As the automotive industry shifts towards sustainable mobility, electric vehicles have gained prominence due to their zero-emission capabilities and energy efficiency. However, with the widespread adoption of electric SUVs, comfort-related issues such as noise, vibration, and harshness (NVH) have become critical factors influencing consumer satisfaction. In this analysis, I address a significant vibration problem observed in a specific electric SUV during rapid acceleration, where pronounced shaking occurs between 50 and 70 km/h. This issue not only affects driving comfort but also highlights the unique challenges posed by electric powertrains, such as high torque output and reduced mass compared to traditional internal combustion engines. Through systematic testing and evaluation, I identify the root cause as excessive axial forces in the drive shafts, propose structural modifications, and validate cost-effective solutions to mitigate the problem. The focus is on leveraging engineering principles, including dynamic analysis and component optimization, to enhance the NVH performance of electric SUVs, ensuring a smoother and more reliable driving experience.
The vibration issue in this electric SUV manifests predominantly during hard acceleration, particularly when the vehicle speed reaches 50 to 70 km/h. Under these conditions, occupants experience a noticeable lateral shake, which is more severe in unloaded states compared to fully loaded scenarios. To quantify this problem, I conducted tests using specialized NVH measurement equipment, such as LMS Test.Lab 16A software, with accelerometers placed at key locations like the seat rails. The data acquisition involved capturing time-domain signals and converting them to frequency-domain representations through Fast Fourier Transform (FFT). The resulting colormap analysis revealed a distinct vibration pattern correlated with motor speed, indicating that the shake is primarily driven by a 0.36-order vibration component. This order corresponds to three times the fundamental drive shaft rotation order, suggesting an involvement of tri-pot joints in the drive shafts. Further investigation into the electric SUV’s powertrain configuration—a front-wheel-drive system with a high reduction ratio—confirmed that the vibration excitation originates from the axial forces generated in the drive shafts during high-torque events.
To delve deeper into the mechanism, I analyzed the relationship between the drive shaft geometry and the resulting forces. In electric SUVs, the drive shafts often use tri-pot constant velocity (CV) joints, specifically the GI-type, which are prone to generating significant axial派生 forces (GAF) due to their sliding friction characteristics. The axial force can be modeled as a function of the joint angle and applied torque, as described by the following equation:
$$F_{axial} = \mu \cdot T \cdot \sin(\theta) \cdot k$$
where \( F_{axial} \) is the axial派生 force, \( \mu \) is the coefficient of friction, \( T \) is the torque transmitted through the shaft, \( \theta \) is the drive shaft angle, and \( k \) is a geometric constant specific to the joint design. For the GI-type joint, the friction arises from point contacts between the tri-pot and the outer race, leading to higher axial forces as the angle increases. In this electric SUV, the drive shaft angles vary with load conditions; unloaded states exhibit larger angles, resulting in amplified axial forces and more pronounced shaking. The 0.36-order vibration is a third harmonic of the fundamental drive shaft order (calculated as motor order divided by gear ratio, i.e., \( \frac{1}{8.28} = 0.12 \)), which aligns with the tri-pot joint’s three-ball structure that introduces triple-frequency excitations. This correlation underscores the need to address the joint design to reduce vibrations in the electric SUV.
Next, I performed a transfer path analysis to identify how the vibrations propagate from the drive shafts to the vehicle body. The primary paths include the drive shaft connections to the wheels, the motor mounts, and the suspension components. By monitoring vibration levels at various points—such as the left and right drive shaft ends, motor mount attachments on the body, damper connections, and seat rails—I constructed a comprehensive model of energy transmission. The data indicated that all paths contributed relatively equally to the overall shake, with no single resonance peak dominating the response. This uniformity suggests that the issue is source-dominated rather than path-specific, reinforcing the focus on modifying the drive shaft joints. The table below summarizes the vibration levels (in m/s²) measured at key locations during acceleration for the unloaded electric SUV:
| Measurement Point | Vibration Level at 0.36 Order (m/s²) | Contribution Rating |
|---|---|---|
| Left Drive Shaft End | 0.25 | High |
| Right Drive Shaft End | 0.22 | High |
| Motor Mount (Body Side) | 0.18 | Medium |
| Damper (Body Side) | 0.15 | Medium |
| Seat Rail | 0.20 | High |
The data clearly shows that the drive shaft ends and seat rails experience the highest vibration levels, confirming the axial force as the primary excitation source. Given that adjusting the drive shaft angles was not feasible due to packaging constraints in the electric SUV, I prioritized optimizing the joint design to reduce the axial forces directly.
For the optimization phase, I proposed replacing the standard GI-type joints with AAR-type (Advanced Angular Roller) joints, which feature an additional bearing inner race that allows for rolling friction instead of sliding. This design change minimizes the axial force by distributing loads over a larger contact area and reducing frictional resistance. The axial force for an AAR-type joint can be approximated by:
$$F_{axial, AAR} = \mu_{roll} \cdot T \cdot \sin(\theta) \cdot k_{AAR}$$
where \( \mu_{roll} \) is the rolling friction coefficient, significantly lower than the sliding coefficient in GI joints, and \( k_{AAR} \) is a modified geometric factor. Comparative analysis between the two joint types reveals that the AAR joint maintains lower axial forces across a range of angles, as illustrated in the table below, which lists axial force values (in N) for typical operating conditions in the electric SUV:
| Joint Type | Axial Force at θ=5° (N) | Axial Force at θ=10° (N) | Axial Force at θ=15° (N) |
|---|---|---|---|
| GI-Type | 150 | 300 | 450 |
| AAR-Type | 50 | 100 | 150 |
After implementing the AAR-type joints on both drive shafts of the electric SUV, I conducted follow-up tests to evaluate the improvement. The results demonstrated a substantial reduction in the 0.36-order vibration, with seat rail vibrations decreasing by over 70% compared to the baseline. Subjectively, the shaking was no longer perceptible, confirming the effectiveness of the modification. The vibration levels before and after optimization are compared in the following table, based on average measurements during acceleration:
| Condition | Seat Rail Vibration at 0.36 Order (m/s²) | Improvement |
|---|---|---|
| Original (GI Joints) | 0.20 | — |
| Optimized (AAR Joints) | 0.06 | 70% |
However, replacing both drive shafts with AAR-type joints incurred higher costs, prompting an investigation into a more economical approach. I evaluated three scenarios: replacing both shafts with AAR joints, replacing only the short shaft, and replacing only the long shaft. Testing revealed that the short shaft, which experiences larger angles due to its geometry, contributed more significantly to the axial forces. Thus, optimizing only the short shaft with an AAR joint provided similar vibration reduction as replacing both, at a lower cost. The vibration data for these scenarios is summarized below:
| Scenario | Seat Rail Vibration at 0.36 Order (m/s²) | Cost Impact |
|---|---|---|
| Both Shafts AAR | 0.06 | High |
| Short Shaft AAR Only | 0.07 | Low |
| Long Shaft AAR Only | 0.15 | Medium |
This cost-effective solution not only resolves the shake issue in the electric SUV but also minimizes project expenses without compromising other performance aspects, such as durability or efficiency. Further validation through endurance tests confirmed that the optimized short shaft maintains reliability under various driving conditions.
In conclusion, the vibration problem in this electric SUV during hard acceleration stems from excessive axial forces in the drive shafts, exacerbated by the GI-type joint design. By switching to AAR-type joints, specifically on the short shaft, I achieved a significant reduction in vibrations, enhancing overall comfort. This approach demonstrates the importance of targeted component optimization in electric SUVs, where high torque and unique powertrain characteristics demand careful NVH management. Future work could explore advanced materials or control strategies to further refine performance, ensuring that electric SUVs meet the highest standards of driving pleasure and reliability.