Kamui Kobayashi says diffuser rule changes hurt Sauber in second half of 2011

The Hinwil-based team started the year strongly, but the FIA banned off-throttle diffusers from the British Grand Prix, and Sauber decided to stop its development.

The ruling body then backtracked and returned to the original regulations, leaving Sauber on the back foot, according to Kobayashi.

"We had a promising winter testing and a strong start to the season," said Kobayashi. "In the beginning we had every reason to be happy with our performance.

"Later, I would say from the British Grand Prix onwards, which was the ninth out of 19 races, we struggled due to the decisions made about the rules and our own technical development.

"We didn't follow the direction of the 'off-throttle exhaust blown diffuser' any further. Instead we worked hard on the direction of development we had decided to go in, but we could not make up for the disadvantage that came from not having the same technology as the other teams.

"I can be happy that I still scored points in the final two races. This was a good achievement under the circumstances."

The Japanese driver had to assume the role of leading driver at Sauber this year, as he was partnered with rookie Sergio Perez this year.

Kobayashi conceded it was a challenging year for him, but he feels he will be stronger next year as as result.

"Certainly it wasn't easy for me. I needed to improve a lot in various areas," he said. "Not only in driving, but also in making decisions with the engineers. The fact that we had to deal with the new tyres from Pirelli, which were different in character compared to what we had before, was challenging for me.

"It was important to find out what was best for the race and how to handle qualifying. There was a big difference in performance between Saturday and Sunday. I think I came to understand many things during this season, and I have got a clearer picture for next year."

Kobayashi finished the season in 12th place with 30 points.