Buffeting (Interior Acoustical Reverberation) - With interior stripped
 

Over the winter I stripped my interior out. In the rear there is no interior. In the front I have seats, dash, door panels, and center console. At highway speeds I get crazy buffeting in the car. In my rear view mirror, I could see my rear glass vibrating a significant amount (I could see the defrost lines moving probably 1/8" in and 1/8" out). The sound pressure level was so high I could hear the hatch vibrating open and closed flexing the latch. Luckily I had ear plugs in or I swear I would have blown my ear drums.

So, how to fix. I have two ideas. I'm picturing the inside of my car as a ported enclosure basically. The higher the volume, the lower the natural frequency. The frequency was super low, barely audible, but you could feel it strong. I can't really change the excitation (window shapes, etc).

1. Decrease the interior volume. This should raise the natural frequency. I could easily do this. I already have a foam block made from acoustical absorbing foam to fit into the spare tire well. i could put that back in, and add additional thickness of the foam (basically fill the trunk up a few inches). I could make it all cut to fit, so it looks nice an flat in the back. The foam also has the advantage that it not only decreases the volume, but it also absorbs sound energy.

2. Install an additional flapper vent (like those found on cars already, usually in the quarter panel area). This should help vent out air in the rear.

Has anyone else with stripped interior had this issue? What was the fix? I'd really like to be able to have my windows down when its 90 degrees (no AC).

Remove/cut off the bottom section of weather stripping on rear glass to vent air?

i don't understand why stripping the interior would have any effect?

My stock FD has always done what you seem to be describing, maybe not to that magnitude, at high speeds any time I have both windows open, somethimes even with just the driver's window open depending on the prevailing wind. I have taken to keeping the windows up when traveling fast and just running the A/C., you may not have that option if you have also removed the A/C for lightening. Maybe a deflector at the A-Post would disrupt the airflow that is contributing to the resonance or aeroelasticity effect. Maybe the guys that track or race FD's with the windows removed have a solution such as window nets affecting the airflow enough to prevent the buffeting.

I may be wrongly assuming that this is happening with the windows down, the condition I experience it in. Regardless, i would be interested in any solution you find.

Cheers, Philip

Removing the interior changed the airflow characteristics as the air flows inside inside past the b pillars. Removing the interior also changed the interior volume, which would change the natural frequency. Removing the interior also removes any damping (no carpet, foam, etc).

I hate to try something exterior to the car, but I will at last resort. Tomorrow I think I'll try a few tests. I'll fill up the back of the car with blankets or something that will take a bunch of volume, and maybe add some damping. If that works, I'll just put a shit ton of acousitcal absorbing foam back there since it weighs next to nothing, and I never put anything back there since the only time I drive it on the way to a race. I load up my tire trailer with all my gear.

You're on the right path with the volume. On any race car you try to lower the volume of the interior as much as you can within the rules. It will help your aero over all and change the frequency of the resonance.

Look at a Trans Am tube frame car there is a wall behind the main hoop that will have a window in it so you can see out the back of the car.

it might be that you could add a little thing to the B pillar to stop it, most race cars have a cage there

I have a roll bar there as well. But I could certainly experiment with some other things in the general area.

great info, thanks

Has this been solved?

I'm about to strip my entire interior down to only the dash and door panels for weight reduction and roll cage. Do not plan on rolling around w/ windows up as the A/C is removed. Someone surely has found a way to eliminate the buffeting?

A simple vent in the hatch may work?

This thread definitely needs an update. I do not have the answer, but I can tell what did not help, and what did help to some extent.

1. Filling with blankets. No change.
2. Passenger in the car. Same buffeting but a 5-7 mph higher speeds.
3. Adding side panels that fill the space between the seat and b pillar. No change.
4. Complete bulkhead just rear of the seats. With only a small rear view mirror cutout. Same buffeting, but at 5-7 mph higher speed.
5. Taping over all and any holes in the interior sheet metal with the super thin metallic ducting tape. Buffeting still exists, but not as severe. The rear glass still shakes visibly at ~100 and above. This is how the car sits. I usually just drive with the Windows partially up on the highway.

Wow, I'd never heard of this and know of several stripped track RX7's. Maybe I'll just leave my interior in.

Stripped my interior completely except dash and door panels. No buffeting issues at any speeds w windows and roof open

Why don't you look at adding those partial window air deflectors at the leading edge of the window opening? If you could reduce the amount of air getting in to the interior, it would also reduce the buffeting. Thats why partially rolling up the windows helps as well. To release the air from the interior of my racecar, I opened up the area at the back of the car behind the rear license plate and then put the plate on a hinge ( I dont have a license plate, but I do have a Zoom Zoom plate). At speed when the air tries to build up in the interior, the plate opens up abd releases the air pressure.

Just a thought.

Eric