The Collaborative International Dictionary of English v.0.48:

Treadwheel \Tread"wheel`\, n.
   A wheel turned by persons or animals, by treading, climbing,
   or pushing with the feet, upon its periphery or face. See
   Treadmill.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tread-wheel
    n 1: a mill that is powered by men or animals walking on a
         circular belt or climbing steps [syn: treadmill,
         treadwheel, tread-wheel]