As an automotive engineer specializing in noise, vibration, and harshness (NVH) and buzz, squeak, and rattle (BSR) issues, I have encountered numerous challenges related to abnormal sounds in vehicles. With the rapid adoption of electric SUVs, the absence of traditional engine noise has made even minor irregularities, such as braking noises, more noticeable to occupants. In this article, I will share my first-hand experience in diagnosing and resolving a specific low-speed braking noise issue in an electric SUV, utilizing a combination of NVH and BSR methodologies. The problem involved a persistent buzzing sound during initial acceleration after braking, which was traced to the brake caliper’s rectangular seal ring and its interaction with the piston seal groove. Through detailed analysis, including objective testing and structural modifications, we successfully eliminated the noise, highlighting the importance of integrated approaches in electric SUV design.
The proliferation of electric SUVs has heightened consumer expectations for cabin quietness, as the reduction in powertrain noise unmasked previously subdued sounds. BSR anomalies, characterized by transient and irregular noises like buzzing, squeaking, or rattling, often stem from vibrational excitations in components such as the braking system. Unlike NVH issues, which may correlate with engine speed or vehicle velocity, BSR problems in electric SUVs are frequently triggered by environmental factors like road surface variations, temperature fluctuations, or humidity changes. In this case, the electric SUV exhibited a low-frequency moan or buzz during starts after braking at low speeds, typically following several thousand kilometers of driving. This not only affected passenger comfort but also raised concerns about vehicle safety, necessitating a thorough investigation.
To begin, I will describe the braking system’s operation in electric SUVs, as understanding the fundamental mechanics is crucial for diagnosis. Most modern electric SUVs employ disc brakes, specifically floating caliper designs, which consist of a brake disc, caliper assembly, piston, rectangular seal ring, brake pads, and related components. During braking, hydraulic pressure from the master cylinder actuates the piston, forcing the brake pads against the disc to generate friction and deceleration. The rectangular seal ring plays a dual role: it seals the hydraulic fluid and facilitates piston retraction upon brake release, ensuring proper clearance between the pad and disc. This retraction mechanism relies on the seal’s elastic deformation, which can be quantified by the retraction distance ΔL. The relationship can be expressed as:
$$ \Delta L = \frac{F_e}{k} $$
where \( F_e \) is the elastic force exerted by the seal, and \( k \) is the stiffness coefficient of the seal material. Over time, wear and material degradation in electric SUVs can reduce this retraction, leading to residual drag and noise. The self-adjusting gap compensation is vital for maintaining performance, as it accounts for pad wear by allowing additional piston movement beyond the seal’s deformation limit during braking.
In the affected electric SUV, the noise manifested as a brief, low-frequency buzz (around 200 Hz) during the first few seconds of movement after braking, particularly in straight-line or turning maneuvers at low speeds. Subjectively, the sound was more pronounced in the rear seats, suggesting a rear brake issue. Initial evaluations pointed to a moan-type noise, often associated with temporary pad-disc contact due to incomplete retraction. This was peculiar because the problem only emerged after approximately 20,000 km of driving, indicating a component degradation issue rather than a design flaw in new vehicles.
To objectively analyze the noise, I employed NVH techniques, installing accelerometers on key brake components—such as the caliper, bracket, knuckle, and fender—and a microphone inside the cabin. Data acquisition focused on vibrational responses and acoustic emissions during test drives. The results revealed significant vibrations in the X and Z directions of the caliper, correlating with the noise occurrence. For instance, the vibration spectra showed peaks at 200 Hz during the 4–6 second interval post-brake release, aligning with the buzzing sound. This confirmed that the caliper was experiencing excessive movement due to friction between the pads and disc. A summary of the vibration data is presented in Table 1 below.
| Component | Direction | Vibration Amplitude (m/s²) | Frequency Range (Hz) |
|---|---|---|---|
| Caliper | X | 5.2 | 180-220 |
| Caliper | Z | 4.8 | 180-220 |
| Bracket | Y | 2.1 | 150-250 |
| Knuckle | X | 3.5 | 170-230 |
Further NVH analysis involved measuring the brake drag torque and disc thickness variation (DTV) to rule out common causes. The drag torque for both rear brakes was within 3–4 N·m, meeting design specifications, while DTV values were below 8 μm (left: 1.98 μm, right: 5.63 μm), indicating no significant disc wear. Additionally, structural simulations of the caliper bracket and knuckle showed adequate stiffness, and modifications like adding mass or reinforcement plates yielded no improvement. This led me to explore BSR methodologies, which emphasize subjective evaluation and root cause analysis of irregular noises.
Using BSR approaches, I conducted repeated road tests to replicate the issue, noting that the noise predominantly occurred after brake release and diminished with subsequent driving cycles. This pattern suggested that the piston was not fully retracting, causing temporary pad-disc contact. A fishbone diagram helped visualize potential causes, such as seal degradation, hydraulic issues, or mechanical wear. After eliminating factors like pedal return problems or vacuum booster malfunctions, I focused on the rectangular seal ring. In electric SUVs, these seals are typically made of ethylene-propylene-diene monomer (EPDM) or similar elastomers, which must maintain high elasticity and oil resistance. The force required for retraction can be modeled as:
$$ F_r = \mu P A + F_s $$
where \( F_r \) is the total retraction force, \( \mu \) is the friction coefficient between the seal and piston, \( P \) is the residual hydraulic pressure, \( A \) is the piston area, and \( F_s \) is the seal’s spring force. Over time, material fatigue in electric SUVs can reduce \( F_s \), decreasing ΔL and leading to insufficient retraction.
The seal groove geometry also plays a critical role. As shown in Figure 18 of the reference, the groove’s dimensions—such as diameter, length, base angle, and corner radii—affect the seal’s deformation and recovery. For instance, a larger base angle increases the retraction distance by providing more volume for seal expansion. The relationship between groove parameters and retraction can be expressed using the following empirical formula derived from CAE simulations:
$$ \Delta L = C \cdot \left( \frac{V_g}{W_s} \right) \cdot \tan(\theta) $$
where \( C \) is a material constant, \( V_g \) is the groove volume, \( W_s \) is the seal width, and \( \theta \) is the base angle. In the faulty electric SUV, the original groove design had a base angle of 15 degrees and a length that limited seal movement, contributing to performance decay after extended use.
To address this, I collaborated with suppliers to redesign the seal and groove. The optimized seal ring had a slightly larger cross-section, while the groove was widened with a base angle increased to 30 degrees. This enhanced the elastic recovery, as calculated by the retraction distance formula. Prototypes were installed on the electric SUV, and extensive testing over 20,000 km confirmed the elimination of the noise. Comparative vibration data before and after the modification is summarized in Table 2, demonstrating a significant reduction in caliper vibrations.
| Condition | Average Vibration Amplitude (m/s²) | Noise Occurrence |
|---|---|---|
| Before Modification | 5.2 | Frequent |
| After Modification | 1.8 | None |
This case underscores the importance of a holistic approach in electric SUV development. By integrating NVH’s quantitative measurements with BSR’s qualitative assessments, engineers can efficiently pinpoint and resolve elusive issues. For electric SUVs, where background noise is minimal, proactive design for components like seal rings is essential. Future work could involve predictive modeling using finite element analysis (FEA) to simulate seal behavior under various operating conditions, ensuring durability and performance. The formula for seal life estimation might include factors like stress cycles and temperature:
$$ L_s = \int_0^T \frac{1}{N(\sigma, T)} dt $$
where \( L_s \) is the seal lifespan, \( N \) is the number of cycles to failure as a function of stress \( \sigma \) and temperature \( T \), and \( T \) is time.
In conclusion, resolving low-speed braking noise in electric SUVs requires a deep understanding of both NVH and BSR principles. My experience with this electric SUV highlights how minor component wear can lead to significant acoustic issues, emphasizing the need for robust sealing systems. As the automotive industry shifts toward electrification, continuous improvement in these areas will enhance customer satisfaction and brand reputation. For engineers working on electric SUVs, adopting interdisciplinary methods and early validation testing can prevent similar problems, ensuring a quieter and more reliable driving experience.