When dogs chase me in a threatening manner (sloppy bouncing labs with big dopey smiles and wagging tails don't count) I proceed in this order:

1) I STOP my bike immediately. I get off and quickly swing my bike so it's a barrier between me and the dog. Stopping takes away the excitement of the chase and puzzles the dog. (there is usually no way I'm going to outrace a dog anyway, unless I'm already going 30 mph) Usually they will then stop too and stand there barking at me.

2) next step: I BELLOW at the dog and SHAME it- "NO! BAD DOG!! GO HOME!!!" has worked every single time for me. A deep commanding voice....LOUD. (hopefully the owner in their house will hear it too) The dog screeches to a halt and/or usually stops barking and feels shamed and and confused and eventually slinks off if I keep at it. They don't like my yelling at them.

3) If the stopping and bellowing doesn't work and the dog looks like it's going to actually bite me and attack, I have no qualms at all about grabbing my pepper spray (at the ready on the outside of my saddlebag) and spraying the dog right in the face. Twice I have actually practiced grabbing my spray quickly and and shooting it at a target. Hopefully I would have time to grab my spray before being seriously bitten, but I have my doubts. Dogs are fast.
(I know a biker whose calf was ripped open very badly by a rottweiller, and he only got the dog to let go of his shredded leg by grabbing his frame pump and beating the dog over the head and face with all his might. He had to have several operations and he sued the dog owner. The dog came up behind him and did not bark, so he had very little warning but managed to stay standing over his bike)

So far I have never had to get to step 3....steps 1 and 2 have worked about a dozen times for me like a charm, even with somewhat scary dogs like boxers.