Brake Pedal Vibration in EVs: A Deep Dive into Noise and Vibration Solutions
In the rapidly evolving world of electric vehicles, where silence and smoothness are paramount to the driving experience, even the smallest mechanical imperfection can become a major point of concern. As automakers push the boundaries of electrification, autonomy, and digital integration, one area that continues to challenge engineers is the subtle yet impactful realm of brake-related noise, vibration, and harshness (NVH). For drivers of pure electric vehicles (EVs), the absence of engine noise at low speeds means that previously masked mechanical sounds—such as faint clicks, pulses, or pedal vibrations—can suddenly become prominent, leading to discomfort and dissatisfaction. This growing sensitivity has placed immense pressure on automotive NVH teams to refine every aspect of the braking system, particularly in the context of advanced electronic hydraulic brake (EHB) systems.
Among the latest contributions to this critical field, a comprehensive study by Jun Zhang from Geely Automotive Research Institute (Ningbo) Co., Ltd. offers a revealing look into the root causes and practical solutions for an increasingly common issue: brake pedal vibration and abnormal noise in low-speed braking scenarios. Published in the April 2024 issue of Noise and Vibration Control, the research provides a systematic analysis of a real-world NVH problem in a compact pure electric sedan, combining rigorous testing, theoretical insight, and engineering pragmatism to deliver a solution that balances performance, comfort, and cost.
The vehicle in question, equipped with a highly integrated “three-in-one” front electric drive system and a non-decoupled EHB setup, exhibited a noticeable shudder in the brake pedal accompanied by a distinct “clicking” sound during light to moderate braking at speeds below 30 km/h. Notably, this occurred without any vehicle lurching or instability, indicating that the issue was not related to wheel lock-up or traction control. Instead, the problem was purely sensory—felt through the pedal and heard inside the cabin—making it a classic NVH challenge that directly impacts perceived quality.
What makes this case particularly complex is the integration of regenerative braking. In modern EVs, the braking system is no longer a simple mechanical or hydraulic affair. It is a sophisticated coordination between electric motor torque for energy recovery and hydraulic pressure for friction braking. This cooperative regenerative brake system (CRBS) must seamlessly blend these two sources of deceleration while maintaining a consistent and natural brake pedal feel. However, as Zhang’s study reveals, the transition between these modes—especially when energy recovery diminishes at lower speeds—can introduce transient pressure fluctuations within the hydraulic circuit, leading to unwanted vibrations.
To diagnose the issue, Zhang and his team conducted a series of on-vehicle tests using a carefully orchestrated sensor array. Accelerometers were mounted on key components including the electronic stability controller (ESC) unit, hydraulic lines, brake master cylinder, pedal assembly, and body firewall. A microphone was placed near the driver’s left ear to capture cabin acoustics. The test protocol involved accelerating to 30 km/h, coasting, and then executing a rapid brake application, all while recording synchronized data.
The results were telling. The most intense vibration was detected at the ESC unit itself, with peak accelerations approaching 7 g—a significant level for a passenger vehicle. The hydraulic lines connected to the master cylinder also showed pronounced oscillations, peaking around 2 g, while other structural points registered much lower levels. This disparity immediately pointed to the ESC as the primary source of excitation rather than a secondary resonator.
Further analysis through time-frequency spectrograms revealed that the dominant frequency content at the ESC was centered around 2,000 Hz, a range typically associated with high-speed internal components such as solenoid valves or piston pumps. In contrast, the brake pedal’s vibration spectrum was lower in frequency and slightly delayed in timing, consistent with a transmission delay through fluid and mechanical linkages. This phase lag confirmed that the pedal was not the origin of the disturbance but rather a receiver of energy propagated through the system.
With the source tentatively identified, the next step was to investigate potential transmission paths. Could the vibration be entering the cabin through structural connections between the ESC and the chassis? To test this, the team performed a series of isolation experiments: removing mounting bolts between the ESC and the front longitudinal frame, disconnecting hydraulic line brackets from the body, adding rubber dampers between the brake booster and firewall, and even attaching mass blocks to alter local stiffness. None of these modifications had a noticeable effect on the pedal vibration or cabin noise.
However, a crucial breakthrough came when the team replaced the rigid metal brake lines connecting the master cylinder to the ESC with flexible rubber hoses. Under this configuration, the severity of the vibration and the audible clicking were significantly reduced. Additionally, variations in brake fluid filling procedures—particularly the level of vacuum in the system—also influenced the subjective perception of the issue. These findings strongly suggested that the problem was not structural but fluid-dynamic in nature: pressure pulsations within the hydraulic circuit were the root cause, and their transmission was highly sensitive to the compliance and damping characteristics of the fluid path.
This led to a deeper investigation into the internal dynamics of the hydraulic control unit (HCU) within the ESC. The HCU relies on a DC motor-driven eccentric piston pump and high-speed switching solenoid valves to generate and modulate brake pressure. Both components are known sources of flow-induced vibration. The piston pump, due to its reciprocating motion, naturally produces flow pulsations that scale with motor speed and eccentricity. Similarly, solenoid valves generate transient hydraulic forces during opening and closing, especially when there is a pressure differential across the valve. These forces, governed by nonlinear fluid dynamics, can excite resonant modes in the hydraulic lines and connected components.
While ideal solutions might involve redesigning the pump or valves—such as using multi-piston configurations, optimizing valve seat angles, or incorporating internal dampers—such changes are often impractical during late-stage vehicle development. They require extensive revalidation, supplier coordination, and carry significant cost and timeline implications. Instead, Zhang’s team focused on a more feasible engineering approach: control strategy optimization.
The logic was straightforward. If the excitation originates from pressure transients during the transition from regenerative to hydraulic braking, then modifying the timing and intensity of that transition could mitigate the issue. Specifically, the original CRBS strategy disengaged full energy recovery at around 15 km/h, at which point the hydraulic system had to rapidly compensate for the loss of motor braking. This sudden handover created a spike in hydraulic demand, triggering the pump and valves to operate in a high-transient mode, thereby amplifying pressure fluctuations.
The proposed solution was to shift the energy recovery disengagement point from 15 km/h to 30 km/h. By doing so, the vehicle relies more heavily on friction braking at higher speeds, where background noise from road and wind helps mask any system-induced sounds. More importantly, it eliminates the abrupt transition at very low speeds, where the cabin is quietest and human perception is most acute. Additionally, the team reduced the operating speed of the ESC motor from 1,800 rpm to 1,200 rpm, thereby lowering the frequency and amplitude of flow pulsations generated by the piston pump.
The results of this software-based calibration update were striking. Subjective evaluations by experienced test drivers confirmed that the brake pedal vibration and clicking noise were no longer perceptible. Objective measurements showed a clear reduction in vibration levels at the ESC and pedal, particularly in the frequency bands associated with hydraulic excitation. Crucially, braking performance and pedal feel remained within acceptable limits, and the impact on overall energy efficiency was minimal, given that regenerative braking at speeds below 15 km/h contributes relatively little to total energy recovery.
This case study underscores a broader trend in automotive engineering: as vehicles become more electrified and software-defined, the line between hardware and software performance blurs. What once might have been seen as a mechanical defect is now often resolved through intelligent control algorithms and calibration tuning. It also highlights the importance of system-level thinking in NVH development. Rather than treating components in isolation, engineers must consider the interactions between powertrain, braking, chassis, and controls, especially in transient operating conditions.
Moreover, the study emphasizes the value of early integration and testing. As Zhang notes, many of these issues could be mitigated during the design phase through hardware-in-the-loop (HIL) simulation, where control strategies can be validated against realistic models of hydraulic dynamics before physical prototypes are built. This proactive approach not only improves quality but also reduces the need for costly and time-consuming fixes late in the development cycle.
The implications of this research extend beyond a single vehicle model. As the global EV market continues to expand, consumer expectations for refinement and comfort are rising. Automakers that fail to address subtle NVH issues risk damaging brand reputation, even if the underlying functionality is flawless. In this context, studies like Zhang’s provide not just a technical fix but a strategic framework for managing the complex interplay between electrification, performance, and user experience.
It also reflects the growing sophistication of Chinese automotive engineering. Geely, as part of the Zeekr and Volvo ecosystem, has been investing heavily in core technologies, including advanced braking systems. The ability to diagnose and resolve such nuanced problems in-house demonstrates a maturation of technical capability that positions domestic manufacturers as serious competitors in the global EV arena.
Looking ahead, the challenges will only grow more complex. With the advent of brake-by-wire systems, where the brake pedal is fully decoupled from the hydraulic actuator, engineers gain greater control over pedal feel and system response. However, this also introduces new failure modes and sensory mismatches that must be carefully managed. Future research may explore the use of active vibration cancellation, predictive control algorithms, or even haptic feedback systems to further enhance braking comfort.
In conclusion, the work presented by Jun Zhang offers a compelling example of how meticulous testing, deep system understanding, and smart calibration can solve real-world NVH problems in modern electric vehicles. It serves as a reminder that in the pursuit of silent, smooth, and sustainable mobility, attention to detail is not just a luxury—it is a necessity.
Jun Zhang, Geely Automotive Research Institute (Ningbo) Co., Ltd., Noise and Vibration Control, DOI: 10.3969/j.issn.1006-1355.2024.02.046