Robert Kubica says his Williams is a "completely different" car than the FW42 allocated to George Russell despite the two drivers running identical set-ups.
Bahrain confirmed the depressed pace of Williams' 2019 contender with Russell and Kubica making up the rear by a wide margin in Friday's practice sessions.
However, Kubica revealed that a comparative analysis of data pointed to different aero characteristics between the Polish drivers' chassis and the FW42 allocated to Russell.
"At least we understood that in Australia I ran a different set-up than George and we could clearly see the different characteristics in the cars," Kubica said.
"Here we started with the same set-up and the characteristic is still different.
"I thought in Australia that it was maybe caused by some damages to the floor, and maybe a different set-up, but at least after the first session here we got an answer.
"It's clear that we are driving two different cars with the same set-up, so that is something we need to understand. It's quite obvious on the data. So now we need to try and find the reason."
Kubica's first conclusion was that his damaged floor was perhaps the source of the aero discrepancy, but subsequent repairs eliminated that theory.
"The floor is in a good state here, and we though we are using the same baseline set-up as the other car we have different characteristics, it is quite significant," he added.
"Maybe it is something within the aero which is upsetting a lot the handling of the car and the general grip of the car. So we have to wait and see and try to find the answers."
Formula 1's returnee concluded Friday's evening session over one second adrift of his team mate, a deficit that did nothing to improve Kubica's confidence.
"It's looking strange, to be honest, and it's putting me in a very difficult position to drive the car," he lamented.
"Additionally, if I try to follow it up with some balance shift I have to reduce a lot the grip, and the potential of the car.
"Neither way is good. I make it more drivable, I am slow. I make it let's say as it should be and it's undriveable. It's a very, very difficult situation.
"I just hope that we will be able to solve it and if we are able to solve it will probably be the first time I am driving the car with better potential."
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