Does reducing ground clearance REALLY reduce drag?
 

[EDIT: this thread split from here... http://forum.ecomodder.com/showthread.php?p=18669]

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Yuck! Those wheels look like fans that will pull disruptive air flow into the path of the car.

Also, lowering the car does not make it more aerodynamic!!! Why does everyone insist on this old school thinking?

cbergeron: where do you get the idea that lowering doesn't improve aero? (Point to a credible source maybe?) It's accepted in practice & theory that it does help for the passenger cars I've seen it applied to.

coyote: underbody airflow is typically a lot less energetic by the time it reaches the back of the vehicle. More turbulence from all the bits hanging down & uneven underbody surfaces, so that behaviour is normal.

MetroMPG,

Here's an "authoritative" reference (Road Vehicle Aerodynamics, mentioned in my Intro), page 51:

"The effects of ground clearance would be different for very rough or smooth undersides. Some researchers suggest that a very rough underside and a large clearance, as in trucks and lorries, actually causes an increase of drag by creating conditiions of greater freedom for the formation of eddies in the underside flow[34]. On the other hand, a general increase in ground clearance leads to a more unobstructed airflow which by bleeding air from the other flow regions produces, in effect, a decrease in the aerodynamic force."

He then shows a diagram that indicates that for a rough underbelly, the Cd increases with increasing ground clearance, while for a smooth underbelly, Cd decreases with increasing ground clearance.

He then goes on to say "These diagrams were produced for cars with an average underside roughness and show that vehicles with bad aerodynamic styling, characterised by large total drag coefficients, display a slight increase in drag while those of good aerodynamic shape display a rapid decrease in drag with increasing ground clearance."

I would also note that both the Insight and the EV1 do not have particularly low ground clearances (5" or thereabouts) and still manage to achieve Cds of .19 and .25 respectively.

One thing I'm picking up from my reading is that it is difficult to generalize about this stuff--it is all very interrelated and what you do in one area greatly influences what happens in other areas (e.g., underbody vs. front air dam vs. air around the tires) and that what works for one car may well be disaster for another.

--Steve

I just don't think lowering the car makes it more fuel efficient. A vacuum is created beneath the vehicle which I think creates drag. Formula F1 race cars are low to the ground so they can turn at high speed because this vacuum keeps the car on the ground.

I know it's accepted in theory but that doesn't mean it's correct. I firmly believe that raising the car up with thinner wheels provides a better Cd (with a smooth underbelly that is).

I have a feeling it's part of the reason that the Aptera achieves higher than 200 MPG.

I'm not trying pick a fight, I'm just trying to get everyone thinking. ;)

In a partial vacuum, there is by definition less air, which in turn means less air dragging on the car.

Interference drag, on the other hand, results when two bodies are too close to each other, such that the flow around one interferes with the flow around the other, with total drag being more than the sum of that from the respective parts. This phenom. became especially apparent with the development of the DC-3 transport plane in the 1930s. A streamlined fuselage had drag X, and a streamlined wing had drag Y, so arithmetically, X + Y should have meant total drag Z. Turned out, total drag was ~63% more than the sum of the individual fuse and wing. Wing fillets reduced the interference drag somewhat. Studies similarly showed that external stores (i.e., bombs) hanging off airplane wings should not be too close to the wing, but rather ~.4 diameter removed from it, to give the air sufficient space to flow past. HPV guys should take this into account, as many HPV streamliners may be running too close to the pavement.

Moral of the story: Lowering the car sandwiches dynamic air between it and the pavement. Could be, interference drag is at work when the gap is too small. Form drag happens when the gap is too large. Look for the sweet spot.

I should probably split this off into its own thread, because some good points have been made. Most notably, the famous "it depends" seems to be arriving on the scene.

The reason I snapped to attention to the "old school" comment is because I can, off the top of my head, name 5 manufacturers of modern, road-going passenger cars who claim reduced drag specifically from reduced ride height in special "efficiency" variants of ther cars: Volvo, Mercedes, Renault, Lexus & VW.

While I'll gladly agree that the effect of lowering will vary from vehicle to vehicle depending on various factors, I find it hard to believe that with the amount of money these companies throw at R&D, they're merely suffering from group-think, and trapped in old-school thinking!

When I see situations like this, I basically figure "it's time for more research". I don't think the author of my book was wrong nor do I think the auto companies are lying. I think, rather, that there are conditions (unstated, of course) in which lowering ride height helps and there are conditions where it doesn't. I think it's entirely possible that the companies you mention have found things which in combination with reduced ride height, makes a difference. That is, if they had just reduced ride height without doing some of those other things, they wouldn't have seen an improvement. Our job is to figure out what those conditions are.

Given the contradictory information, it seems to me that slavishly following a "rule" with something as tricky as aerodynamics isn't likely to lead to good results, except by accident. That said, I don't wish to dissuade anyone from experimenting and reporting their results--I'm only suggesting that there is likely more here than meets the eye.

I hope I run across something in the dozens of SAE papers I'm wading through (slowly). Aerodynamic theory may have something to say about this as well. Thanks for the links, by the way--it gives me (us) some cars to go look at and see what they did.

--Steve

very good!
I have think this hone, but I do not have read, until to hour :)

Hucho's book says something similar:

There's a corresponding chart which shows drag for several vehicles relative to ground clearance:

Citroen DS 19, Cd @ clearance: .405 @ 90 mm; .379 @ 160 mm; .385 @ 280 mm

Note the plot isn't linear for the Citroen. Drag increases as clearance changes in either direction from around 160 mm.

Porsche 914, Cd @ clearance: .34 @ 140 mm; .357 @ 165 mm; .367 @ 180 mm

But the Impact/EV1 which was used to set the electric drive speed record apparently had a reduced ride height from the production version. (Based only on images I've seen - I don't have figures). Doesn't prove anything related to Cd, but I'd bet it was done to reduce it.

So a big part of the "it depends" issue would seem to be quantifying what counts as a "smooth" vs. "rough" underbody.

Based on the 5 production examples I linked to above, it would seem those vehicles may fall into the "rough" underbody class, since lowering aided each one.

Also, we can't forget that setting ride height optimally for miniumum drag in production vehicles is probably in competition with practical considerations (e.g. speed bumps), and handling considerations.

Sooo, putting a smooth undertray on lesbaru (9 inches under the lowest part) might actually hurt drag? Seems like its worth testing.

Everybody missed the point. Lowering the car reduces the frontal area by hiding more of the tires inside wheel wells. Read "under car aerodynamics, part 1" on autospeed. Lowering the car at highway speeds is a trick used by luxury car makers all the time.

tjts1,

With all due respect (and speaking only for myself), I do in fact realize that lowering the ride height does hide more of the wheel. However, as I've said above, there is a lot going on under the car and I think people like Hucho and others who have actually *measured* the Cd for various ride heights probably know what they're talking about.

As I said above, many times the underlying assumptions are not stated explicitly, so it is very hard for us novices (and I'm assuming none of us are professionals in the auto aerodynamics field) to make judgments about the conditions under which a particular change produces a particular result.

As before, my plea is mostly for additional research into the literature to try to find out what's really going on. Speaking of which, I am not sure which autospeed article you are referring to. I've read all the articles I could find on that site and none (that I found) were entitled "under car aerodynamics, part 1". I did find (and quickly reread) "Modifying undercar airflow" parts 1 and 2 and saw nothing in there that referred to ride height (though maybe on my quick re-read I missed it). Got a link?

--Steve

The rougher the underside of the vehicule, the more benefit there will be from a reduction in ride height. On the other hand, if the underside of the vehicule is a very good aerodynamic shape, increasing ride height will reduce the drag coefficient. If I remember right, an airfoil can have a Cd of roughly .05 up high in the air. Bring this airfoil in ground proximity and the Cd now jumps to .1 or the like due to ground effect. There is a reference of this in Hucho's book. The aptera is also a good example of this IMHO.

Generally, your average car will benefit from ride height reduction.

The frontal area argument is not really relevant. Say you drop 2 inches in a 22 square feet car, the net result is a reduction in frontal area of roughly 0.75%.

It sure helped for the Ford Fusion Bonneville team.

( Plus, it makes the car look cool ! :) )

Re the salt flats racing, here is an excellent example of what I am referring to when I mention "unstated assumptions". It may well be that Hucho et al are assuming "normal highway speeds" (or maybe even "normal around-town speeds") whereas the folks out on the salt flats are running at much higher speeds and may therefore be getting different results.

However, the racing guys will often do things that increase drag in order to get better results in other areas like handling and traction so they may be lowering the car for reasons other than drag--reasons that would have nothing to do with improving mileage figures.

--Steve

ride height
 

I'm going to also weigh in with the "it depends" bunch.Dr. Michael Seal's Western Washington University's Viking series of 100-mpg cars used an increased ride height to channel air under their reverse airfoil bodies.Ford Motor Company used active suspension to lower their Probe series hyper-milers.Generally,my thoughts as regards to members cars would be that lowering,as mentioned,veils part of the tire/wheel from the air stream,effectively reducing frontal area,which usually reduces drag,arithmetically,as a percentage of area reduction.Also,lowering increases the fineness-ratio,something Hucho's book also addresses in the boattail modifications to the Mercedes C-111 research vehicle.Since road vehicles all suffer from the "mirroring" effect of the ground,anything which can be done to increase fineness-ratio is a shoe-in for lower drag.,and I believe there is no dispute with regards to this.Dr.Seal attempts to move up out of ground-effect into "clean" air,where fineness-ratios are doubled,Ford uses active suspension to lower cars on the highway,where ground clearance demands are lower than in an urban environment.A significant portion of Ford's low drag is attributed to "lowering".The recent article on the fuel cell Fusion also attributes "lowering" as a significant (0.08 off Cd ! ) to the cars drag reduction.With exception to W.W.University,all my accounts of drag reduction,aside from general coachwork,everyone lengthened the vehicle,lowered the vehicle,or did both.Since there are exceptions,we probably need to every vehicle on a case-specific basis.P.S.,also,in Hucho's book you'll find Dr. Morrelli's body which he developed at Pininfarina,which exhibits very low drag in "clean" air,and looks exactly like the Aptera,however suffers a drag increase as it is lowered into ground-effect.

Maybe you mean something different by "fineness ratio" than I understand it to mean, so perhaps you should provide a short definition. Mine is "width/length" (or, for a cylinder, diameter/length). I therefore don't understand how lowering a car can alter its fineness ratio.

Also, would you be so kind as to expand on your comment that "Since road vehicles all suffer from the "mirroring" effect of the ground,anything which can be done to increase fineness-ratio is a shoe-in for lower drag". What does fineness ratio have to do with the ground effect? I could see it if you were *narrowing* the car, but lowering it doesn't do that.

Thanks,
--Steve

tasdrouille -

I'm gonna stick with this definition as a general rule-of-thumb.

As far as I am concerned the Aptera is a "small-plane" body shape with fixed wheels. Since it doesn't fly (yet), there isn't any reason to make them retractable, ;) .

We really really really really need open source wind tunnel software.

Orrrrrrrr, maybe DIY coin-op car washes could be adapted for wind tunnel testing. Get some big fans and have a "dirty mist" or "misty mud" mode on the car wash control dial. Use the dirty mist mode to "find" the bad aero spots on the car. Wash the car off (more coin for the car wash owner), make another aero mod, and repeat. Pay as you go in 1:1 scale :D !!!!!

CarloSW2

I believe I read somewhere where wheels and tires are around 10-15% of drag. lowing the vehicle causes the wheels to be hidden more, and they don't stick out as much.

It also seems worth mentioning that lowering or raising the ride height can change the critical Reynolds number. The flow at higher heights will be closer to a free flow conditions and lowering it will approach a more constrained flow situation and cause the transition to turbulent flow to occur sooner. For rough undersides this is impossible to predict but for smooth undersides this is something that should be considered.

OK, just to confuse things further, here's something I ran across late today. It's from an SAE paper, 951906, "Vehicle Design Strategies to Meet and Exceed PNGV Goals" by Timothy C. Moore and Amory B. Lovins, found here

https://www.rmi.org/images/PDFs/Tran...nStategies.pdf

The following comes from page 11:

"Rather than smooth the underbody and attempt to tuck chassis
components up out of the flow, the industry strategy has
tended towards air dams below the front bumper to force much
of the flow around the vehicle rather than under. This needlessly
increases frontal area and leads to the erroneous notion
that achieving very low aerodynamic drag requires extremely
low ground clearance[5]."

Footnote [5] says:

"If chassis components are streamlined or otherwise covered
by a smooth floorpan to prevent interference drag, there is
little reason, beyond the limited exposure of more of the tires’
frontal area, to prevent the airflow from passing under the car
(P. MacCready, AeroVironment, personal communication,
April 1995). Allowing the airflow to pass under the car can
actually aid in eliminating lift-induced drag."


In case you are not familiar with "P. MacCready" or AeroVironment, see

http://en.wikipedia.org/wiki/Paul_MacCready

If you look at the Viking cars by Dr. Seal, mentioned by aerohead above, you can see that he took pains to deflect air from in front of the wheels, but did so without front air dams or lowering ride height. The entire collection of cars can be seen from here (click on the boxes to see photos):
http://vri.etec.wwu.edu/cars.htm

Sadly, I have not found anything in the way of technical descriptions of these cars beyond the one paper here:
http://vri.etec.wwu.edu/hybrid_paper.htm

About 3/4 of the way down the page is a nice paragraph on aerodynamic drag reduction which does not include lowering ride height as one of their strategies.

Now, you will notice that nothing I've quoted here says that lowering ride height *won't* decrease drag--it just says you don't need to lower the ride height to achieve low drag.

--Steve

I got this brillant idea a couple years back that my windshield was too steep and needed to be tilted back, so, I cut a couple inches of an old set of rear springs and put them in. (under body stock)

Well it just killed my top end and mileage, so I put the stock springs back in.
Mileage and top end back to normal.

So I wonder if I raise the rear an inch what will it do?

GenKreton, that is certainly an explanation that would fit the observation that lowering ride height on a smooth-bottomed vehicle increases the drag--the smooth underbody would have laminar flow for some distance before the critical Reynolds number would be reached which would turn the flow to turbulent. Anything that would move that point closer to the front, such as creating a more constrained flow condition, would presumably increase the drag.

Thanks for mentioning that....

--Steve

So... I did some CFD runs for the HPV fairing... Rolling wind tunnel, 30mph, 68 degree dry air..... Then I lowered it, from 2 inches to 1 inch.... And I got a decrease in drag... About .3 pounds... Significant for the amount of total drag on the fairing ;) No wheels or anything getting hidden - purely height change ;)

The fairing is a streamlined body - flow never goes turbulent....

Just food for thought ;)

Soon, I'll have a nice models of production cars to test :D

trebuchet03,

I checked out your build thread for the HPV fairing (very cool project, BTW) and noticed that you claimed a Reynolds number of 184. Not 184,000 but 184. Unless I'm totally misunderstanding what you're reporting, that really doesn't seem reasonable--using a characteristic length of 60" and a speed of 30 mph in 20C air I get a Re of 1.4E6. Just to make sure, I did a little looking around the net and even the Re for a bicycle helmet is 1e5.

What am I missing? I ask, because if the Re is really out of line with cars, then your ride height experiment might not be transferable to a car.

--Steve

I'll have to see if I can pull the characteristic length out of the model.... It's not like a chord of an airfoil - there's a lot of compound curves in there (and I really don't want to start hand calculating that) :/ But I'll double check :) <-- mind you, runs I do personally are with software that doesn't give Re# data :(

I'm not looking for transferability - but there was a blanket statement made that applies to vehicles with streamlined undersides.


All that aside - we should have tuft testing data next week :thumbup: So if there's some obvious flow issues, we'll see them :D

diesel_john -

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

Another advantage of lowering a car is that the front fender well gaps are smaller with the wheel sitting higher up in them. This keeps some of the wind that the tires are blowing forwards from getting into the air on the sides of the car. You can see that on a rainy day the front wheels throwing water forward and out of the fender wells.

Thanks for the link! I'm an avid areo paper reader :D

Well, that is totally understandable! Lets suppose you have a given vehicule that looks like an aerodynamic wreck underside. What are you going to do? Redesign everything so you get a more or less streamlined underside? Or stick a plastic dam up in front of all that mess and acheive decent results? Accountants usually have the most convincing arguments when comes the time for management to make decisions.

Since it actually reduced drag, I don't think you can say it needlessly increased frontal area. The guy seems to forget the answer depends on the vehicule you start with.

I completely agree with that. However, I am inclined to beleive almost all the cars on the road today do not have the level of smoothness of the underside required not to benefit from reduced underside airflow.

It is clear in my mind that the average daily driver will see a drag reduction if lowered.

It is also clear in my mind that a streamlined concept vehicule will see a drag reduction if it is rised.

The only gray area that's left is for ground vehicules akin the lotus elise. Cause it's not just the smoothness that counts, the shape is at least as important.

Yes, but the statements I quoted were originally in the context of cars and trucks. And this thread started out being about cars. Sorry if that wasn't clear. :) Not saying your comment about lowering the height wasn't interesting or useful--just wanting to make sure I really understand the import of it.

tasdrouille: If you only look at the extremes (avg daily driver vs. streamlined concept car) maybe things are as clear as you suggest. But what do you tell the person who has put a smooth belly pan on his car and wants to know if he should also raise or lower the height? Also, many cars coming out today have smoother undersides than cars of ten years ago--what do you tell their owners? (And while I admire the spirit of "just try it", some of us have limited time/resources and need/want to put our time/money where we'll get the most bang for our buck.)

CoyoteX: if you're really interested in interactions between the wheel and the wheel well, have a look at SAE paper 2002-0105209. Reducing drag in that area isn't straightforward--even the guys with the moving floor wind tunnels and CFD technology take a lot of trial and error to improve it. The message from that paper (to me) was: just putting the wheel higher in the well doesn't necessarily improve overall Cd.

--Steve

fineness ratio
 

Steve,good question and I'll try and do it justice.From experimental investigation,it was discovered,back in the 1920s,that a body of revolution (say a Zeppelin body/fuselage) which had a drag coefficient of X in free air,would have a drag coefficient of 2X near the ground.The drag behaved as if you had placed the object on a mirror,and the air was striking both images,hence "mirroring".In early windtunnel photographs you will actually see car models joined at the wheels,one right side up,the other upside down,hanging in the air stream.When viewed from the side,if you drew a line which enveloped the double-car combination,hitting all the high points and spanning the voids,the line might describe an egg,as in the VW Beetle.The length of the "egg" divided by the height of the egg would define it's "fineness ratio".Its kinda like the aspect ratio of a wing,where the double-car is the airfoil.Kinda weird! Anyway,since all vehicles are in ground-effect,they all suffer from this virtual "mirroring" effect.So if you divide the length of your car by its height and say its 3-to-1,which would otherwise be very efficient,as far as the air is concerned,its only 1.5-to-1,an extremely blunt body,and not a good candidate for low drag.Hucho's first English translation edition from 1987 shows this whole affair on page 200.By lowering a car,you effectively stretch out the "egg",improving it's fineness as you approach 6-to-1,the ideal for anything in ground-effect.If you go longer,profile drag improves at a lower rate than skin friction which begins to climb,canceling out the benefit of any additional length.A wing has extremely low profile drag but extremely high skin friction and is unfit as a model for car bodies.A symmetrical airfoil of 6-to-1 aspect ratio,cut in half lengthwise,would describe the lowest drag car body.Not really practical because of the necessary length,however it is the "pumpkin-seed" form chosen for world-class energy sippers.Our challenge is to get as close as we can to the efficiency of the pumpkin-seed while addressing all the other parameters of automotive operation.

This is a tricky issue because it is a balancing act. Without proper testing, I can't imagine any of us offering more than an intuitive suggestion.

My intuitive suggestion is to lower the ride height until ~6 inches of clearance exists between the road and frame, fair everything in sight as smoothly and thoughtfully as possible, and install stiffer springs that keep the car stable at an AOA (angle of attack) of 0 degrees.

The only thing I remember having read on the subject is this online chapter that deals with race car aerodynamics. Here is the paragraph in question with its respective graph:

Fig. 6.18 - Cd vs. Ground Clearance
http://www.bentleypublishers.com/exc....nov.06.sd.gif
Lift and drag coefficient increments as a function of ground clearance (based on 1/5-scale wind tunnel test and model with smooth underbody and fixed ground plane).

I do believe decreased ground clearance will eventually have a detrimental affect on overall drag, but I have no idea at what point. All modern aerodynamic cars, from Shell Ecomarathons to car prototypes, have gone either way. I think experimentation is the only true answer.

- LostCause

Gotcha :thumbup:

I'm still working on a converting a pretty complete model of my car... Once done, I can do the same test :thumbup:

CarloSW2[/QUOTE]
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""

aerohead,

Thanks for the explanation--well done! I did know about the mirroring concept, but in a totally different context (nearly all of my knowledge/experience is with objects well above the ground). The bit about considering the ratio of length to height of the car plus its image was the part I was missing. When I (finally) get my copy of Hucho, I'm sure a lot of things will be cleared up.

--Steve

Just so folks understand where I'm coming from on this, I draw a distinction between what I call "blind" testing and "educated testing". Theory (or what we can understand of it) transforms the first into the second and helps us understand what tests are likely to prove fruitful. My model for this is the Wright brothers vs the other clowns that were, in some cases, killing themselves by just throwing a bunch of stuff together and jumping off of things. While the Wrights did a ton of testing, they also did a lot of thinking about theory, even developing new theory about such things as how a propeller works--they spent a lot of time with pencil and paper before they even built models.

If you have limited time/budget (and we all do) then you have to pick the *right* tests and the only way I know to do that is to absorb as much theory as possible and let that guide you. Of course, one can take that to the opposite extreme and never get around to trying anything, so there has to be a balance.

And, of course, it may be that the theory we would most like to have is all proprietary and we won't have access to it for another twenty years. But speaking personally, I have to start somewhere and since I can't start building my conversion for a while, I may as well learn as much theory as I can.

Besides, I really enjoy learning this stuff! When someone like trebuchet03 reports an Re of 184 and that seems to me to be an odd number, I figure I'm about to learn something--after all he's got access to this cool software and testing apparatus, so he probably knows a thing or two that I don't.

--Steve

SteveP, I do agree with what you said. I just think everything we develop here needs to be understood under the pretext of "it depends." I like these types of threads...they make you think. :)

I think we'll best understand variances in drag due to ride height by making some simplifications. If we assume a catamaran configuration with completely smooth sides, we have essentially created a wind tunnel, which have been studied immensely.

Smooth Bodied Catamaran
http://i264.photobucket.com/albums/i.../Catamaran.jpg

I can calculate transistion points and total drag forces on flat plates easily, but I've never solved for ducts. I will try to determine how, but others studying aerodynamics have probably had to solve such a problem in at least one class.

If we vary the height of the duct, we can see when the flow starts to break down. Once we understand the "ideal" state, we can better infer how normal vehicles might fare. This is the most logical approach in my mind.

- LostCause

I'm glad this thread was formed... I didn't want to ruffle any feathers, but in a world where 90% of the [leading] populations have been led to believe that 19 guys with box-cutters destroyed the USA; it's time to re-think things from the ground up - the way Thomas Edison did things.

He once asked his students to determine the volume of a lightbulb and they scribed formulas on their notepads for hours upon hours on end. Eventually a solution was reached on paper. To wit - Edison took a lightbulb, unscrewed it and poured water into it until it was full. He then, poured the bulb full of water into a flask and measured the volume of a lightbulb - not by measure, but in practice (experience); and in a matter of seconds.

"In science, inducing controversy is the greatest way to achieve progress; particularly when great minds are involved."
-Chris Bergeron (unless it's been said before, that is ;)