UpSide Down forks (USD forks)
One of the main advantages in USD forks are fixed tubes of larger diameter than the sliding tubes and the very long path of the sliding tubes through the fixed tube. The USD system serves to raise torsional rigidity and at the same time cannot twist in the fork bridge.
However, the unsprung masses in a USD fork are not smaller than in a conventional fork. The sliding tubes running inside the USD system are still made of heavy steel which makes the USD system not lighter than the old conventional forks.