tailieunhanh - BRAKING DYNAMIC PERFORMANCE

The study of braking on straight road is performed using mathematical models similar to those seen in Chapter 23 for longitudinal dynamics. But in this case, the presence of suspensions and the compliance of tires are neglected and the motion is described by the longitudinal equilibrium equation () alone m¨ = x ∀i Fxi . Apart from cases in which the vehicle is slowed by the braking effect of the engine, which can dissipate a non-negligible power (lower part of the graph of Fig. ), and by regenerative braking in electric and hybrid vehicles, braking is performed in all modern vehicles on all. | 24 BRAKING DYNAMIC PERFORMANCE The study of braking on straight road is performed using mathematical models similar to those seen in Chapter 23 for longitudinal dynamics. But in this case the presence of suspensions and the compliance of tires are neglected and the motion is described by the longitudinal equilibrium equation alone m. r 2 Fx. Vi Apart from cases in which the vehicle is slowed by the braking effect of the engine which can dissipate a non-negligible power lower part of the graph of Fig. and by regenerative braking in electric and hybrid vehicles braking is performed in all modern vehicles on all wheels. Subscript i thus extends to all wheels or when thinking in terms of axles as is usual for motion in symmetrical conditions on all axles. BRAKING IN IDEAL CONDITIONS Ideal braking can be defined as the condition in which all wheels brake with the same longitudinal force coefficient ax. The study of braking forces the vehicle can exert will follow the same scheme seen in Section the only obvious difference being that braking forces like the corresponding longitudinal force coefficients and the longitudinal slip are negative. Normal forces between road and tires can be computed using the equations seen in Chapter remembering here as well that the acceleration is negative. G. Genta L. Morello The Automotive Chassis Volume 2 System Design 231 Mechanical Engineering Series Springer Science Business Media . 2009 232 24. BRAKING DYNAMIC PERFORMANCE The total braking force Fx is thus Fx I1-. FZi where the sum extends to all the wheels. The longitudinal equation of motion of the vehicle is then dV Vi XiFZi 2pV2SCx - .1 vj FZi mg sin a dt m where m is the actual mass of the vehicle and not the equivalent mass and a is positive for uphill grades. The rotating parts of the vehicle are slowed directly by the brakes and hence do not enter into the evaluation of the forces exchanged between vehicle and road. These parts must be .

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