The Brompton system is ... weird. Owing to the fact that the bike folds and the length of the chain line changes dramatically when you fold it, a traditional pa

The Brompton system is ... weird. Owing to the fact that the bike folds and the length of the chain line changes dramatically when you fold it, a traditional parallelogram linkage derailleur wouldn't work. Instead, they have a crazy custom thing that runs the chain through a little pivoting fork that shifts the chain left and right on the two speed freewhatever[0]. The six speed adds the three speed internally geared hub. You can also get one or the other alone. I believe they developed that combination when the supply of 5 speed hubs that fit their highly non-standard read hub spacing dried up. I have a two speed Brompton and love it, but their engineers are next-level crazy in the best possible way. [0] I wouldn't even care to guess. Everything they do is insane.