Some BMW owners have experienced cracking with this particular design of alloy wheel. Given the age of the wheels and the vehicle we would not expect BMW to be of any particular help. It is possible that a combination of the thin-spoke design, run flat tyres and the suspension setup of the Z4 makes them more prone to cracking than other designs, so maintaining the same setup even with brand new wheels may end up with the same result.

Runflat tyres have stiffer sidewalls to provide the the necessary strength in the event of a puncture or deflation, so switching to conventional tyres should improve ride quality as well as potentially sending less energy through the wheel when encountering bumps. As for handling, a wider tyre will provide more grip whereas a narrower tyre will potentially offer more precise steering response. The height of the sidewall also has a part to play in this balance, as a taller sidewall should offer greater bump absorption and better ride quality, but means more sidewall deflection when cornering hard, so it is a matter of deciding if you want to keep the ride and handling balance as they are or make small changes to the tyre specification to achieve a better compromise.

The staggered wheel set up means additional grip at the rear, whether you feel that is necessary is a personal choice - if you frequently find the traction control light flashing during spirited driving then it may be wise to keep the wider rear tyre arrangement. The OEM wheels available for the E89 Z4 had an offset of 27-31 at the front and 38-42 at the rear, so you would need to choose either an alternative BMW design matching this offset or look for an aftermarket wheel that matches this. Even if you are just fitting new tyres we would recommend a four-wheel alignment, so it's a good idea to have this done when fitting new wheels.