SRAM’s HammerSchmidt system replaces a twin-ring crank and derailleur by packing a two-speed planetary gearbox into a crankarm.
The low gear gives direct drive through the single 22- or 24-tooth chainring. Click the shifter and you get overdrive mode, with the ring spinning 1.6 times for every revolution of the cranks and thus behaving like a 36 or 38.
Shifters for Hammerschmidt are sold separately – we had a SRAM X9 unit, but there’s a marginally lighter X0 shifter available too. They look identical to the derailleur shifters but pull different amounts of cable.
They also work back to front, with the pull lever shifting to direct drive and the release lever shifting to overdrive. Shifting is immediate, whether you’re pedalling or not, making last-minute panic shifts halfway up slopes a simple affair.
The system has its quirks, though – in overdrive, the chainring can freewheel, which minimises chain slap but can result in some slack to take up when it’s time to get the power down. There’s noticeable drag in overdrive mode too.