Does reducing ground clearance REALLY reduce drag?

[Otto]

Originally Posted by 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.

Lowering the rear end is probably the worst thing you could do: It just makes the underbody a trap for air, acting essentially like a boogie board that kids use to skim along the shallow water at the beach. If you're going to lower anywhere, lower the front of the vehicle, adding some rake so the vehicle wedges under more air (slightly negative angle of attack) and gets some downforce rather than lift. Go with the front air dam, too, then undertray and/or wheel fairings.

On BMW forums is an ongoing debate about cutting one coil or so off the front springs, giving the car a bit of rake. Some say, only go with expensive aftermarket springs. Others, who've actually tried it, say cutting ~1-1.5 coils off the bottom of the front springs lowers the front by ~1-1.5" , stiffens the spring rate a bit, yet gives essentially the same ride quality. That said, don't cut springs with a torch, which ruins the metal temper. Keep them cool, cut with cutoff wheel or Dremel disk, lubricating and cooling the cut with cutting fluid or oil. (Wrap the spring just above the cut with a wet rag, so the metal does not heat much.) Obviously, first check the spring perch to be sure it will mate with the diameter and pitch of the spring after it's cut. If you do this, proceed at your own risk, consult an engineer, and don't sue me.

[twinair]

Originally Posted by Otto

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.

That's a good point and I can see how this could still be explained with shear forces. As the air velocity between those two bodies would be accelerated (similar to a venturi nozzle), it would generate much higher shear forces than if one simply calculated with the aircraft velocity.
And in this case it is worse than with a car, since both bodies are moving and not just one surface and if one compares the Hagen-Poseuille equation for two moving parallel surfaces against just one moving surface the 2 moving surfaces have a 12 times higher resistance (if everything else is kept equal and again assuming laminar flow, which is not the case here).

A rounded nose aerodynamic vehicle would also increase shear forces, as the rounded nose would accelerate the air moving underneath the car and therefore disproportionately increase shear forces (which would to some extent also explain why a pitched down round nose is more efficient than a pitched up nose).
Having higher air velocity would also get air to reach a turbulent regime sooner which would increase shear forces even more.
However, I think that this wouldn't play a significant role with a passenger car, as the air would be turbulent almost from the start and the nose isn't usually very round either.
So I still think that any practical lowering (>3" above surface), wouldn't be detrimental as long as the back end is not lowered more than the front end.

Originally Posted by aerohead

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.

Thank you for the tip and Happy Thanksgiving. I ordered the 2013 edition of the German-Hucho-Version (I live in a German speaking region) and look forward to receiving it.

[radioranger]

Thanks, that's why I was thinking of just lowering the rear a bit and adding to the air dam from the factory and radiusing it outward if I can get it level with the front tires, seems like it wont mess anything up ride wise, as the front takes most of the hit .anybody with the money or sskill can do major stuff, I'm a little shy of both . so after max results per buck and relatively easy .On the original shelby mustangs they had a cable if I remember right to prevent the rear shock from over extending. maybe i could do something along those lines although rear springs might be easier and refresh struts at the same time

[twinair]

Here are two pictures of the Renault Vesta 2 which apparently managed to consume only 1.94 l/100 km (121 mpg) from Bordeaux to Paris at an average speed of 100.9 km/h (1987): https://fr.wikipedia.org/wiki/Renault_VESTA_2
I would argue that they went as low as practically feasible.

[Superfuelgero]

That car lowered at speed actually.
Renault VESTA 2 - Wikipédia
As did its counterpart
http://www.citroenet.org.uk/prototyp...eco2000/03.jpg
A good thread on this 80's trend that died
http://www.forum-auto.com/automobile...ujet156015.htm

[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?

I came up with 3-pages of notes with respect to lowering.
*The first thing which concerns your formula is that the boundary layer is turbulent and typically the flow is turbulent.Shearing forces aren't really present because of the thickening boundary layer.
The turbulence itself is the issue,as the rotational kinetic energy cannot be converted to useful pressure.
*Ground clearance effects will be affected by:
# degree of underbody roughness (R.G.S.White of MIRA allowed a rating of 1-5 as a degree of roughness)
# body inclination
# body camber
# presence of an airdam
# presence of a diffuser
# variability of wheel/tire/wheel covers
# presence of wheel fairings
# wheelhouse volume
# cooling system extractors
# rocker panel depth
There are just too many variables within the product mix to make 'one-size-fits-all' high confidence predictions.
------------------------------------------------------------------------
Historically,the lowest drag concept cars have full underbody paneling as well as active suspension to lower the car at highway speed.
The Cd 0.152 Ford Probe-IV extends its active airdam, lowers its nose to 3.1-inches G.C., then hikes its booty to achieve the ideal body inclination for lowest drag.
The C.G. is benefited as an aside.
I suspect that the 1985 Cd 0.137 Ford Probe-V is doing likewise although no published data exist so far on that one.

[user removed]

If someone was that close to my arse at 100 KPH, they would regret it.

regards
Mech

[radioranger]

Just for a try I was thinking of running two space saver spares on the rear of the escort and try a few runs on my 65 mile commute, also noticed the picture car has the one mirror well back on the door, was thinking of doing the same as well maybe mount it on a extension of metal rearward.

[euromodder]

The mirror seems to have a streamlined cap in front of it.
But you really want to streamline the rear end !

Check out Vekke's fully streamlined mirror solution in his Lupo 3L to 2L thread.


MetroMPG has tested donut spares on his Metro, and the results were not what you may expect. Though narrow, they don't seem to roll very well, and turned out worse than his regular tyres.

[justme1969]

Radio R
I will tell ya Ive played with this alot and I seen where you final plan is for lowering rear and spoiler scouped front.
You should go with the whole car down first then work on belly pans and front spoiler not so much in that order.
I did find extra benefit from my rear being dramatically lower then front but you might not.
And if you go this route I can testify the side skirts will become a necessity at some point. My car drops 2.5+ inches at 70 mph.
I chose to retain this effect from wore out front struts only because my spoiler and skirts are 3" off the ground and 60% of my driving is free way at 65 -77 mph.
the car becomes more stable at higher speeds but it is a false reaction and any disruption in road smoothness will make the car dangerous to handle after 90 mph.
thus it does what I designed it to do well but that is all And I would never allow anyone else to drive it because of its characteristics.
Feel free to lower but realize all the effects it causes not just the obvious ones you planned.