I've experienced exactly the same thing after having my rear tyres replaced on my BMW 325i Sport touring. Originally I had a different make of tyre (to that of the fronts) fitted , but the handling was soooooo wayward that I took the car back after about 1,000 miles and demanded another set of (same make as the front) tyres fitting. This was done at my expense, as I was convinced the original tyres I had fitted (Falken FK451's) were faulty. I had the Falkens sent back to Dunlop (who make them) for testing, but the results came back negative for defects.
This wasn't exactly a surprise in the end, as the replacement new tyres (Dunlop sp sports) were just as bad!! I checked everything, and experimented with what must have been 8 to 10 different tyre pressure combinations between front and rear tyres in an attempt to restore my car's once faithful handling characteristics.
I've now done about 3,000 miles on the new rear tyres and the situation has only improved marginally. The sensation is almost like the rear end being on castors! Very entertaining, but not good for the nerves! At low-ish speeds (up to 80 mph) the car feels ok, but above these speeds when you really need to start trusting your cars behaviour it's a nightmare! Bizarrely, this affliction seems worse on the left rear tyre than the right rear tyre i.e. through right hand bends. The DSC system, which I normally turn off because I hate it, simply cannot cope.
I'm beginning to suspect it might, just might, be a balancing issue, although I can't detect any discernible vibration. A friend has suggested the problem arises when there is a large differential between the amount of tread-block movement between front and rear tyres, but this doesn't explain the difference between left and right hand corners.
The car is going back to BMW soon (for good!) as I'm replacing it with a 993 (unlucky dogs!), but I would still be very interested in hearing any opinions on what, it would seem, is quite a common issue.
I'm just sorry I can't shed any further light on the subject.
Dean.
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