http://www.pcgamer.com/gta-onlines-transform-races-are-crazy/

GTA Online’s wacky races now include 15 new 'Transform Races' that manage to have a bit of something for everyone, from speeding through the skies in a jet to bouncing down a mountain on a bike, all in the same race. See, these races change your vehicle at set points, so one moment you might be jumping off a ramp in a supercar, but then you’re suddenly in a helicopter. 

The 15 races feature different vehicles and elaborate tracks, all of which you can check out here. If you click on the links in their names, you’ll be able to bookmark them, making it easier to access them in-game.