AUSTIN, Texas - Lewis Hamilton will have a new chassis for Sunday's US Formula 1 GP here because Mercedes discovered two large cracks in the car he was scheduled to drive.The 2008 champion, winner of the first race in Texas in 2012 for McLaren, has been off the pace of team mate Nico Rosberg in the last two races and finished only seventh in Abu Dhabi. Rosberg was third and was on the podium for the previous two races.Team principal Ross Brawn told the autosport.com website that Mercedes had been puzzled by the difference in performance and had decided to strip down Hamilton's car for a detailed analysis.'SHOCKING FORM'"We had a very good look at the car and we found that the chassis was quite badly damaged - more than we thought," he said. "We knew he had had some excursions over kerbs and things but when we started to strip everything the damage was pretty significant. It may even be at a level that was affecting the car's handling."Hamilton told reporters in Austin that his form since the end of the August 2013 break had been "shocking" and wondered if it was something to do with the direction taken in setting up the car. He said after Abu Dhabi, where he suffered a rear wishbone failure as he ran over a kerb in the final qualifying session, that the car felt like it had an extra 50kg on board."In the previous race the qualifying didn't help and it spiralled down from then," he said. I hope this weekend to put myself in a better position."