Does reducing ground clearance REALLY reduce drag?

[DifferentPointofView]

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.

[Gone4]

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.

[SteveP]

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

[diesel_john]

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?

[SteveP]

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.

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

[trebuchet03]

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

[SteveP]

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

[trebuchet03]

Originally Posted by SteveP

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 So if there's some obvious flow issues, we'll see them

[cfg83]

diesel_john -

Originally Posted by diesel_john

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?

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

[Coyote X]

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.