Car aerodynamics, as we know, play a major part in determining fuel consumption, particularly at higher speeds. The virtues of good aerodynamics were probably best exemplified in the post-war mass car market by the Volkswagen Beetle and small Saabs, although there’s plenty of evidence from earlier times, such as Rumpler’s Tropfenauto (teardrop car) of 1921, that car designers understood the basic principles, and also that good aerodynamics often made for attractive body shapes. But compromises are always demanded of body designers in relation to practicality and housing the underlying chassis and mechanics, and these generally hinder the quest for purity of design, and fuel efficiency.
But hybrids, electric cars, and fuel-cell power are definitely changing the game. Cooling of a conventional engine requires significant volumes of air to be directed into the engine compartment, usually passing through a cooling radiator, and often an intercooler, and then allowed to exit in some way that generally affects the optimum aerodynamics. But electric cars don’t require vast amounts of cooling air and, like rear-engined cars, offer the possibility of cleaner and more aerodynamic frontal shaping. Good aerodynamics that consume less power to drive the car through the air are also more critical to pure electric cars, in terms of maximising their battery range, and on them you’ll see some aerodynamic features and refinements that don’t feature on more conventional cars. Take a close look at a Renault Fluence, for instance, and you’ll see all manner of detail aerodynamic refinements around the bonnet and windscreen areas that show the care that’s been taken in design. Using similar measures, the amazing Tesla Model S (with a drag co-efficient of just Cd 0.24) requires only 14bhp to cruise at 70mph, 4bhp less than the 0.25 Cd Fluence. But it’s always a pleasure to recall the underrated 1989 Vauxhall Calibra’s drag coefficient of just 0.26, a figure matched by very few of today’s cars, although the Peugeot 508 stands out as a relatively unremarkable shape that, with clever attention to detail, hits a Cd of just 0.25.
Moving around to other body areas, take a look at the 1938 Panhard Dynamic and, whilst you wouldn’t like to tackle a roadside wheel change, their covered front and rear wheel arches certainly make a stunning design statement, as well as improving their aerodynamics. Honda’s original Insight, and the 60-year old Citroën DS and ID show the use of aerodynamics around the rear wheel arches, which are faired in to reduce turbulence and drag. On more recent eco-design cars you’ll see aerodynamically superior wheels and wheel trim designs that save a little bit more fuel. BMW’s skinny, yet handsome, 19-inch wheels on their i3 may be the beginnings of a healthy trend away from the almost inevitable fat alloys, and using larger diameter, but narrow, wheels and tyres to reduce both rolling resistance and aerodynamic drag. Rear wheel steering, both active and passive (using clever suspension bushes), has featured on some fairly recent Hondas, Mazdas and Nissans, and may yet be seen again; by reducing the front wheel steering angles required, it allows closed in front wheel arches that improve aerodynamics in an area where there is significant turbulence and drag.
Elsewhere, common features like door mirrors, which generate significant aerodynamic drag, could become extinct with the adoption of rear-view cameras, and varied techniques of reducing windscreen wiper drag are being progressively introduced. We’ve already seen innovative door handles engineered into rear window frames, and it’s not beyond possibility that electric door opening and closing could, in time, totally eliminate door handles as we know them. Another feature often encountered on eco model variants is lowered ride height, using shorter road springs, where significant reductions of drag can be achieved, often along with attachments to reduce under-body turbulence. Citroën employed their variable hydropneumatic suspension to reduce the car ride height at higher speeds, such as in motorway cruising, and that’s a technique that’s not beyond conventionally sprung active suspension systems that are becoming more widely adopted. So things are certainly not standing still in car aerodynamics and, in reality, there’s possibly more scope for gains in real life fuel economy from such developments than from further improvements in internal combustion engine efficiency.
Victor Harman