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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.

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.

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.

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.