Brake System and Subsystem Design Considerations for Race Track and High Energy Usage Based on Fade Limits 2008-01-0817

The friction material is arguably at the heart of any brake system, with its properties taking one of the most important roles in defining its performance characteristics. High performance applications, such as race track capable brake systems in high powered vehicles, exert considerable stress on the friction materials, in the form of very high heat flux loads, high clamp and brake torque loads, and high operating temperatures. It is important, for high performance applications, to select capable friction materials, and furthermore, it is important to understand fully what operating conditions the friction material will face in the considered application.

Vehicle dynamic effects during testing on the race track and the resultant effect on braking traction available at each wheel can significantly influence the distribution of braking energy within the brake system, often driving individual rotor temperatures significantly higher than most simplified (non vehicle dynamic-dependent) models would predict. Brake force distribution, front vs. rear brake fade behavior, tire traction, and chassis controls behavior can significantly affect the front to rear braking energy distribution. Similarly, differing rotor cooling behavior in effects such as vehicle slip angle and front wheel steer angle will affect the distribution of cooling ability in the vehicle. Within a given brake corner, deflection of the brake caliper, rotor, and pads under braking clamp and torque loads and the resultant changes in pad to rotor pressure distributions can drive a substantial temperature gradient over the surface of the brake rotor. A temperature or heat-flux related issue on any single brake corner can significantly reduce overall brake system performance, in the form of pedal travel increase, fade, thermal roughness, and/or rotor cracking. A successful race-track oriented brake system design accounts for all of these effects and avoids these issues on even the most heavily loaded (from an energy standpoint) brake corner.

This paper first covers a theory of brake fade behavior, which serves to set operating condition limits (a ‘fade envelope’) on the brake corners and therefore define the design space for the brake system. It will cover a proposed modeling approach involving brake system and brake corner models utilizing 1-D thermal models. In this approach, a 2-dimensional vehicle dynamics/brake system model is used to predict brake rotor bulk temperatures during race track usage. Next, selected braking events are focused on at the brake corner subsystem level to predict the temperature distribution in the friction interfaces, subject to the operating conditions and design parameters of the brake corner. The results of these analyses are then compared to the fade envelope of the friction material to insure that it is operating below its limits in the intended design. In future work, a full 3-dimensional, lumped parameter vehicle dynamics model will be used to give a more accurate prediction of brake rotor temperatures and temperatures distributions during race track operation.