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 Deceleration rate, D x Stopping time, t s Stopping distance, SD Energy (braking) Power (braking)

FBD of vehicle during braking θ c WfWf F bf R xf WrWr F br R xr x z haha DADA W/g D x Wcos θ Wsin θ W R hz R hx h h L dhdh b

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

Stopping time, t s

Stopping Distance

Stopping distance w/drag

Energy and power dissipated See examples on pages 57-58

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.

Brake Pedal / Master Cylinder Forces

Calculating brake force, Fb & brake gain G

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

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

Braking forces, coefficients, efficiency (handout) K1K1 P f, P r P a PrPr PfPf

Braking forces, coefficients, efficiency cont’d

(see web page)

Braking forces change due to weight shift

Conclusions Non-skid deceleration affected by weight shift If tires skid, use μ kinetic/sliding Skidding deceleration may be unstable