brake lines during application of the brakes. During release of the brakes, however, the fluid returning from the rear brakes takes an excessive amount of time to pass back through the orifice causing the brakes to drag and overheat.