To make your motorcycle not have a rider, knock him off by crashing into something. Pause the game while he is off the bike. When you resume the game, there will be no rider on the bike. When you crash you will be back where you fell off, with the man back on.
When riding a motorcycle, crash into something that will make you fly off the bike. While you are still on the ground, pause the game then resume. Your rider will stay on the ground, but you will be able to drive the bike riderless, as long as you do not crash into anything too hard. You can even double back and push the rider around.
When on the Nousagi or any other bike, when your damage is full and the bike is going to explode, quickly press Start, then select "Continue". The bike will be indestructible, and the rider will be gone.