Complex sentence: The two front spoked wheels were nearly 9 metres (27 feet) in diameter; the back one was a smaller (only 2 metres (6 feet) high) double wheel, to ensure maneuvrability.
Split into simpler sentences: The two front spoked wheels were nearly 9 metres (27 feet) in diameter. The back one was a small ("only" 2 metres (6 feet) high) double wheel, to ensure maneuvrability.