The Sopwiths originally employed a French Clerget 9 cylinder rotary engine, but towards the end of the conflict the W O Bentley (of subsequent Le Mans fame) designed a superior although similar engine which was then utilised, it was known as the BR1. Later, an advanced and larger engine was available, the BR2. In his early days, Bentley and several of his brothers all rode various motorcycles and covered astonishing long distance rides on them! He also raced in the Isle of Man races in 1909 and 1910. I believe they had Indians, Rex's, Quadrants and Triumphs. During World War 2, he used a Francis Barnet. He also raced at the Isle of Man post 1st world war with 2 other Bentleys gaining the team prize (on the same course as the bikes)
I had better get my coat!