A variable ratio power split device comprising: radially inner and outer races, each comprising at least two axially spaced parts (100A, 100B); a plurality of planetary members arranged for rolling contact between said inner and outer races; a planet follower carrier engaging with said planetary members; a first rotatable power element spindle which is connected with said planet follower and operable to couple power between said planet follower carrier and a first power element comprising an internal combustion engine; a second rotatable power element spindle which is connected with said inner race and operable to couple power between said inner race and a second power element comprising a regenerative power assembly; a third rotatable power element spindle which is connected with said outer race and operable to couple power between said outer race and a third power element comprising a vehicle transmission assembly, wherein at least one of said power transmission assembly and said vehicle transmission assembly is operable to store energy, and wherein said at least one of said power transmission assembly and said vehicle transmission assembly is operable to reapply said stored energy; means for adjusting an axial separation of said axially spaced parts of at least one of said races to vary a power split ratio between said first, second and third rotatable power element spindles; at least one sensor operable to determine current operating conditions; a set-point unit configured to determine, with reference to stored data indicative of predetermined efficiency characteristics of said first, second and third power elements under predetermined operating conditions, an axial separation of said axially spaced parts of at least one of said races to provide a power split ratio which causes substantially no power to be coupled to one of said first, second and third power elements under said current operating conditions; wherein said set-point unit is operable in any one of a number of modes and, when in a regenerative mode, is configured to determine, with reference to said stored data indicative of said predetermined efficiency characteristics, an axial separation of said axially spaced parts of at least one of said races to provide a power split ratio which causes minimal power to be coupled to said internal combustion engine and power from said vehicle transmission assembly to be provided to said regenerative power assembly at a speed which improves an operating efficiency of said regenerative power assembly under current operating conditions. radially inner and outer races, each comprising at least two axially spaced parts (100A, 100B); a plurality of planetary members arranged for rolling contact between said inner and outer races; a planet follower carrier engaging with said planetary members; a first rotatable power element spindle which is connected with said planet follower and operable to couple power between said planet follower carrier and a first power element comprising an internal combustion engine; a second rotatable power element spindle which is connected with said inner race and operable to couple power between said inner race and a second power element comprising a regenerative power assembly; a third rotatable power element spindle which is connected with said outer race and operable to couple power between said outer race and a third power element comprising a vehicle transmission assembly, wherein at least one of said power transmission assembly and said vehicle transmission assembly is operable to store energy, and wherein said at least one of said power transmission assembly and said vehicle transmission assembly is operable to reapply said stored energy; means for adjusting an axial separation of said axially spaced parts of at least one of said races to vary a power split ratio between said first, second and third rotatable power element spindles; at least one sensor operable to determine current operating conditions; a set-point unit configured to determine, with reference to stored data indicative of predetermined efficiency characteristics of said first, second and third power elements under predetermined operating conditions, an axial separation of said axially spaced parts of at least one of said races to provide a power split ratio which causes substantially no power to be coupled to one of said first, second and third power elements under said current operating conditions; wherein said set-point unit is operable in any one of a number of modes and, when in a regenerative mode, is configured to determine, with reference to said stored data indicative of said predetermined efficiency characteristics, an axial separation of said axially spaced parts of at least one of said races to provide a power split ratio which causes minimal power to be coupled to said internal combustion engine and power from said vehicle transmission assembly to be provided to said regenerative power assembly at a speed which improves an operating efficiency of said regenerative power assembly under current operating conditions.