Strain Gauge Application at Pad Backing Plates to Investigate Friction Force and NVH Issues

Over the last decades the importance of NVH in the vehicle and brake development process has been growing. Unwelcome and annoying vibration and noise phenomena like creep groan or moan can additionally be misinterpreted as functional brake failure. In order to avoid them, it is important to identify their characteristics and excitation mechanisms, e.g. friction induced vibration due to stick-slip. For this purpose, it can be useful to evaluate the dynamics of interface forces close to the tribological contact during a vibration occurs. Moreover, these forces can be applied to estimate a brake system coefficient of friction (CoF).
Experiments with an entire automobile front suspension corner have been performed. It was attached on a combined drum driven suspension and brake test rig. The vehicle setup contained a fixed caliper brake type within a double wishbone axle system including bushings, ball joints, rim and tyre. This article deals with a miniature strain gauge application on both brake pad backing plates which is suitable to measure the forces in the interfaces between backing plates and their caliper pins. Thereby, typical NVH related problems at low brake pressures can be investigated and the brake systems CoF can be estimated. Various operating points have been set in order to observe creep groan and moan vibrations.
The sensor equipment has confirmed its linear system behaviour during sensitivity assessment on a special calibration device. When implemented in the brake system, the reaction forces between each pad and monobloc caliper can be quantified in two spatial directions independently. The measurement system has high time and amplitude resolutions and therefore, it is able to display force variations during very short vibration cycles. Thus, the signals can reveal a typical stick-slip behaviour in the case of creep groan. When considering general brake parameters like predefined effective friction radius or measured brake pressure, an evaluation of the brake torque and a corresponding CoF is possible, whereby expectable values for both parameters are reached.
Due to the functional principle of the applied strain gauges it was necessary to weaken the body of the backing plates around their pin guiding holes. Therefore, the measurement system is limited to brake pressures below 10 bar and must not be used in vehicle road tests.
Experimental studies are an important and proven NVH task during brake development. Of course, simulation tools are getting more accurate, but novel sensing devices can definitely help to solve critical issues. Anyway, a better understanding of contact and friction mechanisms between brake components is still necessary. A method to measure the interface forces between pad backing plates and pins of fixed calipers has been developed and tested.