Originally Posted by
JRat
Most chainring spiders are made such that one chainring can be fitted on the inside and another on the outside with the spacing correct for a front derailleur). The Shimano Nexus hub has a relatively narrow chainline (the shift mechanism is between the sprocket and the drop-out) which explains why the cranks and chainring are moved to the left.
Yes, most spiders are made for two chainrings but the Birdy is supposed to be a single chainring bike. On the Brompton, also single chainring, the spider is made to accept only one chainring.
Alfine are the same, for the same reason, also narrow chainline hubs.
The total opposite to the Rohloff that has a wide chainline (the original application considered by Rohloff was MTB but the hub was never a success for that purpose and found its market in high end everyday and travel bikes).