Michael Fernie profile picture Michael Fernie 4 years ago 75
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What Are Wheel Spats, And Why Don't We Use Them Anymore?

Once used in both production and racing car design, wheel spats have their aerodynamic advantages but definitely have some shortcomings

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What Are Wheel Spats, And Why Don't We Use Them Anymore? - Blog

Aerodynamics have been a priority within the automotive industry since the Second World War, with many former aircraft manufacturers turning their attention towards creating products for the everyday punter. The sudden shift towards sleek, rounded car designs in the 1930s and 40s showed that the automotive and aeronautical industries could be closely linked, with many fundamentals still being used in modern production car design today.

One component that managed to trickle down through aeronautics was the wheel spat, an aerodynamic guard for aeroplane wheels that was drafted into car design and stayed put until the 2000s. So how do these simple, sleek yet sometimes dangerous components work?

The early noughties Honda Insight was the last production car to feature wheel spats as standard
The early noughties Honda Insight was the last production car to feature wheel spats as standard

Wheel spats are effectively just wheel arch covers that are either integral to the car’s body or are bolt-on metal sheets that enclose the rear wheels. 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.

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 GM – that applying spats (also known as fender skirts) increased fuel economy but sadly at a higher production cost and with some inherent drawbacks along the way.

Some cars can pull them off, some definitely can't...
Some cars can pull them off, some definitely can't...

Firstly, using spats meant that the rear tyres had to be narrower to allow for a larger clearance, 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 force between the tyre and the road builds up 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).

Going back to the 1940s, SAAB decided it would be a great idea to apply spats to the front wheels, enclosing them like the wheels on the aeroplanes that they had been designing during WWII. Unfortunately, this led to some fairly catastrophic results – especially in countries where snow was prominent like its native Sweden – as the front arches would clog up with compacted snow and grit, locking the steering in place.

The SAAB Ursaab was the first concept car produced by the Swedish company and featured a deadly set of front wheel spats
The SAAB Ursaab was the first concept car produced by the Swedish company and featured a deadly set of front wheel spats

Although spats have been all but banished from production car design due to their inherent drawbacks, they managed to create a definitive foothold in motorsport. Most famously used during the epic Group C endurance racers of the ‘80s and early ‘90s, wheel spats were implemented 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 Muslanne Straight through drag reduction.

At the same time, the spat maximised the Venturi Effect of the air entering and leaving the wheel arch. The Venturi Effect is 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.

The XJR-9 is one of my personal hero cars. Video via Definition Media

One of the prettiest and most effective applications of wheel spats came in the form of the Jaguar XJR-9, driven by British motorsport legends Andy Wallace and Martin Brundle. Winning at both Daytona and Le Mans in 1988, the 720bhp V12 Jag used wheel spats on the rear wheels that gave it an imposing look, seemingly lengthening the wheelbase of the car.

The  Alfa Romeo SE048 was another Group C prototype that used spats like the Jag, but the car never made it to any competitive racing
The Alfa Romeo SE048 was another Group C prototype that used spats like the Jag, but the car never made it to any competitive racing

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 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. They look wrong and clumsy on some cars, yet others would seem naked without them.

What’s your favourite application of wheel spats? Do you wish they were integrated back into car design or should they be kept to the classics? Comment with your thoughts below!