Does reducing ground clearance REALLY reduce drag?

[radioranger]

I read a Ford Racing article somewhere that stated a 4 mph top speed increase for every inch lowered

[aerohead]

Originally Posted by diesel_john

CarloSW2

I was thinking something similar, aka only lower the front end. Are you thinking that the "larger volume" open space at the rear would aid in air exiting faster at the rear? CarloSW2[/QUOTE]



good point. cfg83,
But, i didn't think of that until i started thinking about a smooth underbody.
I found lowering the back end increased the drag (with a stock underbody), so would raising the back end decrease drag (with a smooth underbody). So now that i have smooth under, I will try to raise the back.

Probably each car should be optimized for front to rear height.
We say the shape of the rear is more important, so does that mean I can raise the rear ?to enhance the tail and while blunting the windshield, and still reduce drag, inexpensively.
Angle of attack effects so many things.
the grill to hood flow
hood to windshield flow
windshield to roof flow
roof to rear deck flow
on the bottom lift or downforce
under flow exit

lowering the front would compromise my alignment.

aerohead, thanks for the explanation. It gave me the answer to my question. What is optimal?
""A symmetrical airfoil of 6-to-1 aspect ratio,cut in half lengthwise,would describe the lowest drag car body""[/QUOTE]

diesel_john,sorry for this very late response,I just now caught your question.
For airfoils in ground effect Abbott and von Doenhoff caution that none of the tabulated data they published will be valid.Ground effect violates the 'flight conditions' under which the wings were evaluated.Also,wings are tested at infinite span and aspect ratios impossible to use with a vehicle.Spanwise flow is off limits also,so we're s.o.l.on that one.Finally,free flight presumes laminar air and we don't get that either do to the viscous shearing of strata of air as you move from zero velocity at the road boundary upwards.
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As to sections themselves,I believe that a 4:1 cord/thickness ratio delivers the lowest drag per unit length.That's from Sighard Hoerner.(there's a pictorial drag table for this section at the Aerodynamic Streamlining Template thread.
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The research done for road vehicles has shown that derivatives of 3-D streamline bodies of revolution,'half-bodies' specifically,produce the lowest drag with conventional 'door-slammer' type designs.
We're cautioned to never exceed 22-23 degrees with the aft body roof/body contour,which puts it in the territory of the 'Template,'which is based on a 2.5:1 Length/Diameter fineness ratio streamline body which never exceeds 22-degrees.Hucho reports this body in free flight at Cd 0.04.As a half-body in ground effect it yields Cd 0.08.When 'skinny' tires are added,the drag goes to Cd 0.12.

Jaray,Lay,Fachsenfeld,Kamm,Heald,Elliot,Prandtl and others all leaned on the streamline body during development of their work.

[twinair]

I just realized that one can easily calculate/estimate the power requirements to overcome the pure air shear forces underneath the car. https://en.wikipedia.org/wiki/Hagen-Poiseuille_equation

(It is also known as Coutte flow: Couette flow - Wikipedia, the free encyclopedia)
(I was just too lazy to actually run the numbers before).
Assuming you have a car with a width of 1.7 m and a length of 4 m and decide to lower it just down to 8 cm above ground (anything lower than this wouldn't make any practical sense).

At 120 km/h one ends up with:
P = F*v = 18 * 10^-6 Pa*s * 33 m/s * 4 m * 1.7 m / 0.08 m * 33 m/s = 1.67 W
to overcome the shear forces underneath the car.
(Viscosity of air is: 18 * 10^-6 Pa*s Viscosity - Wikipedia, the free encyclopedia )
(1.67 W is about 0.01% of what is needed to overcome the rest of the resistance at that speed).

Granted that this is assuming laminar flow and one would actually deal with more resistive turbulent flow, but one would simply not reach a practical point where lowering has a detrimental effect, since the beneficiary effects of having less frontal area and lesser exposed wheel-wells would be higher.

[aerohead]

Originally Posted by twinair

I just realized that one can easily calculate/estimate the power requirements to overcome the pure air shear forces underneath the car. https://en.wikipedia.org/wiki/Hagen-Poiseuille_equation

(It is also known as Coutte flow: Couette flow - Wikipedia, the free encyclopedia)
(I was just too lazy to actually run the numbers before).
Assuming you have a car with a width of 1.7 m and a length of 4 m and decide to lower it just down to 8 cm above ground (anything lower than this wouldn't make any practical sense).

At 120 km/h one ends up with:
P = F*v = 18 * 10^-6 Pa*s * 33 m/s * 4 m * 1.7 m / 0.08 m * 33 m/s = 1.67 W
to overcome the shear forces underneath the car.
(Viscosity of air is: 18 * 10^-6 Pa*s Viscosity - Wikipedia, the free encyclopedia )
(1.67 W is about 0.01% of what is needed to overcome the rest of the resistance at that speed).

Granted that this is assuming laminar flow and one would actually deal with more resistive turbulent flow, but one would simply not reach a practical point where lowering has a detrimental effect, since the beneficiary effects of having less frontal area and lesser exposed wheel-wells would be higher.

I can't imagine any self-respecting aerodynamicist who wouldn't caution against making generalized statements about streamlining.
Every vehicle will have discreet architecturally specific differences than other vehicles and each should be approached on a case-specific basis as has already been mentioned by others earlier in the thread.

[radioranger]

So Maybe with a bigger air dam and lowering just the back of my escort 4 dr would help and not really hurt the Handling or ride like lowering the front, since the air dam would send some of the air around the car instead of under it .and would have the effect of lowering the whole car some . A lot easier to do also .

[twinair]

Originally Posted by aerohead

I can't imagine any self-respecting aerodynamicist who wouldn't caution against making generalized statements about streamlining.
Every vehicle will have discreet architecturally specific differences than other vehicles and each should be approached on a case-specific basis as has already been mentioned by others earlier in the thread.

My statement isn't about streamlining, it's about the effect of the distance between road surface and bottom of a parallel surface on pure shear forces (and that these shear forces wouldn't be substantial, since it is not practical to lower the car to a point, where these shear forces would start to play a significant role).

Maybe there are other detrimental effects than increased shear forces which may start to play a role when lowering a car.
Can you show any data that shows that lowering a passenger car has increased aerodynamic resistance?

[aerohead]

Originally Posted by twinair

My statement isn't about streamlining, it's about the effect of the distance between road surface and bottom of a parallel surface on pure shear forces (and that these shear forces wouldn't be substantial, since it is not practical to lower the car to a point, where these shear forces would start to play a significant role).

Maybe there are other detrimental effects than increased shear forces which may start to play a role when lowering a car.
Can you show any data that shows that lowering a passenger car has increased aerodynamic resistance?

Copy-Pro,here where I use the computer is closing for the Thanksgiving holiday,so it will be next week before I can bring anything.
If you can get a look at any of Hucho's books,you'll find a section on 'Effective-Bluffness','Effective-Fineness Ratio',which examines the role ground clearance plays with Cd.The Schl'o'rwagen is investigated completely for G.C. effects.
You'll notice that,for instance,the 'Aerodynamic Streamlining Template' would have a Cd 0.08 on the ground with it's wheels completely retracted into it's body,rather than Cd 0.13 or so with ground clearance.
*Lowering effectively increases fineness ratio.
*It also reduces the frontal area by virtue of shielding the wheels/tires.
*It can affect body drag optimization by virtue of inclination/rake effects,as with both Ford Probe-IV and V which are both lowered and 'raked' for lowest drag.
I'll catch up next week.
We are cautioned to investigate vehicles on a case-specific basis,so we'd be remiss to attempt blanket statements.

[Otto]

Originally Posted by twinair

My statement isn't about streamlining, it's about the effect of the distance between road surface and bottom of a parallel surface on pure shear forces (and that these shear forces wouldn't be substantial, since it is not practical to lower the car to a point, where these shear forces would start to play a significant role).

Maybe there are other detrimental effects than increased shear forces which may start to play a role when lowering a car.
Can you show any data that shows that lowering a passenger car has increased aerodynamic resistance?

Consider Sighard Hoerner's Fluid Dynamic Drag book, specifically his section on external stores (i.e., bombs and drop tanks) and the interference drag created when two otherwise streamlined shapes operate in close proximity. I think this may be instructive re. your problem regarding shear forces. Basically, the flow past the wing gets tangled up with the flow around the drop tank, increasing drag well beyond the arithmetic sum of the respective bodies. Further, absent an undertray and wheel fairings, the bottom of the car is not nearly as smooth as the underside of a wing or shape of a drop tank, so the car bottom traps air and floats with extra lift. Not good.

FWIW, I suspect the HPV streamliner bikes at Battle Mountain are virtually all running too close to the surface--the air under them has no place to go, so generates more drag than it otherwise would. They too should consider Hoerner, per above.

Back to cars: Surely the most bang for the buck is a $7 Home Depot garden edging air dam, smoothing the airflow laterally past the wheels, which are normally in deep yaw and act like buckets or sea anchors. Push much or most of that air past the wheels in barge-like fashion and get most of the benefit of undertray, wheel fairings, etc. at a tiny fraction the cost of materials and effort, then call it a day.

[radioranger]

Well I was going to go a bit more radical, and wondered if anybody had coments on lowering just the rear a bit, since it should be cheaper and not mess up the alignment too much , just go down to full load height and of course extend the factory spoiler down and around the corner of the undertray area.

[Superfuelgero]

Originally Posted by aerohead

Copy-Pro,here where I use the computer is closing for the Thanksgiving holiday,so it will be next week before I can bring anything.
If you can get a look at any of Hucho's books,you'll find a section on 'Effective-Bluffness','Effective-Fineness Ratio',which examines the role ground clearance plays with Cd.The Schl'o'rwagen is investigated completely for G.C. effects.
You'll notice that,for instance,the 'Aerodynamic Streamlining Template' would have a Cd 0.08 on the ground with it's wheels completely retracted into it's body,rather than Cd 0.13 or so with ground clearance.
*Lowering effectively increases fineness ratio.
*It also reduces the frontal area by virtue of shielding the wheels/tires.
*It can affect body drag optimization by virtue of inclination/rake effects,as with both Ford Probe-IV and V which are both lowered and 'raked' for lowest drag.
I'll catch up next week.
We are cautioned to investigate vehicles on a case-specific basis,so we'd be remiss to attempt blanket statements.

I've seen positive results on the vx by lowering the front only (2"). Unfortunately its difficult to do ABA with the time and alignment, so I won't post numbers. It is significant though, and completely changes the wind noise pattern.