TBR Technical Corner: Effect of Sub-Frame Boundary Conditions on Vehicle Judder Performance (Part 3 out of 3)


Source: Applus IDIADA

This TBR Technical Corner is the third in a series of articles on vehicle judder by: Juan Jesús García, PhD, Product Manager, Braking Systems in Applus IDIADA on this topic.

The first part of this series can be viewed by clicking HERE.
The second part of this series can be viewed by clicking HERE.

ADELANTO, Calif. –As explained in the first part of the article, this work is based on a comparative analysis of cold brake judder behavior on a SUV with two different chassis. Results obtained from V-Chassis 1 were already detailed in the second part of the article.

Vehicle performance with the Rigidly-Bolted V-Chassis 2

Conclusions reported in the previous article suggest that the option of stiffening the sub-frame bushings can effectively reduce the sub-frame movement shown in figure 6.

Figure 6: Test vehicle with configuration V-Chassis 1 – wheel rotation 2nd order @ 980 rpm  – Lateral view. Maximum vertical deflection of sub-frame: up movement (Left); maximum vertical deflection of sub-frame: down movement (right).

This option was materialised by installing four steel bushings. The objective of this validation exercise is to show the effect that the rigid bushings have on the judder performance of the test vehicle.

Figure 7: Rigid bushings (steel) used in the tests (left). Sketch of the installation of the rigid steel bushings on the sub-frame for the configuration V-Chassis 2 (right)

The braking test matrix used with the rigidly mounted sub-frame corresponds to the one already mentioned for the configuration V-Chassis 1 (see the first article part). The brake application selected for the vibration analysis of the test vehicle with the rigid sub-frame (V-Chassis 2) was equivalent to the one used for the judder analysis of the vehicle with the suspended sub-frame (V-Chassis 1).

Effect of Sub-Frame Bushing Stiffening on Vehicle Judder Performance

Figures 8 to 10 show the difference in the acceleration levels measured in the test vehicle with the suspended and the rigidly-mounted sub-frames. The three points considered to assess the effect are the seat rail, the brake pedal and the steering wheel. We observe that stiffening the sub-frame bushings (V-Chassis 1) reduces the acceleration levels in the X and Z direction on the seat rail during judder while the acceleration on the brake pedal does not change significantly. However, Figure 10 shows that the effect on the steering wheel vibration is quite important in the X and Y directions. This positive effect on the steering wheel has a big impact on the subjective perception of judder.

Figure 8: The effect of the sub-frame bushing stiffening on the seat rail vibration of the test vehicle during judder with DTV of 20µm.

Figure 9: The effect of the sub-frame bushing stiffening on the pedal vibration of the test vehicle during judder with DTV of 20µm.
Figure 10: The effect of the sub-frame bushing stiffening on the steering wheel vibration of the test vehicle during judder with DTV of 20µm.

Effect of Sub-Frame Bushing Stiffening on Sub-Frame to Body Vibration Transmission

In this section we show the main effect produced by the bushing stiffening on the transmission of acceleration from the sub-frame to the vehicle body during judder. Figure 11 shows this relationship for the front joint points of the suspended sub-frame to the vehicle body. We observe that under judder conditions, the suspended sub-frame produces a reduction in the acceleration of the body side of about 6dB. Figure 11 also shows the relationship between the acceleration level under judder conditions of the vehicle body and the sub-frame rigidly bolted. The result proves that the acceleration levels on the body side of the front joints of the sub-frame are very similar and slightly lower on the body side. We note that the acceleration along the Z direction is lower on the body side.

Figure 11: Relationship between the acceleration levels on the front joint points of the sub-frame and body side for the suspended and rigidly-bolted sub-frame configuration

Figure 12 shows the relationship between the acceleration levels at the rear joint points of the suspended sub-frame. We note that, in this case, the accelerations along the Y and Z directions are similar in level and rpm distribution, whereas the one along the X direction has a lower level on the body side. Figure 12 also shows the relationship between the acceleration of the rear joint points of the rigidly-bolted sub-frame. We deduce that, in this case, the acceleration levels on both the vehicle body and the sub-frame are very similar in the Y and Z directions. However, the acceleration on the body side along the X direction is considerably lower.

Figure 12: Relationship between the acceleration levels on rear joint points of the sub-frame and body side for the suspended and rigidly-bolted sub-frame configuration.

Effect of Sub-Frame Vibration on Steering Wheel Response

The running mode analysis reported in Figure 6 shows that, during judder, the sub-frame of V-Chassis 1 exhibits a vertical vibration at the front joint points combined with a rotation on the axis lying on the rear joint points (see Figure 13).

Since, for the test vehicle used in this work, the steering system is directly bolted to the sub-frame, the rigid body vibration of the latter has a direct effect on the vibration of the steering wheel in the x-y plane. This is clearly shown in Figure 14. This figure shows that the relative acceleration between the joint points of the sub-frame and the vehicle body is quite large for the suspended sub-frame (Z-direction) and negligible for the rigidly bolted option. Therefore, the restriction of the mobility of the sub-frame during judder loads reduces considerably the acceleration of the steering wheel in the X and Y directions.

Figure 13: Vertical oscillation of the front joint points of the suspended sub-frame and rotation about the axis shown exhibited by the sub-frame under judder excitation.
Figure 14: Relationship between the relative sub-frame rigid body vibration and the steering wheel overall vibration under judder conditions.

Interpretation of the Effect of Bushing Stiffening on Chassis Vibration

The vibration data acquired during judder conditions for the test vehicle with the sub-frame rigidly-bolted to the body has been analysed using 3D animations of the frequency data.

Figure 15 shows the geometry used in the 3D animations for the test vehicle with the two mentioned configurations of sub-frame attachment points. The animations show that the rigidly bolted sub-frame cancels its relative movement with respect to the vehicle body. This effect is desirable since it means that the overall vehicle stiffness is increased. Also, in our case, this reduction in the relative movement between the sub-frame and the body reduces the overall vibration of the steering wheel under judder conditions.

Figure 15: (Left) Configuration V-Chassis 1 (rubber bushings) – Wheel rotation 3rd order. The chassis exhibits a vertical deflection of the front points with up and down movement. (Right) Configuration V-Chassis 2 (rigid bushings). The chassis and vehicle body exhibit no relative movement.

Conclusions

An objective assessment of the relative movement and displacement of the sub-frame with respect the body under judder conditions reveals the mechanical integrity of the structural assembly defined by both systems and explains those directions in which the sub-frame is not adding stiffness to the body. The results suggest that these directions of high relative mobility can be the cause of high vibration levels in target cabin points, such as the steering wheel, for example. Relative vibration movement between the sub-frame and the vehicle body under brake judder loads determines the additional dynamic stiffness provided by the sub-frame to the vehicle body. This relative movement has an important effect on the vehicle judder performance and might be the direct cause of the vibration exhibited by in-cabin target points under judder, such as the steering wheel or the brake pedal. Also, the operational vibration of the steering wheel and the seat pedal, show good correlation with the maximum relative displacement exhibited by the sub-frame with respect to the vehicle body.

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