Does reducing ground clearance REALLY reduce drag?

[MetroMPG]

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.

[brucey]

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

[tjts1]

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.

[SteveP]

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

[tasdrouille]

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%.

[Cd]

It sure helped for the Ford Fusion Bonneville team.

( Plus, it makes the car look cool ! )

[SteveP]

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

[aerohead]

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.

[SteveP]

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

[cfg83]

tasdrouille -

Originally Posted by tasdrouille

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%.

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 !!!!!

CarloSW2