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Braking

Braking. Brake components Disk brakes (HSW) Drum brakes (HSW) Master cylinders (HSW) Braking performance (Eng. Char., SEP) Equilibrium forces Weight shift confounds analysis! Sequence of events during braking Brake pedal/cylinder forces Method of calculating . Braking Performance Measures.

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Braking

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  1. Braking • Brake components Disk brakes (HSW)Drum brakes (HSW)Master cylinders (HSW) • Braking performance (Eng. Char., SEP) • Equilibrium forces • Weight shift confounds analysis! • Sequence of events during braking • Brake pedal/cylinder forces • Method of calculating

  2. Braking Performance Measures Deceleration rate, Dx Stopping time, ts Stopping distance, SD Energy (braking) Power (braking)

  3. Rhx hh Rhz DA ha FBD of vehicle during braking x W/g Dx Wsinθ Wcosθ W h Rxf z Fbf θ Rxr b Fbr c Wf L dh Wr

  4. Axle Loads during Braking On level road, with no hitch or drag loads… To find the axle loads we need to know the deceleration … but we can’t calculate the deceleration without the braking force on the tires and the braking forces can’t be calculated until we know the axle loads! Huh? A different approach is required! ASSUME deceleration rate Dx

  5. Stopping time, ts

  6. Stopping Distance

  7. Stopping distance w/drag

  8. Energy and power dissipated See examples on pages 57-58

  9. What really happens during braking? • Driver pushes on brake pedal (and brake lever) • Brake lever pushes on push rod of master cylinder • Hydraulic fluid becomes pressurized • Hydraulic line pressure pushes on caliper piston • Caliper piston pushes on brake pads • Pads clamp rotor/drum causing friction forces & torque • Brake torque causes braking force at contact patch • Weight is shifted to front axle • Tires grab and slow vehicle, OR • One or more tires skid (too much braking, “brakes locked”), also slowing vehicle, not as much though.

  10. Brake Pedal / Master Cylinder Forces

  11. Calculating brake force, Fb & brake gain G

  12. Skidding= F(brake)>F(road friction)

  13. Rear wheel lockup 1. Front wheels turning and steering 2. Little braking force on rear wheels “unstable equilibrium” “spin out”

  14. Pf Pf, Pr Pr Pa K1 Braking forces, coefficients, efficiency (handout)

  15. Braking forces, coefficients, efficiency cont’d

  16. (see web page)

  17. Braking forces change due to weight shift

  18. Conclusions • Non-skid deceleration affected by weight shift • If tires skid, use μkinetic/sliding • Skidding deceleration may be unstable

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