Lewis Hamilton has described the rear of Mercedes' new W12 as not "particularly great" which has left the team struggling to find its sweet spot.
Mercedes has ordinarily proven bulletproof in pre-season testing but this year at the Bahrain International Circuit the team has shown itself to be fallible.
Aside from a gearbox issue that confined Valtteri Bottas to the garage for almost all of the first morning session on Friday, the rear of the car has appeared to lack instability.
Whilst the wind has played its part, there have been several off-track moments, with Hamilton, in particular, spinning into the gravel during his Saturday morning session.
It is clear Mercedes has yet to fully exploit the new downforce regulations that have affected the floor and the rear.
Suggested to Hamilton the W12's rear still appeared to be sketchy, Hamilton said: "The wind did a 180 as the track is a lot different.
"In some places, you can push where you couldn't push yesterday, and it's very gusty, as I found out into turn 13 [where he spun].
"The rear doesn't feel particularly great with this new regulation change, and we're trying to find the sweet spot."
As to the overall feeling from the car, Hamilton added: "It pretty much felt the same, not really much different.
"The track feels a bit better, so the balance was [also] perhaps. Slightly better but we're still working through certain things."
Hamilton completed 58 laps, with his best a one minute 33.399secs, set on the hard compound. Overall, the Briton has managed exactly 100 laps across his two half days in the car.
"Compared to the past it's not been particularly useful," said Hamilton of his running to date. "I did 60 laps [58], something like that, so it's not a lot compared to other test days we've had, but we're trying to be as efficient as we can be.
"We have less mileage than some of the others so far, like the Red Bull. We're just trying to stick to our programme and manage the amount of laps we have."
Despite the problems encountered by Mercedes, Hamilton insists there is no cause for alarm yet.
"It's day two of testing so we're just focused on doing our job, so nowhere near [being worried] at the moment," insisted Hamilton.
"We're just focused on trying to understand the car. There's no point getting worried just yet. Everyone is focusing on their programme and that's what we are doing."
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