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Stop it Already
Innovation has changed the automobile significantly over the last 25 years. From computer-controlled engine and transmission controls to stop/start technology, driving a brand-new vehicle, in many ways, is a different experience from driving a car built in 1991. While brakes remain the same to a large degree, mechanically, the braking system has benefited from innovation, too. It would have been difficult back then to imagine an automatic braking system that could pump the brakes safely many times more than humanly possible, especially on snow and ice. And, this technology is also used to improve traction in those conditions.
Let’s take a look at braking basics and the technology that has made brakes better than ever before.
For decades, all automobiles used drum-style brakes on all four wheels (Figure 1). A drum brake assembly consists of (1) a wheel cylinder with two pistons, (4) forward and rear brake shoes and (8) a cast iron drum. The top of the brake shoe backing plates fit into the outer stub on the wheel cylinder pistons.
When the brake pedal is pushed, pressurized brake fluid inside the wheel cylinder pushes the two pistons outward, forcing the brake shoes outward and against the rotating brake drum. This is the braking force that stops the vehicle. The major drawback with this design is heat buildup. As the drum became hot from continued use, it expands, requiring a greater force to push the shoes outward. This phenomenon is known as brake fade.
Disc brakes were initially used on race cars. Although simpler in design than drum brakes, disc brakes were more expensive, limiting the use on passenger cars (Figure 2). A disc brake assembly consists of (6) a cast iron rotor, (2) an inboard shoe, or pad, (3) an outboard shoe, or pad, and (not shown) a caliper that fits over both pads.
When the brake pedal is pushed, pressurized fluid inside the caliper pushes a piston against the inboard pad, forcing it against the rotor. In addition, this force slides the caliper in order to push the outboard pad against the rotor. In other words, the two pads are pinching the rotor, providing braking force to stop the vehicle. Performance vehicles, such as the SRT8®, use calipers with multiple pistons to provide greater braking force.
The advantage of the disc brake over the drum brake is the absence of brake fade. As the rotor becomes hot, it expands, but moves closer to the brake pads. This action is the opposite of what occurs with drum brake use.
FCA US LLC was one of the first car manufacturers to use disc brakes on production automobiles. Initially, this design was limited to the front brakes because the majority of braking force is required on the front wheels. Over time, disc brake use became widespread on all four wheels.
FCA US LLC’s entire product lineup uses front disc brakes. With few exceptions, the product lineup in 2016 consists of vehicles with four wheel disc brakes. Rear drum brakes are still standard equipment on the Jeep® Wrangler. Also, rear drum brakes were standard equipment on models such as the Dodge Neon and Chrysler PT Cruiser, with rear disc brakes being optional.
Parking, or emergency, brakes are standard equipment on all FCA US vehicles. Two different designs are used for rear disc brakes. The first design is the top hat design in which a shallow drum is incorporated onto the back side of the rotor and is fitted with smaller scale drum brake assembly. This assembly works in the same manner as a drum brake system (Figure 3), except the braking force is mechanical (parking brake cable) instead of hydraulic (brake fluid).
The second design is known as the Integral Parking Brake (IPB). The rear brake caliper combines a mechanical parking brake function with a conventional rear caliper. This caliper features a lever attached to a shaft on the outside of the caliper. The lever uses a ball-ramp mechanism to mechanically apply piston clamp force to the rear rotor when the parking brake lever is applied. As the lever and shaft rotate, the ball-ramp interface causes the shaft to lift and the piston to travel toward the rotor.
The IPB includes an automatic adjuster mechanism that keeps the mechanical park brake mechanism in contact with the piston as the pads wear. The auto-adjuster is linked to the piston through a friction clutch that is used to detect pad wear.
ANTI-LOCK BRAKING SYSTEM
The purpose of the Anti-lock Brake System (ABS) is to prevent wheel lockup under braking conditions on virtually any type of road surface. Anti-lock braking is desirable because stopping a vehicle without locking the wheels retains directional stability and some steering capability. This allows the driver to maintain greater control of the vehicle during braking on wet and icy roads.
ABS is installed on vehicles with four wheel disc brakes. While an option on some vehicles built 10 to 15 years ago — such as the Chrysler PT Cruiser, which had rear drum brakes as standard equipment — it is standard equipment across the board. The exception is the Jeep® Wrangler.
Here’s how ABS works. The Hydraulic Control Unit (HCU) provides individual pressure control to each front and rear brake. During normal braking, the HCU solenoid valves and pump are not activated. The master cylinder and power booster operate the same as a vehicle without an ABS brake system. During anti-lock braking, solenoid valve pressure modulation occurs in three stages: pressure increase, pressure hold and pressure decrease.
Electronic Variable Brake Proportioning (EVBP) is used to balance front-to-rear braking in place of a traditional rear proportioning valve. The EVBP system uses the ABS system to control the slip of the rear wheels in partial braking range. The braking force of the rear wheels is controlled electronically by using the inlet and outlet valves located in the integrated control unit (ICU). EVBP activation is nearly invisible to the customer since there is no pump motor noise and minimal brake pedal feedback.
The Brake Assist System (BAS) is designed to optimize the vehicle’s braking capability during emergency braking maneuvers. The system detects an emergency braking situation by sensing the rate and amount of brake application and then applies Optimum pressure to the brakes. This can help reduce braking distances. The BAS complements the anti-lock brake system (ABS). Applying the brakes very quickly results in the best BAS assistance.