Tyre Pattern Features and their Effects on Passenger Vehicle Drag

In light of the drive for energy efficiency and low CO 2 emissions, extensive research is performed to reduce vehicle aerodynamic drag. The wheels are relatively shielded from the main flow compared to the exterior of the passenger car, however, they are typically responsible for around 25% of the overall vehicle drag. This contribution is large as the wheels and tyres protrude into the flow and change the flow structure around the vehicle underbody. Given that the tyre is the first part of the wheel to get in contact with the oncoming flow, its shape and features have a significant impact on the flow pattern that develops. This study aims at identifying the general effects of two main tyre features, the longitudinal rain grooves and lateral pattern grooves, using both CFD and wind tunnel tests. This is performed by cutting generic representations of these details into identical slick tyres. Combinations of the two resulted in four physical tyre patterns that are tested on both a production and a closed rim. The test setup is reproduced in CFD taking the tyre deformation under loading into account. Due to the tyre's deformation, Moving Reference Frame - grooves (MRFg) was used to model rotation, while the rim spokes were modelled with the sliding mesh approach. The results indicate that the rain grooves play a significant role in reducing drag when introduced on a slick tyre both in test and simulations, while the results from adding lateral grooves were less consistent dependent on the rim-tyre combination. The interaction between the longitudinal and lateral grooves could be observed on the overall vehicle drag. In general, CFD is able to predict the drag changes for different tyre patterns with good accuracy for the open rim, however the closed rim case proved to be more challenging.