The hub automatically shifts at (approximately) 15 km per hour (28" wheel).

Specifications of the rear hub:
Brand: Sturmey Archer
Model: A2(L)
Color: Silber
Number of spoke holes: 36GTS
Type of brake: Rim brake
Number of gears: 2S Automatic
Type of axle: Vaste as 180 MM
Axle included: Yes, with nuts
Type of bearings: Ball bearings
Average weight (GR): 1003
Intended use: City

Data for spoke length calculations using the Sapim spoke size calculator:
Total length of the hub (MM): 132
Non-gear side (MM): 35
Gear side (MM): 40
Hub diameter non-gear side (MM): 67
Hub diameter gear side (MM): 67

Link to spoke calculator: www.sapim.be/spoke-calculator