Axon's automotive anorak: Having a spat

The 1949 Cadillac Series 62 Sedanette on the Cartier Lawn, along with the early Jaguar XK120 Roadster and a bunch of Citroen 2CVs, could be found proudly sporting spats and part-covered rear wheels. This was not unusual given the late 1940s timings when each of these cars were launched, with rear wheel spats being at their height immediately post-war.

Much more of a surprise though was the brand new Pagani Zonda Barchetta – currently the world’s most expensive new car at c.£12.5million+ – making its global dynamic debut at Goodwood on the FOS Hill and in the Supercar Paddock, and proudly brandishing a pair of spats over its rear wheels, a feature not seen on modern cars for some time.

Wheel spats were a component that managed to trickle down through aeronautics, an aerodynamic guard for aircraft wheels that was drafted into car design and stayed put until the late 20^th Century.

Wheel spats are effectively just wheel arch covers that are either integral to the car’s bodywork (as on the Citroen 2CV or the part-covered wheel 1991 Chevrolet Caprice) or are bolt-on metal sheets that enclose the rear wheels (as found on the early Jaguar XK120 and new Pagani Zonda Barchetta). They were introduced initially as aerodynamic aids, blocking air from entering the wheel arch and instead forcing it to flow smoothly over the bodywork, reducing the overall drag coefficient of the vehicle. Without spats, incoming air is able to force its way inside the wheel arch where it swiftly becomes turbulent as it bounces between the rotating wheel and the bodywork. 

So, as it turns out, covering the rear wheels with spats (or fender flares, or fender skirts for any North American readers tuned-in) can indeed go a long way towards making vehicles more aerodynamic, quieter and fuel efficient, as well as potentially more stylish and distinctive in many cases.

Aerodynamic drag harms fuel economy, so multiple manufacturers decided to enclose the rear wheels to eke as much efficiency from their vehicles as physically possible. It was tested – and proven by General Motors for its pioneering aerodynamic electric GM EV1, for example – that applying spats help improve fuel economy, but sadly at a higher production cost and with some inherent drawbacks along the way.

However, the costs and drawbacks associated with applying rear wheel spats could now be are too steep and complex for mass-production. For starters, manufacturers of modern cars might have to run narrower tyres at the rear of the vehicle to allow for a larger clearance so as not to scrape the spats, possibly decreasing handling prowess at the same time, which intrinsically affected the handling and overall dynamics of the car.

Also, any air that does inevitably make its way under the spat and into the wheel arch is essentially trapped. This will lead to a high pressure within the wheel arch along with a build-up of heat as the friction between the tyre and the road converts somewhat to sound and heat energy. This will invariably increase the pressure of the tyre itself, thus risking a possible blow-out if the pressure was allowed to reach over normal operating figures (normally 30-36 psi). Changing a rear wheel would also be more complex and time consuming too, resulting in more bruised knuckle and foul language!

So, although spats have been all but banished from recent modern production car design due to some of these inherent drawbacks, they managed to create a definitive foothold in motorsport, most famously used during the epic Group C endurance racers of the 1980s and early ‘90s, such as those found on the Le Mans-winning Jaguar XJR9 and wonderous (but stillborn) Alfa Romeo SE048. Wheel spats were implemented for Group C racers to do two main jobs.

Firstly, they were used to reduce the amount of unwanted water and spray entering the wheel arches and disturbing the driven wheels from their operation, along with allowing the track monsters to reach upwards of 200mph on the banking at Daytona and the Mulsanne Straight at Le Mans through drag reduction.

At the same time, the spats maximised the Venturi Effect of the air entering and leaving the wheel arch, the Venturi Effect being when air is accelerated through a deliberate constriction in the path of the airflow. A small but deliberate air inlet allowed the rear tyres and brakes to stay cool and within their operating temperature ranges, but with an outlet to reduce the build-up of unwanted high pressure that would cause inevitable tyre blowouts in long endurance races.

So, it’s safe to say that wheel spats do have their aerodynamic uses when it comes to both performance and economy motoring, but in the same way that many manufacturers use unnecessarily large wheels in even their most-docile production cars, the main reason wheel spats have been consigned to the history books is because of styling trends. The world has now become obsessed with cramming the largest wheels possible into the arches, therefore covering them up to save a small amount of fuel or drag force is simply not on the agenda.