Aerodynamic effects of tyre deformation
The aerodynamics on a Formula One car are refined to the nth degree, with engineers chasing the smallest gains to trim an extra hundredth of a second off lap times. As a result, no stone is left unturned in the quest for improvements.
Being open-wheeled vehicles, the tyres of a Formula One car play a major role in its aero performance and, as such, understanding their impact here is an important part of optimising the car’s overall aero package. Tyres are dynamic entities though, so their shape and size changes when loaded by the car on track. Along the straights, they will grow in diameter and get narrower; in corners, vertical and horizontal loads will cause them to deform laterally.
To ensure that the impact of this deformation is accounted for in aerodynamic testing – both in CFD and the wind tunnel – requires some clever methodologies. Most Formula One teams keep such testing secrets close to their chest; however Honda has revealed some of the techniques it used during its last stint in the sport.
For scale-model wind tunnel testing, scale rubber tyres are used to replicate the behaviour of their full-scale counterparts. In most wind tunnels though, it is only possible to apply vertical forces to the tyres, and perhaps a small amount of side force by yawing the car on the wind tunnel’s moving belt. To combat this problem, Honda devised various testing methods based on deformation measurements taken from its cars on track. This data could then be fed into its CFD simulations to gauge the impact of this deformation on overall aerodynamic performance.
The actual data for deformation was gathered using a tyre test rig, with the inputs for side loadings derived from load cell measurements of the car’s suspension when on track. The measurement of the deformation was achieved by scanning the tyres as they were on the test rig, and using this information to create 3D models of the tyres. These models could then be fed in to Honda’s CFD simulations. As an example of the level of deformation present, a side loading of 7000 Nm on the tyres’ tread caused a deflection of the side wall of about 20 mm.
The impact of this deformation on the flows around the tyre were considerable. Looking at the front tyres, CFD simulation showed that as the tyre deformed, the separation point of the flow at the base of the tyre sidewall moved backwards. As a result, flow that moved around the tyre when it was not deformed, started to flow under the car, reducing the effectiveness of the underfloor aerodynamics.
With this new-found knowledge, Honda went on to replicate the tyre deformations as accurately as possible in the wind tunnel, using both scale and full-sized vehicles. It was verified, through force measurements and PIV (particle image velocimetry) visualisation, that the changes to the flow in CFD correlated with the real-world effects. It is interesting to note that one method suggested by Honda to more accurately replicate tyre deformation in the wind tunnel involved fitting a roller inside the test wheels that could be used to load the sidewall of the tyre to produce the same deformation as seen on track.
It is more than likely that current teams have various other methodologies for assessing problems such as tyre deformation, no doubt helped by advances in CFD and other simulation methods. However, Honda’s study provides an interesting example of the amount of effort needed to accurately quantify just one area of a Formula One car’s overall aero behaviour.
Written by Lawrence Butcher