Article by: Narcís Molina, Project Manager, Braking Systems in Applus IDIADA
Creep groan is a low frequency self-excited brake vibration, typically less than 200 Hz, whose generation process is the result of a stick-slip intermittent motion at the friction interface. Different essays have discussed the mechanisms leading to self-excitement of vibrations. It is believed that the stick-slip may occur when the static friction coefficient is markedly greater than the kinematic one. Several authors have presented a number of methods for reducing, or even preventing, stick-slip: typically, by increasing the damping of the brake assembly and the actuation system stiffness, but also by modifying the composition and the particle size of the friction material. Another comprehensive study had suggested a map —where various parameters such structure stiffness, normal force, sliding velocity and static/kinetic coefficients of friction intervene— to be used as a guideline in the reduction of stick-slip, thus agreeing with other essays that have revealed that the stick-slip phenomenon is the result of a combination of effects: the deformation of the actuator’s structure, as well as the friction material formulation.
While stick-slip events start the process, the brake by itself cannot generate enough noise, thus needing the vehicle structure or suspension to respond to the input. The path, therefore, is purely structural: the mechanical energy travels into the suspension and into the vehicle structure, the steering system and other components up to vehicle interior panels. Some works have approached the problem by tracing the flow of vibro-acoustic energy through the different paths as the creep groan occurs.
This article recognizes the importance of the stick-slip phenomenon, as an incipient mechanism that triggers creep groan. But most importantly, it proves the existence of a positive feedback loop, induced by a self-sustaining excitation as a result of a front axle chassis resonance, which increases exponentially the noise and vibration amplitude response.
Methodology
The vehicle under study is a medium-duty commercial vehicle equipped with an air brake system. The main front brake specifications are:
Firstly, a subjective test matrix is performed aimed at identifying the contribution of the following parameters on the low frequency vibration that shakes the whole vehicle:
- Hill Slope: 12%, 16% and 20%
- Vehicle Direction: Forwards and Backwards.
- Brake Lining Temperature: Cold (~30 ºC) and Hot (~120 ºC)
- Gear: Reverse and Neutral
The worst case scenario is found to happen with the vehicle rolling backwards on a 12% slope, in reverse and with cold brakes. Hereinafter, thus, all operational measurements are conducted under these test conditions.
The figure below depicts the main components of the brake actuation system, along with the terminology that is used throughout the article.
From the chassis dynamics point of view, creep groan is characterized by means of the acceleration of 95 points, whose measurement layout is selected in order to identify all possible transfer paths for a quarter vehicle: from the intrinsic source of the problem —the brake assembly, host of the triggering stick-slip motion— till the receiving points in the cabin that define the interface between the driver and the vehicle.
The following figure shows the local sensors layout within the brake drum, along with a picture of some of the accelerometers. These are glued with thermally-resistant adhesives on the backplates of both upper and lower shoes, the S-Cam and the anchor abutments. Two rows of five accelerometers are distributed over each shoe, thus detecting their bending and torsion movements. The figure also includes the brake actuation system, with accelerometers placed on the
The instrumentation, on the other hand, is listed in the table below:
Under operational creep groan conditions, this experimental set-up allows characterizing the shoes and brake actuator displacements and rotations, as well as the energy distribution within the brake, as the vibration evolves from a local triggering phenomenon to a global unstable shake. Vehicle modes and displacements are respectively explored by means of a Running Modal Analysis (hereinafter RMA —aka Operational Deflection Shape, ODS) and Real Time Animations (hereinafter RTA).
In order to understand the dynamic response of the truck front axle, under static conditions, on a flat surface and with no brake pressure applied, the stub axle is impacted with a hammer, so the FRFs between the mentioned excitation and the truck response are obtained. An Experimental Modal Analysis (EMA) of the whole vehicle corner is also performed to investigate the global vehicle modes and find the response in the frequency of interest.
Further studies are conducted with the isolated brake system —the assembly, decoupled from the rest of the vehicle, includes the actuation mechanism (from the brake chamber till the S-cam) and the friction couple (shoes and drum). A modal analysis of the assembly —with and without brake pressure— is obtained under free-free conditions (see the figure below; left), so to explore the local modes without mechanical restrictions imposed by the truck. Similarly, the local modes are also investigated after having fixed the assembly on a rigid bench (idem; right) —again, with and without brake pressure—, thus emulating the stub axle stiffness and the restriction imposed by the track rod and the drag link arms.
The following table summarizes all mentioned tests:
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