Proportioning Valve Buyer's Guide
A proportioning valve (also called a brake bias valve) controls how much brake pressure goes to the rear brakes compared to the front. Its job is to prevent the rear wheels from locking up before the front wheels, allowing you to maintain control even during hard braking. Brake bias describes the balance between the front and rear braking forces. By finely tuning brake bias using a proportioning valve, you can optimize your vehicle's braking. This guide walks you through everything you need to know about proportioning valves including why to use a proportion valve, how they work, and the theory behind them.
Why Use a Proportioning Valve?
When braking hard, you ALWAYS want your front brakes lock up before your rear brakes. If your front brakes lock up first, you skid but generally travel straight. If your rear brakes lock up first, your vehicle becomes uncontrollable and you start spinning. To avoid this, vehicles can use a proportioning valve to deliver less braking force at the rear wheels so they do not lock up first. This ensures that you remain in control of your vehicle even during hard braking. Modern vehicles use the anti-lock braking system (ABS), or another related braking system, to prevent rear wheel lock up but proportioning valves remain very common on older cars and in performance applications.
How Proportioning Valves Work
Under low braking forces, the proportioning valve does not interfere. The hydraulic pressure flows equally to the front and the rear brakes. However, under high braking forces, the proportioning valve begins partially restricting the pressure delivered to the rear wheels. The pressure at which the proportioning valve begins limiting pressure is called the "knee point" or the "split point" and is usually adjustable on most proportioning valves. The reduction in pressure transmitted, aka the proportioning ratio, is usually fixed and typically somewhere in the range of 40-60%.
Ideal Brake Curve
To demonstrate the advantage of proportioning valves, let's examine what engineers call the "Ideal Brake Curve". The ideal brake curve shows the maximum braking force from the front vs rear brakes based on ideal conditions. It's important to note that the curve's shape is not dependent on the brakes present on the vehicle. The curve's position and shape is determined solely by the vehicle's weight, weight distribution, center of gravity, and wheelbase (distance from front wheels to the rear wheels). The curved shape of the line comes from the idea that the nose of the vehicle will dive down during more aggressive braking, shifting more of the braking load onto the front wheels.
The diagonal lines on the diagram below represent the friction coefficient of tires on different surfaces (asphalt ~0.8, ice ~0.2, etc). By following the diagonal lines to the intersection of the ideal brake curve, you can read the front braking force from the x-axis and the rear braking force from the y-axis based on ideal conditions. Give it a try with the example curve above.
Now let's see how actual brakes would perform on this graph. With no proportioning valve, the ratio of front braking to rear braking remains constant so the actual brake curve is linear.
The point where the actual brake curve intersects with the ideal braking curve is called the Z-Critical Point. Past this point, the rear wheels will lock up. The goal of any brake system is to get as close to the ideal brake curve as possible to maximize braking force without passing the Z-Critical Point.
Since the curves are fundamentally two different shapes, it would be impossible to optimize braking without the use of a proportioning valve. We can adjust the strength of the front vs rear brakes, but we will always overshoot or undershoot different areas of the graph without a proportioning valve.
However, with a proportioning valve, you can split the actual brake curve into two distinct linear regions to better approximate the ideal brake curve. This allows you to optimize your braking force in a greater range of friction conditions.
Key Features and Specifications
The two key specifications for a proportioning valve are its activation pressure and its proportioning ratio. Adjustable proportioning valves allow you to adjust the activation pressure with a knob - this is the preferred option. As for the proportioning ratio, this is vehicle dependent and nearly impossible to know the "perfect" ratio. Choose a ratio and dial in your braking bias using the knob.
Conclusion
Proportioning valves are essential for safety and braking performance. They help ensure that the rear wheels do not lock up under hard braking. For most drivers, the factory proportioning valve is properly calibrated for stock vehicle configuration and requires no attention beyond ensuring it's functioning correctly. However, if you have modified your suspension or braking components, an adjustable proportioning valve will allow you to fine-tune brake bias to match your current setup. You can find all the parts you need and plan your build on RideBuilder.
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