Buffeting With Sunroof Open

[Chief]

If you're driving with your sunroof open, especially as you approach highway speeds you may notice what feels like a pressure change in the cabin, almost like the air is being pumped in and out.

When you open a sunroof on any vehicle an area of low pressure is created. Air inside the cabin tries to come out through the opening but is pushed back by the rushing force.

To solve this problem you only need to crack open one or both of your back windows. This allows air to flow out and breaks the pressure seal.

This isn't a defect with your Chevy Avalanche, as Scottie on the old Star Trek use to say, "you cannot change the laws of physics!"

 

[Scott Minyard]

I didn't notice this problem until I recently had a wind deflector mounted above the sunroof. What effect, if any, does the deflector have on the situation?

 

[Chief]

The add-on deflector (I'm going to take an educated guess that it is smoke colored plastic and similar to a vent visor in shape) raises the airflow higher over the opening of the sunroof and reduces this pressure effect.

Without the deflector in place think of the roof of our car as an air foil. The air in your cabin is not moving at all, and thus creates an area of higher pressure. The air streaking by your roof at 60 MPH creates a zone of low pressure - this is called the Bernoulli Principle (well sort of - although the opening of your roof does not exactly create the shape of a wing - but it does create these pressure differences).

Because nature hates a vacuum, the low air pressure above your roof wants to be filled. As the pressure continues to drop it eventually takes a gulp of cabin air (and creates the buffet). Now the process begins again, and the faster you go, the more frequent and more severe the buffeting becomes.

The auxilary deflector you've added has broken the air foil shape by lifting the moving air over the sunroof opening. The pocket of higher pressure is now located immediately behind this deflector, up on your roof, and the air pressure in your cabin remains equal.

Ya - I can be a geek sometimes...

 

[menacng]

I took the advise of another post and adjusted the "wind deflector springs". When it comes from the factory the springs are not "sprung" enough.
If you look at each end of the wind deflector you will see 2 squeeze clips that hold the wind delector to the spring assembly. Carefully squeeze those clips with needle-nose pliers and release the deflector from the spring assembly on both ends. After doing this, the entire spring assembly will slide out. There are 2 prongs on each spring, with pliers and needle-nose pliers move the prongs a fraction of an inch to make more tension. Carefully, It doesn't take much.
This will work great. I can go from 0-100 mph and nothing but the sound of the wind. Good luck!

ShapeShifter Cleaned up extraneous text in subject.

 

[ShapeShifter]

Chief has a good discussion as to why this happens. Another way to look at it is to picture a flute: it is a hollow tube with a hole on the side of it. You blow across this hole and it sets up a "buffeting" that creates a standing pressure wave inside the tube. The resulting vibrations create a note. By covering and uncovering holes in the tube, you change the length of the pressure wave, and the pitch of the note changes. The longer and larger the tube, the lower the note. This is why a piccolo is so much higher in pitch than a flute: it is much smaller, therefore vibrates faster.

Now, back to the Av: With the sunroof open it is basically a very large flute. The air passing over the opening creates the same sort of pressure wave inside the truck. The big difference is the pitch: Instead of being a couple of thousand times a second like in a flute, the frequency is down in the range of once a second or so, due to the large interior volume. Basically, a super-bass super-loud flute.

Opening the windows changes the tuning (pitch) of the flute, and prevents it from hitting that resonant frequency that can cause pain to your ears.

Raising the deflector breaks up the flow of air across the opening, and creates enough turbulance that it disrupts the buffeting action.

It's just like blowing across the mouth of a bottle. Hit it just right, and the air inside vibrates and generates a tone. The bigger the bottle, the lower the tone. Change the way you blow across it just a bit, and the note is gone.

-- SS

 

[nh_mark]

I've had my Av since 7/02 and I noticed this problem right away. ?This is my first vehicle with a sunroof so I figured that just the way it is. ?No big deal since I usually never have just the sunroof open anyways. ?But recently my wife got an '03 TrailBlazer with a sunroof and she can open her sunroof with absolutely no buffeting. ?It's kind of nice to be able to open only the sunroof, specially on days like today (45 degrees).

Now I understand and I agree with Chiefs original post but this makes me wonder why the T.B. is ok and the Av is not? ?Could it be that the distance between the sunroof opening and the back of the vehicle is shorter in the Av than it is in the T.B.? ?She has the EXT model.

-Mark