Vehicle Characteristics

Vehicle Characteristics - Vehicle Characteristics Tractive...

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Vehicle Characteristics Tractive Effort available force (lb) Resistance forces impeding vehicle motion (lb) Major Sources of Vehicle Resistance: -Aerodynamic resistance 85% due to turbulent flow of air around the vehicle body 12% due to air friction 3% due to air flow through vehicle components -Rolling resistance 90% due to tire and roadway interaction 4% due to weight and pavement type 6% due to slippage of tire on the roadway and air circulation around the tire and wheel -Grade resistance due to gravitational force acting on a vehicle R a R rLf F = ma F f Wsin g Wcos g g Fig. 2.1 W R rLr F r g Forces acting on a road vehicle R a = aerodynamic resistance (lb) R rLf = rolling resistance of the front tires (lb) R rLr = rolling resistance of the rear tires (lb) F f = available tractive effort of the front tires (lb) F r = available tractive effort of the rear tires (lb) W = total vehicle weight (lb) g = angle of the grade (degrees) F’= deceleration force or force required to accelerate (lb) m = vehicle mass (slugs) a = rate of acceleration or deceleration (ft/s 2 )
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+ F x = 0 F f + F r = ma + R a + R rLf + R rLr + R g EQ. 2.1 F = F f + F r => sum of available tractive effort ma = deceleration force or force required to accelerate R a = aerodynamic resistance R rLf + R rLr = R rL => sum of rolling resistance R g = W sin g => grade resistance due to gravitational force of the weight of the vehicle F = ma + R a + R rL + R g EQ. 2.2 Aerodynamic Resistance : R a = ( /2) C D A f V 2 EQ. 2.3 R a =aerodynamic resistance (lb) = air density (slugs/ft 3 ) Table 2.1 (Ranges from 0.001755 to 0.002378) C D = Coefficient of drag (unitless) Table 2.2 & Table 2.3 (Ranges from 0.25 to 1.8), f(of vehicle design and shape) A f = frontal area of vehicle (ft 2 ) V = speed of vehicle (ft/s), relative speed to prevailing wind speed Power required to overcome aerodynamic resistance: P Ra = (Force)(speed) P Ra = (( /2) C D A f V 2 )(V) Since 1 hP =550 ft-lb/s, then hP Ra = (( /2*550) C D A f V 3 ) hP Ra = ( C D A f V 3 )/1100 EQ. 2.4
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Rolling Resistance: R rL = (f rL )(W cos g ) R rL = rolling resistance (lb) f rL
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