Jump off the bike and use it to block the dog from attacking you while yelling at it and acting a fool.
When a dog is chasing you, that's the best bet IMO. It's worked for me. Either the dog is just chasing you and they quit once the game is over, or if they are aggressive, they'll back off if you meet them with the same aggression. Basic fight or flight stuff.
Trying to fiddle with a can of mace/pepper spray, if you even have time to get it out of your jersey pocket, is better in theory than in practice.