Under Body and Fuel Eco

[Echoshill]

I had a friend send this to me today and I thought it an interesting topic. Any of you guys have thoughts on this? Admittedly the 33% number sounds high and at the very least opens the door for any self respecting engineer to say….”ok so how did they arrive at that number”, but that aside; here it is.

***************I have read that around 33% of a vehicle's drag comes from air traveling under the car. Why don't auto manufacturers work on the underneath of production cars? I'm thinking a flat bottom would do wonders for drag and aero stability by reducing lift. I know you may not be able to get a true flat bottom but I'm thinking you could do allot better than they do now. I think the closer to zero lift you get the safer these cars become especially on the highways.*********

 

[evelrod]

Most modern mfgr's DO spend a great deal of effort (limited for sure by the 'bean counters') at cleaning up the underside of their cars. There is a lot of room for improvement...that I'll grant. This is not news as race car chassis builders, especially NASCAR, spend as much or more time 'underneath' as on top...It pays big dividends in drag reduction. Jim Hall and Smokey Yunick, to name a couple leading aero specialists in race car design, were into the chassis clean up in the early 60's that I personally witnessed.

Rod

 

[patprimmer]

Like Rod said.

Most engine bays have an underskirt now, and most front bumpers are designed to reduce the air going under the car to some extent.

A fill underskirt would:-

Cost a lot.
Not be visible to the buyer.
Be VERY prone to damage.
Significantly reduce ground clearance.
Cause overheating problems of exhaust system and adjacent components.
Increase risk of some mechanical noise transmitting into the cabin, but may reduce some other noise, like wind noise.

Regards

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[GregLocock]

"Cause overheating problems of exhaust system and adjacent components."

That's the killer

Plus to be honest at 0.3 the Cd is not a big contributor to fuel consumption, all things considered.

And the lift arguments are spurious.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[evelrod]

Greg, did you notice the latest VW recall...for failure of underside heat shield?

Rod

 

[carnage1]

having worked as a trans mechanic I can assure you that the underside of the newer autos are much more streamlined than older autos. one thing to remember is that the cooling air going through the radiator has to go mostly under the car as they don't want to vent that air across the outside of the front windshield.

 

[Rob45]

Friend of mine is a heat protection engineer working on Ford light trucks. Three or four years ago, they had him working with a belly-pan that was tested and found to give a huge improvement in fuel economy: .005 mpg...

That's 26 feet farther per gallon of fuel.

Sanity ultimately prevailed, and the idea was dropped.

 

[3star2nr]

Actually automakers do take the under-body of cars into consideration. Case in point Porsche, have a look under a 911 and you will see its perfectly flat.

True this is a supercar company, but other lower companies, like Toyota, Mitsubishi, Subaru, and Honda also do the same thing.

Under many lower sports cars, the under-body is covered by plastic covers, to give the under-body a flatter surface.

My car for example (Mitsubishi 3000GT), has these plastic covers. I removed them one day to see just how big an effect it would have on the cars handling and it was quite significant! From as low as 60 mph the car's "nose" got very susceptible to cross winds, and at speeds over 100, the car was very unstable, steering wheel shaking etc, and the car kept 'wandering'.

Put the covers back on and the car would climb to 135mph and remain as straight as an arrow. Also the time to get to those speeds was significantly less than without the covers.

Also note in NASA Time attack classes, you are penalized 7 points for running a skid plate or flat bottom floor, because it makes such a big difference to the cars aerodynamics.

I am currently working on a project to fabricate an aluminum skid plate to replace the plastic covers, and make the under-body of my car as flat as possible, I also want to make an under-body shield for the gas tank that would incorporate a diffuser, to add even more downforce.

 

[evelrod]

Yes, the 'flat bottomed car'.
Been around a while. Right off the top of my head, the GM Corvair, the VW beetle, the Austin Mini...I'll bet several more. I doubt the designers of these cars were thinking much about aero, obviously...they were FLATBOTTOMED, though.

Rod

 

[patprimmer]

The Porsche is relatively flat bottomed for the same reasons a VW Beetle is. Packaging and ease of construction for the floor pan chassis design.

Regards

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