TBR Technical Corner: Effect of Subframe Boundary Conditions on Vehicle Judder Performance (Part 2 out of 3)

Source: Applus IDIADA

Second in a series of articles on vehicle judder by: Juan Jesús García, PhD, Product Manager, Braking Systems in Applus IDIADA on this topic.

Part one can be read by clicking this sentence.

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 layouts and design exhibiting unequal dynamic stiffness: V-Chassis 1 and V-Chassis 2 versions. V-Chassis 1 is assembled to the vehicle with four rubber bushings; V-Chassis 2 is the same subframe, but using four rigid (steel) bushings in the same locations (as shown in figure 1 above.

In this article we identify the main vibration transmission factors that determine the judder perception in the vehicle with the two chassis boundary conditions. It is important to emphasize that the selected brake application was based on a poor judder subjective perception.

Vehicle performance with V-Chassis 1

Figures 2 and 3 show the operational acceleration values of the steering wheel and brake pedal that the test vehicle with the V-chassis 1 configuration exhibits under severe judder conditions.

Figure 2: Braking event with severe judder conditions. Steering wheel
(colourmap and Overall & wheel rotation orders 1 to 5 vs wheel rpm). We observe high levels of response associated with the presence of multiple harmonics. The green circle denotes similar acceleration levels to those suggested by data base values.


Figure 2 shows the operational acceleration values of the steering wheel during a brake application with severe judder, for the X, Y and Z directions:

  • Steering wheel – X direction: we note that, the acceleration level of the test vehicle is higher by 2 dB with respect to a reference value for similar vehicles.
  • Steering wheel – Y direction: the acceleration of the steering wheel of the test vehicle in the Y direction is considerably higher over a wide range of rpm values (5 dB) with respect to the reference value.
  • Steering wheel – Z direction: the acceleration in the steering wheel shows similar levels to the reference value. The acceleration for the tests vehicle is dominated by the 1st and 2nd order.

Figure 3 shows the operational acceleration values of the brake pedal during the same brake application with severe judder:

  • Brake Pedal – X direction: the test vehicle shows almost the same level of variability and level of acceleration as the reference data (+1dB). The response is dominated by the 1st wheel rotation order.
  • Brake Pedal – Y direction: the reference data shows higher vibration level (2.5 dB) than the ones for the test vehicle. The acceleration is dominated by a combination of wheel rotation orders.
  • Brake Pedal – Z direction: very similar acceleration levels to database. The acceleration in the test vehicle is dominated by the 1st wheel rotation order

Figure 4 summarizes the acceleration levels and spectrum generated at various points on the subframe and the vehicle body for the vehicle test with configuration V-Chassis 1. The figure shows how the wheel rotation orders evolve as one goes from the wheel knuckle (point 4) to any point in the chassis structure (point 3, for example) and to points located on the body longitudinal members (point 1 and 6).

Figure 3: Braking event with severe judder conditions. Brake pedal (colourmap and Overall & wheel rotation orders 1 to 5 vs wheel rpm). The green circle denotes similar acceleration levels to those suggested by data base values

An interesting effect shown by the results is the big change in the structure of the wheel orders in the Z direction with respect to the X direction at points 1, 2 and 3. This suggests that the main excitation produced by the artificially generated DTV (20 , peak) that corresponds to the 1st wheel order, is transformed in a multiple-order vibration on the subframe (points 3 & 2) and on the body (points 1 & 6) at high frequencies. This suggests that the behaviour of the subframe under judder exhibits a non-linear behaviour at the attachment points which, for this configuration, are materialised by rubber bushings.

Figure 4: Holistic view of the operational acceleration of the test vehicle with the configuration V-Chassis 1 (rubber bushing) with severe judder. The figures indicate the bands dominated by multiple wheel rotating orders.

The vibration data acquired during judder conditions for the vehicle with V-Chassis 1 has been analysed using 3D animations of frequency data. The most relevant animation that shows the dynamics of the front suspension and the front subframe of the vehicle due to torque brake fluctuations is shown below.

Figure 5 shows the geometry used in the 3D animations for the test vehicle. This geometry includes the front suspension, the calliper, the front subframe with its rear reinforcements joining it to the body, and the corresponding parts of the vehicle body where the suspension and the subframe are attached.

Figure 5: Geometry for the running mode analysis animation: Test vehicle (SUV). Meaning of colors: (Blue), Suspension arms; (light green), subframe; (dark green), body; (red), knuckle.

Figure 6 shows the relative amplitude of the oscillating movement associated with the wheel rotation for the 2nd wheel order @ 980 rpm, of the various subframe and suspension parts affected by brake judder.

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

The conclusions inferred from this animation are that:

  • The front subframe exhibits a vertical movement (Z direction) rotating about the line between the rear bushings. This vertical movement causes a high variability in the vertical distance between the subframe-upper point (see figure 1) and the body and, therefore, will produce vertical forces onto the body
  • The vertical connecting arms of the subframe exhibit no deformation as the subframe oscillates. This is due to the effect of the front bushings
  • The lower control arms move in phase due to oscillation of subframe.

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