Complex sentence: Rootes'intention for the engine was to produce a new range of Commer trucks with the modern "cab forward" design, which required an engine low enough to mount under the driver's cab, rather than in front of it as previously.
Split into simpler sentences: Rootes'intention for the engine was to produce a new range of Commer trucks with the modern "cab forward" design. This required an engine that was low enough to mount under the driver's cab, rather than in front of it as previously.