Measuring trunk hatch buffeting with an iPhone barometer

[EVChris]

I took delivery of a 2015 S85 on Tuesday. It's a very nice car with some warts that I'm going to have to remove. One of them is that the ride is extremely harsh and another (related) is that the trunk hatch buffets. Indeed driving on the stretch of 580 from Hayward to Oakland, about 13 miles, I'm practically throwing up by the end. This is jointed concrete which I've never even noticed in the 100s of times I've driven in multiple cars.

Yes, there's a fix for the buffeting, but we can measure it first. Starting with the iPhone 6, the iPhone has a barometer, a Bosch BMP282 Digital Pressure Sensor. The specs say that it can be sampled at 150 Hz but the app I used, SensorLog ($3.99 in the AppStore) only lets you sample at 100 Hz. There is some elevation gain+loss on the drive. Joints are about 15 feet apart and I was doing maybe 65 mph or about 95 fps or about 6 hz or so. So the 100 Hz sampling rate seems high enough.

Unfortunately, the BMP282 has an IIR filter to smooth over the effects of slamming of a door or window. And the CSV data really looks like that. I've sent a note to the author of the app seeing if it's possible to turn the IIR smoothing off.

The noise floor, sitting parked, looks like:

Driving on 580 looks like:

 

[cab]

You can try to measure it with just an audio recording. In the worst cases, it might sound a bit static-like or like when "wind" hits a singers mic at a concert (or the singer is right on the mic). If you can find a washboard surface close to home that will replicate it, that's your best test. Unfortunately, unless you can A/B two cars back-to-back on the same road surface with identical equipment, I'm not sure what you'll get out of it.

I recommend starting with adjusting the hatch adjusters on the body and hatch lid and going for a test drive.

 

[EVChris]

+1 on the hatch adjusters although part of me wants to leave this unadjusted until the service on Thursday.

I measured the air pressure just because I thought it would be interesting and it certainly registered. But when I went to look at a higher sample rate to see if it tracked the road joints, I noticed it was filtered data. Bummer and the app author hasn't gotten back to me yet. I know that others have measured the bumps with an accelerometer.

 

[cab]

On the adjusters, you can always just turn them (count the turns), test it, and then put them back before your appointment Thursday. It's all pretty low tech. As an aside, there are 3 pairs of adjustment points:

1. On the ends of the hatch itself - two rubber legs.
2. On the lower body - The two little pentagonal round discs the "legs" on the hatch sit on. As a side note, I'm not 100% sure these can be lowered and raised on every Model S. They may have been "fixed" in some years.
3. On the upper body, middle area of the hatch - two rubber adjusters.

Good luck!

 

[FUSKED]

Your issue sounds like mine. However, my situation progressed and the adjustments are now moot. The catalyst of your buffeting may be linked to the threading that’s fastened to the lower shim on the body under the liftgate. Photos attached. Let me know how your threading looks. How’s the spacing of your liftgate to it’s adjacent panels? Thanks