Clearly, what we require is a mechanism that effectively adjusts the brake bias as a function of the cars deceleration in order that the front and rear traction-fraction curves remain reasonably similar to each other. While not a function of acceleration, a brake proportioning valve is a device that regulates the rate-of-change of hydraulic fluid pressure to the rear callipers as a function of the rear hydraulic line pressure. Since the deceleration of the car is a function, ultimately, of the fluid pressure, the effect is similar. A brake proportioning valve is an adjustable mechanical device that is typically installed in the rear fluid line between the rear M/C and the rear calliper. It is usually installed within reach of the driver in order that it may be adjusted while driving. The graph below shows the effect of including a brake proportioning valve whose ”aknee pressure”a has been adjusted to 500psi. It should be noted that the inclusion of a brake proportioning valve has a significant effect upon the choice of M/C sizes. In this case a 3/4”a M/C is installed in-line with the front callipers, and a 5/8”a M/C in-line with the rear. The bias-bar is adjusted 55% rearwards.
As in the case of the previous graph, you can still only expect to achieve roughly 1.1g of deceleration (largely dictated by the tyres). However in the latter case it will take approximately 8kg more of pedal force to achieve this deceleration. The advantage, however, is in that both the front and rear axles contribute almost equally to the deceleration of the car across the full range of deceleration.
Take note that the maximum required pedal force will drop from roughly 50kg to 35kg in the event that the stock 4.25:1 ratio pedal is replaced with a 6:1 ratio pedal.
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