Technical F1 Unlocked
TECH TUESDAY: The ‘plank’ has been in F1 since 1994 – so why did Mercedes and Ferrari get caught out in Austin?
The disqualification of Lewis Hamilton and Charles Leclerc from their respective second and sixth places in the United States Grand Prix has brought the underbody plank into the frontline of F1 news for the first time since Michael Schumacher’s Benetton was excluded from victory at Spa 29 years ago. What is the plank for and why has it suddenly come back into the news after so long? To fully understand that requires the telling of some F1 history.
The compulsory fitment of the plank to the centre of the underfloor was part of a raft of reactions to the fatal accidents at Imola 1994 of Roland Ratzenberger and Ayrton Senna. A contributory cause of the Senna accident was believed to be the underbody airflow stalling as the car ran low on under-pressure tyres following several slow laps behind an under-powered Safety Car.
These were flat-bottom cars. F1 regulations had outlawed ground effect venturi floors and stipulated flat bottoms from 1983-2021, but even so ground effect could not be un-invented and the flat-bottom cars generated it by running the front of the floor low and the rear higher. As the air rushed to fill the tiny gap between the road and the front of the floor, so its pressure was reduced which increased its speed.
The lower the pressure, the greater the difference to the atmospheric pressure above and the harder the car was sucked into the ground. A diffuser at the back of the floor gave a big expansion chamber for the air, pulling it through that tiny gap at the front of the floor even faster. The principle is the same as the venturi-floor cars of 1977-82 and 2022-23, but they use their heavily-shaped tunnels either side of a flat central area to accelerate the air faster than a full flat bottom could – and thereby create more underbody downforce.
Whether it’s a flat-floor or a tunnel car, the part of the floor sitting closest to the ground is the ‘throat’ and it’s around that point that the car is being sucked down hardest, as that is where the lowest air pressure will be. Under the flat bottom regulations, that throat was at the front of the floor. With the tunnel cars, it’s towards the back just before the diffuser. The closer to the ground that throat point runs, the more downforce is created – and the increase is exponential in the last few millimetres.
So with the early flat bottom cars there was a competitive imperative to run the floor as low as possible at the front. But too low and it could stall completely, giving a dangerously sudden and big reduction in downforce. The underbody plank was stipulated in ’94 to counter this possibility. It ran up the centre of the underfloor and was 10mm deep, which meant that the gap at the front of the floor would in theory never close completely.
It reduced the amount of downforce it was possible to generate but it ensured the floor could not fully stall. Anticipating that teams would try to simply wear the plank away on the track so as to run as low as possible, the governing body stipulated that the plank should still be at least 9mm deep along its length if measured after the race. A very hard-wearing wood called ‘jabroc’ was used for these planks initially, but today they are made from a glass-reinforced plastic composite.
Schumacher was excluded from Spa ’94 when the front of the plank was found to be between 7-8mm deep after the race. The team appealed on the grounds that this was from kerb damage after a spin. Although the appeal was rejected, the incident led to a more robust way of mounting the plank, using metal mounting points at specified parts, so giving more protection to the plank in such incidents. This had the effect of switching off the plank problem immediately.
Furthermore, the way aerodynamic development proceeded led to designers seeking to increase the volume of air being fed to the underfloor and so that gap no longer needed to be quite so tiny. Other ways were found to accelerate the airflow harder without the on/off peakiness of the tiny gap – such as barge boards and enhanced ‘coke bottle body’ profiles which accelerated the air along the outer floor and converged it behind the diffuser, so pulling harder on the underbody airflow. As such, plank wear became something of a non-issue for decades.
But the reintroduction of venturi tunnelled ground effect cars in 2022 (as part of F1’s efforts to make a cleaner aerodynamic wake so the cars would be more raceable) brought the plank back into the reckoning. With the throat now towards the back of the floor – and the volume of underbody air no longer limited by the height of the front of the floor – teams were trying to run their cars as low as possible to the ground at the back to maximise how hard the tunnels were working and the plank was therefore potentially a limitation once again.
This might have reared its head last year except almost every team found they could not run their cars as low as they’d expected – because they ran into porpoising problems even at ride heights greater than those imposed by the 10mm plank. For every car except Red Bull, the plank was not usually the limitation in how low they could run; porpoising was.
Red Bull, with a floor design far more tolerant of very low ride heights and compliant rear suspension, generally averted the porpoising problem and from the start it could run lower than any other car without introducing any bouncing or porpoising. This was – and remains – a major part of its downforce advantage over the field. But that meant that for Red Bull, plank wear did become a potential issue, particularly at very fast or bumpy tracks when the car is fully compressed on its suspension.
So Red Bull became very aware of the need to ensure plank legality. With the increased downforce of this year’s RB19 that has been even more the case – and the team has clearly become very adept at predicting what the static ride heights need to be to minimise the plank grounding but retaining as much downforce as possible.
Other teams are only just now getting to the level of aerodynamic development where plank wear – and not porpoising – is the limitation, and this seems to be what has caught Mercedes and Ferrari out around the bumpy COTA track, with the Sprint format providing just a single practice session before parc ferme.
They were just a little optimistic with where they pitched that rear ride height and it did appear that the Red Bull was running higher at the back than either of those cars. Could that have contributed to Max Verstappen’s reduced level of dominance? The coming races should give a clearer answer to that question.