Lewis Hamilton says his Mercedes W12 was not quite underneath him in the way he wanted throughout both of Friday's practice sessions at the Belgian Grand Prix.
Hamilton was third fastest in the second practice session, just 0.072 seconds behind title rival Max Verstappen who crashed at the end of FP2.
"We were trying to navigate [through the day] the best way we can," said Hamilton. "At least it was dry for FP2 so we got some laps in.
"But the car wasn't quite underneath me today, so I've got to do some work tonight and try and figure that out."
Hamilton admits he is still trying to find the right compromise between running a low downforce or medium downforce rear wing.
The seven-time World Champion says he was either very quick in the first and last sector, or fast only in the twister middle sector at Spa-Francorchamps, depending on his rear wing level.
"It's a massively challenging circuit to try to find that balance," added Hamilton. "This morning, we had it one way. We changed it this afternoon. It was very, very similar [for the overall lap time].
"It's very bumpy now through Eau Rouge. Something's happened and they kind of ruined it a little bit. There's a massive bump right at the compression point which we've never had there before. It's really, really sharp and you feel it on the backside.
"But I'm sure they'll fix it, I think it has something to do the mudslides, or the rain."
Video: The secrets of the Spa-Francorchamps F1 circuit
Follow Rudy van Buren as he takes you on a flying lap around Spa-Francorchamps while explaining what makes the track so exciting.