I've got 18" 320's with Pirelli CINTURATO P7 RFTs on my X1. On bumpy roads it's a hard drive. Hit a pot hole and you know it! I don't think it's so much the size of the wheel, but what tyres you have. As much as the RFTs are great on smooth roads and are very grippy in the corners, non RFTs (IMHO) will give a much better overall ride.