Originally Posted by Winfried

A Shimano Nexus/Alfine adds about 700g when retroffiting a six-speeder

How much heavier are hybrid brakes + disks compared to Brompton's road calipers?

Replacing the square taper BB with a hollowtech BB, the pedals and the saddle with lighter parts wouldn't cost much considering Brompton sells ten's of thousands of bikes a year. And possibly the steel seatpost with a titanium part.

Wouldn't that stick to the ~1,800€$ the six-speed C line currently sells for?

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Edit: The Nexus SG-C6001-8R is announced for 1735g while the — disk only, but it can be used with rim brakes — Alfine SG-S7001-8 is said to weigh 1670g.
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Edit: With this "Driveline-crank for road Birdy Brompton, 5holes, 130bcd 170mm", the crank is announced for 680g while its BB is 90g — as a reference, the Shimano BB-UN55 118x68mm is 300g.