Humor me, I have lowered my trucks Cd to .32?
I think the Cd on my truck is .32.
.32, down from .39 factory, with just a half tonneua cover and lowering. Using the Aerodynamic and rolling resistance tool, I figure my Cd is .32 Here's the variables I used- Vehicle weight: 1950.4 kg / 4300 lbs Crr: .008 Cd: .32 A: 2.7 m2 / 29.5 ft2-Down from stock 29.75 Fuel energy density (Wh/US gal.): 33557 Engine efficiency: .26 Drivetrain efficiency: .97 Parasitic overhead (Watts): 0 rho: 1.225 kg/m3 Varables that I know to be true- Truck weighs >4300 pounds with me in it Frontal area stock is 29.75 advertised Cd stock is .39 I took a 762 mile round trip, using 10% ethanol, 87 octane gas traveling @ 70-73 mph ( measured by GPS) I got 27.14 mpg. Does lowering the truck decrease frontal aera more than just .25 sq foot? I figured that by less tire tread being exposed, because it's now further in the wheeel well. So is my Cd really .32 now or I'm I tripping. |
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"Likely"
I was commenting on the 'I' for 'am'. Sorry Excuse me? |
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I think you can basically ignore all the calculations (BS in = BS out) and just look at the magnitude of changes required on real, properly-measured cars to see that change in Cd. |
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I say congrats on improvements! |
It would be hard for me to believe my simple mods reduced drag that much. Thanks for the comments.
One thing for sure, lowering the truck did drastically improve the gas miliage. Which begs the question-"Why does Toyota make them so high off the ground, when people buying midsize trucks are looking for fuel efficency? |
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* E10 reformulated gasoline is 111,836- Btu / gallon * 32.777 kWh/ gallon * 6.128- pounds /gallon mass * A non-turbo'd, non-Atkinson cycle, ICE engine, at steady speed cruise, might be in the vicinity of 36% thermal efficiency, rather than 26%. * Driveline efficiency will vary with transmission type. * 92 % driveline efficiency might be more realistic ( if a manual overdrive transmission ). * 2% accessory losses under the hood. -------------------------------------------------------------------------------------- * It would be good to have a baseline mpg for the un-modified truck, under the same test conditions. -------------------------------------------------------------------------------------- * Your mpg and velocity would yield gallons/ hour. * Gallons / hour yields Btu / hour. * Btu / hour divided by 2546 Btu/ bhp-hr = Brake Horsepower Hour ( gross) * Bhp-gross X thermal efficiency = Bhp-hour actual * Bhp X driveline efficiency = Road Load Horsepower @ test velocity ----------------------------------------------------------------------------------- * Calculate your aerodynamic- hp from the aero power formula. * Subtract this from the Road Load -hp = Rolling resistance hp. * R-R hp = velocity/ torque -to-power factor ( test weight X Cf rr ) * Solve for Coefficient of rolling resistance. -------------------------------------------------------------------------------------- * With your new ( modified ) mpg, solve for bhp. * Solve for new road load hp * Subtract the rolling resistance hp ( it hasn't changed unless you've introduced a significant weight change or different tires ) ------------------------------------------------------------------------------------- * The remainder is your new ( modified ) aero drag power * Solve for the new Cd using the modified frontal area. It just falls out of the math. NOTE : you're presuming a constant BSFC ( the nature of the beast ) The drag could actually be different due to same gearing, moving the engine to a less efficient island on the engine map. You'll see a trend. Absolute values will be elusive unless measured in a lab. If you're considering coast-down testing, have professionals do it. They'll have the equipment, experience, venue, and expertise to get as good a numbers as are available that way. Wind tunnel? |
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