Chris J. Talbot

The Measurement and Analysis of the Disc/Pad Interface Dynamic Centre of Pressure and its Influence on Brake Noise.

This paper discusses the measurement of the dynamic centre of pressure (CoP) of a brake pad during a normal braking event using a modified 12-piston opposed calliper. The modifications allow the centre of pressure to be controlled both radially and along the length of the pad, inducing a leading or trailing centre of pressure as desired. The technique is unique in its design and implementation. Both the centre of pressures of the in-board and out-board pads are recorded simultaneously with varying pressures and speeds. The results, which include pressure and force maps, show the position of the centre of pressure to vary considerably during a braking event, both radially and axially along the pad. The CoP offset is related to the calliper mounting geometry which is subsequently compared to the effective “spragging angle” and the generation of brake noise. It is seen that by inducing a leading offset noise may be generated and subsequently eliminated if a trailing centre is then induced. The results suggest that by careful selection of the backplate abutment friction level the CoP may be controlled to always fall within the “stable envelope” region and so resist noise generation.

Application of the pseudo-spectral method to 2D eigenvalue problems in elasticity

A pseudo-spectral approach to 2D vibrational problems arising in linear elasticity is considerede using differentiation matrices. The governing partial differential equations and associated boundary conditions on regular domains can be translated into matrix eigenvalue problems. Accurate results are obtained to the precision expected in spectral-type methods. However, we show that it is necessary to apply an additional “pole” condition to deal with ther=0 coordinate singularity arising in the case of a 2D disc.

Rotor asymmetry used to reduce disc brake noise

Asymmetry is applied to a heavy-duty commercial twin caliper disc brake rotor as a means to alleviate an undesirable high amplitude noise. The problematic frequency is 2400 Hz, the rotor blade exhibiting a 5- diametric mode order of vibration. The asymmetry is introduced by drilling sets of radial holes into the disc rim. Modal analysis is carried out over a range of frequencies using added masses applied magnetically to the rim of the rotor. This shows the amplitudes at set frequencies to reduce considerably when asymmetry is introduced. When a set of 5 masses is added to the rotor the vibration amplitude at the troublesome frequency is seen to be considerably reduced. Finite element analysis complements the experimental results. The analysis of a plain disc initially shows the two normal modes at very close frequencies but when asymmetry is introduced, by drilling holes in the rim of the disc, there is a noticeable frequency decoupling of the 2 normal modes. This is also accompanied by a distinct positioning of the antinodes with the antinodes of one mode positioning at the sets of drilled holes, the other mode antinodes being between the holes. When an asymmetric disc is fitted to the vehicle the noise is eradicated. Significant testing of the vehicle has since been undertaken without noise being generated

Thermal Brake Judder Investigations Using a High Speed Dynamometer

This paper is concerned with addressing the problems experienced with the thermo-elastic behaviour of the disc - that of optimum heat dissipation, and equally important, even heating of the disc blade. The primary objective is to develop a more temperature-stable brake disc. The work presented approaches the problems of thermal judder through benchmarking the current situation. This is approached by modelling the current brake and its validation by means of vehicle and laboratory testing. The empirical work is centred on a bespoke high speed brake dynamometer which incorporates the full vehicle suspension for an accurate yet controlled simulation of brake and vehicle operating conditions. The dynamometer is housed in a purpose built laboratory with both CCTV and direct visual access. It is capable of dynamic measurement of DTV, caliper pressure fluctuations, disc surface temperature and vibration measurements at discrete points about the rig. This information is presented and supported by thermal imaging of the brake during a heavy brake application and subsequent thermal judder. The results also include surface scanning of the disc which is carried out at appropriate stages during testing to identify disc deformation including disc warping, “ripple” and the effects of “hot spotting”. Disc run-out measurements via non-contacting displacement transducers show the disc taking up varying orders of deformation ranging from first to third order during high speed testing. The state of cold deformation of the disc is also shown to vary with the disc returning to first or second order deformation upon cooling. Thermal images of the brake disc have shown vane patterns to show through to the disc surface identifying uneven heat distribution.

Fourier analysis of holographic data from a noisy disc brake and its implication for modelling

Abstract Double pulsed laser holography is used to obtain images with fringe lines giving detailed information about the mode of vibration of a braking system generating high-frequency noise or squeal. If the images obtained are digitized, it is shown to be possible, using image processing techniques, to develop algorithms that determine the displacement field of the exposed part of the disc surface. Using least-squares mathematical approximations it is then feasible to generate Fourier series type representations of the out-of-plane motion of the vibrating disc. Holograms of the disc rim can also be analysed to show how the vibrating motion varies throughout the thickness of the disc (in-plane motion). The results demonstrate the existence of travelling waves as well as significantly large amplitude and quite complex in-plane vibrations. Implications for the construction of mathematical models of disc brake noise are discussed.

Animations of a Disc Brake Generating Noise

Previous work on generating animations from real disc brake systems generating noise (squeal) has been consolidated and developed. Using the method of double pulsed laser interferometry a series of holograms (typically ten per half cycle) can be recorded from the brake during a cycle of excitation. From these holograms a considerable amount of data can be obtained about the vibration of the disc and pad surfaces. Standard methods from image processing and algorithms developed to investigate hologram fringe lines can be used to generate three-dimensional representations of the surfaces. Furthermore although part of the disc surface and even more of the pad surface are obscured by the calliper, etc., it has been possible to form a reliable numerical reconstruction of the whole disc and pad surfaces partly by using standard mathematical approximation techniques and partly by intelligent extrapolation of the available data. From these an extension of previous animations to show the full disc and pad surfaces undergoing vibration can be obtained. Using a Fourier analysis of the numerical data indicates a possible way to construct reliable mathematical models of real disc brake systems.

The measurement of the absolute displacement of a noisy disc brake

Abstract The investigation of in-plane vibration of a noisy disc brake is problematic because it is difficult both to measure and to verify. Because of the disc structure and the inability to visualize disc in-plane vibration, there has been reluctance by researchers to accept the contribution of a displacement parallel to the surface of the object, or in-plane displacement, to noise generation. In addition to measuring absolute displacement, it has been difficult to isolate the in-plane and out-of-plane components of displacement using either non-contact or conventional displacement measurement techniques. This paper investigates absolute displacement of a brake disc during noise generation. Double-pulsed holographic interferometry is used to record a series of time-related images of the brake head from three different angles of observation. Because each image views the brake head from a different perspective, each of them records a different degree of in-plane and out-of-plane displacement. By careful analysis of the three images, it is possible to isolate the in-plane displacement from the out-of-plane displacement. The time-related series allows the displacement to be investigated over a full cycle of excitation and so create an animation of the mode of vibration. It is seen that the in-plane displacement is complex and that its amplitude may be about twice that of the out-of-plane displacement.

An Investigation of the Pad/Disc Dynamic Centre of Pressure using a 12 Piston Opposed Caliper

Pressure-sensitive film embedded into the body of a pad friction compound is a unique technique used to measure the dynamic center of pressure at the pad/disc interface during a normal braking operation. This paper uses of a modified 12 piston opposed caliper where the initial center of pressure may be varied both along the pad and radially. Results show a very definite movement of the center of pressure towards the center of the pad as the brake pressure is increased. In addition it is seen that a leading center of pressure (CoP) will result in noise whereas a trailing CoP gives a quiet brake. Equally a CoP towards the inner edge of the pad increases noise propensity whereas towards the outer edge a quiet brake. The results also show little influence on the CoP with disc speed.

Numerical Modelling of Three-Dimensional Thermal Surface Discharges

Abstract: The use of canal water for cooling is an important opportunity for companies to reduce their carbon emissions and save on energy bills. The available models used to evaluate the possibility of using canal water are too complicated and their applications are difficult. As such some potential opportunities for using canal water for cooling are being lost. This paper addresses current concerns to produce an interactive numerical model that will produce a three dimensional representation of the temperature distribution of heated water discharged into a still water region. The interactive model may be manipulated by non technical personnel to evaluate different discharge scenarios and ensure that the proposal does not infringe the stringent regulations imposed by the Environmental Agencies. The proposed model makes use of information gathered from real on-site testing using a thermal camera and grid measurements in addition to a laboratory experimental tank that duplicates the various on-site options. The parameters of temperature, velocity and boundaries were determined in both practical processes using a predefined grid mesh that covered the mixing zone. These practical results gave the turbulent diffusivity which was then used in the formulation of the model that was optimised to represent the real system results. It is shown that the results from the on-site testing, the laboratory results and the 3-D model all give complementary values so validating the final model.

Measurement of the dynamic centre of pressure of the disk/pad interface during a braking operation

This paper demonstrates the influence of the trailing end of the piston, or inboard, pad on the propensity of a brake to generate audible squeal. The unique technique was employed to measure the dynamic CoP at the pad/disk interface during a normal braking operation. This novel technique uses an embedded pressure sensitive film within the pad. The paper also presents the co-planar analysis of the pad/caliper contact regions and its influence on the position of the disk/pad CoP. The analysis shows how the trailing end of the piston pad influences the position of the CoP and goes on to explain why the centre moves as the pad abutment vibrates against the abutment. The paper includes additional work with a 12-piston opposed caliper where the initial CoP may be varied both along the pad and radially. Results show a very definite movement of the CoP as the brake pressure is increased.