Deciphering Brake-by-Wire Technology: From Friction Brakes to EHB & EMB

• Published on: July 14, 2026
• Author: Johnny Liu, CEO at Dowway Vehicle
• Category: Automotive Engineering / Next-Gen Mobility / Software-Defined Chassis
• Estimated Reading Time: 12 minutes

About the Author

Johnny Liu is the Chief Executive Officer at Dowway Vehicle, a pioneering automotive technology firm specializing in next-generation intelligent chassis systems and drive-by-wire solutions. With over two decades of automotive powertrain and chassis development experience, Johnny leads teams building software-defined electric vehicle (SDV) systems.

Executive Summary

The automotive industry is shifting from traditional hydraulic systems to intelligent drive-by-wire solutions. Braking is transitioning from vacuum-assisted friction mechanisms to fully decoupled Brake-by-Wire (BBW) systems. This guide traces the development of braking systems, from drum and disc brakes, through active safety systems (ABS/ESP/AEB), to Electro-Hydraulic Braking (EHB) and Electro-Mechanical Braking (EMB).

1. Foundational Brake Architectures: Service and Parking Brakes

To understand how modern digital braking works, we must first look at the mechanical and hydraulic foundations of classic vehicle systems.

1.1. Service Brakes (Foot Brakes / 行车制动)

The service brake is the primary driver-operated stopping system. It uses two main physical designs:

A. Drum Brakes (鼓式制动)

• Definition: A drum brake is a friction-based braking setup that pushes brake shoes outward against the inside of a spinning drum to slow the wheel.
• How it works:
  1. When you press the brake pedal, the hydraulic or mechanical booster (助力器) transfers force to the internal brake shoes (制动蹄).
  2. This force pushes the brake shoes outward.
  3. The friction linings (摩擦衬片) press tight against the inner wall of the rotating brake drum (制动鼓内壁).
  4. This direct friction stops the drum from spinning, slowing the wheel down.
  5. Release Phase: When you lift your foot, a return spring (制动蹄复位弹簧) pulls the brake shoes back to their original position. The friction lining separates from the drum, and the braking force disappears.

B. Disc Brakes (盘式制动 / 碟刹)

• Definition: A disc brake slows a wheel by squeezing a spinning metal disc between two brake pads using hydraulic pressure.
• How it works: Disc brakes use a brake caliper (制动卡钳) to clamp a flat, spinning metal brake disc/rotor (制动盘). The friction from this clamp turns kinetic energy into heat, stopping the car quickly and shedding heat much better than drum brakes.

1.2. Parking Brakes (Handbrakes / 驻车制动)

The parking brake holds the vehicle in place when it is stopped. This technology has shifted from manual cables to electric motors.

A. Mechanical Handbrake (机械手刹)

• How it works: This system uses a drum-type parking brake structure (鼓式驻车制动器结构) to lock the rear wheels.
• When you pull the handbrake lever, it pulls a steel tension cable (拉线), which moves a pull rod (拉杆).
• A mechanical clearance regulator (间隙调节器) then pushes the friction plates (摩擦片) outward against the rear brake drum, holding the vehicle steady.

B. Electronic Parking Brake (EPB / 电子手刹)

• Definition: An EPB is a computer-controlled system that replaces the physical handbrake lever with a dashboard “P” button and uses electric motors to lock the wheels.
• EPB Designs:
  1. Cable-Pull Type (拉索式): Uses a standard steel cable but pulls it with an electric motor instead of a hand lever.
  2. Caliper-Integrated Type (卡钳式): Removes cables entirely. High-torque electric motors sit directly on the rear brake calipers.
• How it works:
  1. You press the “P” button on the dashboard.
  2. The Electronic Control Unit (ECU) receives the parking command.
  3. The ECU commands the motors on the rear calipers to spin.
  4. The motor runs through a reduction mechanism (减速机构) that multiplies torque, pushing the caliper piston forward to clamp the brake pads tightly onto the rotor.

2. Active Safety and ADAS Integration

With the rise of Advanced Driver Assistance Systems (ADAS), braking moved from passive control to active safety (主动防御).

2.1. Anti-lock Braking & Electronic Stability Programs (ABS/ESP / 防抱死与电子稳定系统)

To stop wheels from locking during hard stops and keep the car stable:

1. When you press the brake pedal, a 真空助力器 (vacuum booster) multiplies your physical force.
2. The booster sends a pressure signal to the ABS/ESP control system (控制系统).
3. The control system monitors wheel speeds and automatically adjusts pressure to individual wheel calipers, helping you retain steering control.

2.2. Autonomous Emergency Braking (AEB / 自动紧急制动系统)

AEB uses sensors like millimeter-wave radars (毫米波雷达) and cameras (摄像头) to monitor the road ahead and assess collision risk.

To keep stops safe and comfortable, the system uses a 3-stage graded intervention strategy (分级干预策略):

1. Stage 1: Warning (预警): The car sounds an alarm and flashes visual warnings if a hazard is detected ahead.
2. Stage 3: Partial Braking (部分制动): If you do not react, the car automatically applies moderate brakes to slow down and prepare the brake lines.
3. Stage 4: Full Braking (全力制动): If a crash is unavoidable, the system immediately applies 100% braking power to stop the vehicle or reduce impact speed.

3. Decoupling the Brake Pedal

The main shift in Brake-by-Wire (线控制动) is separating the physical brake pedal from the wheel actuators.

Electric Vehicles (EVs) have no engine vacuum to run traditional boosters. Brake-by-wire solves this by converting pedal force into a digital instruction.

3.1. Reading Driver Intent

In a decoupled setup, pressing the pedal does not physically push brake fluid. Instead:

• A pedal position sensor (踏板位置传感器) tracks how far and how fast you press the pedal.
• The system converts this movement into an electrical signal (电信号) and sends it to the central controller.

3.2. Creating the Right Pedal Feel

Drivers need to feel resistance to judge how hard they are braking. To prevent a lifeless, spongy pedal:

• A pedal feel simulator (模拟踏板发生器) is built into the pedal box.
• Using springs, rubber dampeners, and control algorithms (算法), the simulator pushes back against your foot to match the vehicle’s deceleration rate (踏板反馈感).

4. EHB vs. EMB: Two Paths to Digital Braking

Brake-by-wire technology splits into two designs: Electro-Hydraulic Braking (EHB) and Electro-Mechanical Braking (EMB).

4.1. Electro-Hydraulic Braking (EHB / 电子液压制动)

• Definition: EHB is a hybrid brake-by-wire system that keeps hydraulic fluid lines but replaces the old vacuum booster with an electric motor to generate stopping pressure.
• Design: It uses standard calipers, brake fluid, and lines, but replaces the physical booster link. Modern “One-Box” systems integrate the motor booster and stability control into one compact hydraulic block.
• Backup: If the electrical system fails entirely, a mechanical valve opens, creating a physical hydraulic backup link between your foot and the brakes.

4.2. Electro-Mechanical Braking (EMB / 电子机械制动)

• Definition: EMB is a fully “dry” braking system that completely removes hydraulic fluid, master cylinders, and lines, using individual electric motors on each caliper to press the pads.
• Design:
  • There is no hydraulic fluid.
  • Each wheel has its own high-torque motor mounted directly on the caliper.
  • These motors convert electricity into clamping force, turning a ball screw to press the brake pads against the rotor.
• Technical Outlook: EMB cuts response times to under 50ms, removes toxic fluids, cuts weight, and works with 48V redundant power setups in highly automated vehicles.

5. EHB vs. EMB Comparison Matrix

This comparison highlights the core differences between the two main Brake-by-Wire designs:

Feature/Metric	Electro-Hydraulic Braking (EHB)	Electro-Mechanical Braking (EMB)
Medium	Wet (Hydraulic Fluid)	Dry (Pure Electrical)
Actuator Mechanism	Central Motor + Hydraulic Lines	Caliper-Mounted Motor + Ball Screw
Response Time	~120 – 150 ms	< 50 ms
Chassis Weight	Moderate	Lightweight (saves up to 100kg on heavy vehicles)
Maintenance	Needs regular fluid changes	No fluid, zero fluid maintenance
Fail-Safe Mode	Hydraulic physical connection	Electrical circuit & battery backups
Cost Profile	Lower, widely used today	Higher, moving to early production

6. Author Perspective: Why Brake-by-Wire Matters for Automated Driving

At Dowway Vehicle, we view Brake-by-Wire and pure EMB not as minor updates, but as necessary building blocks for automated driving.

Old hydraulic lines have too many physical parts that can leak, plus a mechanical delay that slow down computer controls. A system that stops a car up to 9 meters shorter from 100 km/h (due to EMB’s fast <50ms response) can save lives. As we look ahead, pairing brake-by-wire with steer-by-wire will allow unified chassis control, making travel safer and more efficient.

7. Frequently Asked Questions (FAQs)

Q1: What is the main difference between EHB and EMB in braking?

EHB uses hydraulic fluid to push the brake calipers, while EMB is a fluid-free system that uses electric motors to clamp the brakes. EHB acts as a hybrid bridge that replaces the vacuum booster with an electric motor. EMB is a fully dry system that places independent electric motors directly on each wheel caliper, removing all brake lines and fluid.

Q2: What happens if a Brake-by-Wire system loses electrical power?

EHB systems open a backup hydraulic valve so you can stop manually, while EMB systems rely on redundant electrical wires and backup batteries. In EHB, your physical foot pressure still pushes fluid to the wheels if the power cuts out. EMB handles safety through backup power lines, dual-winding motors, and independent backup batteries.

Q3: How does a pedal simulator work in Brake-by-Wire?

A pedal simulator uses physical springs, rubber dampers, and digital feedback algorithms to mimic the natural feel of a hydraulic brake pedal. Because the pedal is physically disconnected from the brakes, the simulator pushes back against your foot. This ensures you still feel familiar resistance when stopping.

Q4: Why are manufacturers moving to Electronic Parking Brakes (EPB)?

EPB systems save interior space by replacing the mechanical hand lever with a button and using motors on the rear calipers to secure the car. An integrated electronic controller runs these small caliper-mounted motors through a reduction gear. This allows the car to release the brake automatically when you drive away and assists with safety features like hill starts.