The dark red you remember was most likely red lead oxide primer (minium). Red lead was the standard anti corrosion primer for steel from roughly the early 20th century to the 1980s. It bonds well to bare steel and passivates the surface. Chassis frames were primed and often left in that primer colour or given a thin topcoat over it, so the bordeaux tone was the primer showing through. Red lead is highly toxic. Lead based coatings were progressively banned or restricted across Europe through the 1980s and 1990s. That removed the historical reason for the red colour. Modern frames are treated differently. Two things drive the dark grey: Cathodic dip painting (e-coat or KTL in German usage). The whole frame is dipped and an electric field deposits an epoxy coating evenly, including inside box sections. The standard e-coat colour is dark grey to black. It is far better at corrosion protection than a brushed primer. Zinc based systems. Some frames use zinc rich primers or galvanising, which also read as grey.
The shape of a truck is one of the largest single influences on its fuel consumption at motorway speed, where aerodynamic drag accounts for a major share of the energy a tractor unit has to provide. The underlying physics of that drag is covered separately; this section describes the technology used to manage it, namely the devices and cab design features fitted to reduce drag, and the regulatory framework that now allows manufacturers to reshape the cab itself.
Aerodynamic performance is built up from many small contributions rather than a single feature. The principal technologies are:
For decades the cab itself could not be optimised freely, because Directive 96/53/EC capped the overall length of a combination at 16.5 metres for a standard articulated outfit, so any length spent on a rounded nose was length lost to the load. Directive (EU) 2015/719 amended that rule by inserting Article 9a, which allows a cab to exceed the maximum authorised length where it delivers improved aerodynamic performance, energy efficiency and safety, provided the extra length does not increase the load capacity of the vehicle.
The cab must be type approved under the EU framework. The type approval requirements had to be in place by 1 November 2019, and cabs built to them could enter the market from 1 September 2020. This is the legal basis for the rounded, longer nosed cabs that began appearing in the early 2020s, and it is why a tractor fitted with such a cab is legal even though the combination measures more than the familiar limit.