Does reducing ground clearance REALLY reduce drag?

[cbergeron]

[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]

(duplicate post removed - caused when Darin split the thread)

[MetroMPG]

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.

[SteveP]

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

[cbergeron]

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.

[Otto]

Originally Posted by cbergeron

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.

[MetroMPG]

Originally Posted by cbergeron

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

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!

[SteveP]

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

[fabrio.]

Originally Posted by SteveP

MetroMPG,

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

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.


--Steve

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

[MetroMPG]

Originally Posted by SteveP

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.

Hucho's book says something similar:

p. 219: The effect of ground clearance (e) on lift and drag is less clear-cut... In "normal" vehicles, ie., vehicles with structural roughness on the underbody, the drag decreases as a car is set closer to the ground.

However, vehicles with a smooth underbody, in this case the Citroen DS 19, have the opposite tendency. For these vehicles the drag increases with reduced ground clearance, in the same way as for a streamlined body [example given is the Schlor car - on right, in image below - which has an entirely smooth underbelly with cutouts for the wheels.]



This rise in drag can be traced to the increasing effective thickness of the body with reduced ground clearance... This thickness effect is more than offset on vehicles with rough underbody, as the (high) underbody drag decreases when the flow between vehicle and road is impeded.

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

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.

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.