That E46 roof spoiler

IIRC the theory behind roof spoilers is that the downforce is applied to the balance centre of the car, so if the car is nicely balanced then you get equal downforce at both ends.

It's all cobblers, of course, since unless you're on a race track and doing about 150 mph, it's going to make bog-all difference to the handling. You could try removing it and seeing what happens. If you immediately crash, I was wrong.

I think you''l find they start becoming effective at far lower speeds than that. ISTR it's at something like 50-60mph. Mike.

I doubt very much if 99 out of 100 drivers would notice at those speeds.

But since a blind test is unlikely, we'll never know.

Thanks everyone.

And what has that got to do with my comment? I also think your explanation of their purpose is incorrect. AIUI they are intended to reduce drag, rather than contribute any downforce. Mike.

The inference was, by your use of the term "start becoming effective", that it would have a noticeable effect.

It may very well be. If you look, I said I wasn't sure.

So in other words, you don't know either.