Complex sentence: McLaren driver Keke Rosberg, who was a close friend of de Angelis, retired at the end of the 1986 season, although contrary to rumour, de Angelis' death was not the reason Rosberg retired.
Split into simpler sentences: McLaren driver Keke Rosberg, who was a close friend of de Angelis, retired at the end of the 1986 season. Though contrary to rumor de Angelis' death was not the reason Rosberg retired.