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Brake judder is one of the most serious problems in automotive-brake systems. It is basically a forced vibration caused by the friction-surface geometry of a brake disc, and therefore, disc rotors play a significant role in judder. There are two types of judder: cold and hot. Hot judder is caused by the thermo-mechanical deformation of a brake disc due to high-speed braking. There are several shapes of deformation, e.g., coning and circumferential waviness. Circumferential waviness is caused by thermo-mechanical buckling and typically found as a butterfly shape in a 2nd rotational-order and hot-spotting. In a previous paper, two groups of disc castings with different material homogeneity were machined intentionally to have two kinds of dimensional variations.

Brake judder is one of the most serious problems in automotive-brake systems, and brake discs play a significant role in judder. There are two types of brake judder: cold and hot. Hot judder is caused by the thermo-mechanical deformation of a disc rotor due to high-speed braking. There are several causes and shapes of the deformation, e.g., coning and circumferential waviness. Circumferential waviness of brake discs is typically found as a butterfly shape in a 2nd rotational-order and corrugation (or hot-spotting) around a 10th order, which are caused by thermo-mechanical buckling. The author focused on the effects of material and dimensional homogeneity on the transient and permanent wave-like deformation of ventilated discs in low rotational-orders during repetitive high-speed braking. The tested discs were in two groups that had the same design and gray-cast-iron class but were cast in two foundries by using horizontal- and vertical-molding machines, respectively.

There are various processes for finishing the friction surfaces of a brake disc, which affect the braking effectiveness of a vehicle in the early stages of use in some cases. To examine the interaction between the disc surface texture, rotational direction, and friction material, a series of experiments on a tribotester using small-scale specimens was conducted. In a previous paper (2013-01-2056), the results from the first series of experiments, which involved of thirty disc surface textures and a less aggressive non-asbestos organic (NAO) friction material in on-brake-drag conditions combining constant speed and normal-load, was reported. Disc surfaces were finished by the following finishing processes in two rotational directions: turning under four cutting conditions, roller burnishing after turning, turning with a wiper insert, and grinding with two stones. Contact-pressure dependency of friction and wear was confirmed.

The surface texture of a brake disc in some cases affects the braking effectiveness of a vehicle in the early stages of use. Brake discs usually turn in one rotational direction during their finishing process but are turn in two directions on a vehicle. This causes a difference in friction or wear between two wheels. Directional surface textures of brake discs finished by turning or roller burnishing may cause this interaction to become more severe than those finished by grinding. Full-scale tests using actual friction pairs are effective for estimating the total braking performance of a full vehicle or its corners. However, they are exposed to various factors and different brake-disc locations creating different friction and wear histories. The author, therefore, concluded that fundamental experiments using small-scale specimens are necessary to examine the details of the interaction between the disc surface texture, rotational direction, and friction material.

This paper focuses on the interaction in friction behavior between the surface texture of brake discs, rotational directions in braking operations and metal-cutting process, and friction materials with different degrees of aggressiveness. A factorial experiment for front brake discs was conducted by combining eight discs with directional surface finishes, two rotational directions, and two NAO friction materials on a brake dynamometer. The author analyzed several test results, such as the friction coefficients, disc wear, roughness, and the correlation between them. An assumed mechanism describing the friction behavior is discussed using the experimental results and by introducing the contribution of the aggressiveness and adhesiveness to the friction and confirmed by the test results.

Reducing vehicle weight for promoting a sustainable global environment is one of the most significant challenges in the automotive industry. It is difficult to replace cast iron with lighter brake-disc material for ordinary vehicles. Material homogeneity also affects the thermal strength of brake discs. In our previous study, we established an integrated system for developing and manufacturing homogeneous brake discs to reduce judder. With our system, we maintained the thermal strength of a lightweight brake disc by improving its material homogeneity. As a result, we can optimize the brake disc design for reducing a disc's weight and contribute toward sustaining our global future.

Brake squeal is a critical issue for automotive brake systems and its propensity significantly depends on the natural frequencies of brake discs. The variation in natural frequencies is caused by various factors in the disc manufacturing process, from foundry through machining. To reduce this variation, we analyzed the factorial effects of material and dimensional properties on natural frequencies of brake discs with various configurations by conducting intensive computer-aided engineering experiments. These experiments were performed accurately and quickly with the help of our original brake disc design system. As a result, we determined the critical factors affecting the natural frequencies of brake discs and their contribution.

We studied the relationship between disc thickness variation (DTV) and casting material properties and clarified that the circumferential homogeneity of brake discs has significant effects on brake judder. As a solution, we established an integrated system for developing and manufacturing homogeneous brake discs to reduce judder by making the most of our integrated business process from R&D through manufacturing, including our in-house foundries. The system we developed consists of the following elements: foundry engineering to obtain homogeneous discs, an original flake graphite structure index (K-FGI) to measure the homogeneity of our products, and accelerated wear tests on a brake dynamometer to estimate the potential on-brake DTV growth.

The necessity of a high-precision disc to reduce brake judder was reconfirmed from the three viewpoints: a literature review, experiments, and bench marking. Geometric accuracy is the critical issue that brake disc suppliers must address to improve the NVH performance of automobiles. We have achieved disc lateral runout of less than 20 micrometers after rough and semi-finish turning operations by increasing the rigidity of lathe components and tool holders, improving work-holding devices, selecting appropriate cutting insert material and geometries, and optimizing cutting conditions. With an additional grinding process, by selecting the most suitable abrasive wheels, optimizing grinding conditions, and keeping the alignment of the work-piece and grinding-wheel spindles accurate through productive maintenance based on the in-line measurement data, we can correct the geometric dimensions of the disc friction plates to obtain higher accuracy.

The friction surface of a brake disc is generally finished through several machining processes such as grinding and turning. They have gains and losses in braking effectiveness, cost, etc. In some cases, effectiveness is not stable under a certain condition during running-in. This phenomenon is becoming more apparent as non-asbestos organic lining materials have come to be used more widely. In Part I of this study, brake discs finished through two processes were burnish-tested together with five lining materials that had various levels of aggressiveness. These tests show that a more aggressive lining material needed fewer burnish stops but, as expected, caused more wear. The behavior of friction and wear was found to depend both on lining materials and finishing processes. In Part II, many discs finished through various processes were tested together with one of the lining materials that showed the largest difference between the finishing processes described in Part I.

A disc rotor is one of the friction pair in a brake system, but its behavior as friction material is seldom studied. The authors had found that some used discs in the field with large DTV wore locally at the same angular position on both the in-board and out-board friction surfaces. This phenomenon suggests that these discs have non-uniformity in the casting material properties such as the graphite structure and/or matrix. To identify the relationship between local wear and casting material properties, a design of experiment (DOE) is performed. Surrogate discs with non-uniform material properties are intentionally cast and tested on a chip-on-disc type friction tester. Local wear is reproduced and it is found that circumferential non-uniformity in the casting material has some correlation with disc local wear. The local wear depth also depends on the friction materials and gray cast iron grades.