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I just noticed something strange while driving my spider with the top down. When driving around 50mph, I noticed the wind is spinning around me but when I sped up to 80mph, the only wind is on my hair. How can that be? Can anyone explain the physic behind this?
 

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As your speeds increase, higher air pressure against the windshield causes the air to arc over the top and around the side areas of your car. You're head is in a lower pressure area causing your hair to swirl around.

Since the air is "unattached" to the bodywork, this creates more aerodynamic drag. Deck lid spoilers in addition to creating downforce at the rear of the car, "reattach" the air, smoothing out the airflow as it leaves the body. Dirty rear windows, especially in station wagons, is caused by this swirling disturbed air picking up road dust. If your Alfa is leaking any oil (like mine) the droplets deposit on the rear panel. I had a 1965 Corvair in the Seventies and Eighties, and the leaking oil did the same thing.

Contemporary cars reduce this detached air along top by arcing the roof body design. Notice how new car no longer have flat roofs?

Here are a couple of links to aerodynamic tuft test on both an early and late model Corvair (scroll down to Corvair Aerodynamics:

Welcome to the GNRT Web Site

While a late model Corvair visually may seem more aerodynamic, the boxier early Corvair actually is more aerodynamic. Notice how all of the wool tufts are "disturbed" on the '66 rather than lying straight & lat along the bodywork as in the '64. Putting the rear bumper back on the '64 would probably clean up the air slightly coming from under the body and sides.
 

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NOTE: Despite links show "Browser Warning", they do work with no evidence of computer harm on my Mac!

Additional link showing aero tuft tests on some contemporary cars.

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Smoke tests in a wind tunnel. Scroll about halfway down to see the effect of convertible in the top down position:

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It is good to also note that at a speed of over 45 MPH you will not get wet in a light rain. The harder it rains, the faster you must go to stay dry. IT REALLY IS A LAW OF PHYSICS, AS STATED ABOVE...
 

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Spiders wil do 80??? One aside always of note to this untrained observer is the form the top takes when "up" as a car runs down the road.
 

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It is good to also note that at a speed of over 45 MPH you will not get wet in a light rain. The harder it rains, the faster you must go to stay dry. IT REALLY IS A LAW OF PHYSICS, AS STATED ABOVE...
You beat me to it. I've done it for short trips to the store in light rain with the top down, and it works fine. Until you park.
 

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I would drive from Santa Clara to the San Francisco Financial District in an earlier Alfa Spider in heavy rain and not get wet. Yes, with the top down!!!! People probably thought I was "nuts" but I did not get wet in a heavy rain around 80+ miles per hour. I only got a little damp on Third Street coming off the 280. Great way to clear one's mind and really enjoy a "true sports car" - Alfa Romeo Spider!!!

Steve Hurt
 

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We carry an umbrella to put up at traffic lights!
 

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Those days are over now. My spider is dry weather only now and in fact, in another month or so it'll be time to bring it out! Washington summer starts a little after July 4.
 

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I would drive from Santa Clara to the San Francisco Financial District in an earlier Alfa Spider in heavy rain and not get wet. Yes, with the top down!!!! People probably thought I was "nuts" but I did not get wet in a heavy rain around 80+ miles per hour. I only got a little damp on Third Street coming off the 280. Great way to clear one's mind and really enjoy a "true sports car" - Alfa Romeo Spider!!!

Steve Hurt
Good post. I do that all the time. You will stay dry if you drive fast enough.
 

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Yep, I could even pull off this "staying dry" trick in my Triumph TR-4! Which means that you don't have to be going 80 mph for it to work. Of course, the TR-4 has an aerodynamic profile of a shoe box, so not a direct analog of a Spider (fortunately).
 

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I personally think that my Spyder is a great little rocket. Took her down some twisties nearby and had my buddy chasing me in his 400+ whp mustang... asked him later how fast we went (85 mph speedo...), and he said we hit about 114. Considering it was slightly up hill and the staight was maybe a quater mile after a 45 mph turn, I'm pretty happy with that. Quite a fun little bugger when you get on her!
 

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But how did he know?
If the corner was marked 45, you should have been going through at closer to 60~65 if not faster and he should have fallen off the road at 55, so at the end of that 1/4 mile he wouldn't have been there to compare against anyway :)
 

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Darren- haha, very true. It was a blind turn so I stayed at 45 until the straight, and he's heavily modified his car not to handle like a Mustang any more. :D Even with all his upgrades and power though, I'm pretty sure I could still lose him on some tight twisties. Some red springs and yellow shocks wouldn't hurt my odds...
 
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