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ABSTRACT Among the performance concerns in brake design, drag and fluid displacement are getting more attention in the requirement definition. High drag not only affects fuel efficiency and lining life, it is also a contributing factor to rotor thickness variation and brake pulsation. In this paper, a system approach to drag performance of a disc brake caliper is presented. A one-dimensional simulation model, which considers all the significant factors, including lining stiffness and hysteresis, housing stiffness, seal/groove characteristic, and stick-slide behavior between the seal and piston,46848
is developed to capture the interactive impact of each parameter to caliper drag performance. The
system model is validatedA with experimental measurements for caliper fluid displacement and piston retraction. A parameter study is then conducted to investigate the component interactive impact to the drag performance. Finally, a case study is presented to demonstrate the merit of the one-dimensional model as an effective analysis tool to guide the brake engineers in their brake design.
1.INTRODUCTION
A typical disc brake system is composed of a caliper assembly and a rotor. The caliper assembly consists of housing, piston, friction pads, and bracket(not shown). Within the piston bore of the housing is the seal groove. The rubber seal is an elastomer used to seal the brake fluid. During a brake apply, hydraulic fluid forces the piston and friction pads against the rotor surfaces. The clamping action of the friction pads creates torque to slow down and stop the vehicle. At brake release, the energy stored in the assembly generates a spring brake effect that retracts the piston. However, due to the energy loss such as friction, lining and rubber hysteresis, the piston is not able to retract to the original position. When the piston is not retracted sufficiently, it causes the lining to be pressed against the rotor and the residual pressure results in a drag torque in the brake that degrades vehicle fuel efficiency and reduces the lining life. A prolonged drag torque can exacerbate the development of disc thickness variation and possibly cause brake pulsation.
A comprehensive understanding of how the system interacts dynamically is essential to achieving a viable caliper design that possesses superior drag performance. A reliable analytical approach, which comprehends the interaction of the system components and the resulting caliper performance, presents an opportunity for brake engineers to develop a caliper with minimal prototyping efforts and product lead-time.
Recently, a study has been conducted on seal/groove modeling. Analytical studies using FEA to evaluate seal/groove performance and piston retraction behavior have also been reported. Those attempts successfully characterized the impact of seal/groove design to caliper performance. The time-consuming implicit FEA model, however, prohibits a thorough and quick parametric study for brake engineers to achieve a brake design with optimal drag performance. It is our intention, in the present paper, to focus on a system approach to the study of brake caliper drag performance, and establish an understanding of parametric impact to critical caliper design issues. Our objective is to develop a simple simulation tool that includes all the critical contributing parameters, such as housing stiffness, lining compliance and hysterisis, suspension and shoe slide force, as well as piston slide force and seal/groove characteristics, to predict caliper drag performance at the early stage of a program development.
In the following sections, the dynamics and drag performance in a disc brake system are briefly discussed. A simplified one-dimensional system simulation model is then presented to characterize the caliper drag performance. The simulation tool is validated with experimental results. It is also used for a parametric study which explores the trade-off among the contributing parameters, including housing stiffness, lining compliance, seal/groove design and piston sliding. A case study is then presented to demonstrate the model as an effective analysis tool in guiding caliper designs. Finally, improvements to the one-dimensional model and the potential of the model for more complicated brake problems are discussed.