CFD development for the Hadron Aero wheel line expansion.
Following the success of the Hadron 625 development, Swiss Side has taken to expand their aero wheel line. The target was to develop additional models to maximize aerodynamic performance over the complete spectrum of wind conditions and rider requirements seen in the real world.
Although the rider is responsible for the majority of drag, it’s the wheel aerodynamics, in particular, the stability which can have the biggest influence on reducing the rider drag.
Since the rider is responsible for 75-80% of the total drag, producing aerodynamically stable wheels, that allow the rider to keep in the aero position for the highest proportion of time is the most important factor. Hence, based on the wind conditions and the individual riders’ personal requirements, a range of wheels is necessary.
The Hadron 625 was developed to be the most broad reaching and versatile wheel offering extremely low drag with a heavy focus on maintaining low side force and steering torque sensitivity in windy conditions. The Hadron 625 was not designed to achieve absolute minimum drag levels on the wheel but minimum total drag on the entire system in windy conditions. However, the requirements can differ, not only, depending on the wind conditions but also based on the rider needs. For completely wind-still conditions or for riders experienced with handling deep section wheels, a deeper profile section can achieve even lower drag levels (relative to the Hadron 625) if less weighting is given to the side force and steering torque sensitivity performance parameters. Similarly, for high wind conditions or for riders who are very sensitive to the steering torque response (e.g. lighter weight riders), a shallower section profile can result in even further reduced steering torque sensitivity and side force in exchange for a small drag increase.
For developing the new shallow and deep section Hadron wheel profiles, the Swiss Side aerodynamics team, once again, undertook an extensive CFD program, applying the same pioneering simulation and mathematical prediction methods, which the team brings from their vast experience in Formula 1 racing engineering.
The complete details surrounding the Swiss Side CFD development methods can be found in the previously published reports: ‘Update 4’ and ‘Update 7′
Shallow & Deep Section Rim Development Process:
In the first step, the key geometric parameter ranges were defined for investigation. For the shallow profile, the chosen depth range was 35 – 50mm and for the deep profile, 70 – 90mm. In the second step, the weighting systems defining the overall performance were defined for each the shallow and deep section profiles. As each design is simulated over a broad range of yaw (cross-wind) angles, the weighting system is defined to give a single performance figure over the complete yaw range. Following this, using a combination of iterative CFD simulations combined with mathematical prediction methods, the frontiers of possible performance were mapped. The key performance measures considered for mapping these ‘development windows’ are the drag and steering torque.
Building on the Hadron 625 project, the following graph shows the ‘potential performance windows’ resulting from this extensive simulation process. The Hadron 625 window (shown in red) was already presented in ‘Update 7’ and serves as the reference for the new profile development. The deep section profile performance window is shown in green and that for the shallow section profile in blue.
The final step was to define the performance targets in order to determine which point within each performance window offered the best overall performance. Once the point of interest was selected, finer iterations and further sensitivity checks (for tyre shape for example) were conducted around these target points to attain the final optimized designs.
Results & Visualisation:
Shallow Section Rim:
The target for the shallow section rim was to offer a further step in reducing steering torque and side force compared to the Hadron 625, whilst minimizing the associated drag increase. The ‘potential performance window’ showed that this target was achievable. Note however, that the absolute lowest possible drag solution was not the one selected, as the target was to further reduce the steering torque relative to the Hadron 625.
Based on the overall performance targets defined by the Swiss Side aero team and final optimization iterations, the resultant best configuration for the shallow section aero wheel delivered a profile depth of 48.5mm, to be known as the Hadron 485.
Deep Section Rim:
The desired step in drag reduction was achieved for the deep section wheel relative to the HADRON 625. However, it was clear from the start that to achieve the lowest possible drag, the magnitude of steering torque and side force was going to extend far beyond the original threshold set for the Hadron 625 wheel. This is ultimately the key tradeoff of deep section aero wheels against their shallower counterparts. This is clearly visible in the ‘potential performance windows’, with the significant step in steering moment magnitude. However, it’s important to mention that as part of the sensitivity checking, the steering torque ‘characteristic’ with yaw angle, is also closely surveyed to ensure that a desirable, aerodynamically stable response is maintained. Note again, that the absolute lowest drag solution was not the one selected. It would have been possible to further reduce the wheel drag slightly but for an unacceptable compromise in steering torque and stability.
Based on the overall performance targets defined by the Swiss Side aero team and final optimization iterations, the resultant best configuration for the deep section aero wheel delivered a profile depth of 80mm for the front wheel and 85mm for the rear wheel, to be known as the Hadron 800+.
Prototypes of the Hadron 485 and 800+ wheel sets would be produced for wind tunnel evaluation. CFD yaw scans for these final configurations are shown in the following results. The Hadron 625 CFD data is always included for reference. Note that accurately predicting the stall point in CFD is very challenging and is not completely representative of the reality. Experience comes into play in this respect when interpreting the results. In reality, the stall point is dictated largely by tyre effects on the laminar to turbulent boundary layer transition. The CFD methods used here assume a purely turbulent boundary layer model. Therefore the CFD results are used to predict trends and characteristics, rather than defining absolute performance figures.