The riders had their special time trial bikes out yesterday. Most were riding with solid back wheels. The idea is that they have less drag. Drag must be caused by the air rushing around the spokes. There is the possibility of vortex shedding on a small scale, I suppose. The solid wheels don't have this problem. However you don't see them on ordinary days because if the wind it blowing from the side they present a bigger target and put sideways forces on the bike, making it hard to control.
