If there isn't a source then it's a joke from a friend or original wit.
Isabell required on your bike?
You know you're addicted to cycling when you have more bikes than the days of the week.
Q. Do you know what is the hardest part of learning to ride a bike?
A. The pavement. (source)
Q. Why did the lion chase the cyclist?
A. He heard that bikes are good for you. (source)
"I've really had it with my dog: he'll chase anyone on a bicycle."
"So what are you going to do - leave him at the dog's home? Give him away? Sell him?"
"No, nothing that drastic. I think I'll just confiscate his bike."(source)
Q: Why did the boy take his bike to bed with him?
A: He didn't want to walk in his sleep.
You know you're addicted to cycling when the surgeon tells you need a heart valve replaced and you ask if you have the choice between presta and Schrader.
Q: Why don't bikes stand up by themselves?
A: They're two-tired!
You know you're addicted to cycling when you catch your self using turn signals while navigating the grocery store isles.
Anyone else have a good bicycle related joke?