His parents may have had him christened Hermann Paul Müller, but he was invariably known as ‘HP’, or, less formally, as ‘Happi’ Müller.
Born in 1909 and brought up in Bielefeld, Germany, his first interest was in flying, attending flying school in Münster where he gained his pilot’s licence. But then motorcycles appeared in his life, and he began road-racing in 1929 on a 500cc Imperia, then dirt-tracking on a 250cc JAP-engined UT.
In 1931 he returned to road-racing as a member of the Victoria factory team, and won for them the 1932 German Championship in the 600cc sidecar class, returning to the ranks of privateer in 1934, riding his own 350cc JAP-engined Victoria.
Early in the 1935 season he was offered a factory DKW, and in the following year…
