I've suffered 5 of these now (all rears)...and BMW simply don't want to know. That's a cost of about £4k!!
The first time I contacted them the guy on the suport desk claimed he had never heard of a cracked alloy from BMW before (yeah right!!) and as it turns out, he didn't even log my call!!
3 were MV4 (style 225)
2 were Twisted Spoke (style 230)
I've moved away from RFTs, and dropped the pressures slightly. I've even been taking the same route to work for 4 years and know where all the potholes are etc and make sure I don't hit any of them...makes no difference. The only thing I have noticed, is that the colder the weather, the more likely the crack as the alloy hardens in the cold.
BMW are offering to 'test' my wheel but the last time I did, they said it was out of tolerance and charged me for the testing. Of course it's out of tolerance..it's cracked!!
how is it that Porsche, Audi, Mercedes and even Ford don't have this problem yet drive on the same roads as we do??