Cannondale’s Lefty forks…. WHY!? There’s no need and no point and they just look s**t.
There is actually an engineering reason for this. In most forks the lowers move up and down on bushings which can be prone to stiction. So Cannondale thought about this and realised if they had two square tubes sliding inside each other they could run on needle roller bearings with zero stiction (a concept I also invented back in college, but never implemented 🙂 ). This ends up being quite heavy, but then they realised that if the tubes were square they could get away with only one leg thereby saving weight – because it can’t twist. Guessing the headshock comes from the same idea.
And Lefties are indeed very plush with great small bump sensitivity, although these days lubrication, materials and design have caught up mostly with traditional forks. They have made some pretty light ones in the past too. The current top model is a touch heavier than a SID but I’ll bet it’s got better small bump compliance. But then, it’s apparently not that important to the market since you don’t see that many being ridden.