Ground Clearance vs Aerodynamic Drag

[aerohead]

Here's a visual on a Porsche 914 that was tested:
The 914 was tested at:
*180mm
*165mm
*140mm
*zero

Here is a pictorial of how the 914's effective fineness ratio varies between the two G.C. extremes.

Here we see VW's Flow Body drag vary with G.C.,and also with the addition of wheels.



Here,Ford gets a four-for,lowering their 1983 Probe-IV,lowering a deployable airdam,then hiking the rear end to finally achieve the inclination which achieves the lowest Cd and also reduces frontal area.

*The Probes official ground clearance does not take into account the effect of the fully-deployed airdam which actually reduces G.C. to about 1.92" (48.76mm)
*Lowering alters the fineness ratio from 3.702,to 3,80 (2.6%).
*The computerized attitude control moves the max. roof camber position 9.54" ( 242.3mm) aft.
*The attitude also alters the backlight-to-boot angle,from 18.5-degrees,to 16.5-degrees,a 10.8% change.
*The attitude also converts the entire belly to a 2.5-degree diffuser.
*Lowering submerges the tires into the body,however,the 3" (76.2mm) extension of the airdam raises the frontal area,leaving the car at essentially a constant 20.5 sq-ft (1.905 m-sq) frontal area.
*From scale drawings it is very difficult to discern that the final height is any lower than the 'urban' setting due to the lifted tail.
*The final initial design came in at Cd 0.17.
*Ford had anticipated Cd 0.18.
*With a lot of work on the side-view mirrors and other tricks she came in at Cd 0.152.
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The 1937 Schl'o'r (Pilbug) car
Neil mentioned this one and it's good for us,as Hucho included it in his book with a drag breakdown based upon ground clearance.
I did a photo-enlargement of Hucho's table and solved it graphically for the test data points included.

*The Pilbug is 1,275mm high.
*Based on this height and length we're given the following info
- @ G.C.= 3,315mm ----------------------------- Cd 0.1057 (smooth belly)
- @ G.C.= 1,730mm------------------------------ Cd 0.1086 ditto
- @ G.C.= 923mm-------------------------------- Cd 0.116 ditto
- @ G.C.= 529mm-------------------------------- Cd 0.1306 ditto
- @ G.C.= 268mm-------------------------------- Cd 0.141 ditto
- @ G.C.= 232mm-------------------------------- Cd 0.158 ditto
- @ G.C.= 232mm-------------------------------- Cd 0.186 ('Standard underbody')
- @ G.C.= 0mm---------------------------------- Cd 0.201 (guestimate) with smooth belly
- The smooth bellypan is good for a 14.67% drag reduction,or without it,the drag rises 17.72%.
I've got pictorials in the works,but Copy-Pro is closed this weekend for Easter and it will be next week at the earliest before I can progress.Also got other pans on the burner.
I'll add material as its generated.

[California98Civic]

That's a really big reduction on the basis of ground clearance for that porche. But the benefit of lowering would be less significant if the car had first been fitten with an under tray and/or airdam, no?

[Cd]

Doing it wrong :
Still doing it wrong :
Just right ...


We now return to our regularly scheduled topic :

Thanks for the info Phil. Dropping a car real has a big difference in drag, eh ?

I keep thinking back on that Fusion that ran at Bonneville and the amount of drag that was reduced by the drop

Here is the image :

[ksa8907]

So dropping the cts 1.5" all around may give a measurable gain?

[California98Civic]

Here is the image : [/QUOTE]

I can't help but wonder if they tested this fusion's bellypan improvement after they had already lowered it all the way down to 2" GC. The bellypan benefit was less than mirror removal, which seems counterintuitive.

[rollercoaster]

Originally Posted by California98Civic

That's a really big reduction on the basis of ground clearance for that porche. But the benefit of lowering would be less significant if the car had first been fitten with an under tray and/or airdam, no?

I would think the combination of a full belly pan and lowering would increase drag because of the pressure differentials from top compared to bottom. In essence, you're creating suction -yes?

OP -Any data on lowering in conjunction with a full bottom pan?

[serialk11r]

Wow those are pretty big % changes, no wonder after lowering my old MR2 I could see higher vacuum at cruise.

[Vekke]

I have also studied this long time ago and if I remenber correctly if you go lower than 50mm clearance it does not help anymore :/.

You can try to find details of the Bugatti veyron as it lowers at high speeds.

[WD40]

While I was recently searching the motor vehicle act for British Columbia, I read that there is a law regarding road clearances.
The strange part was that all it mentioned was nothing can be lower than the bottom of the wheel rim itself, so those of you with low profile tires can go the lowest.
The reason I was looking was for the front and rear wheel spats that I "had" on my VX.
One of the things the cop mentioned to me was that they were to low to the ground, well in fact they were still within the legal area.
Know your local rules and regulations regarding your car.

[ennored]

I've always though that lowering a car had more to do with lessening frontal area and hence drag than actual drag coefficient.

I know that when measuring small changes, like mirror removal, they don't recalculate the frontal area, just take it as an improvement in drag. Maybe that's what's going on here? Probably never know for sure, but anyone have any solid facts?