Do wheel skirts really work?

[theclencher]

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[cfg83]

theclencher -

Originally Posted by theclencher

I think Bill is on the money.

As for why aren't skirts in general use: they are extra cost and weight, they interfere with checking tire pressure, they interfere with tire changing and other wheel service, and last but not least, some of the lowest drag cars in existence don't have 'em!!! So they must not be that critical.

And maybe they're not in fashion anymore? While I have nothing against them, I have always identified them aesthetically with great big Detroit-dinosaur cars.

CarloSW2

[landspeed]

I suppose the frontal area also counts, because the Cd is the drag for the shape, while the CdA is the actual drag for that size car .

So, the insight might end up with better aero in the end due to smaller frontal area than the other cars.

Another thing not really considered is the effect of wind speed. I know that, on aeroplanes, the faster you go, the flow seperates earlier from the aerofoil. So, maybe this also happens on cars?. It might be that, at 25mph, my car has good aero from the rear hatch, while at 50mph, the Cd increases (in addition to the actual drag increasing), as the air is flowing too fast so it seperates at the roofline.

[trebuchet03]

Originally Posted by landspeed

Another thing not really considered is the effect of wind speed. I know that, on aeroplanes, the faster you go, the flow seperates earlier from the aerofoil. So, maybe this also happens on cars?. It might be that, at 25mph, my car has good aero from the rear hatch, while at 50mph, the Cd increases (in addition to the actual drag increasing), as the air is flowing too fast so it separates at the roof line.

This happens on anything with flow moving around it - the transition point can be found with the Reynolds number equation set to the transition Reynolds number - then solve for the characteristic length. Everything after that point can be considered a turbulent zone and the boundary layer starts getting bigger

No so easy on complex shapes like cars -- so experimentally, you can use tuft testing or smoke testing

[silverinsight]

I'm considering closing up the front wheel arches on my Insight, something like the ice in the first picture.

Attachment 328

After looking at the other two pictures, it looks like my wheel skirts add drag to the car

Attachment 329
Attachment 330

[MetroMPG]

Originally Posted by silverinsight

After looking at the other two pictures, it looks like my wheel skirts add drag to the car

Can we conclude that from water/debris flow patterns?

When I look at the bottom pic, I think we're seeing the effect of water getting channeled upwards and backwards in the skirt's forward seam, and reaching some point where some factor (volume of water?) causes it to spill out of the channel and continue being swept rearwards.

[MetroMPG]

I like Houston Bill's idea of using a video camera to record an analog gauge for coastdown testing. It would permit you to remove most of the human variable of coordinating the stopwatch with a moving target (needle). Tougher is ensuring you're always starting the coastdown from the same speed. This could be accomplished with cruise control & pressing "cancel" at a predetermined point though.

Originally Posted by theclencher

Still, if the effect is that minute, doesn't that say something right there?

Yep: to me it says that aside from a fully tapered boat tail, there's no magic bullet in aerodynamics. Successful aero is the sum effect of a lot of small, but effective, individual elements.

[Bill in Houston]

Originally Posted by MetroMPG

Tougher is ensuring you're always starting the coastdown from the same speed.

I think that you would start from above, and time as you coast down through the speed range. I think...

[silverinsight]

Originally Posted by MetroMPG

Can we conclude that from water/debris flow patterns?

When I look at the bottom pic, I think we're seeing the effect of water getting channeled upwards and backwards in the skirt's forward seam, and reaching some point where some factor (volume of water?) causes it to spill out of the channel and continue being swept rearwards.

I think I tend to agree. Maybe I should tape the seams and drive through a few puddles

[omgwtfbyobbq]

Yeeeesssssss. Lemme see. When I'm coasting down, I notice a significant difference depending on passengers, temp, etc... So it seems doable. We'll have something (All in metric or we crash into Mars) like Weight(Crr)+.5(ro)(Speed^2)(CdA)-Weight(gravity)sin(theta)=Force, where theta is the angle corresponding to the grade. A 10.5% grade is a 6 degree angle, so that means the potential energy component of a 14,500N car is about -15,000N. Assuming Crr=.015, the rolling friction coefficient is ~220N, so, for a car with CdA=.7m^2, the fluid friction coefficient must equal ~12,800N. Plug'n'chug, and we get the Speed is ~122m/s. Way fast. But not surprising considering the grade. Since we're sane, we leave it in gear, and end up going much slower since the spinning engine/trans provides drag. Lets say in gear we end up going 38m/s (85mph). This means we have ~1,240N from air drag slowing us down, and the other ~11,560N comes from the engine/trans spinning at whatever rpm. Lets say we drop the CdA from .7, to .68. Now we *still expend (not exactly) ~1,240N for fluid friction, but our speed increases in order to do this. With the .68 CdA we're going ~38.6m/s, which is a ~1.3mph increase in terminal velocity. Granted, there's still the tires slipping a bit, and the *engine/trans drag probably isn't linear, but it's still a nice result imo. I bet a much nicer grade (~2-3%?) with the car in N would yield similar behavior.

So, my BS seems to match up with Coyote X's experience. As long as we find a smooth enough, or steep enough highway hill, it looks like those of us w/o scanguages can figure out what improves CdA the most. Lothar approves!

I should add, that imle it's way easier to notice what the top speed on a decent is because highways tend to have pretty even grades, and the top speed will be held for at least a few seconds. A coast down test otoh, requires the driver to start coasting and a specific point and specific speed. Then accurately give the speed the instant they get to the other location. Or, make note of the location where they see the speed. Either way, there's more room for human error imo. Whereas with terminal velocity, we don't do anything except keep on eye on the speedo for a top speed, which should be present for at least a few seconds.