Mystery or Magic? The Science Behind Porpoising In F1 Today

Aerodynamic solutions.
Modern racecar aerodynamics – particularly in Formula 1 are complex systems; without looking in detail at CFD or wind tunnel data it would be impossible to definitively know the probable causes of a specific car.  It could be that Ferrari’s wide side pods help produce higher-energy and better-directed flow into the tunnel throat.  Mercedes uses a narrow side pod, perhaps this leaves them heavily dependent on a rear beam wing being more vulnerable to rear ride height effects. Or the ‘floor sealing’ vortices could be at fault as many have suggested, but I can see that happening in both the way suggested and the complete opposite.

For example, let’s say you made the ‘floor sealing vortices’ less effective, introducing more turbulent flows to the diffuser at low ride height. You may have stabilized a situation between low and high ride height performance: what was once an overly-aggressive geometry for low ride height now continues to function without decay. Conversely, you could concentrate on the floor sealing vortices being more effective at high rear ride height. That will increase performance at high ride but also converge the optimum geometry between high and low ride.  Complementing strong performance at low ride again reduces a relative performance decay between the two kinematic states.  So both “better” or “worse” floor sealing vortices can stabilize behaviors across ride heights depending on how you define them. Which choice will result in a higher or lower performing car overall might follow a trend, but there are no absolutes given the complexity of a total package.

In not being a fly on the wall inside all the teams, I am left with more questions than answers.  Each team will map out their best ideas and test them until they find the least compromised solution.  Looking at the changes made up until race day or into the first few races may provide further clues.

Often in practice, you don’t need to change the tunnel behavior necessarily.  A modified geometry on any part of the car could potentially change the performance trend.  The outboard rear floor edge for example could be designed to produce more downforce on the top side with lower ride height.  Many other examples exist, if you can stabilize the total forces on the suspension through the vehicle’s attitudes of interest, you solve the problem.  As the challenge involves solving for a range of downforce outcomes across many vehicle attitudes the best overall solution can sometimes be a compromise between various aerodynamic systems and surfaces producing more or less downforce as needed to allow flows through the tunnels to be optimized across relevant vehicle conditions.

Heave stiffness, the nuclear option.
Of course, you can avoid all this by simply cranking up the suspension heave (vertical spring) stiffness until the car no longer gets close enough to the ground.  This, however, comes with a cost, the added stiffness will likely compromise performance in other parts of the track. Additional heave increases the propensity for sudden loads into the tires reducing grip, shortening tire life, increasing tire failure risk, increasing tire temperatures, and more drag.  In addition to all this, the heave spring will be at a progressive rate and so damping is a compromise between low and high rates.  Leaving the car under-damped and more prone to porpoising without even further handling compromises.

At the top end of motorsport, a team of engineers are pouring over aero maps, simulations, and making tiny adjustments to milk the aero map utilization.  These small gains in downforce can have an enormous impact on lap time against the rest of the field in a sport this dependent on aero.  Putting the car into a constrained attitude mechanically just to survive down the straight is not an easy pill to swallow: best compromises in vertical spring forces are so important that modern F1 suspensions employ a dedicated heave spring.

Illustration of an F1 suspension, the heave spring controls the vertical stiffness of the rear axle ref: Craig Scarborough

The difficulty of replicating low ride heights during development.
Whilst every team likely foresaw this risk some may have under-estimated it. At any rate, there exists some difficulty in replicating the situation during aerodynamic development.

A Formula One car on the circuit will often come into contact with the ground on the straight.  The rest of the time near top speed it will be very close.  The better job you do at walking this fine line, the quicker you go.  This is difficult to replicate in aerodynamic testing.  The central skid or plank is the absolute limit to minimum ride height.  That also means that where many other motorsports must control their ride height to keep from destroying the aero, an F1 car will impact the plank and survive. Sparks flying are an indication that the car has made contact with the ground.

F1 plank made of a highly compressed wood material ref: wikimedia commons

Modern wind tunnels utilize a moving belt floor and a model suspended from the roof by a device called a sting. The rotating tires are supported from the sides or by carefully designed attachments to the car that isolate it from the belt vibrations. The moving belt floor below the car is spun by large drums. The model of the car is equipped with extremely sensitive instrumentation both inside the model and in the sting.

Concept F1 tunnel car in the wind tunnel ref: F1.com