There's some jumps at the base that you might be allowed to hit and they may even let you bring it up the gondola but save yourself the pain and rent a downhill bike if you're going to Mammoth. Even if you can ride the 20" down the trails, you wont be enjoying it... It's just too rough, you need suspension and the rolling advantage of the larger diameter 26" wheels. You'll enjoy the whole thing way more than on a 20".
