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Old 06-27-2012, 06:44 PM   #102 (permalink)
orbywan
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Join Date: Jul 2010
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Quote:
Originally Posted by aerohead View Post
I'm afraid that my answer won't be as good as your question but here's what comes to mind.And I hope some of the other members chime in.
*with the belly pan,the air furthest from the ground should be moving close to the velocity of the air along side the vehicle.If so,then there would be no spanwise flow.Strakes,as developed for aerospace,are to work as a flow fence to sequester air and prevent it from flowing sideways to a region of lower pressure,where it would spin up into an attached vortex.This pressure differential would not be great on a road vehicle.One of the recent land speed record motorcycle streamliners developed in the GALCIT tunnel at Cal Tech exhibited no under body flow disturbance except very close to the wheels,which self-corrected a small distance downstream.They used nothing under the bike except nice smooth bodywork.
*Air under and beside the vehicle is basically flowing back in a straight path,with the lowest kinetic energy unlike over the roof of a car.
*Low-drag vehicles will have wheel fairings to mitigate pressure spikes in the region of the wheels and provide for sectional density of the overall vehicle,allowing only very gentle changes in body cross-section.
*I'd be inclined to go after the wheels before I went into micro-managing airflow.
*get all the pick and shovel work done first,then go after the minute details.
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I do have 'strakes' under my diffuser,but they are structural reinforcements for a long sheet metal span and were never intended as a streamlining device.
They could actually hurt in a crosswind.Don't know.
I was hoping you would say that, I know just enough to be hazardous, but I agree. When I figure out the shape for the fairings I'll run that by everyone for some feedback before I build them.

Driving in a crosswind was one of the reasons I don't like the idea of strakes, seems like that would cause a serious amount of turbulence the entire length of each strake. That plus I didn't want to mount them on top of the belly pans. There's enough stuff down there as it is.

I know this will make everyone laugh, but my goal is 20mpg, straight and level, no load, no A/C. If I can improve the numbers by about 18% I'm there so I'm hoping some redesign work on the the front, plus the front air dam, plus the wheel fairings will put me in the neighborhood. If they do, then I'll do the last mod which will be 3:55 gearing or as close as I can get. Maybe an under drive if the final drive works out to 3:55 or close but I think it will be much lower than that, which I don't think will make for improvements.

Actually the last consideration will be HHO and/or adding propane. The propane is questionable for sure, but I achieved a consistent 20% increase in mpg with my Tundra with an HHO system, but the paraphernalia and increased 'figit' factor required to deal with the truck's ECU was not worth it to me. Not to mention the fear of burning valves from running it in the 17 to 18:1 mixture range. The best results were in the 16:5 to 17:1 mixture range with minimal HHO flowing believe it or not. The 7.3 doesn't have the O2 sensors to deal with so it should be much easier.

Anyway, it's all hot air until you do it, so I'll shut up until it's done.
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