The science F1 must defy to prevent Spa shambles repeat

As we look back at what was an extremely exciting 2021 Formula 1 season involving the closest inter-team competition that we have seen for many years, several events will stand out. Most of these will be positive as even the circuits that normally struggle to produce good racing seemed to throw off the mantle of dullness. Unfortunately, the one negative that will be discussed for some time to come is the events that occurred at Spa.

The circuit is renowned for wet weather, with prevailing westerly winds picking up moisture as they transit the sea and then meeting the hills of the Ardennes, causing them to drop that moisture as rain. We have raced at Spa for years and always dealt with it, but last year the sheer intensity caused the race to be effectively abandoned.

So what are the complexities of racing in the rain? It comes down to two things: aquaplaning and visibility. Grip is reduced considerably, but this in itself is not a problem. The drivers adjust and it actually increases the differentiation between the very best drivers and the rest of the field.

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Drivers dread aquaplaning as they have no control over it. For a given tyre it is a function of speed and depth of water. Below a critical speed, a tyre will clear water and above it the tyre will ride up on a wedge of water causing a total loss of grip. Tread design is secondary to these other factors in determining the propensity to aquaplaning.

Visibility is a much more complex problem, as it involves not just the tyre but also the aerodynamics of the car. Like aquaplaning,
 it is really only something of concern when on the full wet tyre. The intermediate tyre, by definition, is dealing with much less 
severe conditions and not ones that generally concern the drivers.

A full wet tyre displaces an enormous amount of water. At 155mph, a front tyre displaces 50,000cc of water per second; a rear tyre displaces 70,000cc per second when running through water 2.5mm deep. To put that into context, an average bath is filled to around 100 litres, so the four tyres on an F1 car displace enough water to 
fill that bath in well under half a second.

Poor visibility resulting from wet weather racing is a serious problem for F1 to tackle

Photo by: Erik Junius

As a tyre rolls over the water on a track, there are several mechanisms of fluid flow in place. In front of the tyre, a proportion of the water is expelled forward of the tyre contact patch in the form of a bow wave. Side waves are also formed by the water that is squeezed out sideways from the tyre.

The side wave is a plume of water that encounters a wall of water at the edge of the contact patch which absorbs some of its energy but also changes its direction, throwing it upwards. The wall of water that is outside the area of the contact patch has now had this energy imparted to it and undergoes the same process.

The result is that the apparent displacement of water is actually much wider than the tyre footprint itself. Both the bow and the side waves start off with relatively large droplets, maybe 3-5mm in diameter, but when they interact they are broken into a fine mist that is carried aloft as spray. Spray droplets are typically well below 1mm in diameter and hence are more easily moved by the airflow around the car.

It may be that the 2022 cars, with reduced outwash, perform slightly better for following car visibility, but until we understand and can replicate the physics of this complex subject it is difficult to be sure

A further amount of water is actually picked up by the tread and ejected behind the tyre at an angle of around 30 degrees. The final contributor to the spray is a film of water that adheres to the tyre by capillary adhesion and is stripped from the tyre by the airflow as a very fine aerosol.

With an F1 car it is not just the tyres that are contributing to the spray. At a certain speed, the low pressure generated under the car by the floor and diffuser will be enough to overcome the surface tension that is holding the water on to the track and will suck it up into the rooster tail.

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We can understand that an enormous amount of water is lifted off the surface by the car, and what goes up must come down. Luckily a combination of crosswinds and natural outwash of flow from the cars will displace an amount of water sideways, leading to the racing line drying out – but the water will still be held off line and, if there is any banking on the track, can continue to flow into the path of the cars.

Aerodynamic pressures, in addition to tyres, also contribute to creating the spray effect

Photo by: Mark Sutton / Motorsport Images

At F1 we are undertaking some fundamental research into the formation and dispersion of spray in wet weather. It is a complex subject involving the simulation of what are termed ‘multi-phase’ flows – that is flows that have both a liquid and a gas component.

Fernando Alonso has told me his experience of LMP1 cars in the wet is much better and so I am sure, even without enclosing the wheels as is done in sportscars, we can do things to mitigate the visibility problem that exists in F1 today.

Our research will not be the matter of a moment, as some new techniques need to be evolved, but similar approaches have been employed in other industries. For example, it is important that an aircraft engine running on a wet runway does not ingest too much water and this is investigated by modelling as well as practical tests.

It may be that the 2022 cars, with reduced outwash, perform slightly better for following car visibility, but until we understand and can replicate the physics of this complex subject it is difficult to be sure. What I am sure of is that there is an answer to the problem and we will eventually make F1 considerably safer in the rain.

F1 is working on solutions to make wet races safer in future

Photo by: Mark Sutton / Motorsport Images