1. Field of the Art
The present invention relates in general to a hydraulically-operated anti-skid brake system for a motor vehicle, and more particularly to improvements in consistency of braking effect of right and left wheel brake cylinders upon anti-skid brake application while right and left wheels are running on road surfaces of different conditions.
2. Related Art Statement
Such an anti-skid brake system uses, for example, an anti-skid device of a so-called closed-circuit type or variable-volume type as disclosed in Japanese Patent Application which was laid open under Publication No. 58-26659, or an anti-skid device of a so-called recirculating type as disclosed in Japanese Patent Application which was laid open under Publication No. 57-80956. In the closed-circuit type, a variable-volume chamber is provided in communication with a part of the fluid passage which communicates with a wheel brake cylinder, but is disconnected from a master cylinder of the brake system upon activation of the anti-skid device. The pressure in the above-indicated part of the fluid passage is regulated by changing the volume of the variable-volume chamber by an actuator. This actuator is operated by a pressure generated by a hydraulic pump, a vacuum pump or other pressure source provided independently of the master cylinder. In the recirculating type, the brake fluid is discharged from a wheel brake cylinder to lower the pressure in the brake cylinder upon an excessive rise in the braking pressure therein, and the discharged fluid is pressurized to a higher level of pressure by a pump. The thus reserved pressurized brake fluid is recirculated into the brake wheel cylinder when it becomes necessary to raise the braking pressure in that brake cylinder. In either one of the above two different types, solenoid-operated control valves are generally provided for the right and left wheel cylinders, and are controlled independently of each other by a main controller whose major part consists of a computer. For example, the main controller is adapted to receive signals from slip sensors which detect slippage of the right and left wheels of the vehicle on the road surfaces. Based on these signals, the main controller controls the solenoid-operated control valves so as to hold the slip ratios of the right and left wheels within an optimum range.
3. Problem solved by the invention
However, the hydraulic brake system equipped with an anti-skid device indicated above suffers from a problem of reduced straight-line running stability of the vehicle in the event that relatively high braking pressures are applied to the brake cylinders while the right and left wheels are running on road surfaces having different coefficients of friction. Described more specifically, the solenoid-operated control valve corresponding to the wheel running on the road surface with a comparatively low coefficient of friction starts an anti-skid control of the braking pressure in the corresponding brake cylinder, at an earlier timing than the other control valve, so as to hold the braking pressure in that brake cylinder at a comparatively low level. On the other hand, the other control valve corresponding to the other wheel running on the road surface with a higher coefficient of friction is left in its non-operated position, allowing the braking pressure in the corresponding brake cylinder to be raised at a higher rate. As a result, the right and left brake cylinders have different braking effects on the corresponding wheels of the vehicle, whereby the straight line stability of the vehicle is reduced.