Quote:
Originally Posted by ennored
First thing that popped into my head after knowing that the test was over and read some of the results was: How did it match your predictions? What did you think the CD would be? Based on what? Does the measured number match your predictions based on fuel economy? Coastdown testing? Tuft testing? Don't know that you had any specific predictions for this exact configuration, but you still must have had a number in your head that you thought it would do, right?
Your list of improvements seems to go after small stuff. Are there any BIG things that you can't really go after? Plan taper on the bed may be larger than I'm thinking? Maybe ride height? Lots of little stuff adding up?
I selfishly vote for a large vehicle to the wind tunnel next. Some sort of RV or box truck. Love to copy the Dryden van and try a few different tails.
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Predicting was going to be problematic before I left on the trip.
*The design velocity was going to be lower (65-mph) for which I have little data.The trip did indicate the highest mpg I've seen at 65 (over 37.5 mpg),so I felt like the most recent mods had a chance for 'showing.'
*Crashes and way-points forced 'shorter' distances between top-offs.There were some overnight stays with cold-restarts the next morning which is typically a no-no for testing.
*The wind never stopped blowing for the entire trip (hurricanes Dolly,Norbert,and Odile),so the trip was not indicative of past testing.
*Rain and wind from Odile killed me from Southern California to New Mexico.
*Air density can be 12% lower at elevation depending on temp and local baro.pressure,so it's going to skew the numbers.
*In 2012 I failed to reach terminal velocity at Bonneville,and without a top speed it would be impossible to use Hucho's 30%/10% rule.And the salt was so wet,even If they let me run the 5-mile course,the salt rolling resistance (Cf 0.055 vs 0.008) would have corrupted the top speed.
*Shooting from the hip,I thought I might see as low as Cd 0.18,do to the fineness ratio,but the lack of plan-taper of the truck box and incongruity of its shape,with respect to the aero shell seems to doom it to higher drag since it screws up all the tumble home and radii you'd get without the 'boxy' sides.I might be losing over 16% right there according to Kamm and Fachsenfeld.
*The diffuser is too steep also and that would require some effort to correct in a 'roadable' version.
*Maybe Cd 0.203 measured at A2 with a custom bed.
*Maybe Cd 0.182 with new bed and camera system as measured at A2.
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I can't think of any big-ticket items I could go after.
Torsion-bar suspension makes active (lowering) suspension problematic.
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If we want to do larger vehicles we should talk to the tunnel guys.Hucho says we can't exceed 30% blockage ratio.We'd want to run the numbers.