Nikita Mazepin says he is driving a heavier car than the one raced more successfully by his high-profile teammate Mick Schumacher.
"I wouldn't say it's unacceptable, it just complicates my job," said the Russian rookie, whose Formula 1 debut this year has been tumultuous so far.
"It does have quite a big impact on the weight distribution and the setup, but I'm surrounded by great engineers and I'm sure the situation will get better."
However, he will not get a new and lighter chassis until after the summer break.
"When there are a lot of long straights and your car is heavier than the other one, it's difficult stay ahead," Mazepin insisted.
The explanation for Mazepin's heavier car is that the monocoque is older than the one used by Schumacher.
"It's a year old, and usually you do a new car every year. And sure, a chassis doesn't get lighter after a year," admitted team boss Gunther Steiner, referring to the fact that constant patch-up jobs add to the weight.
"But the difference is not gigantic. It's less than 20 kilos, or not even 10. Obviously a heavier car doesn't make you faster.
"Nikita will get a new chassis at Spa," he added.
The story may only now be breaking because of rumours that Mazepin's father Dmitry - the team's title sponsor via his company Uralkali - offered to pay more if Haas agrees to let his son drive Schumacher's car.
Auto Bild even claims that billionaire Mazepin snr would like Oliver Oakes - his son's manager - to take over from Steiner as Haas team boss.
When asked about his teammate's heavier car, Schumacher said in Austria: "You have to ask the team."
