Tech Draft: Rain, wet tyres and turbulent wake

wet spa- belgian grand prix

It is quite understandable to be confused about why getting the very wet 2021 FIA Belgian Formula 1 Grand Prix at Spa-Francorchamps underway on Sunday was near impossible.

After all, motor racing events are held in the rain regularly and have been for decades.

Nevertheless, what transpired at Stavelot was an unusual set of circumstances that made the circumstances unsafe and the perfect storm, if you pardon the pun.

At no point during the period from 1500 through to the point at which the race result was declared was the rain intensity level so high that the circuit was unable to drain effectively, nor to such a level that standing water had accumulated making the risk of aquaplaning dangerously high.

However, it was wet enough that the spray, primarily from the rear tyres of the cars, even at the low speeds they were travelling at whilst under the control of the safety car, was settling at a slower rate because of the still conditions, particularly in the more sheltered areas such as the stretch along Kemmel from Eau Rouge through to Les Combes.

Additionally, combined with the water on the track surface, the ambient temperature of 12-13°C and track temperature of 18°C not only would have made it difficult to generate adequate operating temperatures in the braking and PU systems, but the low speeds required to safely navigate the spray whilst following another car would barely have generated enough energy to activate even the full wet compound.

By means of comparison, whilst a full dry slick F1 tyre offers 100% contact patch purchase with an operating contact surface temperature of approximately 100-120°C with no effective water clearing capacity, the intermediate will offer 75% purchase with a temperature range of 60-70°C and a 30 litre per second water clearing capability at 300 km/h, and the full wets a 65% purchase, 40-60°C temp range and 85 L/S at 300 km/h.

Even if the regulations did not mandate following the safety car in those circumstances on full wets, the track surface was sufficiently wet enough to prevent the intermediates from pumping enough water away from the contact patch and from generating enough energy to activate the tyre compound, even though the amount of spray generated would have provided better visibility for following cars.

It is true that F1 races have been held many times in the past in just as equally or even more atrociously wet conditions, but it is important to remember that the safety issue of F1 driver visibility in the wet is a function of the amount of water being thrown rearwards and the turbulent wake.

It is unfortunate that in the context of Sunday, F1 is in an era where the combination of a high drag flat bottomed rear diffused floor and high drag rear wing producing large turbulent wakes, coupled with the largest water clearing capacity full wets F1 has ever had, resulting in the highest and longest wet weather spray throw, or rooster tails in F1 history.

In theory, whilst there is no reason why the characteristics of the new 2022 larger diameter Pirelli tyres will be any different, in theory, a ground effect underfloor aerodynamic device should have less energetic and more laminar flow exiting rearwards, resulting in a lower and shorter spray throw.

We can only hope so.

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