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-   -   Video: tuft testing front wheel skirts (Geo Metro) (

MetroMPG 10-04-2008 09:25 PM

Video: tuft testing front wheel skirts (Geo Metro)
(See also:

And -

Next step: Designing/building front wheel skirts v.1.0 (Geo Metro))

Basjoos, you're going to have company soon!

FYI, two EM members that I know of have installed front skirts: see

No video yet. I'll post it tomorrow probably when I've got the "skirt on" (cardboard) footage.

But a few screen grabs of the stock situation to whet your appetite:

0 km/h:

60 km/h:

The video pretty much shows what you'd expect to see (detached flow in the first 2-3 rows aft of the wheel well, and noticeable wiggling in the tufts the rest of the way back).

What will be interesting to see is how much calmer those rear-most tufts will be with the skirts on.

You should see the camera rig I set up to capture this stuff. :)

Daox 10-05-2008 09:16 AM


Originally Posted by MetroMPG (Post 65256)
You should see the camera rig I set up to capture this stuff. :)

Yes, we should. However, WE can't post pics of it. :P

Good stuff Metro! :thumbup:

Also, would you mind doing some even higher speed test (100 km/h / 60 mph) for us that drive faster?

MetroMPG 10-05-2008 11:00 AM

1 Attachment(s)
Regarding speed: tuft testing doesn't require high speeds. It's just flow visualization. Speed matters more when trying to quantify some aero mod, where 100 km/h may reveal something you can't detect @ 60 km/h.

Also... my camera rig probably wouldn't permit very steady pics @ 100. Or stay attached to the car, for that matter! :D

Julian @ AutoSpeed seemed to like 70-80 km/h for his tuft testing experiments. I was aiming for 60 km/h.


I captured the "skirts-on" video this AM, and again I was surprised by the results!

Just as I was surprised at what the tufts showed on the rear window/trunk of the Corolla after sticking AirTabs on the roof, sticking cardboard wheel skirts on the Firefly had a bigger apparent impact than I expected.

Here's a still captured from the "skirts-on" video (I'll post that later today):

What I saw was ALL tufts streaming back cleanly/quietly, with the exception of the one on the bottom row immediately aft of the skirt (the curve at the aft end of the skirt was too great = flow separation). Occasionally the tuft aft of that one would wiggle too, but not as much.

The biggest surprise to me were all the other tufts on the door. Not nearly as much wiggling as in the first vid. The noticeable effects of an open wheel well extend much further back than I would have guessed.

More pics & vid coming...

Bicycle Bob 10-05-2008 11:24 AM

I've been thinking that the best way to hinge a tight front skirt is from the top of the fender, so that it stays roughly parallel to the airflow when it opens. I think it would be adequate to let the tires rub on the skirt to push it open, although rolling elements could be added. At low speed, when you would usually want the skirts to open, only light retaining springs would be needed. To keep them closed at speed, it might be nice to supplement the spring with a large bellows fed by ram air. Speed-controlled latches would not allow skid recovery on ice. Mechanical interlinks with the steering system might have unfortunate feedback, but servos controlled from the steering wheel could be ideal. No rubbing; you'd just have to remember the extra width in tight spots.

In general, I think that fender skirts would benefit by overlapping the body somewhat, rather than filling the existing hole.

MetroMPG 10-05-2008 12:08 PM

3 Attachment(s)
Bicycle Bob:

I'm planning to hinge mine at the top, as you suggest and as basjoos did on his Civic:

Though once I start construction, I'll probably wish I'd followed the "rubber membrane" method Newton used on his Mira:

I also like your idea of servos for hinge control (hadn't thought that there might be feedback if there were a direct link to the steering bits).

But I'm going to keep it simple and go with a low friction plastic stuck on the back side of the skirt which the tires will contact when they are steered. basjoos went with low profile metal rollers.


For chuckles, some pics of the camera rig, or more technically, "web cam on a metal stick, secured with duct tape, a nylon strap, rope and a bungee":

The boom is an extruded aluminum U-shaped member (the track from a large vertical blind), secured to a roof rack. Between the "skirt off" and "skirt on" video sessions, I added the rope that goes from near the end of the boom to the bottom of the passenger door, tensioned with a bungee cord to the rear of the car. That put some pre-tension on the boom and reduced the camera bounce/wiggle compared to the runs without it.

It stuck out about 4 feet from the side of the car.

I was careful not to overtake cyclists or drive close to sidewalks or utility poles! ;)

Bicycle Bob 10-05-2008 12:24 PM

Wow, thanks for the details. I had thought that Basjoos' skirts were fixed. So much to read. . .

Cd 10-05-2008 07:32 PM

Thank you for doing this !
I'm amazed that you did not get pulled over and ticketed for having something extending that far away from your car !

Very impressive.

( What surprises me is that your hubcaps are totally smooth, and your wheel well gap is tiny compared to most cars, yet there was such a large effect to be seen ! Imagine the effect on other cars ! )

Hasbro 10-05-2008 09:26 PM

While this smooths out the side more is it possible more turbulence is created underneath due to air not escaping from the rear of the well?

gascort 10-05-2008 10:26 PM


Originally Posted by MetroMPG (Post 65344)
I was careful not to overtake cyclists or drive close to sidewalks or utility poles! ;)


Funny - a roof rack being used to help FE (in the long run). Nice setup.

Blue Bomber Man 10-06-2008 02:46 AM

I havent started designing my car yet, but I gave some thought to the wheel skirt problem for the front. I was thinking of having a flexible fabric of some sort and have a guide bar that was attached around the wheel to the steering mechanism, that would push the fabric clear of the wheel during turns.

Once I get my copy of SolidWorks I will try and draw up something for you to see. (Probably about two months out still, upkeep on my car just set me back $900)

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