McQueen also owned a Vincent, not the high performance twin, just one of the lowly 500CC Comet singles, also known as "half a Vincent"
Famous Vincent racer and record breaker Marty Dickerson (South Cal Vincent Owners Section)mentions how someone with a familiar voice offered some help to him one day while he was working on his Vincent Sprint bike in the pits. He turned to see that it was Steve McQueen talking to him. McQueen suggested that Dickerson's problem (clutch slippage) was in the clutch cable lower end which was pinching the cable inner and was causing clutch slippage. This turned out to be correct.
Both the Comet and the twin have more or less the same cable arrangement and McQueen mentioned he had experienced the same problem with his little Comet.
On a side note\, Dickerson also met and befriended Burt Munro (World's Fastest Indian) and made many reel to reel tapes of Munro discussing all of the modifications he had made to his 200 MPH 1922 Indian.
Anthony Hopkins used these tapes extensively in researching the role and getting into the part. Dickerson was listed as a consultant for the movie.