Wheel spats and their contribution to vehicle aerodynamics

Michal Sura [email protected]

Climate change threatens people with extreme heat, increased flooding, food and water scarcity, more disease, and economic loss. This threat has led to stricter regulations for greenhouse gas emissions and put more pressure on the automotive industry to develop more efficient vehicles. The automotive industry has begun to produce zero-emission vehicles, but there are still many opportunities to reduce propulsion energy consumption. The aerodynamic drag of the vehicle significantly contributes to the previously mentioned energy consumption. The significant contribution to overall vehicle aerodynamic drag has been the aerodynamics of wheels and surrounding wheelhouses. In this analysis, we will focus on possible improvements in wheel aerodynamics using wheel spats.

The primary goal of vehicle aerodynamic improvements is to reduce drag. Figure 1 illustrates aerodynamic drag ratios that occurs on different parts of a vehicle.

Figure 1.

From experiments in both research (Cogotti 1983) and development (Mercker et al 1991) it can be concluded that wheels and wheel wells contribute approximately half the drag of a low-drag car (1). Wheels and wheelhouses account for approximately 25% of passenger car aerodynamic drag (Figure1) (2). Because the underbody and wheelhouses are responsible for a larger part of the aerodynamic drag force, reducing the aerodynamic drag of these parts would reduce consumption of propulsion energy and thus extend the range of vehicles. A wheel spat is one of the components that would be able to improve the aerodynamics of these parts (Figure 2).

Figure 2.

During WW2, aircraft aerodynamics advanced significantly. It was found that improved aerodynamics significantly increased aircraft speed. After WW2, the knowledge gained in aircraft aerodynamics began to be applied to vehicle aerodynamics. Wheel covers that were used on aircraft without retractable landing gear to improve aerodynamics (Figure 3). Similar attempts were made by car designers, who used spats to cover the wheels.?

Figure 3.

In the 1940s, SAAB came up with the idea of applying spats to the front and rear wheels and enclosing them like the wheels on the planes of the WW2 era (Figure 4). Unfortunately, this had disastrous consequences, especially in areas where snowing was common, such as Scandinavia, because there happened that the front wheel arches were clogged with compacted snow that locked the steering of the vehicle.

Figure 4.

It turned out that it was not a good idea to apply spats to the front wheels. Most car manufacturers have stopped using spats. One of the few carmakers that had been keeping to apply spats to the rear wheels was Citroen (Figure 5).

Figure 5.

The computational fluid dynamic (CFD) method, which is based on mathematical models, showed that applying spat really improves aerodynamic drag coefficient(3) (Figure 6).

Figure 6.

It was clear that spats were more suitable for rear wheels, but using spats had another adverse effect. Any air that does make its way under the spat and into the wheel arch is effectively trapped there. Friction between the tire and the road produces heat, which heats up the accumulated air in the wheelhouse. This will lead to an increase in the pressure of the tire itself and there is a danger of a possible blow-out when normal operating pressure is exceeded. However, car designers were able to deal with this issue. Jaguar's solution is one such example.

The Jaguar XJR-9 (Figure 7), for example, was one of the most effective uses of wheel spats, winning at both Daytona and Le Mans in 1988. The spats in the case of this car helped to maximize the Venturi Effect. This phenomenon reduces the air pressure beneath the car and creates downforce, also known as negative lift (4).

It was found how to overcome the challenges associated with the use of spats. Air inlets were used to keep the rear tires and brakes cool and within their operating temperature ranges, while air outlets were used to reduce unwanted high pressure that could cause tire blowouts in long endurance races.

Figure 7.

Let's take a look at what it's like to use spats nowadays. We found that Dutch startup Lightyear One comes equipped with spats, as we can see (Figure 8). Lightyear One achieved a drag coefficient Cd of 0.20, which is an excellent value. As can be seen in Figure 8, the spat only covers about one-third of the wheel. They appear to have chosen a compromise in order to avoid the issue of wheel and brake cooling.

Figure 8.

At the end of this analysis, we would like to present Mercedes-Benz Vision EQXX concept (Figure 9), which is a technology demonstrator. Yes, you can see well, there were no applied spats to the rear wheels, but engineers were able to achieve a drag coefficient Cd of 0.17. It's an incredibly low value and Mercedes' aerodynamic engineers did an excellent job.

As can be seen, good aerodynamic resistance values can be achieved using spats or without them. The decision will be made by the car designers.

Figure 9.

References:

1, https://www.yanfabu.com/resources/editupload/files/2013112216461820.pdf

2,https://www.researchgate.net/publication/237827393_Flow_analysis_around_a_rotating_wheel

3, https://iopscience.iop.org/article/10.1088/1757-899X/444/7/072005/pdf

4, https://rmsothebys.com/media/33041/xjr-9_oct_140.pdf