October 23, 2002

Dynamic Rear Proportioning

With almost every introduction of a new vehicle in the last couple years, the words "Dynamic Rear Proportioning" have been touted as one of the key features of the brake system. What is Dynamic rear proportioning, and why does it offer advantages over regular hydraulic brakes?

First, let's look at how a vehicle stops during normal braking. When the brake pedal is pressed, the pistons in the master cylinder push brake fluid out to the wheel calipers or wheel cylinders where they apply the brakes. With disc brakes on the front, and drum brakes on the rear, the front and rear need different brake pressures to stop the vehicle smoothly in a straight line. The difference in hydraulic pressure is accomplished by adding proportioning valves to the vehicle's brake system.

Vehicles with disc brakes all around also need proportioning valves to control rear brake pressure. On all types of brake systems, the brake force required at the wheels varies by the rate of deceleration, the weight transfer of the vehicle, the design of the suspension, and loads inside the vehicle.

In rear-wheel-drive vehicles, the brake lines from the master cylinder are typically routed so one part of the master cylinder feeds the front brakes, while the other half of the master cylinder feeds the rear brakes. A proportioning valve could be mounted as part of the master cylinder, but many vehicles incorporate the valve into a 'combination' valve that is also used to control minimum front brake pressure and the brake warning light.

Front wheel drive vehicles typically use a front/rear combination connection to the master cylinder. The left front and right rear wheels are fed from one part, and the right front and left rear are fed from the other part. This allows more balanced braking if one hydraulic circuit fails. The rear brakes on front wheel drive vehicles may only provide up to 10% of the total braking force, so this combination arrangement ensures there should be at least one front brake working in the event of a leak. Proportioning valves on this system may be at the master cylinder, but are often found in the rear brake lines.

Inside a proportioning valve, there is a spring and a valve. With zero or low pressure in the brake line, the valve is open, allowing brake fluid to flow easily through the valve. As the brake pedal is pressed further, line pressure increases and the proportioning valve closes, limiting the flow of fluid to the rear brakes. This prevents the rear brakes from locking up as the weight of the vehicle is transferred to the front and rear wheel traction decreases.

With passenger cars, the design of the vehicle makes predicting loads and weight transfer relatively predictable, so the spring tension in the proportioning valve could be set at the factory for normal operation. Trucks and vans were more difficult because of the variety of loads placed in these vehicles. Some of these vehicles used a height sensing or load sensing proportioning valve. These valves had a moveable arm connected to the rear axle and as the height and load of the vehicle changed, the arm placed more or less pressure on the proportioning valve spring. It worked fine until parts became damaged or corrosion seized the valve.

So now, vehicle manufacturers are switching to dynamic rear proportioning. This is an electronic system integrated into the vehicle's ABS system. Dynamic proportioning will vary the rear wheel brake pressure by reducing brake pressure only as much as necessary for each brake application. This provides optimum braking on the rear wheels and keeps the vehicle stable during braking.

When the brake pedal is pressed, full hydraulic pressure goes to the back wheels. The computer monitors deceleration rates for each wheel and operates solenoids to reduce pressure at the rear wheels before they reach the point of lock up. In reality, this is very much like ABS system operation, but the programming reacts differently and a wheel does not have to lock before activation occurs. Dynamic is the key word, with the brake pressure continually changing to stop the vehicle the best. Regardless of load, traction, or braking speed, Dynamic Rear Proportioning prevents the rear wheels from locking, and provides optimum braking; yet another advantage of today's electronic vehicle systems.

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).

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