[pw4]

The G42 does a remarkable job of vacuuming up dirt and general gunk from the road with its Kamm tail. This is my freshly-cleaned car after just a half-day on wet roads, with a couple of small patches of dirt from farm driveways.

[Bandit009]

Originally Posted by pw4

The G42 does a remarkable job of vacuuming up dirt and general gunk from the road with its Kamm tail. This is my freshly-cleaned car after just a half-day on wet roads, with a couple of small patches of dirt from farm driveways.

This is true of my car too! The back end loves to get dirty.

[ZeusHorkios]

Yeah, I've noticed this too!

[Coconut]

All my cars have had filthy backends. Something to do with low pressure zones at the back

[pantdino]

Originally Posted by Coconut

All my cars have had filthy backends. Something to do with low pressure zones at the back

Yes, a low pressure zone develops behind the rear which pulls up dirt from the roadway.

I can't find a video example, but I've seen what looks like a dust-laden horizontal whirlwind following along behind the car.

https://en.wikipedia.org/wiki/Kammback

[BimmerVinyl]

Same problem! LOL

[VegasG42]

Yep. I went to reverse the car out of the garage after the previous week of rainy weather. I was surprised at how bad the rear camera image was until I looked at the rear end and saw how much dirt and water spots were all over the rear of the car

[Evilmoretea]

Guess the diffuser isn’t just for looks

Tbf I think this is common with most cars and one of the reasons hatchbacks usually have a rear wiper whereas coupes don’t.

The floor of the G42 is incredible flat as well which probably accelerates the effect.

I’m not a scientist don’t quote me, just a fool rambling.

[RockCrusher]

Originally Posted by VegasG42

Yep. I went to reverse the car out of the garage after the previous week of rainy weather. I was surprised at how bad the rear camera image was until I looked at the rear end and saw how much dirt and water spots were all over the rear of the car

My 230i back up camera gets pretty dirty. I've come home and before I can back the car into the car have had to get out and wipe the camera lens clean so I can see the back up image.

[RockCrusher]

Originally Posted by Evilmoretea

Guess the diffuser isn’t just for looks

Tbf I think this is common with most cars and one of the reasons hatchbacks usually have a rear wiper whereas coupes don’t.

The floor of the G42 is incredible flat as well which probably accelerates the effect.

I’m not a scientist don’t quote me, just a fool rambling.

The diffuser and the rear spoiler and to some extent the body's shape.

First car I really noticed this with was my 996 Turbo.

My MINIs (hatchbacks all) did this. I didn't drive my Challenger Scat Packs and Hellcat in bad weather. But even in good weather they all collected some dirt (and really black dirt which is tire "dust").

Don't drive the M2 in bad weather. But its back end -- the top of the rear bumper especially -- collects dirt and some of that black tire dust. But not as bad here compared to CA as the traffic here isn't nearly as heavy.

The 230i I drive in bad weather. So yeah it collects grime and even grit as the roads are salted and sanded here.

[kwikit356]

I wonder if the people who have installed larger (higher) rear spoilers have as much of a problem as the rest of us do?

I have a '16 981 (not a 918!!) Spyder that has a much larger built-in spoiler (that is also more vertical). The rear of that car stays a lot cleaner.

[RockCrusher]

Originally Posted by kwikit356

I wonder if the people who have installed larger (higher) rear spoilers have as much of a problem as the rest of us do?

I have a '16 981 (not a 918!!) Spyder that has a much larger built-in spoiler (that is also more vertical). The rear of that car stays a lot cleaner.

FWIW my Boxster's rear didn't collect dirt nearly as bad as my Turbo. The Boxster had the factory spoiler which deployed automatically at IIRC 70mph. (My 996 Turbo likewise had a factory spoiler which deployed at 70mph.)

Something about the shape of the Boxster vs. the Turbo -- and they were considerably different at the rear -- had the Turbo hoovering up/collecting fine dirt/tire dust compared to what collected on the rear of the Boxster.

[Fronzdan]

I’d seen this thread in passing but have now lived it for the first time. Yesterday was the first time I drove on wet highways during a drizzle. Holy crap that ass is filthy. It looks just like that but even more noticeable in Brooklyn Grey. Going to have to do a quick wash later if temps are above freezing.

[kwikit356]

I hope someone with a taller spoiler will let us know if that makes any positive difference???

[rons e92]

Originally Posted by kwikit356

I hope someone with a taller spoiler will let us know if that makes any positive difference???

I had fhe OEM CF spoiler on my G42 and it still sucked up an insane amount of dirt in the rear. Significantly more than the F30 330i I had before that, and more than the 2020 WRX I had before that, and more than the 2009 MB C300 I had before that.

Strangely enough, it seems like my G87 with the same spoiler sucks up slightly less dirt in the rear. It still does it, but it takes a bit longer to reach the same level of dirt.

I know not quite the "high" wing you were looking for, but hopefully it helps.

[kwikit356]

I wonder if something that actually directs airflow from the top of the trunk in a downward direction could possibly disrupt what is causing the rear of the car to attract so much dirt. One possible problem is that we certainly do not want to create any lift. There is also the possibility that something underneath the rear of the car could redirect airflow. I'll try a few things with tape and cardboard- not very sophisticated!

[Rdurty2]

Just wash the car more often

[kwikit356]

Originally Posted by Rdurty2

Just wash the car more often

I do wash my car more often when the weather is warm enough to do it at home. It's even easy to rinse off the dirt on the rear. During the winter cold months getting the accumulated dirt/salt off the rear end without damaging the paint is more involved.

The bigger issue for me is that the dirt and salt accumulates so quickly on the rear that it can actually be somewhat of a safety factor, particularly if I'm driving any significant distance in messy conditions.

I wonder if anyone on this forum has a '22-'24 VW Golf GTI or R with the changed rear spoiler. The rear spoiler on my '18 Golf R was a simpler version and allowed quite a bit of extra dirt to cover the rear of the car- not as bad as the BMW G42/G87 series, but definitely a negative issue. If the newer Golf spoiler helps keep dirt off the rear that could give us some guidance.

[Littlecoupe]

Besides the dirt accumulation, the other day during a snowfall and fresh snow on the road, my backup camera was CAKED in snow. The entire rear, in fact, was caked in snow, more than any prior car I can recall.

The benefit is photo radar can't photograph a license plate caked in snow