MPH: Diamond in the rough - is this why Red Bull was fastest by far?

Sepang last weekend saw the roughest track surface of the season by far. That was significant for much more than just tyre behaviour. It’s not widely appreciated that how rough or smooth a track surface is has a major impact upon the aerodynamic performance of the car. Especially under these technical regulations. At Sepang – over a surface which had deteriorated a lot since it last had F1 cars running over it in 2017 – untangling and controlling the aero effects of that surface almost certainly had a major bearing on the competitive order.

There are any number of parameters determining the competitive hierarchy between the cars from track-to-track and the relative importance of each parameter varies according to the venue. At Sepang, track roughness became one of the most important of those parameters, all the more so because no-one had anticipated it accurately and so there would be a greater variance between the best and worst simulations than usual.

It’s fair to say that the roughness of the track took the teams by surprise when they arrived here. Pre-event simulations were essentially obsolete and so much of the intense preparation at the factories went out the window, to be replaced by an on-the-hoof analysis and simulation.

“It’s ridiculous how sensitive these cars are here to the slightest change you make,” said Lewis Hamilton after qualifying his Ferrari on the front row following an initial struggle to get a good balance. “You make the smallest change and it’s ridiculous the difference it can make from one run, one session, to the other. One click of front wing, or one millimetre ride height, a quarter of a degree of camber, or 0.1-degree of toe [angle], or something like that. These little things make such a big difference to balance, and it’s literally a moving target that you’re trying to work around with set-up and the [cockpit] controls that we have.” Track roughness is a major reason for that extreme sensitivity.

Just how the aerodynamics of a car could be so disturbed by the granular surface of a track probably isn’t immediately obvious. But drilling down into that phenomenon is illuminating.

There are at least two mechanisms at work here. One is about how much airflow the underfloor receives. The other is about the dynamics of that airflow – ie how it behaves. The former is crucially important to the car’s grip; the latter to its grip balance front to rear.

The greater the volume of air you can feed through that underbody and the faster you can make it flow, the greater the downforce. So, how does a difference in track roughness change how much airflow the underbody receives? It’s all about something called the boundary layer of the airflow. The boundary layer is the layer of slow-moving air which attaches itself to the surface of the car (in this case, the floor and the track surface). When a fluid (such as air) encounters a solid surface (such as the underfloor), the molecules of air stick, but subsequent molecules glide over the stationary molecules. Up to a point (the boundary layer), the further out from the surface, the faster the air moves.

The fast-moving air creating the downforce slips past the slow boundary layer. The thickness of the boundary layer balloons with speed – and with the roughness of the track surface (because of how disturbed the rough surface makes the air molecules). The thicker that boundary layer, the less air volume can be fed through the underbody. By a factor way bigger than the actual physical difference between a smooth and rough surface.

The movement of any fast-moving fluid (and air is considered a fluid by nature) greatly amplifies any natural effects in that they are not arithmetical but typically exponential. In other words, a small difference in track roughness equals a much bigger difference in underfloor airflow volume. In turn, a small difference in airflow volume equals a much bigger difference in downforce. You can appreciate how the process is feeding upon itself.

So much for the surface’s impact upon aero grip. How does it affect balance? That seems even less obvious. This is where the ’26 generation of flat-bottom car is much more susceptible to surface roughness than the previous ground effect car. The ground effect cars featured tunnels either side of the central plank area of the floor, which manipulated the air pressure spectacularly. It was like having two downforce-generating engines within the floor and the air pressure difference between there and the rest of the floor was immense, allowing the tunnels to effectively suck the air through. The air knew exactly where it was going. It had little choice.

With a flat-bottom car, that’s less the case. Now, when the car rolls, pitches, dives, slows, accelerates, it has a much bigger impact upon the distribution of airflow no longer being forced through low-pressure tunnels. Combine this with how a rough track surface tends to induce vertical oscillations in the suspension, which can break the aero seal of the floor, and the airflow can go on the wander. When it does so, the whole aero balance of the car changes, sometimes quite suddenly. Rapid aero balance shifts are a nightmare for the driver. So teams now have aero simulation programmes for track roughness. The results of those simulations have a major bearing on set-ups.

A specialist F1 aerodynamicist might argue about the precision of some of the words used for the purposes of communicating with an interested layman, but the above is broadly factual; that is just how the aero works related to track surface roughness and it applies to everyone. Now let’s take that knowledge and use it for informed speculation.

Red Bull. A troubled car, but the RB22 is finally hitting form. One of its key problems has been what Max Verstappen describes as ‘car degradation’. It has not been able to retain its peak aero performance over more than a few laps, a quite distinctive problem. Many of its updates have been concerned with trying to prevent excessive movement of key components from breaking the aerodynamic seal around, for example, the brake drums and the connection between the front of the floor and the car’s monocoque.

Earlier in the season, downforce-inducing winglets were even removed from the rear brake ducts in an attempt at reducing the load upon them, trying to restrict their movement so that the important aero seal wasn’t broken.

What if a crucial part of the concept of the car is to retain it in a tiny window of ride height, so as to keep the aero platform from being too disturbed by track surface roughness? This has required super-stiff suspension. Which would make it poor over kerbs (it is) and potentially induce component-damaging vibrations. But much better than others on a rough track surface. And on the roughest track of the season, it’s suddenly the fastest car on the grid. By far.

“We are puzzled a little by the fact that in high speed we are no longer the best car,” said Andrea Stella of the McLaren in Sepang, “like we seemed to be in Madrid or Zandvoort. So there are some aspects whereby those who are doing a better job in development seem to be taking a performance advantage. The way in which it unfolds is quite complicated.