The GT series is an important category of motorsports due to its popularity. It became, or always was, a kind of paid racing series, because it is economically supported by gentleman drivers. GT racing is a clear example of a series that is a kind of automotive propaganda to sell cars. Nowadays FIA regulations define three categories for GT racing, just one of them runs with prototypes which is the GTE. This one reunites the best of grand touring (GT) cars, the best drivers and these cars are also deeply modified relative to the road going version. Consequently, this category is the most expensive of the three.

Balance of Performance (BoP)

Although there are political reasons for BoP, the objective is to avoid long periods of dominance of one manufacturer. Actually, GT racing is a kind of racing series that works as propaganda for the manufacturers. Hence, BoP tries to avoid that one car can run at a different pace relative to another. In addition, it also avoids that project miscalculations could represent a lost season for a manufacturer. BoP from the aerodynamic point of view is done based on wind tunnel test sections. Hence, all models are defined by the regulations, then the modifications allowed are defined according to the wind tunnel results for these cars. A window of BoP is a range which downforce and drag for each car can vary. For GT racing these windows are simpler, defined based on drag and engine power correlation, downforce and efficiency, which indirectly is also related to drag, and engine power with vehicle weight and aerodynamics. Since BoP is a window, it is also possible for the teams to explore some adjustments, in limited amounts, even though. The difference between BoP windows of GT series is the size of those windows, GTE is the biggest one, while GT3 and GT4 are smaller. Each car can only compete if it is inside the BoP window. As an additional BoP procedure, the federation can adjust the weight and engine power if a car is exhibiting an advantage. Actually, these two parameters are the most used to balance, because aerodynamics depends too much on the bodywork and this is more difficult to control. BoP is a continuous adjustment during the championship.

GT4

GT4 (Figure 2) is the most basic regulation. The cars are more similar to road cars. Aiming in cost reduction, cars are homologated just once in a defined window. Hence, no modifications are allowed until the next homologation.

GT4 guidelines

Similar to GT3 cars, in GT4 racing there are just guidelines that define the allowed modifications. Concerning aerodynamics, the only modifications are the front splitter, the inclusion of dive planes and a rear wing. The front splitter must have a ground clearance not less than 40 mm, it must not protrude from the bumper more than 100 mm at car sides and 80 mm respectively to the car centerline. One dive plane is allowed per side, but must not protrude from the splitter line. The rear wing, the main aerodynamic device, must have a maximum chord of 300 mm and not exceed the car shape lines when seen from the top view. In terms of height, the rear wing must not be higher than 180 mm from the lowest point of the deck lid. The maximum width of the wing is the same distance between A-pillars.

GTE

GTE car modifications are based in boxes, which are imaginary zones that define the spatial range of the modification. In terms of size, it defines height and width of specific sections of the car. The body shape can not be outside of boxes. According to the regulations, it must be provided a reference car that fits the regulation. This must have 2 doors, 2 seats or 2+2 seats, closed or open top and, mainly, must be approved for road use. Hence, the car must be available on the market and its manufacturer must be certified by FIA. There are limitations according to materials. For instance, titanium parts are only used in wings and wheels. However, it can be used if the chassis is of carbon monocoque type. This gap is probably due to the fact that some carbon monocoques use titanium in their carbon fiber composition.

Boxes

GTE is the most important GT championship, and is the only one that has specific technical regulations which are the boxes of modification of the car. These are the side, the front bumper, the front splitter, the flat bottom (Figure 4), the rear wing and the rear diffuser volumes. The side volumes define a maximum width of 2050 mm. However, as can be seen on Figure 4, the modification on the surface can not exceed the box volume. The front bumper must not exceed a height of 350 mm from the flat bottom. The front splitter volume can only go further relative to the original body work by 100 mm. In addition, it must be a continuous lower and upper surface, without wings or openings. The splitter must be at a height of 150 mm from the flat bottom. Actually, the flat bottom is also constrained. The underwing must be flat throughout the entire length of the car and must not protrude laterally exceeding the perimeter of the bodywork.

The rear wing is composed of only one element, so no flaps. Its dimensions must not exceed 300 mm of chord and must be inside a box volume of 1800x450x150 mm. In addition, the rear wing is constrained in terms of height, it must not be higher than the car roof and no element of the wing can exceed more than 100 mm from the rear overhang. The rear diffuser must not protrude more than 100 mm from the car perimeter, wing profiles are forbidden. In addition the rear diffuser is constrained in height from the flat bottom, 260 mm and its maximum width is 1,400 mm. The rear bumper is defined according to the vertical projection of the original car rear bumper, the shape design is free, but the only constraint is the height, it must not be higher than 350 mm from the flat bottom.

GT3

GT3 is similar to GTE, but it does not compete in the World Endurance Championship (WEC). This category is usually dedicated to young drivers. It is more affordable since the cars are not deeply modified. GT3 does not have a too restrictive regulation, while GTE has. Actually, this one has the so-called Balance of Performance (BoP). This approximates all cars in terms of performance. For instance, if the car is faster in corners than rivals, thus it is added more weight through ballasts. In the same way, if this car is faster on straights, its engine power is reduced. GT3 has too simple rules, a kind of guidelines.

GT3 guidelines

The GT3 guidelines that refer to the aerodynamic devices define that the front splitter must have a maximum overhang of 100 mm from the front bumper perimeter, the rear diffuser overhang must go until the rear bumper overhang. The limitations regarding the flat bottom are the only limitation that it must not go beyond the perimeter of the car. However, it is free in terms of flat bottom length. The rear wing position is fixed vertically and relative to the overhang point, which is 100 mm. The louvers are allowed on fenders.

Aerodynamic analysis

The adjustment allowed for the teams in terms of aerodynamics are based in the front underwing and in the rear wing. The first one is a closed system (Figure 7) which means that it is not in connection with the flow field. Actually this is a thermodynamic denomination, because it is not connected with the external flow field. The only point which could be a connection are the splitters or the louvers. These are openings on the wheel archers. There is the exit boundary condition for the front underwing, which guides the flow to the flat floor and to the openings.

Although it changes the pressure conditions at the region, these devices have a small effect in the general aerodynamics. Actually, in GT cars the aerodynamics are governed by the flat floor and diffuser. The effect of this configuration is a car that is aerodynamically unstable, because the efficiency of the splitter is connected to the vehicle ride height. Therefore, GT cars are very sensitive to the ride height and the main aerodynamic indicator became the ratio between front balance and ride height (F_bal/mm). These cars are 2x more sensitive to RH than other closed wheels cars, as LMP prototypes and old DTM ones. All the air that is running under the car, either is running under the flat floor or beside the wheel archers, which is where the air gets out of the car. The problem with closed systems is the promptness to lift, because the splitters in the wheel arches cause a pressure field that increases the downforce. However, in cases of wavy tracks, a subtle RH variation can lead the front underwing to be connected to the external flow field, thus making the front prone to lift. To avoid this situation, louvers are placed at the top of the wheel archers, thus creating a connection with the upper flow field. The louvers connect the inner part of the wheel archers to the external flow (Figure 7 and 8), the objective is to change the pressure condition inside them. For instance, part of the air running inside the wheel arch is sucked by the external flow field through the louvers. Hence, any variation of the pressure at the top will change the pressure inside the wheel arch and this is felt by the front splitter. When the car goes up, the pressure flow field acts as a compression bag, which lifts the front of the car. The louvers are necessary, because it creates air evacuation for the air inside, thus reducing the lifting effect at the front. The problem with the using of louvers is that it also increases drag. However, these can be regulated in terms of the amount of closed slots. Open louvers reduce the car sensitivity to RH, but it generates more drag, because air flowing through the louver gets out under turbulence, thus creating wake and drag. Another problem is the connection to the outer surrounding air (Figure 7), which means that this is a fixed boundary condition on the outside. For instance, since the outer pressure is fixed, the generation of the downforce will be more constant with respect to the ground clearance. The amount of air sucked by the louvers depends on their geometry and position. In any case, as large the louvers, larger is the size of the wake and bigger is the drag. It is possible to introduce Gurney’s flap to improve the sucking effect at the wheel archers, while it also increases pressure at the front. Therefore, the use of louvers increases drag, because it induces a wake in the vehicle aerodynamics. This wake affects the rear wing efficiency, thus any modification on the front underwing must be carefully taken in order to guarantee a proper aero balance.

Conclusion

Figure 9 illustrates two histograms that compare high performance and GT racing cars aerodynamics. Basically, the behavior is the same, the difference is that GT racing cars are aerodynamically more efficient. The drag is most produced by the car body, while the downforce is most generated by the underbody. Hence, in terms of downforce, the rear wing has a minor effect. Wheels and cooling openings only increase drag. Actually, the cooling blanks are usually not entirely open, depending on the situation the full opening can lead to more drag, but at the cost of some engine cooling.

Bottom line, GT racing cars are very sensitive to ride height, not only because of the use of front underwing, but also because the downforce of the car depends entirely from the underbody (Figure 10). The rear wing is a good add for the downforce, but its efficiency is too low when compared with the underbody. The top of the car has a minor impact on downforce, it generates more drag than downforce, the only section of a significant improvement on downforce is the rear wing one. However, the efficiency (C_zS/C_xS) is higher on underbody, mainly at the front underwing region.

References

• These article is based in my lecture notes from Industrial Aerodynamics lectures that were hold in Dallara Accademy.