John D. Fieldhouse

Double pulsed holography used to investigate noisy brakes

The vibrational characteristics of a noisy passenger car disc brake have been studied using the double pulsed holographic technique which has been developed to allow three orthogonal visual images of a vibrating brake system to be recorded simultaneously. These images show the disc to be vibrating in a bending mode whereas the pad is seen to be excited in a variety of modes such as bending, torsion, and often a combination of both. The development of the technique includes alternative ways of triggering the laser and typical results from the application of these differing methods are also included along with mechanical signals which confirm the visual interpretations. Final results, using a laser trigger delay technique, show that the disc mode waveform rotates about the disc at a rate equivalent to the frequency of vibration divided by the diametral mode order. Early work on a passenger car drum brake is also introduced, this complementing commercial ‘noise fix’ solutions and a proposed theoretical model.

A proposal to predict the noise frequency of a disc brake based on the friction pair interface geometry

The mode of vibration of a noisy disc brake is always diametral with a noise frequency marginally less than the free mode of vibration of the disc. Wheel speed does not affect the frequency but if brake pressure is altered then the noise frequency changes accordingly~an increasing pressure resulting in an increasing frequency over a specified range. Such observations have been made of a variety of different disc brake designs from single piston sliding fist type calipers to four piston opposed rigid calipers with it being possible to relate the noise frequency to the free mode of vibration of the disc in all cases. If the characteristics controlling this behavior can be identified then the same principles and criteria may be used to predict the noise propensity of any brake at the design stage. The paper proposes, and shows, that the preferred frequencies of excitation of any disc brake system may be related directly to the free mode frequency of the disc. In addition it is shown that the interface geometry of the pad, the effective rubbing diameter of the disc and the effective working length of the pad are all important features regarding the frequency generated. It is demonstrated that this relationship allows the probable instability frequencies of a disc brake to be predicted and shows that the frequency will vary over a computable range as a result of the brake pressure. The principle is successfully extended to consider a braking system arrangement employing two calipers working on one wheel. The proposal suggests that brake noise be as much a basic design problem as a material selection problem and answers questions such as why chamfers on pads changes the propensity of a noisy brake to generate noise. A case study of a heavy vehicle using a four opposed piston caliper on the rear and a pair of four opposed piston calipers on each front wheel goes some way to validate the proposal. The study and tests show that angular positioning of the front wheel caliper pair has a significant influence on the propensity of the brake to generate noise and the frequencies that were subsequently emitted

An optimization study of a multiple-row pin-vented brake disc to promote brake cooling using computational fluid dynamics

Abstract Brake disc cooling is an important area of research for high-performance brake disc manufacturers, users as well as academia. In high-demand braking applications, vented discs are increasingly being used as these are considered to have high heat-dissipating characteristics. The cooling efficiency of ventilated brakes depends on three key characteristics: the mass flowrate through the disc, i.e. the pumping efficiency of the rotor, the average heat transfer coefficient on the surface of the disc, and the wetted area of the rotor. Recent research has shown that the pin-vented discs have high heat transfer rates because of an increase in turbulence which results in a higher heat transfer coefficient. The pin-vented discs also have a higher resistance to thermal deformation owing to the more even distribution of material, resulting in lower thermal stress build-up within the rotor. The pin-vented discs in general have multiple rows of pins. In this paper an optimal configuration of various rows has been found for the maximum heat transfer rate. It has also been found that the ratio of wetted area of the disc to the frontal area of pins defines the heat transfer rate from the disc uniquely and can be used as a design parameter for the optimal design of a brake disc.

CFD Investigation of a Novel Fuel-Saving Device for Articulated Tractor-Trailer Combinations

Abstract Enhancing the aerodynamic efficiency of commercial vehicles is gaining increasing importance as it helps to reduce both overall fuel consumption as well as emissions. This paper investigates the advantages offered by a novel fuel-saving device for such vehicles. This device uses a moving surface to impart additional kinetic energy to flow near the roof surface. The additional momentum in the flow modifies the flow field, thereby significantly reducing pressure drag. Distribution of drag shows that the front and rear faces of the semi-trailer and the tractor cab are the dominant contributors to drag. The overall reduction in drag has been quantified by the distribution of pressure on the surface as well as the individual contribution to drag by each surface of the vehicle. The newly developed device has been shown to be very effective in reducing the fuel consumption of tractor-trailers.

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.

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

A study of brake noise and the influence of the centre of pressure at the disc/pad interface, the coefficient of friction and calliper mounting geometry

Abstract It is generally accepted that the coefficient of friction between the friction pair, the pad and the rotor plays a significant role in the propensity of a brake to generate noise. Because of the ease with which changes may be made to the pad compound material it is this that is more often the subject of change when there is a problematic noisy brake with both the rotor and the calliper tending to remain ‘isolated’ from examination. This paper investigates the propensity of a brake to generate noise over a range of temperatures and pressures under conditions that allow a mechanically induced offset centre of pressure between the pad and rotor to be varied. It is shown that for a high-volume production car brake there is an increased tendency for it to generate noise when a very specific leading offset centre of pressure is engineered. With this condition it is shown that a situation is promoted whereby the brake becomes mechanically unstable, with system changes such as brake pressure and temperature variations having little influence on the brake to alter its tendency to generate noise. Furthermore, it is shown that the critical offset centre of pressure may be related to the coefficient of friction between the friction pair and the mounting geometry of the calliper. Confirmation of the findings are supported by a repeat investigation of the vehicles alternative brake calliper. It is suggested that the source of a noisy brake may lie as much in basic mechanical design as inappropriate material choice. Further confirmation is demonstrated through the study of a commercial sliding fist-type brake, where the brake is significantly noisier when the centre of pressure and calliper mounting arrangement provide an appropriate spragging angle related to the friction coefficient.

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.

A computational fluid dynamic analysis on the effect of front row pin geometry on the aerothermodynamic properties of a pin-vented brake disc:

Abstract Increasing demand from the consumer for higher levels of refinement from their passenger vehicles has put considerable pressure on the automotive industry to produce ever quieter cars. In order to prevent the occurrence of many forms of brake noise, especially judder and drone, excessive heating of the brake disc must be avoided, while minimizing temperature variations across the rotor. In order for this to be achieved the brake rotor must be designed such that it ensures sufficient uniform heat dissipation and thermal capacity. In high-demand braking applications, vented discs consisting of two rubbing surfaces separated by straight radial vanes are normally employed, as they utilize a greater surface area to dissipate heat. Within this paper the effects of changing the geometry of the first row of pins on aerothermo-dynamic properties of a pin-vented brake rotor are investigated using computational fluid dynamics (CFD). The validated CFD model shows that decreasing the thickness of the first row of pins by 10 per cent improves the mass flowrate through the rotor by 14 per cent and the heat transfer rate by 6 per cent. The results obtained can be used for the design of brake discs which are efficient with respect to heat dissipation.

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.