Because European road regulations limit total vehicle length, manufacturers placed the cab on top of the engine to preserve cargo volume: that is how the cab-over design was born. In the American market, where the length limit is defined by the trailer, tractors evolved in the long-nosed (conventional) form.
Two Philosophies, Aerodynamically
The long-nosed cab offers a natural form that guides the flow gradually; the bonnet and mudguards act as transition surfaces. The cab-over cab, with its upright front face, generates higher pressure drag; it compensates for this loss with roof spoilers, side flaps and an aggressive reduction of the cab-trailer gap. In practice, a well-matched cab-over combination can compete with its long-nosed rival as a total package.
The New Direction in Regulation
Updates to European legislation are bringing the two worlds closer together by permitting extended cab forms for aerodynamic and safety purposes; rounded front faces and streamlined nose designs are now possible on European roads too. On the trailer side, the lesson remains unchanged: whatever the cab, tractor-trailer matching and gap management remain the decisive factors in total drag.
References
- Van Raemdonck, G. M. R. & Urila, I. (2019). A Study in Options to Improve Aerodynamic Profile of Heavy-Duty Vehicles in Europe. Sustainability, 11(19), 5519. DOI: 10.3390/su11195519