I would imagine weight - especially for road bike tyres. For comparison, a lightweight Zipp wheelset comes in a just over 1Kg. A mid range lightweight set will

I would imagine weight - especially for road bike tyres. For comparison, a lightweight Zipp wheelset comes in a just over 1Kg. A mid range lightweight set will be in the 1.4Kg range. There are other ways to make a road bike handle bumps and road buzz, e.g. specifically constructed carbon seat post, or flexible seat stays. A good race to watch for these kind of things is the Paris Roubaix, some of the bikes had rubber inserts to the seat stays a few years ago, but now have enough damping just from carbon layup of that section.