Minimizing brake drag of disc brakes in passenger cars is mainly driven by efforts aiming to improve the energy efficiency especially for BEVs. Upcoming restrictions (EU7) for brake particle emissions increase the necessity for low-drag-/ zero-drag-brakes further. Since research indicates that residual friction contacts can contribute significantly to the number of airborne particles emitted by disc brakes. Furthermore, grinding brakes during the off-brake phase come along with increased (taper-) wear and reduced heat dissipation. The common countermeasures aim to generate a larger airgap between pads and disc. Whereby “large” is meant relatively referring to space sizes comparable to the thickness of a human hair. Such small magnitude is necessary to guarantee a direct response of the brake, limit the brake fluid consumption and avoid excessive deposition of dirt or water. Last-mentioned is important in particular for BEVs as recuperation reduces brake applications immensely which usually help to keep the friction contacts clean. In this light the brake development process aims for a trade-off regarding the adjustment of the pad-to-disc airgap – large enough to avoid brake drag and small enough to ensure quick and stable brake torque generation under every driving conditions. State of the art for brake drag evaluation is based on component-level measurement. The corresponding drake dynamometers allow to run different speed profiles while applying vehicle related brake pressure and measuring brake drag during the off-brake phases. On this occasion the brake caliper is mounted to a fixture introducing the acting residual torque into a load cell, thereby an accuracy below 0,5 Nm can be achieved. With this tool brake drag is evaluated while varying certain parameters including speed, pressure, temperature and pre-conditioning-factors (e.g. excessive taper wear). Based on the results it is possible to minimize residual drag torque by different measures for example pad-retracting-springs or increased piston retraction. Usually this happens through iterative cycles of testing and adopting until the torque values undercut the aimed level. So, in an ideal case the brake’s drag is near zero throughout all the different tested cycles on the bench. But does this also apply to all uses cases the brake undergoes when installed into the vehicle? Several research works state that so called vehicle-integration factors like vehicle load or wheel forces can lead to significant misalignments between component-level- and vehicle-level-test results. Hence, some authors aim for an increased transferability of component-tests by adding certain influencing factors like road excitation to brake dynamometer setups. Nevertheless, a holistic approach measuring brake drag directly in vehicle to a wider extent has not been published yet. Therefore, this work takes up an in-vehicle measuring setup for brake drag (previously published by the author, chassis tech 2022, “Real-driving residual-drag-torque of disc brakes”) and presents the application of the system to a BEV along with the measuring results. The purpose is to accompany the vehicle’s corresponding brake setup and it’s drag behavior throughout the vehicle’s development process. Since brake drag behavior is sensitive and even differs between individual brakes of the same type, most of the tests are performed with one assembly during both component- and vehicle-level-tests. The measuring system uses piezoelectric shear-force sensors integrated between brake caliper and wheel carrier to measure residual drag torque. For implementation a few millimeters of the caliper housing are milled off, so that changes in brake drag due to the measurement itself are rather improbable. In addition, pressure sensors in the calipers and temperature sensors for the discs are installed to capture the brake’s conditions. Tests are performed in three main use-cases, on the brake dynamometer, in the vehicle under laboratory conditions (roller dynamometer & 3D-vehicle-dynamics test-bench) and in the vehicle on the road. Whereby last-named covers normal roads (city, rural, highway), racetracks and different proving grounds. To analyze the transferability certain identical measuring procedures are performed on each setup. Results point out an acceptable transferability from component-level to the vehicle on the roller dynamometer, while drag behavior in the vehicle on the road differs especially due to wheel forces. The acquired knowledge from road testing can help to adjust measuring procedures on the component-level and find effective countermeasures.
Mr. Philipp Huchtkoetter, PhD Student, Porsche AG in cooperation with University Stuttgart; Prof. Dr.-Ing. Andreas Wagner, Holder of the Chair of Automotive Engineering, Universtiy Stuttgart, Institute of Automotive Engineering (IFS); Dr.-Ing. Jens Neubeck, Head of Automotive Engineering 1, FKFS