Porpoising explained by the architect of F1 2022's technical rules

A new word was on everyone’s lips after the first test of the radically new 2022 cars. That word was ‘porpoising’ – and actually it wasn’t that new, as the few of us still active in F1 who were also involved in the days of skirted ground effect cars remember the phenomenon well. In fact, this troublesome effect is not just historic, as anyone who has worked with contemporary LMP cars knows. Like the new generation of F1 cars, sportscars have, for some time, incorporated design features which make porpoising prevalent, and engineers of these cars know how to tame it.

But what exactly is porpoising? Many will say it is an aerodynamic effect, but that is only partly true. It is a fact that we only see porpoising when there is an element of ground effect aerodynamics in use, but F1 cars have been running in ground effect for many years. The front wing, for example, is significantly influenced by ground effect and, prior to the advent of high-rake cars over the last 10 years or so, it was common practice to run a set-up that stalled the diffuser at the low rideheights associated with high speeds in order to reduce drag.

So, if front wings and diffusers were running in ground effect why didn’t cars porpoise in the recent past? To answer this question we need to both understand the mechanism of the aerodynamics and also, from a systems engineering point of view, understand how the aerodynamics interact with the chassis dynamics.

Looking at the aerodynamics first, we can see that the 2022 cars are running the rear rideheights much lower than was the case previously. This is because the shaped underfloor is no longer a simple diffuser starting at the rear axle centre line, but is now a full-length device extending from the front of the sidepods to the rear of the floor. Additionally, until this year the outer lateral part of the floor was 50mm higher than the floor on the car centre line. This ‘step plane’ ensured there was little sealing of the floor to the ground. What sealing there was occurred due to the presence of vortices rather than physical bodywork. Now the edges of the floor are much lower and performance is rewarded by low rear rideheights. 

F1 teams are aiming for lower rideheights to increase performance with harming the airflow

Photo by: Davide Cavazza

As the rideheight lowers, which it does as speed increases and the enormous aerodynamic forces try to push the car into the ground, so too does the effective sealing increase. It is commonly thought that this leads to the airflow detaching from the floor due to stall, and to some extent this is true, but it is also true that at lower rideheights, the vortices formed by the strakes at the front of the floor can burst, thereby significantly reducing the flow through the underbody and hence the downforce.

PLUS: The mechanics behind porpoising in F1 - and how to fix it

We now have an unstable system. As speed increases, the car squats lower to the ground and downforce increases at a rate greater than would be predicted by the speed increase alone. It then reaches a critical speed at which the aerodynamics can no longer ‘hang on’ and downforce decreases causing the car to rise slightly. As it rises, the aerodynamics become stable again and start to suck the car down.

This movement is of course linked to the vertical stiffness of the car. If the car had no springs and the tyres were infinitely rigid then, as the speed increased, the downforce would increase proportional to the square of the speed as simple fluid dynamics theory would predict.

Designers are struggling to get cars to the weight limit as a huge amount of stiffness needs to be built into the floor. This stiffness can only be achieved by a substantial, weighty, floor construction. Those who did not pay enough attention to this will suffer worse porpoising

Even stiffly sprung F1 cars are far from rigid, however. They have two fundamental frequencies of vertical motion. The first is the bounce frequency associated with pure vertical motion of the car. This occurs typically around four or five hertz. The second is the pitch frequency associated with the front of the car going down as the rear rises and vice-versa. This is at a higher frequency, maybe up to seven hertz.

If we examine the frequency of bouncing of the cars, which we can do even without access to data by using a stopwatch and examination of video, we see the frequency of porpoising is different on different cars but lies typically between 4.8-5.4 Hz. In other words, the aerodynamics are coupled to the suspension frequency.

Mercedes has been one of the worst teams hit by porpoising

Photo by: Andy Hone / Motorsport Images

So what determines the suspension frequency? The simple answer is: for a car of a given mass, the stiffer the system the higher the frequency. Due to the requirement to run closer to the ground, the stiffness of the suspension springs is much greater. Additionally, the vertical stiffness of the low profile 18” tyres is higher, although not by as much as one might think.

All this assumes that the bodywork itself is rigid and it is here that some teams have specific problems. With the trend to waist the bodywork in as much as possible between the rear wheels, a significant area of the floor is left less well supported than one would like. It is noteworthy that designers are struggling to get their cars to the weight limit and part of the reason for this is a huge amount of stiffness needs to be built into the floor in this area, where it is cantilevered out from the engine and gearbox. This stiffness can only be achieved by a substantial, weighty, floor construction. Those who did not pay enough attention to this will suffer worse porpoising.

A very late change to the regulations to allow a further floor stay to be put in this area allowed stiffness to be increased with a minimal weight penalty. Helpful for those who had not achieved the required stiffness, but annoying for those who built heavy and substantial floors.

F1 floor stays have become a common sight after a late regulation change

Photo by: Giorgio Piola