Page 384 - Automotive Engineering Powertrain Chassis System and Vehicle Body
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Braking systems      C HAPTER 12.1



















                                 2


                             3       1
                                                     Disc

           Figure 12.1-23 Finite element model of 10 segment of vented disc and hub.

           in Figure 12.1-23. Note that in order to accurately  and the vehicle is subsequently left to stand at the end of
           simulate the heat loss from the rotor, it is sometimes  the descent; (ii) a repeated high speed autobahn stop
           necessary to include the wheel and other components in  with the rotor allowed to cool only moderately between
           the model.                                         stops. The former test determines the ability of the
             The heat input to the system is estimated from the-  design to limit temperature build-up in the rotor by heat
           oretical consideration and applied over the rubbing sur-  transfer to the atmosphere whilst the high-speed re-
           face. The heat loss to the surrounding is specified by  peated stop examines the ability of the rotor material to
           convective and sometimes radiative heat transfer condi-  withstand repeated thermal cycling and the ability of the
           tions along relevant boundaries of the model. The tem-  friction pair to resist ‘fade’ under these severe conditions.
           peratures predicted by a thermal analysis can be used as  Friction performance cannot easily be predicted by
           input conditions to a structural analysis in order to pre-  the FE approach and there remains a requirement for
           dict thermal deformations and stresses. If the pad is in-  dynamometer testing to determine the fade-and-wear
           cluded in the model, the contact pressure distribution  characteristics of every new friction pair. The dyna-
           (and hence the distribution of heat input) can be esti-  mometer can either be a full-scale device or a small
           mated leading to the possibility of a fully coupled  sample rig in which the geometry and loading conditions
           thermal-structural analysis (Brooks et al., 1994).  are scaled to give an accurate representation of the actual
             In addition to details of geometry and material prop-  brake. These tests will not only give data on friction
           erties, accurate date on heat loss to other components and  performance over a wide range of conditions but can also
           to the atmosphere are vital to allow accurate predictions  be used to determine the MOT of the pad and rotor
           of rotor temperatures using FE methods. Such data can be  materials by progressively increasing the temperature at
           generated by conducting the so-called ‘cooling tests’ on  the rubbing interface until some form of failure occurs
           actual vehicles fitted with representative brake rotors  (Grieve et al., 1996).
           carrying rubbing or embedded thermocouples. The rotor
           surface is first heated to a predetermined temperature by
           dragging the brakes and then allowed to cool whilst the  12.1.6 Advanced topics
           vehicle is driven at constant velocity. By comparing the
           experimental rate of cooling with that predicted by  12.1.6.1 Driver behaviour
           the FE simulation for different boundary conditions,
           optimized heat transfer coefficients can be derived which  The driver of a vehicle plays a key role during any braking
           are then assumed to apply for different rotor materials  event since his/her reactions to external stimuli have
           and factored for the varying air stream velocity under  a direct bearing on his/her ability to maintain complete
           different test conditions.                         control over the vehicle trajectory and deceleration rate.
             Two very different vehicle brake tests are often sim-  A knowledge of how the driver interacts with these ex-
           ulated to critically examine the maximum temperatures  ternal stimuli and the way in which the vehicle responds
           and integrity of new rotor materials or designs: (i) a long  to the control signals generated by the driver is vital to
           slow Alpine descent during which the brakes are dragged  the future development of safe road transport systems.


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