Complex sentence: His early racing was on BMW R50 sidecars with a chassis of his own design, but after a bad accident in 1961 took time off only to return with his own design URS four-cylinder machine and win the title in 1968.
Split into simpler sentences: His early racing was on BMW R50 sidecars with a chassis of his own design. After a bad accident in 1961, he took time off and returned with his own design URS four-cylinder machine to win the title in 1968.