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CHAP TER 1 2. 1       Braking systems




                                           Heat input





                               Disc  temperature             MOT of material






                                          Heat dissipation
                                           (a) Strategy I           (b) Strategy II     (c) Strategy III
               Figure 12.1-22 The ‘bucket-and-hole’ analogy.


               ‘bucket-and-hole’ analogy in which the rate of water  Strategy III
               flow into the bucket is taken to represent the heat flow  Even smaller diameter bucket but much deeper with
               into the disc and the height of the water level in the bucket  moderately sized hole (see Figure 12.1-22c). This
               represents the maximum temperature of the disc     implies a material with high MOTwhich can be allowed
               surface. A hole in the bucket represents the ability of the  to run much hotter than current designs and so lose
               disc to lose heat to the surroundings. The volume of the  significant amounts of heat by radiation as well as more
               bucket is therefore the heat capacity of the disc whilst the  moderate amounts by conduction/convection. Carbon–
               height is the MOTof the disc material. The question then  carbon composites are a possibility here but, as men-
               is how close does the level of water in the bucket get to  tioned above, these are currently too expensive for
               overflow!                                           mass produced vehicles. High-temperature steels with
                 Consideration of the ‘bucket-and-hole’ analogy and  good strength retention at temperatures well in excess of

               with reference to the typical material properties of Table  1000 C may also be candidate materials under this
               12.1-3, three distinct strategies for brake rotor materials  heading. Such discs could perhaps be made much thinner
               can be identified (Grieve et al., 1996):            and without vents, and therefore also save significant
                                                                  weight. However, there would be concerns over compati-
               Strategy I                                         ble friction materials and heat transfer to other compo-
               Large diameter and relatively deep bucket with small  nents in the underbody wheel arch area if discs were
               hole (see Figure 12.1-22a). This implies a high volu-  allowed to run much hotter than is currently the practice
               metric heat capacity to store heat during braking and  with cast iron.
               a relatively high MOT but only moderate conductivity of
               heat away from the rubbing surfaces. Current GI discs
               represent such a system but some steels may also meet  12.1.5.4 Disc materials/design
               these criteria.                                    evaluation

               Strategy II                                        Ultimately, any new brake material or design must be
               Smaller diameter and relatively shallow bucket but large  validated by experimental trials on actual vehicles to
               hole(seeFigure12.1-22b).Thisimpliessmallervolumetric  allow accurately for model-specific parameters such as
               heat capacity and a relatively low MOT. Hence, it is im-  the effect of body trim on rotor cooling. However, much
               portanttohavehigh conductivitytotransfer heat toother  can be learnt about potential new rotor materials or
               parts of the rotor and then the surroundings in order to  designs by numerical simulations of critical brake tests
               prevent temperature build-up at the rubbing surfaces.  using finite element (FE) analysis. Such techniques re-
               Aluminium MMC may meet these criteria but recent re-  quire the rotor and/or stator geometry to be broken
               search (Grieve et al., 1998) suggests that this can only be  down into a number of small non-overlapping regions
               successfully achieved for currently available MMCs if the  known as elements which are assumed to be connected
               brake rotor is redesigned to increase its thermal mass and  to one another at certain points known as nodes. A 2D
               cooling capability. Other materials that may be successful  axisymmetric FE idealization can be used as a first ap-
               with appropriate development include high reinforcement  proximation but, for more accurate simulation of the
               content MMCs and coated alloy discs but again there are  heat flow and stresses, a 3D model is desirable such as

               manufacturing, integrity and cost issues to be resolved.  the 10 segment model of a brake disc and hub shown

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