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Brake System Performance at Higher Mileage
Abstract:
The purchase of a new automobile is unquestionably a significant investment for most customers, and with this recognition, comes a correspondingly significant expectation for quality and reliability. Amongst automotive systems -when it comes to considerations of reliability - the brakes (perhaps along with the tires) occupy a rarified position of being located in a harsh environment, subjected to continuous wear throughout their use, and are critical to the safe performance of the vehicle. Maintenance of the brake system is therefore a fact of life for most drivers - something that almost everyone must do, yet given the potentially considerable expense, it is something that of great benefit to minimize. Additionally, the performance of the brake system (like the tires) can change over the useful life of the components, realized in the form of changing friction levels, fluid consumption, and drag at a brake corner level, and realized to the driver in the form of changing pedal effort, travel, response time, and fuel economy.
Most studies of brake system performance, and most regulatory requirements that affect the design of the brake system, focus on the “near-new” condition. This is not accidental, the simple fact is that it is extremely difficult, expensive, and time consuming to realistically accelerate wear of brake components so that performance can be assessed in a worn condition. On a well-designed brake system, components in the hands of an average customer can last 5-10 years before wear out occurs, meaning that any practical study of wear effects must be greatly accelerated to occur within a typical vehicle development timeline. Environmental exposure involves many complex and time-dependent chemical reactions, which puts an upper limit on how much simulated field exposure can be accelerated.
The present study is based primarily on evaluation of brake corner performance after vehicle-level durability test exposure. Brake corners from a diverse selection of vehicles (including two hybrid vehicle examples) were retrieved from end-of test vehicles that had received the structural equivalent of 160,000 km of test exposure, along with 10 years of simulated corrosion exposure, and then subjected to performance and residual drag tests. To supplement the findings, lab-based studies of brake hardware with simulated 50% worn use and corrosion exposure are also referenced. Brake corner performance including apparent friction level, fluid consumption, drag, torque variation, and torque hysteresis were studied and related to observations of the physical condition of the parts. The effect of the measured brake corner level performance was then accounted for at a vehicle level in the form of pedal feel, fuel economy, and lining life for representative case studies.