HONDA The Power of Dreams

New regulations introduced to the GT500 class of SUPER GT in 2014 stipulated a “design line,” with aerodynamic development only permitted in the area below that line. The design line was stipulated basically as a line extending across the front face of the car at a height of 275 millimeters (mm) above the flat bottom (floor) of the car, and down the side of the car at a height of 275 mm at the front end and 400 mm at the rear end, with the line curving over the front and rear wheel housings. It was further stipulated as extending across the rear face of the car at a height of 400 mm.

The aim of limiting the area that could be developed was to minimize excessive development. Within the area stipulated by the design line, the area between the front and rear wheel housings is called the “lateral duct” and the left and right corners of the front bumper are called “flick boxes.” Boomerang-shaped aerodynamic devices called “canards” (“flicks”) can be attached to the flick boxes.

In 2014, the first year of the new regulations, updates were permitted only once during the season. In 2015, specifications employed during the opening round had to be used until the end of the season. In 2016, development was frozen to minimize speed increases through aerodynamic and other performance improvements. In 2017, to reduce downforce by 25%, in addition to shortening the front end of the splitter (front underpanel) by 50 mm, the height of the rear end of the diffuser, located at the rear of the car, was almost halved (from 206 mm to 105 mm). A low-drag (low air resistance) specifications rear wing, which was only permitted in the Fuji round previously, also had to be used throughout this season.

Aerodynamic development area

Aerodynamic development was permitted in the area indicated in blue on the side view. The lateral duct, which is the area between the front and rear wheel housings, was not a focus of development in the GT500 class until and including 2013, making it uncharted territory for the development team. The rear wing was a common part for all cars. The underpanel and diffuser were designated as parts that each company could design in line with designated specifications. Some areas of the wheel housings and front splitter could be freely developed.

HRD Sakura (HRC Sakura from April 2022), Honda’s motorsports development site, and Honda’s automobile product development site (Tochigi R&D Center) collaborated on aerodynamic development of the Honda NSX CONCEPT-GT (NSX-GT from 2017).

While wind tunnel testing was mainly conducted at the HRC Sakura wind tunnel facility, the engineers specializing in aerodynamic development were stationed at the automobile product development site (Tochigi R&D Center). This was due to the fact that Honda had traditionally conducted aerodynamic development for its mass production cars and for its racing cars in the same department to enable mutual feedback between designers.

The moving belt wind tunnel at HRC Sakura is set up to F1 specifications. Although the facility is capable of measuring full-scale vehicles, Honda conducts testing with 60% scale models for SUPER GT in view of development efficiency, and costs and lead times associated with manufacturing parts.

The 60% scale wind tunnel model is suspended and supported in the measurement chamber by a strut from the ceiling, with the strut having six built-in electric actuators that enable control of factors such as pitch angle, yaw angle, roll angle, and heave (in-phase vertical motion of the front and rear axes). In the past, one attitude would be tested before stopping measurement, changing attitude, and starting measurement again, which was repeated again and again. At HRC Sakura, on the other hand, the system enables measurement of a series of attitudes without stopping, thereby testing the best attitudes for bringing lap times down through continuous changes in car height, yaw, steering angle, and roll.

Wind tunnel models have a built-in six-component balance that measures axial forces, and their associated moments, along the x-axis, y-axis, and z-axis. Within the moving belt unit, a balance measures vertical load via the air layer supporting the belt. To improve accuracy of measured data, wind tunnel models are equipped with specially designed rubber tires. Wind tunnel models are also equipped with a built-in active suspension system to reproduce an attitude as close as possible to an actual cornering attitude. It is possible to not only measure static pressure across the body surface, but also air speed through the heat exchanger mounted within the body. The environment created at HRC Sakura enables advanced, high-accuracy measurements.

60% scale wind tunnel model

F1-level wind tunnel, with loads measured through a load cell attached to the roof

Computational fluid dynamics (CFD) is employed in desktop studies prior to confirming the effectiveness of development in the wind tunnel. This enables air flows to be reproduced by computer to verify the effectiveness of aerodynamic development. Aerodynamic development of racing cars, including for the GT500 class, uses longitudinal vortexes formed by the property of high pressure air moving to areas of low pressure. Air flow is controlled through the vortexes to increase downforce (force created through an air pressure differential to hold the car body to the ground) and reduce drag (air resistance).

Canards attached to the flick boxes create these vortexes, which together with the lateral duct flickups (rising bow-shaped parts) work to increase downforce. By attaching one canard to the left flick box, for example, air flow will try to wrap around it from the top surface where air pressure is higher to the bottom surface where air pressure is lower, thereby generating a clockwise-rotating vortex from the wingtip.

Air in the generated vortex flows toward the rear of the car, changes direction as it meets the vortex generated by the lateral duct flickup, and flows under the floor. This leads to an acceleration of air flow under the floor and subsequently to increased downforce at the rear.

Results of CFD-based desktop studies

Highly accurate CFD technology is regularly used to accurately visualize the longitudinal vortexes often seen in air flows around racing cars. Images show that the aerodynamic method regularly used by Honda, instead of the general method, achieves clearer calculations of vortexes generated from the front canards and lateral ducts.

The process for ideas that are considered potentially effective starts with using CFD to verify that effectiveness. After sifting through and narrowing down the results, the remaining items are verified in a wind tunnel. With further sifting of those items in the wind tunnel, Honda moves on to field testing. In other words, aerodynamic development repeats the loop from CFD to wind tunnel to the field. Correlations between CFD, wind tunnel, and the field are therefore important for achieving highly accurate development. For instance, a dedicated measurement device called an “aero rake” is used to correlate numbers between field testing and CFD. The aero rake is mounted on the actual car to measure wind speed and direction, and the generated data is compared to the CFD data to improve accuracy.

Aerodynamic performance and speed are not necessarily proportional. No matter how high peak performance is, drivers are unable to draw out that performance if they cannot drive with confidence. Driving simulators can be used to confirm this as they can reproduce, as close to real driving as possible, instantaneous changes in grip and the resulting feeling of instability. They are therefore used for the sensory checks that are difficult to judge from desktop calculations alone.

When manufacturing a car to meet the new 2014 regulations, Honda found that the changes alone increased downforce by 30% compared to the 2013 car. The NSX CONCEPT-GT struggled with cooling around the engine because it had a midship-engine rear-wheel drive (MR) layout using a common monocoque structure designed for a front-engine rear-wheel drive (FR) layout, so Honda was kept busy working on countermeasures throughout 2014.

Nevertheless, working toward the 2015 car, Honda focused development on increasing rear downforce and, reflecting a deeper understanding of the new regulations, it succeeded in improving downforce by close to 7%. As explained above, development was then frozen during 2016, so there were no quantifiable changes in terms of aerodynamic performance.

While downforce was temporarily reduced through new regulations introduced in 2017, it was restored over 2018 and 2019. As one example of such measures taken, while the lateral ducts tended to be closed off until and including 2017, Honda created a much more open design for its 2018 specifications, with improved air flow increasing the front downforce. Prior to that, development was aimed predominantly at rear downforce, but Honda changed policy after finding that generating front downforce had the effect of increasing downforce across the entire car, and this provided a greater contribution to lap times.

For the 2019 car, Honda developed flick boxes to further improve downforce. Using two stacked canards with slits in the top canard dividing it into three, and three fins lined up above that, the structure was quite complex. With record course times achieved in field testing, Honda was very pleased with the result. Numerically, the resulting downforce was the same level as it was in 2015.

However, according to driver feedback from the field tests, when the wind was strong and other cars were driving in front, downforce was lost and the car became unstable and difficult to drive. After examination of the issue, it was decided to delay introduction of this new item even though it was very good numerically. Honda therefore reverted to the 2018 specifications to tackle the 2019 season. This particular episode demonstrated the difficulty with aerodynamic development.

Flick boxes developed for the 2019 season

Complex shape with three fins provided above a triple-stepped canard

2018 specification flick boxes