Does reducing ground clearance REALLY reduce drag?

[twinair]

I wouldn't doubt that different cars react differently to lowering. All that I'm saying is that any practical lowering (>3" above ground) reduces aerodynamic drag (if all other parameters are left the same).

By shear forces I mean frictional forces and frictional forces do play a role.
E.g. frictional forces increase with a 'rough' underbody (which also increases the size of the boundary layer, which then also increases pressure drag).
In the meanwhile I have received the following book: "Hucho - Aerodynamik des Automobils" by T. Schütz et al:
On page 201 it shows pressure and frictional drag for different Audi models (recent models) and frictional drag causes about 10% of the total drag (I believe all these models have a smooth underbody).

There's relatively little information regarding lowering in this book:

On page 169 it has a general statement that Cd is reduced (not just frontal area) by 0.004/10 mm of lowering (doesn't say what this is based on and what range it is valid for).

On page 316 it shows data from another book (Janssen und Hucho 1973) of 4 different cars.
The Porsche 914 (with the lowest Cd of all cars) shows the largest Cd reduction due to lowering before a not-named-car and a VW van. Only one car shows a Cd-increase after an initial Cd-reduction (Citroën ID 19).

On page 317 it shows the only recent lowering data (Audi Q7).
The Cd*frontal-area is reduced by 9% at 50 mm lowering and is increased by 4% at a 24 mm raise.
The data is almost on a linear curve and stops at 50 mm lowering (so much more aerodynamic drag reduction could probably be achieved with more lowering).

However, I don't quite trust the old data on page 316 since this must have been measured before they introduced moving floors in wind tunnels and a moving ground leads to a significant reduction of frictional forces underneath the car.
(As I mentioned before, the frictional drag between two moving surfaces is 12 times higher than of just one moving surface. If you read in chapter 2 'Analytical Navier-Stokes Solutions' of this book: http://homes.nano.aau.dk/lg/Lab-on-C...20lectures.pdf you'll come to this conclusion if you solve equation 2.14 for differential pressure due to frictional forces and do the same with equation 2.58. )
And this factor 12 is based on laminar flow without any boundary layers, whereas in a turbulent flow regime you have two boundary layers if the ground is not moving as opposed to just one boundary layer with a moving ground (factor 12 and one more boundary layer).
The Citroën ID 19 in the example above probably just started to touch the boundary layer on the wind tunnel floor at some point, which then started to disproportionally increase the frictional forces underneath the car which subsequently increased Cd. (In 'Race Car Aerodynamics' by Joseph Katz it is mentioned that the boundary layer on a wind tunnel floor has a thickness of approx. 100 mm - this boundary layer is missing with a moving ground or on the road for that matter. The fact that efficient ground effect race cars (without the use of fans) were not 'discovered' before the mid 1970's was because a moving ground in a windtunnel is an essential part for air to remain attached to the underside surface of the bodywork in order for it to generate a significant downforce.)

Back to the Hucho-book:
All data which is pitch angle related suggest that a lowered nose always reduces Cd and a raised nose always increases Cd.
For instance, the VW 411 had a Cd reduction from 0.45 to 0.42 with a -2 degree negative pitch angle increase and the Audi Q7 a Cd*frontal-area reduction of 5% with only -1.5 degree negative pitch angle.
So, if you choose to lower only one part of the car, you should lower the front or raise the rear but not raise the front.

[aerohead]

here's a tidbit from 1922 which might help.
*The Aerodynamic Streamlining Template along with it's mirror-image as a streamline body of revolution,in free flight away from ground effect has Cd 0.04.
*When the AST is lowered into ground reflection its drag doubles to Cd 0.08.
*Lifting it and adding wheels raises the drag to the neighborhood of Cd 0.13.
*If lowered back down to zero ground clearance,rolling on an ultra-smooth roadway,its drag would be Cd 0.08.This has been proven empirically in wind tunnel tests.
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*We've never altered the body,but it's drag can be reduced by 38% just by lowering.

[aerohead]

The wheel area of passenger cars is so small that drag effects associated with a moving roadway are statistically insignificant.
Open-wheeled race cars suffer much drag from the presence of the wheels and a moving roadway tunnel is a necessity.

[radioranger]

So lowering is the way to go then, just how to do it without ruining the ride and handling, i thought lowering the rear would help because it would alter the roof and hood line as well as the trunk , and would be much easier to add more air dam and cut a loop off the rear springs, I stil think it might help. only way is to try I guess.

[justme1969]

This plus more was discussed last year, and there was another point I did not see brought up here.
The point was some times peculiarities also cause even more cd when lowering.
Depending upon vehicle design boundry layer frontal charged air may condense more acting as said, instead of going under a vehicle only slightly compressed.
Thus you end up with a nose speed brake effect even worse than your typical wind screen area.
" here is where the differance between air dam and frontal pitched spoiler count."Sorry if I muck things up by adding this point but it is relevant.

[LeanBurn]

Cutting a loop always increased the spring rate. The part about cutting is that it was never tested by the engineers that built the car, nor anyone else so there is no telling how it affects handling, so it can be totally unpredictable and can be dangerous.

Going with an aftermarket spring such as Eibach etc, or name brand coil overs is a more better approach as they are designed and tested by engineers for increased handling performance and usually backed by a warranty. The handling performance will be safely increased and predictable.

[twinair]

Originally Posted by radioranger

So lowering is the way to go then, just how to do it without ruining the ride and handling,

Well this is the point: If you somehow managed to go so low to actually increase aerodynamic drag, the car would simply not be driveable anymore (not just from a riding point but more so from not being able to drive over bumps and such).

A movable ground isn't necessary to test a passenger car in a wind tunnel unless effects of lowering are to be measured: If lowering effects are to be measured a moving ground (road simulation) is imminent, since not doing so will increase the frictional drag underneath the car by a factor of 12 (compared to the 'real world') even if a boundary layer on the wind tunnel floor was missing (which it is not).
(And if frictional drag underneath the car wouldn't play a role, car manufacturers and hyper-milers wouldn't install smooth underbody panels).

Race cars with wing like structures on the underside of the surface wouldn't work in a wind tunnel without a moving floor as the wing like surface would simply stall (due to the low air speed underneath the vehicle provoked by the unnaturally high frictional drag including wind tunnel floor boundary layer).



Air is not being compressed at the speeds most cars and especially hyper-milers travel. At 120 km/h dynamic pressure (ram pressure) only reaches 0.6% of atmospheric pressure: Dynamic pressure - Wikipedia, the free encyclopedia
The pressure difference (pressure drag) between front and rear of the car is primarily created by the 'negative pressure region' behind the car (wake). (And the size of the wake is also affected by what is happening in front and underneath the car).

[aerohead]

Originally Posted by twinair

Well this is the point: If you somehow managed to go so low to actually increase aerodynamic drag, the car would simply not be driveable anymore (not just from a riding point but more so from not being able to drive over bumps and such).

A movable ground isn't necessary to test a passenger car in a wind tunnel unless effects of lowering are to be measured: If lowering effects are to be measured a moving ground (road simulation) is imminent, since not doing so will increase the frictional drag underneath the car by a factor of 12 (compared to the 'real world') even if a boundary layer on the wind tunnel floor was missing (which it is not).
(And if frictional drag underneath the car wouldn't play a role, car manufacturers and hyper-milers wouldn't install smooth underbody panels).

Race cars with wing like structures on the underside of the surface wouldn't work in a wind tunnel without a moving floor as the wing like surface would simply stall (due to the low air speed underneath the vehicle provoked by the unnaturally high frictional drag including wind tunnel floor boundary layer).



Air is not being compressed at the speeds most cars and especially hyper-milers travel. At 120 km/h dynamic pressure (ram pressure) only reaches 0.6% of atmospheric pressure: Dynamic pressure - Wikipedia, the free encyclopedia
The pressure difference (pressure drag) between front and rear of the car is primarily created by the 'negative pressure region' behind the car (wake). (And the size of the wake is also affected by what is happening in front and underneath the car).

*Public domain literature reports that any wheel/tire affect can be well represented without a moving-floor wind tunnel.General Motors investigated it as early as the 1950s and found it not worth the trouble.Simple trip strips attached to the tires can create 'moving-floor' flow phenomena as far as drag measurements are concerned.
*Wheel drag,as associated with rotation has been reported at Cdr 0.005 or less for passenger cars.
*The drag coefficient of the spinning tire IS different from a stationary counterpart,but spinning it is not necessary to predict the drag.
*Dr.Alberto Morelli reported from the Pininfarina tunnel that tire/wheel Cd does not vary as the wheel is retracted into the wheelhouse,only the frontal area is reduced.
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*The issue with the underbody of a car is turbulence.Eddies and turbulence are pure entropy.Their kinetic energy cannot be converted to static pressure.
*An airdam can shield underbody parts from source flow,which would otherwise be tripped into turbulence.If the airdam is too deep it can increase drag.Ask bondo.
*The belly pan prevents any turbulence from forming and the integrity of the flow allows for the use of the diffuser which acts synergistically with the pan.
*By shunting air down the sides of the car,less travels underneath,which is beneficial to lift issues,and minimizes the velocity of the undercar flow to near the road velocity,although the convention is to consider this flow at less than the road speed.
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*Race cars with wings are open-wheeled and the frontal area of their tires are enormous in relation to the actual body of the car,minus pods and teams typically will use moving-floor tunnels although 'flow-trippers' can be used in stationary-floor tunnels to good effect to measure accurate drag.
*And bear in mind that not too long ago,an Formula-1 car with Cd 1.2 was considered 'good.'

[radioranger]

Maybe I can go larger size in the rear of car and smaller in front , probably only good idea on 6 speed cars though , allready the escort revs too high on the freeway , how much would I gain if I went 5 % larger on front tire according to tire rack I can go from around 880 revs pper mile to 830 , would I gain that much , off topic a bit .

[kach22i]

I just found this today, looks like it applies to this thread.

Splitter or Air Dam – Which Design is Best? - Mazda Miata
cfd | Hancha Blog

CFD Models

1. Stock 1990-1997 Mazda Miata
2. Stock 1990-1997 Mazda Miata at a 4in Ride Height
3. Small Front Air Dam at 4in Ride Height
4. Small Air Dam with Splitter at 4 in Ride Height
5. Large Air Dam at 4in Ride Height
6. Large Air Dam with Splitter at 4in Ride Height

Note: The air dam and/or splitter is 2 inches off the ground in study 3-6

I think the stock ground clearance for the Mazda Miata is 4.6-inches.

From what I can tell, just lowering the car about 1/2" increases the drag from Models 1 to 2.

However, going down via air dam/splitter to 2-inches gives better numbers than stock (Model-1).

Lowering a vehicle will increase the downforce by increasing the velocity below the vehicle. This does however increase drag.