Maybe it comes from the days when bikes were unreliable and it was a "I'm ok mate" gesture.

I usually nod to other riders,I will wave occaisonally

Dinosaur.

"Oh Man I only ride 'em.I don't know what makes 'em work". Donald "Oddball" Sutherland

"Don't let the bastards get you down". Kris Kristofferson

"I am only paranoid because everyone is against me". Larry [Frank Burns] Linville