In recent weeks much attention has been given to the phenomenon causing F1 cars to bounce down the straights, known colloquially as ‘porpoising’. Behind the scenes, teams of experienced aerodynamicists are working quickly to find solutions. Many have asked: how did they miss this one?
With the growing accessibility of basic CFD testing, more people are trying their hand at vehicle aerodynamic development; accordingly, this is an opportunity to discuss some of the critical points in aerodynamic design. The concepts involved are important both in and outside of Formula One. I hope this article will be of interest to those of you who are enjoying aero as a hobby and serve as an interesting read for those who follow it as a curiosity.
To introduce myself, I’m the director of AMB Aero , an aerodynamics consulting firm based in Sapporo, Japan. We utilize both in-house 40 DP-TFLOP super-compute CFD and physical wind tunnel test facilities. My mentor and partner Yoshi Suzuka was a major contributor to the modern understanding of underbody tunnels. His work shaped them from the early F1 style ‘underbody wings’ to what would later become known as the modern ‘venturi’ style during the development of Nissan GTP cars. At AMB Aero, we have been fortunate enough to develop all sorts of cars – especially many race cars – all over the world. Thanks to classes like IndyCar, LMP, or even Hill Climb and Time Attack that never fell victim to the ‘flat floor dark ages’. We have completed more than ten thousand CFD and wind tunnel tests for tunnel cars alone. By way of relevant contrast, we also have completed work more recently in modern F1, which gives an interesting perspective from which to observe current F1 performance trends. This said I would encourage anyone to view our perspective objectively, and sincerely welcome and enjoy hearing comments, questions, or even enlightening fresh ideas in the comments below from anyone (whether inside or outside F1).
‘Ground effect’ was discovered by accident.
In the late 1970s Lotus first discovered this phenomenon when their wind tunnel model was not sufficiently rigid resulting in the side pods sagging closer to the ground. As they did, downforce increased sharply. Lotus engineers then sought to understand this phenomenon and in doing so determined to close the sides of the floor, leading to the now-famous ‘sliding skirts’ (that were subsequently banned).
So powerful was this ride height effect that after banning skirts, a car was run with completely solid suspension. In this infamous test, the driver commented that the car was quicker set low and solid, but he struggled with vision and vibration. The driver asked for a padded seat and was jokingly suggested to sit on his wallet. Fast forward many years later and even without tunnels, Formula One and many other disciplines have far exceeded the total downforce of F1 in those days. The concept of designing-in ride height sensitivity became deeply rooted in modern racecar aerodynamics. Cars are generally run close to the ground to maximize underbody downforce (among other reasons), and accordingly – much as Lotus found in the 70’s – it is incumbent on designers to manage this phenomenon towards a net performance advantage.
Lotus’s early “ground effect tunnels” a simple side pod wing shape. ref: Giorgia Piola via motorsport.com
The oldest known (to me) concept of an under-wing, dated 1928!
“Ground effect” is a dated term.
In the early days, it was believed that the effect acted similarly to the known behavior of airplane wings wherein lift increases with proximity to the ground. You may have even felt this during a landing as if the plane is somewhat ‘cushioned’ in late descent with the pilot adjusting controls until touchdown. It was thought that we were experiencing the same phenomenon with downforce-generating (inverse) wings with downforce increasing with ground proximity. [https://en.wikipedia.org/wiki/Ground_effect_(aerodynamics)]
Many years later we would come to understand some differences with ground vehicles and that the proximity of underbody wing forms is not the major cause of extreme sensitivity to ride height. At AMB Aero we call them underbody tunnels. In our experience, this sensitivity is primarily due to changing flow field resulting from the relative position between the tire and the sprung body of the car. In the case of Formula cars, this can be conceptualized as the position above vs below the floor. Or for a closed wheel car, a tire is more or less shrouded inside of a wheelhouse.
The difference in tire exposure to underbody between high and low rear ride height. A diffuser era car compresses with speed, effectively pivoting around the forward edge of the plank ref: f1technical.net