Williams ran a radical rear wing on Tuesday morning as the year’s first in-season test got underway at the Barcelona circuit.
Test driver Alex Lynn ventured out on Tuesday morning with an additional rear wing mounted to the existing wing on his FW38.
The design, though, is not legal under F1’s current regulations.
Williams confirmed to F1i.com that they were running it in order “to gain a better understanding of the car aero balance over a lap.”
Meanwhile, Jenson Button, who was trialing minor updates to his Honda engine, was quickest on Tuesday morning, 0.010s up on Sebastian Vettel.
The McLaren driver, however, was using the super-soft tyres while the Ferrari driver put in his best time on the softs.
Nico Rosberg, who spent his 54 laps on the medium tyres, was third ahead of Lynn, Daniel Ricciardo and Manor’s Pascal Wehrlein.
1 Jenson Button McLaren 1:23.753 39 laps
2 Sebastian Vettel Ferrari 1:23.763 0.010 50 laps
3 Nico Rosberg Mercedes 1:25.099 1.346 54 laps
4 Alex Lynn Williams 1:26.071 2.318 41 laps
5 Daniel Ricciardo Red Bull 1:26.086 2.333 31 laps
6 Pascal Wehrlein Manor 1:26.464 2.711 50 laps
7 Esteban Ocon Renault 1:26.530 2.777 47 laps
8 Romain Grosjean Haas 1:26.669 2.916 35 laps
9 Pierre Gasly Toro Rosso 1:27.795 4.042 38 laps
10 Alfonso Celis Force India 1:28.570 4.817 34 laps
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