Go to the top of Mt. Chiliad with a parachute. If you cannot find one, there is one on top of the mountain near a Camper and a Journey. Equip your parachute and jump on the mountain bike. You should trigger the downhill race. Then, jump off the edge with the bike, but before you jump off and deploy your parachute. You should spawn back on top with the bike, but with your parachute still deployed.