Customer Orientation: A Further Target in Brake System Design 2003-01-0599

The attention to the perceived quality, e.g. the quality as it is evaluated by the final customer, is becoming more and more important for the car makers. The paper describes how such aspects of braking system feel quality, joined with brake thermal constraints and interactions with other vehicle subsystems, such as suspensions, tyres and rims, can become manageable by an engineering process. In order to do that experienced designers define: longitudinal brake performance, stability limits in braking in a turn by comparison with referenced cars; brake thermal constraints; brake system constraints due to interactions with other vehicle subsystems. For perceived quality, on the other hand, it's necessary to take a different kind of approach in order to define targets. Some fundamental and different steps are necessary: the definition of measurable performance for the subjective aspects; the identification of the target values for such aspects coherent with product positioning; the deployment of such overall values generally fixed at “system” level into sub targets at subsystem level expressed by measurable and predictable quantities. A design procedure has been developed which allows one to define braking subsystem main parameters (longitudinal tyre characteristics; disc, piston and pad dimensions; coefficient of friction; brake balance; pedal ratio, vacuum-assisted brake booster, pump, lines and calliper characteristics) during brake system design, using at first simple models and then other more and more complex subsystems and vehicle models. The targets of the procedure are objective indexes calculated in the processing of normalised road tests data and correlated with subjective evaluations of normal and professional drivers. These indexes are the output of the simulation since the earliest design phase and become the “virtual subjective evaluation” of the vehicle model. This new approach at car advanced design represents an evolution of the traditional approach because it tries to merge customer orientation with general criteria defined by experienced designers. The approach has been used with success during specification for innovative systems, such as brake by wire. In this case, since the connection between brake pedal and tyres is not mechanical, it is possible to impose the brake pedal feel defining the response of pedal group (travel and effort feel) and the characteristics for corners actuators (deceleration feel). In general, the methodology that will be described has the aims of reducing time to market and guaranteeing a “customer oriented fingerprinting” in braking performance and feel.